I. INTRODUCTION JOURNAL OF CHEMICAL PHYSICS VOLUME 115, NUMBER 1 1 JULY 2001

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1 JOURNAL OF CHEMICAL PHYSICS VOLUME 1, NUMBER 1 1 JULY 2001 Phtdissciating methyl vinyl ether t calibrate O ethylene prduct branching and t test prpensity rules fr prduct channel electrnic accessibility M. L. Mrtn, D. E. Szpunar, and L. J. Butler The James Franck Institute and Department f Chemistry, The University f Chicag, Chicag, Illinis Received 16 January 2001; accepted 13 March 2001 These experiments investigate the phtdissciatin f methyl vinyl ether at 193 nm in a crssed laser-mlecular beam apparatus. We bserve tw C O bnd fissin channels, a minr channel prducing CH 3 CH 2 CHO (X 2A ) and the majr channel yielding CH 3 CH 2 CHO (à 2 A ). Sme f the neutral à state vinxy prduct underges secndary dissciatin t prduce ketene H. These experiments n the phtdissciatin f methyl vinyl ether, which prduce nascent vinxy and methyl radicals cleanly in a ne-t-ne rati, serve tw purpses. First, using the measured phtfragment velcities and prduct branching we calibrate the relative sensitivity f mass spectrmetric detectin t the methyl and vinxy plyatmic radical prducts at the m/e daughter in, taking int accunt the lss f neutral vinxy t ketene H frmatin. This relative mass spectrmetric sensitivity calibratin factr is determined t be Knwledge f this factr allws us t extract the prduct branching rati between the tw majr cmpeting primary prduct channels frm the O( 3 P) ethylene reactin, a branching rati that has been much disputed in the literature. Our results give a CH 3 HCO/H CH 2 CHO prduct branching rati f R , crrespnding t 38% 5% branching t the CH 3 HCO channel. Secnd, we use the result that the channel prducing Ã-state vinxy dminates ver the frmatin f grund-state vinxy t test prpensity rules being develped t help predict what prduct channels may be suppressed by electrnically nnadiabatic effects in chemical reactins. These prpensity rules discriminate between channels that are electrnically facile and electrnically difficult/ prhibitive. We find that a qualitative analysis f the changes in electrnic cnfiguratin alng the reactin crdinates fr the phtdissciatin f methyl vinyl ether crrectly predicts the dminant channel t be the prductin f excited state (Ã) vinxy, the electrnically facile channel American Institute f Physics. DOI: / I. INTRODUCTION This paper reprts a crssed laser-mlecular beam phtfragment scattering experiment n methyl vinyl ether at nm, a study mtivated in part by the need t calibrate prduct branching in the thermal O ethylene reactin. This bimlecular reactin is f imprtance nt nly in the cmbustin f ethylene, but als in the cmbustin f methane and f larger aliphatic hydrcarbns and armatics where ethylene is a key intermediate. Prir mlecular beam reactive scattering experiments 1 by Lee and c-wrkers n the O( 3 P) ethylene reactin detected tw majr prduct channels cnsistent with mst prir wrk, 2 11 CH 3 CHO and CH 2 CHO vinxy H, but met with cnsiderable bstacles in determining the branching between thse tw channels. The reprted branching rati, derived frm the relative signals frm daughter ins at m/e 30 frm HC 18 O and m/e frm CH 2 CHO and using semiempirical estimates f the prducts inizatin crss sectins, did nt agree with that determined using the signal attributed t the methyl and vinxy prducts at m/e. The experiments reprted in this paper vercme these bstacles, using the phtdissciatin f CH 2 vchoch 3 t give CH 3 CH 2 CHO t calibrate the relative sensitivity f mass spectrmetric detectin f the methyl and vinxy plyatmic radical prducts at the m/e daughter in. Determinatin f this calibratin factr allws us t extract the branching rati between the tw majr cmpeting primary prduct channels in the prir wrk n the O( 3 P) ethylene reactin frm the relative signals frm these tw channels at m/e with a methd that in principle circumvents the need t estimate inizatin crss sectins and determine daughter in cracking patterns. This is the first time this methdlgy has been applied t determining prduct branching in a thermal bimlecular reactin, althugh its analg 12,13 has prven useful in a variety f phtdissciatin reactins. In general we expect that this methd, which generates plyatmic radicals in a 1:1 rati in a phtdissciatin experiment t calibrate the mass spectrmetric sensitivity t plyatmic radical prducts, will be useful in determining prduct branching in a wide range f mass spectrmetric and phtinizatin experiments withut resrting t unreliable estimates f ttal inizatin crss sectins. We als became interested in studying the phtdissciatin f methyl vinyl ether because it prvides an pprtu /2001/1(1)/204/13/$ American Institute f Physics

2 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Methyl vinyl ether accessibility 205 nity t further develp ur predictive ability in reactins where branching t primary prduct channels is altered by electrnically nnadiabatic dynamics. 14 If electrnic nnadiabaticity des nt influence the dynamics, then the usual way t predict the branching between prduct channels is t identify what adiabatic prduct channels are energetically allwed, and then cnsider the relative barrier heights and calculated Arrhenius A factrs fr each pssible chemical reactin pathway t estimate which chemical reactin will dminate. Hwever, when the change in electrnic wave functin alng ne r mre reactin crdinates is t large fr the electrns t adjust adiabatically frm the reactant electrnic wave functin t the adiabatic prduct electrnic wave functin, thse reactin channels becme electrnically inaccessible t sme extent, slwing their rate relative t ther channels r allwing access t electrnically nnadiabatic reactin channels. Understanding hw branching t a prduct channel can be hindered by its electrnic inaccessibility is a key advance in develping ur intuitin abut chemical reactivity. The cmpetitin between the tw O CH 3 bnd fissin channels in methyl vinyl ether phtdissciatin, ne frming grund-state vinxy prduct methyl and the ther frming Ã-state vinxy prduct methyl, affrds the pprtunity t test electrnic prpensity rules being develped t help predict what prduct channels may be suppressed by the inability f the electrnic wave functin t change alng the reactin crdinate.,16 In the discussin we classify the tw O CH 3 bnd fissin reactins in terms f thse electrnic prpensity rules such that the channel frming excited-state vinxy radicals is electrnically facile, while the ne frming grund-state radicals is electrnically prhibitive because it requires a tw-electrn change in electrnic cnfiguratin as the mlecule evlves frm reactant t grund-state prducts. Despite the fact that methyl vinyl ether has been used as a phtlytic precursr f the vinxy radical fr use in dynamics and spectrscpic studies, infrmatin n the branching t primary prduct channels r the electrnic states f the phtprducts are unavailable. Wrk in prgress by Su and Bershn 20 has als recently sught t determine the vibratinal distributin f grund-state vinxy prduct frm methyl vinyl ether phtdissciatin at 193 nm. The vinxy radical is f cnsiderable interest nt nly as an imprtant intermediate in cmbustin, but als because f its rle in phtchemical smg cycles, as it can underg fast reactin with NO 2 in the atmsphere. 21 While methyl vinyl ether in its grund state has been f cnsiderable interest due t its gearing-type trsinal interactin f the methyl internal rtatin with the skeletal trsin, 22 infrmatin n electrnic states f this mlecule is relatively scarce. The UV absrptin spectrum was published by Planckaert et al. 23 and tw verlapping transitins near 200 nm were assigned. Methyl vinyl ether has a brad strng * absrptin with a maximum at 188 nm ( 193 nm cm 2 mlecule 1 ) with an verlapping lwer energy vibratinal prgressin that has been assigned t a Rydberg transitin. N theretical studies t date have examined the excited-state ptential energy surfaces f methyl vinyl ether, but within this paper we present a simple cnfiguratin interactin with single electrn excitatins calculatin CIS 24 that yields an scillatr strength f f 0.5 fr excitatin t the secnd excited state, the experimentally accessed * transitin. The mst prbable reactins f methyl vinyl ether phtexcited at 193 nm are the fllwing: CH 2 vch O CH 3 CH 2 CHO CH 3, 1 CH 2 vch OCH 3, C 2 H 4 H 2 CO. 3 If enugh energy is available, the prduct f reactin 1 abve, the vinxy radical, CH 2 CHO, can underg lss f hydrgen t frm ketene CH 2 CHO CH 2 vcvo H. 4 In the experiments undertaken here, enugh energy was available fr this reactin 4 t ccur. Thus, we needed t accunt fr the lss f vinxy t secndary dissciatin t frm ketene H in the calibratin f the mass spectrmetric sensitivity t the vinxy and methyl fragments prduced in the phtdissciatin f methyl vinyl ether. After utlining the experimental methd, we present ur results and analysis in fur subsectins. The first, Sec. III A, investigates the primary phtfragmentatin channels f methyl vinyl ether at 193 nm excitatin and gives the results f the angular distributin measurements. Sectin III B describes hw we used the relative signal levels at m/e 14,, and alng with literature daughter in fragmentatin patterns t determine, and prvide an internal cnsistency check n, what fractin f the nascent vinxy prduct frm the primary dissciatin underwent secndary dissciatin t ketene H. Sectin III C describes hw we calibrated ur detectin sensitivity at the m/e daughter in fr the vinxy and methyl phtprducts, and Sec. III D uses this calibrated sensitivity rati t derive, frm data taken by Schmltner et al., the branching between the CH 3 CHO and the vinxy H prduct channels in the O ethylene thermal reactin. In the discussin we fcus n bth the new O ethylene prduct branching calibratin and n understanding the key experimental result that the frmatin f methyl excited state vinxy dminates ver the frmatin f electrnic grund-state prducts. We analyze that result t test prpensity rules being develped t help predict what prduct channels may be suppressed by electrnically nnadiabatic effects in chemical reactins. II. EXPERIMENT In these experiments, the velcity and angular distributins f the prducts f the phtdissciatin f methyl vinyl ether at nm are measured using a crssed lasermlecular beam apparatus. After phtdissciatin with a pulsed excimer laser, the neutral phtfragments prduced scatter frm the crssing pint f the laser and mlecular beam with velcities determined by the vectr sum f the mlecular beam velcity and the recil velcity imparted by the phtdissciatin event. The fragments that scatter int the 1.5 acceptance angle f the detectr travel 44.4 cm t an 2

3 206 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Mrtn, Szpunar, and Butler electrn bmbardment inizer, where they are inized by 200 ev electrns. After mass selectin by a quadruple mass filter, the ins are cunted with a Daly detectr and recrded with a multichannel scalar with respect t their time-f-flight TOF frm the interactin regin after the dissciating laser pulse. The in flight time cnstant f 4.5 s/amu 1/2 was used t crrect fr the flight time f the in thrugh the mass spectrmeter; figures within shw ttal (in neutral) flight times. Frward cnvlutin fitting f the TOF spectrum determines the distributin f energies released t relative prduct translatin. The plane defined by the mlecular beam and the detectr axis is perpendicular t the directin f laser prpagatin, and mlecular beam surce angles are given with respect t the detectr axis. T frm the mlecular beam, a 9% mixture f methyl vinyl ether in helium was expanded thrugh a 0.10 mm diameter nzzle heated t apprximately 200 C at a stagnatin pressure f 500 Trr. The methyl vinyl ether was btained frm Aldrich 98% purity and used withut further purificatin. The high nzzle temperature reduced cluster frmatin in the methyl vinyl ether beam, which we fund t be a significant prblem with a rm temperature nzzle. The temperature was measured using a chrmel alumel thermmeter that was calibrated using a helium beam TOF. The velcity f the parent beam was measured by rtating the mlecular beam surce t pint int the detectr and raising a chpper wheel int the beam. A typical peak mlecular beam velcity in the number density distributin, N(v), was 1360 m/s with a velcity spread ( v/v peak ) f 19% with v calculated frm the full width at half maximum FWHM f N(v). Pulsed light at 193 nm frm a Lumnics PM-848 excimer laser with an ArF fill phtdissciated the methyl vinyl ether. Fr the time-f-flight measurements, the unplarized, attenuated laser energy in the interactin regin was fcused ta5mm 2 spt size in the interactin regin at pulse energies typically 5 mj/pulse. The intensity f the signal bserved frm the phtdissciatin f methyl vinyl ether was fund t be linear with increased pwer, indicating that nly ne-phtn prcesses cntributed significantly. Fr the anistrpy measurements, we dispersed the unplarized laser light int tw linearly plarized cmpnents with a single quartz Pellin-Brca. The hrizntal cmpnent is utilized, and the plarizatin is rtated int the desired directin with a half-wave retarder. The angular distributins are btained by measuring the change in signal intensity with laser plarizatin angle by finding the area under the peak in the m/e spectrum with the laser set t the specified plarizatin angles. The plarizatin-dependent signal, integrated in many repeated shrt scans and alternating between each laser plarizatin directin shts t a ttal f at each angle, required n additinal nrmalizatin t laser pwer r detectr efficiency. Laser plarizatin angles are defined with respect t the detectr axis, but cunter-rtated frm the surce angle. Signal was bserved at varius angles fr several parent and daughter ins f the primary neutral phtfragments: at m/e 58 parent, at a surce angle f 6, fr clusters nly, C 2 H 2 O, 10 and, (CH 3,10 ), and 14 CH 2, FIG. 1. Experimental TOF spectrum fr the phtdissciatin f methyl vinyl ether, cllected at m/e, at a 10 surce angle and ver 3 millin laser shts. Open circles represent data pints; the ttal fit shwn in a bld line is a sum f cntributins frm channels shwn in dashed lines. This signal is attributed t C 2 H 2 O frm the heavy phtfragments prduced in O CH 3 fissin f CH 2 vchoch 3 with a very small cntributin frm cluster phtdissciatin the lng-shrt-lng dashed peak buried inside the largest peak. The large slw peak is fit with the lwer energy P(E T ) shwn in Fig. 2; this signal cntains cntributins frm bth CH 2 CHO radicals cracking in the inizer t give signal at m/e and frm the secndary dissciatin ketene prducts cracking at parent in. As expected, the secndary dissciatin des nt significantly alter the velcity f ketene frm the vinxy radical precursr, s ne sees the required mmentum match between all m/e signal and that at methyl m/e, Fig. 3. The small fast peak is fit with the higher translatinal energy P(E T ) shwn in Fig. 2 and is assigned t daughter ins f grund-state vinxy radicals prduced in the initial dissciatin event. 10 and. A small amunt f signal was bserved at m/e and, 29 10, 26 10, and 25 10, and was adequately fit within the limits f the pr signal-tnise rati using the mass and cluster distributins. N discernible signal ther than cluster cntributin was bserved at m/e 43 10, 31 10, 30 10, 27 10, r The high natural backgrund at m/e 28 caused by CO prevented cllectin f data at this mass. Fr the measurements in Sec. III B, when we needed t determine the ratis between signal levels at m/e 14,, and, we prceeded as fllws. Systematic errrs including fluctuatins in the parent mlecular beam were averaged ut by alternating data cllectin between m/e (CH 3 ) and m/e (C 2 H 2 O). 500 K shts were taken in each scan and the scans repeated until 2 M shts were cllected at each mass. We cllected data with the same methd f alternating scans f 500 K shts at m/e 14(CH 2 ) and m/e (C 2 H 2 O), with scans taken until 2 M shts ttal at each mass were cllected. Quadruple reslutin fr all measurements in this paper was adjusted t rughly 1.0 amu FWHM at each mass. T characterize the excited state accessed at 193 nm, we ran a cnfiguratin interactin with single excitatins CIS calculatin 24 with GAUSSIAN 94 using a 6-311G* basis set f atmic rbitals. The grund-state gemetry in C s symmetry used fr these calculatins was taken frm a theretical paper by Leibvitch et al. 25

4 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Methyl vinyl ether accessibility 207 III. RESULTS AND ANALYSIS A. Identificatin f primary prduct channels in methyl vinyl ether phtdissciatin The data fr methyl vinyl ether excited at 193 nm shw evidence fr tw cmpeting prduct channels, bth f which are C O bnd fissin pathways leading t vinxy plus methyl (C 2 H 3 O CH 3 ) frmatin, reactin 1. N evidence fr vinyl plus methxy frmatin, reactin 2, nr ethene plus frmaldehyde frmatin, reactin 3, was seen in ur data. Figure 1 shws the TOF spectrum taken at m/e at a surce angle f 10. We attributed this signal t the heavy phtfragment frm O CH 3 fissin, reactin 1, t frm the vinxy radical CH 2 CHO mass 43 CH 3, since the TOF spectrum at m/e has a fast cmpnent with arrival times that clsely crrespnd t that predicted fr the methyl c-fragment using the usual requirement f cnservatin f linear mmentum in the primary dissciatin and the P(E T ) derived frm frward cnvlutin fitting f the m/e TOF shwn in Fig. 2. Hwever, nt all the signal at m/e results frm simple cracking f the mass 43 vinxy radical prduct t m/e in the electrn bmbardment mass spectrmeter. Sme f the vinxy radical prduct is frmed with enugh internal energy t underg C H fissin as a neutral and prduce the neutral mlecule ketene (CH 2 vcvo), which gives sme signal at parent mass upn electrn bmbardment inizatin. As has been seen befre by Schmltner et al., 1 very little r nne at all f the vinxy radical prduced in the interactin regin survives intact thrugh electrn bmbardment inizatin t give m/e 43; we saw n signal ther than clusters at the masst-charge rati crrespnding t the parent vinxy radical FIG. 2. Translatinal energy distributins, P(E T ) s, used t fit the O CH 3 bnd dissciatin pathways f methyl vinyl ether. The tw distributins are derived frm frward cnvlutin fitting the m/e TOF spectra. The faster P(E T ) is assigned t frmatin f grund-state vinxy (X 2A ) and grund-state methyl (X 2A 1 ) radicals. The slwer distributin, which peaks at 8 kcal/ml, crrespnds t a channel prducing methyl excited state vinxy radicals, which we assign as CH 2 CHO (à 2 A ). These nascent à state CH 2 CHO radicals have enugh internal energy that they may dissciate t frm t ketene H, fluresce, r underg internal cnversin and dissciate n the grund state. See the Results and Analysis sectin fr further details. The nrmalized P(E T ) s are scaled accrding t the fllwing: fast slw rprb fast rprb slw 0.35Q vinxy 0.65Q ketene Q vinxy 0.35f vinxy 0.65f ketene 0.35f vinxy. in. As expected, this secndary dissciatin des nt significantly alter the velcity f the ketene frm the vinxy prduct frm which it was brn, s ne sees the required mmentum match between the methyl signal and all the signal at mass, even thugh sme f that signal is nt frm vinxy daughter ins. Interestingly, it appears that nt all the nascent vinxy prduct with enugh internal energy t dissciate t ketene H des see Sec. III B. This may be understd if sme f the vinxy prduct is frmed electrnically excited and lses energy via flurescence befre underging C H fissin. Inspectin f the center-f-mass translatinal energy distributins P(E T ) s fr O CH 3 fissin yielding C 2 H 3 O CH 3 derived by frward cnvlutin fitting f the m/e TOF spectrum evidences tw different O CH 3 fissin pathways ccurring at 193 nm. One channel yields fragments with high translatinal energies; the faster P(E T ) see Fig. 2 extends frm 45 kcal/ml t almst 70 kcal/ml. Because this distributin extends t clse t the available energetic limit (E avail 86 kcal/ml), this pathway is assigned t frmatin f grund-state vinxy (X 2A ) and grund-state methyl (X 2A 1 ) radicals. This will be called the fast channel. The secnd pathway, fit by the slwer P(E T ) in Fig. 2, prduces phtfragments with lw translatinal energy and frm nw n will be termed the slw channel. This translatinal energy distributin extends t nly 45 kcal/ml and peaks at a much lwer energy f 8 kcal/ml, and s the phtprducts are initially frmed with cnsiderable internal energy. It is likely that the initial electrnic state f the nascent vinxy radical prducts frmed in this slw channel is the Ã( 2 A ) state f vinxy, as the zer pint level f the B state lies abut 82 kcal/ml abve the grund state and thus is nt energetically accessible except fr the smallest recil kinetic energies in the slw distributin in Fig. 2. The zer pint level f the Ã( 2 A ) state f vinxy lies nly 23 kcal/ml abve the grund state and s is easily accessible with the available energy. The nascent vinxy radical prduct may dissciate t ketene H, fluresce, r underg internal cnversin and dissciate n the grund state. The secndary dissciatin is cnsidered further in the next sectin. The fits t the m/e TOF spectrum are supprted by fitting the TOF spectra taken at m/e and 14, seen in Fig. 3 and Fig. 4, respectively. In Fig. 3, the mmentum-matched methyl fragments t the tw O CH 3 fissin channels bserved at m/e are the small peak bserved at the fastest arrival time and the bulk f the largest peak which arrives between 100 and 200 s, althugh the cntributin frm daughter ins prduced by cracking f the fast vinxy radicals in the inizer can be clearly seen in this largest peak as well. Finally, the slwer shulder is signal frm cracking f the vinxy radicals frmed via the slw O CH 3 fissin channel which frms nascent radicals with higher internal energy. Althugh sme f the slw vinxy prduct underges secndary dissciatin t ketene H, ketene des nt have an m/e daughter in s thse prducts d nt cntribute t the slw shulder in the m/e TOF spectrum. They d at m/e 14. A miniscule cntributin frm clusters is als included, which is fit in the fllwing manner: Fig. 5 shws the

5 208 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Mrtn, Szpunar, and Butler FIG. 3. Experimental TOF spectrum fr the phtdissciatin f methyl vinyl ether, cllected at m/e, at a 10 surce angle and ver 5.5 millin laser shts. Open circles represent data pints; the ttal fit shwn in a bld line is a sum f cntributins frm channels shwn in dashed lines. The mmentum-matched methyl fragments t the tw O CH 3 fissin channels bserved at m/e Fig. 1 are the small peak bserved at the fastest arrival time and the bulk f the largest peak. Clearly als seen in the largest peak is signal assigned t daughter ins f the grund-state vinxy (X 2A ) prduct. The slw shulder is assigned t signal prduced frm daughter ins f the excited state vinxy (Ã 2 A ) frmed via the slw O CH 3 fissin channel. Nte that this peak is much smaller than the crrespnding ne in the m/e 14 spectrum Fig. 4, as ketene des nt crack t give m/e. This fact was used in the calibratin fr the lss f nascent vinxy radicals t ketene frmatin, as explained in the text. TOF signal bserved at m/e 58 (CH 2 vchoch 3 )ata6 surce angle. Because this signal was bserved at the mass/ charge rati f the parent methyl vinyl ether mlecule, it must arise frm phtdissciatin f clusters present in the expansin. While such clusters were minimized by heating the nzzle, they still represent a small percentage f the species in the expansin. The TOF spectrum shwn in Fig. 5 is fit t determine the center-f-mass recil speed distributin P(v), and as the signal attributable t cracking f the phtprduced mnmer appears at late arrival times fr mst mass/charge ratis, the cntributin frm clusters in the expansin can be accunted fr by fitting the P(v) t the slw signal at the mass/charge ratis f interest. In the TOF spectrum at m/e 14, Fig. 4, what was the slwer shulder at m/e has grwn t the largest peak in the m/e 14 spectrum. This is because at the energies used in this experiment, mre than half f the vinxy frmed via the slw channel underges unimlecular dissciatin t frm ketene H as a neutral. Ketene des nt crack t m/e, but has a strng daughter in at m/e 14. In the next sectin III B we detail hw we estimated what fractin f the nascent vinxy radical prduct in the slw channel underwent secndary dissciatin. This was fund by using the relative signal levels at m/e vs that in the slw shulder at m/e and the relative signal levels f the slw shulder at m/e versus that at m/e 14. The internal cnsistency f the estimate was checked with the m/e 14 data. Angular distributin measurements f the phtfragments frm methyl vinyl ether phtdissciatin were taken FIG. 4. Experimental TOF spectrum fr the phtdissciatin f methyl vinyl ether, cllected at m/e 14, at a 10 surce angle and ver 3 millin laser shts. Open circles represent data pints; the ttal fit shwn in a bld line is a sum f cntributins frm channels shwn in dashed lines, which are scaled assuming a 35% survival prbability fr vinxy radicals frm the slw channel and 100% survival prbability fr vinxy radicals frm the fast channel. The CH 2 daughter ins f the mmentum-matched methyl fragments t the tw O CH 3 fissin channels bserved at m/e Fig. 1 are the small peak bserved at the fastest arrival time and the bulk f the secnd largest peak frm abut 100 t 200 s. The tiny peak buried in this larger ne is signal assigned t daughter ins f the grund-state vinxy (X 2A ) prduced by cracking in the inizer. The largest peak, beginning frm abut 200 s with a lng slw tail, is the m/e 14 signal crrespnding t that seen in the slw shulder in the m/e spectrum; this signal has arrival times crrespnding t the daughter ins f the excited state vinxy (Ã 2 A ) frmed via the slw O CH 3 fissin channel and includes the daughter ins f the secndary dissciatin prduct ketene, which will nt have significantly altered arrival times frm thse frm the vinxy radical precursr. Nte that this peak is much larger than the crrespnding ne in the m/e spectrum Fig. 3, as ketene des nt crack t give m/e. This fact was used in the calibratin fr the lss f nascent vinxy radicals t ketene frmatin, as explained in the text. FIG. 5. Experimental TOF spectrum fr the phtfragments f detected at m/e 58, at a 6 surce angle and ver 2 millin laser shts. Since the signal ccurs at the parent mass, it must result frm phtdissciatin f clusters present in the supersnic expansin. The fit shwn in a bld line is the center-f-mass recil speed distributin P(v) assigned t cluster dissciatin. The slw arrival time and small amunt f signal attributable t clusters ensures that that cntributin t signal at mst mass/charge ratis can be accunted fr by fitting the P(v) t the slw signal at the mass/ charge ratis f interest.

6 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Methyl vinyl ether accessibility 209 FIG. 6. Experimental angular distributins f the slw vinxy methyl channel, measured at m/e (C 2 H 2 O ), with a linearly plarized laser and a surce angle f 10. lab is the angle f the laser electric vectr with respect t the detectr axis. The data pints represent the integrated experimental TOF signal between 204 s and 286 s measured at six different laser plarizatin angles. Line fits shw the predicted change in integrated signal with plarizatin angle btained, after transfrmatin frm the centerf-mass t the labratry frame, with five trial anistrpy parameters; 0.2, 0.1, 0.0, 0.1, and 0.2. The angular distributin is best fit with a 0, crrespnding t a nearly istrpic distributin. at m/e using the prcedure utlined in the experiment sectin. It is well knwn that the angular distributin f prducts frm dissciatin can be described in the center-fmass frame by the classical electric diple expressin, I cm 1/ 4 1 P 2 cs cm, 5 where cm is the angle between the phtfragment recil directin in the center-f-mass reference frame and the electric vectr f the linearly plarized light. In the limit f prmpt axial phtfragment recil, the anistrpy parameter,, is given by 2P 2 cs, 6 where is the angle between the transitin diple mment fr the absrptin and the breaking bnd. Figure 6 shws the experimentally measured angular distributin f vinxy fragments alng with curves simulated using different values f in the prcedure abve. By cmparing the experimental pints with the simulated curves, we see that 0, which means that the phtdissciatin f CH 2 vchoch 3 t give CH 3 CH 2 CHO gives a largely istrpic distributin. Accrding t the CIS calculatins perfrmed, is abut 25, suggesting that the istrpy results because the time scale f dissciatin is lng with respect t mlecular rtatin. Breaking the vinyl methxy bnd, reactin 2, is als pssible at the energies available in this experiment. T check fr this reactin, we tk data at the masses f the different daughter ins that wuld be prduced frm the phtfragments f reactin 2 : m/e 31 (OCH 3 ), 30 (OCH 2 ), 29 OCH, als see cracking frm C 2 H 3 O, 27 C 2 H 3, als might see a small amunt frm cracking f C 2 H 3 O, 26 (C 2 H, als see cracking frm C 2 H 3 O, 25 C 2, als see cracking frm C 2 H 3 O, and 16 (CH 4 ). We saw n evidence fr the prductin f vinyl and methxy radicals as a primary dissciatin channel f methyl vinyl ether at 193 nm: if any signal at all was visible at m/e 31, 30, 27, r 16 it was easily assigned t the cluster cntributin; fr m/e 29, 26, and 25, the signal seen was adequately fit by assuming it arse frm vinxy radicals and using the P(E T ) in Fig. 2. The lack f signal at m/e 31, 30, and 27 als eliminated the reactin R3 prducing frmaldehyde and ethene as a significant primary prduct channel fr the phtdisscatin f methyl vinyl ether at 193 nm. B. Secndary dissciatin f vinxy radical t frm ketene H This sectin first assesses which nascent vinxy prducts have enugh internal energy t underg secndary dissciatin t ketene H and speculates n the mechanism. Then, we estimate frm the pertinent signal intensities at m/e,, and 14 what fractin f the vinxy radicals ptentially subject t secndary dissciatin actually des s rather than lsing the energy via flurescence. The energy available fr translatinal and internal energies f the phtprducts frmed frm O CH 3 cleavage was estimated using bnd dissciatin energies prvided by Ruscic, 26 which were calculated at the G3 level fr methyl vinyl ether at 0 K: E avl h D O CH 3 bnd kcal/ml 61.6 kcal/ml 86.4 kcal/ml. 7 Thus, the internal energy f the methyl vinxy (electrnic vibratinal rtatinal) is E avl E T 86.4 kcal/ ml-e T where E T is the kinetic energy partitined t prduct recil in the primary dissciatin, Fig. 2 and we expect mst f this internal energy t be partitined t the vinxy prduct, nt the methyl. The phtdissciatin spectrscpy and dynamics f the vinxy radical have been studied in detail by Osbrn et al. 27 at selected excitatin energies up t almst 4000 cm 1 abve the B state zer pint level. They fund that upn excitatin t the B state at energies 1400 cm 1 ver the zer pint level, the vinxy radical underges fast internal cnversin (B Ã X ) t the grund state fllwed by dissciatin dynamics that branches t tw prduct channels h CH 2 CHO CH 3 CO H ev kcal/ml, h H CH 2 CO H ev kcal/ml. 9 On the grund-state ptential energy surface, ne expects a large exit barrier t prduct channel 8 t frm methyl CO and a small exit barrier beynd the endthermicity t prduct channel 9. The calculated 27 barriers with zer-pint crrectins t 8 and 9 are 1.78 and 1.63 ev, respectively, with channel 9 having a lser transitin state. Thus, since the internal energy f the nascent vinxy radical prduct in ur experiments is the bulk f 86.4 kcal/ml-e T, there is mre than enugh energy fr either 8 r 9 t ccur fr all f the nascent vinxy prducts in the slw channel if the dissciatin prceeds via internal 8

7 210 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Mrtn, Szpunar, and Butler cnversin t the grund state. Mst f the vinxy radical prducts frmed in the fast channel have lw enugh internal energies t be stable t secndary dissciatin. If the secndary dissciatin ccurs frm the à state f vinxy rather than via internal cnversin t the grund state, Ref. 27 gives a barrier 2.09 ev abve the grund-state zer-pint level fr reactin 9 t frm ketene, but des nt calculate the barrier t reactin 8. We shuld nte that Osbrn et al. fund that upn exciting vinxy radical t the B state that the prduct channel branching was 4( 2):1 fr channel 8 : channel 9, in gd agreement with their RRKM calculatins that presumed that dissciatin ccurred upn internal cnversin t the grund state, but in ur experiments a negligible fractin f the nascent vinxy prduct underges dissciatin via reactin 8. The entire signal at m/e and 14 is accunted fr in the fits shwn in Figs. 3 and 4 by three surces: primary methyl prduct frm O CH 3 fissin, daughter ins f vinxy radicals, and daughter ins f the secndary ketene prduct. We can predict the arrival times f the ketene daughter ins at m/e 14 as they are essentially the same as fr the vinxy reactant frm which they were brn, s are determined by mmentum matching with the primary methyl prduct. A pssible explanatin fr the difference in prduct branching is that the internal energy f the nascent vinxy radical prduct in ur experiments, 86.4 kcal/ml-e T E int,ch3, is nt enugh energy t frm vinxy in the B state fr primary dissciatin events releasing mre than a few kcal/ml t prduct translatin, since the B state rigin lies 82 kcal/ml abve grund-state vinxy radical. Thus, mst f the nascent vinxy prducts in the slw kinetic energy channel in ur experiments are likely frmed in the à state. Because the gal in the next sectin is t use ur data at m/e t calibrate the relative mass-spectrmetric detectin sensitivity fr methyl vs vinxy radicals, we must accunt fr the fact that sme f the nascent vinxy prduct is lst t secndary dissciatin t ketene and s des nt cntribute t the m/e signal. The relative signal levels at m/e in these data frm the slw channel wuld nrmally, if the vinxy did nt underg secndary dissciatin, represent prductin in the interactin regin f ne vinxy radical fr each mmentum-matched methyl radical. Then, applicatin f the methd used in Sec. III D t determine mass-spectrmetric sensitivity t radical prducts and help determine prduct branching in the O ethylene experiments detecting methyl and vinxy prducts at m/e wuld nt invlve any assumptins abut the daughter in cracking fractins r require errr-prne semiempirical estimates f inizatin crss sectins. Hwever, since sme f the vinxy radicals in ur calibratin in Sec. III C are lst t secndary dissciatin t ketene, which des nt crack t m/e, we need t determine what fractin f the nascent vinxy prducts des nt underg secndary dissciatin. We derive the result in the next paragraph that abut 35% f the nascent vinxy prduct des nt underg secndary dissciatin ( vinxy 35%), s the relative signal levels at m/e in these data frm the slw channel represent prductin in the interactin regin f 0.35 vinxy radicals fr each mmentum-matched methyl radical. Unfrtunately, determining this 35% survival fractin des require us t use ratis between selected daughter in cracking prbabilities and semiempirical estimates f the inizatin crss sectins fr vinxy vs ketene, but frtunately the inizatin crsssectin ratis (Q ketene /Q vinxy ) are relatively rbust t the semiempirical methd used t estimate them. Thus, this des nt intrduce significant errrs in the sensitivity calibratin. We als prvide a duble-check belw, shwing that the rati between the m/e and m/e 14 signals is predicted t be within 10% f ur bserved signal intensity rati when we use the literature daughter in cracking prbabilities frm vinxy and ketene and the derived survival fractin f the vinxy t secndary dissciatin. T determine frm ur data what fractin f the nascent vinxy prducts des nt underg secndary dissciatin, we begin by nting that bth the m/e and the m/e 14 signal have cntributins frm daughter ins f bth stable vinxy radicals and secndary ketene, while the m/e signal in the same range f arrival times has n cntributin frm ketene since ketene des nt have m/e daughter in. In general, the bserved integrated signal IS at a particular daughter in frm a reactin prduct in a scattering experiment with electrn bmbardment detectin depends n several factrs 3D velcity and angular distributins f prducts, reactant beam intensities, inizer transit times, inizatin crss sectins, daughter in cracking patterns, etc. Frtunately, we nly require ratis between signal intensities, s the signal intensity factrs relevant t this experiment that differ between vinxy and the secndary ketene prduct are simply: 1 the electrn bmbardment inizatin crss sectins, Q vinxy and Q ketene, f vinxy and ketene at 200 ev; 2 the prbability f the inized prduct appearing at a particular daughter in calculated frm nrmalizing literature daughter in cracking patterns fr vinxy 1 and ketene 28 measured at a similar high energy electrn bmbardment, e.g., f vinxy ; and 3 the rati f quadruple transmissin factrs T min, if signal intensities at tw different mass t charge ratis are being cmpared. Our experiments use a Daly detectr, which is insensitive t the chemical identity f the in. Thus, the integrated signal ver a selected range f arrival times at m/e fr the slw channel can be expressed as the sum f the signals frm daughter ins f the surviving vinxy, IS,vinxy, and frm the parent in f the secndary ketene, IS,ketene, and each f thse can be individually expressed as fllws: IS IS,vinxy IS,ketene C vinxy Q vinxy f vinxy C ketene f ketene where C C. 10 In Eq. 10 abve the multiplicative cnstants C and C cllect tgether all the factrs abve that affect the signal intensity that d nt differ between vinxy and ketene. Letting C C requires nly ne assumptin, that the survival prbability f nascent vinxy des nt vary significantly fr the range f recil velcities sampled in the slw dissciatin channel e.g., that the branching vinxy / ketene between stable vinxy and vinxy that dissciates t ketene des nt

8 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Methyl vinyl ether accessibility 211 depend strngly n internal energy in the range f internal energies relevant t the slw channel. The reliability f this assumptin is supprted by the fact that the slw daughter in signal at m/e has nly cntributin frm stable vinxy, while the signal at m/e has cntributin frm bth stable vinxy and secndary ketene, yet the shape f the TOF spectra in the vicinity f the slw vinxy prduct at m/e is fit well by the P(E T ) derived frm the m/e data. If the survival prbability f nascent vinxy vinxy varied significantly acrss the internal energies represented by that range f arrival times e.g., if the nascent vinxy frm the slw channel arriving at earlier times and thus having lwer internal energies was less likely t underg secndary dissciatin that the vinxy at lnger arrival times, then the shape f the TOF spectra in the vicinity f the slw vinxy prduct at m/e wuld nt be fit well by the P(E) derived frm the m/e data. Nw, deriving the branching vinxy / ketene between stable vinxy and vinxy that dissciates t ketene is trivial using Eq. 10 and the relevant integrated signal intensities signal frm nly the slw channel attributed t the nascent vinxy prduct, sme f which survives as vinxy and sme f which gives ketene at m/e and m/e : IS IS vinxy, IS ketene, IS IS vinxy, 0 IS vinxy, IS vinxy, IS ketene, IS vinxy, f vinxy ketene f vinxy Q ketene f ketene vinxy Q vinxy f vinxy T cunts T cunts ketene Q ketene , vinxy Q vinxy where we have assumed the quadruple transmissin is the same at m/e and m/e since we have adjusted it t the same reslutin at these tw masses and have used the same hi-q head. In actuality, the literature daughter in cracking pattern we are using fr vinxy did nt include crrectin fr quadruple transmissin, s the T factrs abve are buried in the daughter in cracking prbability ratis. Slving Eq. 11 fr ketene / vinxy and using the semiempirical methd f Fitch and Sauter 29 t estimate the electrn bmbardment inizatin crss sectins fr vinxy and ketene FS: Q vinxy Å 2 and Q ketene Å 2, we get ketene / vinxy Thus, the survival prbability f the nascent vinxy prduct is determined t be vinxy % f the nascent vinxy radicals d nt underg secndary dissciatin t ketene H. Frtunately, the derivatin is nly sensitive t the rati f the inizatin crss sectins, because an estimate f the individual crss sectins using the ther cmmnly used semiempirical methd f Center and Mandl 30 CM using sums f atmic plarizabilities 31 gives inizatin crss sectins almst ten times larger fr each CM: Q vinxy Å 2 and Q ketene Å 2, but the rati Q ketene /Q vinxy is similar t that frm the Fitch and Sauter methd. Carrying the calculatin thrugh using the Center and Mandl estimate fr the inizatin crss sectins gives ketene / vinxy 1.65, crrespnding t a survival prbability f the nascent vinxy prduct f vinxy Thus, using either methd fr estimating the electrn bmbardment inizatin crss sectins, we determine that abut 36% f the nascent vinxy prduct is stable t secndary dissciatin and cntributes t the m/e signal at slw arrival times and 64% f the nascent vinxy prduct in the slw channel underges secndary dissciatin t ketene H. Since the determinatin abve f the fractin f nascent vinxy prduct that survives secndary dissciatin is dependent n estimated inizatin crss sectins and literature daughter in fragmentatin patterns, we did try t assess the reliability f the estimate. In particular, we were cncerned that the literature daughter in cracking patterns, since they are determined fr relatively cld vinxy radicals, may nt be a gd apprximatin t that fr the highly internally excited vinxy prduct frm this phtdissciatin experiment, even thugh the electrn bmbardment energy is quite high 200 ev. The high electrn energy helps ensure that the daughter in fragmentatin pattern is nt strngly dependent n internal energy. T assess the reliability f the 40% estimate, we did tw analyses. First, we fllwed the prcedure abve but used the rati f the integrated signal in the m/e 14 data instead f m/e t the m/e data t determine the fractin f nascent vinxy that survived secndary dissciatin. Using the Fitch and Sauter inizatin crss sectins, this gave a value f 35% as cmpared t 36% using the m/e ver m/e integrated signals. We then used that 35% determinatin t try t predict the rati between signal level at m/e and the signal level at m/e 14. While the predicted integrated signal rati btained with literature daughter in fragmentatin patterns was 1.06, the experimentally determined signal rati was Thus, the errr intrduced by these estimates appears t be rughly 10% in the determinatin f signal intensity ratis, r crrespnding apparent crss-sectin ratis. This is smewhat larger than the errr intrduced in the integrated signals by Pissn cunting errr alne. A crrect predictin requires a cnsiderably lwer survival prbability fr the nascent vinxy, but the calculatin invlves subtracting tw similar numbers in the denminatr, s is subject t cnsiderable errr. Thus, we adpt the survival prbability f 35% 5% ( ketene / vinxy 1.86) fr the best determinatin f the fractin f the nascent vinxy prduct that is stable t secndary dissciatin and cntributes t the m/e signal. We shuld clarify the integratin ranges fr the relevant integrated signals at m/e, 14, and used in this sectin. In all cases we wanted t integrate daughter ins frm the same set f neutrals, and the in flight time is different at m/e than at 14 r, s we needed t shift the integratin range depending n the spectrum we were integrating. We chse an integratin range fr the m/e TOF spectrum f channel 88 thrugh channel 0 at 2 s/channel, crrespnding t ttal flight times between 174 and 300 s,

9 212 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Mrtn, Szpunar, and Butler while fr m/e 14 and the crrespnding integratin range needed t be shifted t shrter times by 6 channels r 12 s. In additin, since we nly wanted t include integrated signal frm the vinxy/ketene prducts frm the slw distributin and nt frm the fast distributin r frm methyl prducts, we subtracted frm the ttal integrated signal in these ranges the cntributin frm thse ther surces as determined by the fits shwn in Figs. 1, 3, and 4. At m/e, the signal frm slw vinxy/ketene crrespnds t 95.73% f the ttal integrated signal, while at m/e and 14 it is 24.47% and 79.51%, respectively. The m/e data have a large verlap frm the slw methyl prduct. We repeated the determinatin using the m/e ver m/e integrated signals fr a different integratin range, channel 98 t 160 fr m/e and the 6 channel shifted range at m/e 14 and t assess any errrs. We fund that using the new integratin ranges and the Fitch and Sauter estimates fr the inizatin crss sectins gave ketene / vinxy 1.79 rather than 1.82 and gave ketene / vinxy 1.63 rather than 1.65 when using Center and Mandl inizatin crss sectins. Thus, the derived survival prbability fr the nascent vinxy radicals f abut 35% 5% is, as expected, rbust with respect t the integratin range chsen t make the determinatin. C. Calibrating the relative mass spectrmetric detectin sensitivity t vinxy and methyl radicals at the mõeä daughter in fr use in prduct branching determinatins Determining a branching rati between different reactin channels can be a difficult task, especially when ne r mre f the prducts are plyatmic radical species which can crack int many different daughter ins in the electrn bmbardment inizer. One methd f determining the ttal amunt f neutral parent fragment depends n cllecting data at and summing up the cntributin frm each f the many pssible daughter in masses. This signal is then crrected fr quadruple transmissin if pssible and divided by an estimated inizatin crss sectin. There are ften large surces f errr in this methd, arising frm high backgrund r limited signal that might bscure r reduce signal frm the daughter ins at sme masses such as m/e 28 (CO ) and m/e 16 (O ). There is als errr frm the value used fr the ttal inizatin crss sectin, which is rarely experimentally determined and instead is estimated with ne f tw semiempirical methds, 29,30 neither f which is tested fr plyatmic radicals. The data here n methyl vinyl ether phtdissciatin prvide the essential calibratin factr t circumvent the abve prblems when ne wishes t derive a branching rati in a mass-spectrmetric experiment that prduces vinxy radicals in ne channel and methyl radicals in anther, such as in the bimlecular reactive scattering data f Schmltner et al. n O( 3 P) ethylene discussed in the next sectin. We can use the methyl vinyl ether phtdissciatin data t calibrate 13 the relative mass-spectrmetric detectin sensitivity t methyl vs vinxy radical prducts at the m/e daughter in, since bth the methyl and vinxy prducts frm the slw O CH 3 fissin channel in the 193 nm dissciatin f methyl vinyl ether give a signal at m/e and that relative signal arises frm a 1:0.35 rati f methyl:vinxy neutral prducts. This is nrmally a 1:1 rati, since phtdissciatin prduces ne methyl fragment fr each vinxy fragment, but in the previus sectin we shwed that nly 35% f the vinxy prducts frm the slw O CH 3 fissin channel survive secndary dissciatin and cntribute t the m/e signal. The signal intensities at m/e are fit by varying the relative prbability weightings fr the daughter in signal frm primary methyl prducts and frm the 35% f the mmentum-matched vinxy partners in the slw dissciatin channel that survive secndary dissciatin. These relative prbability weightings, rprbs als termed the apparent crss sectins in Ref. 1, include the usual crrectin fr kinematic factrs in the 3D velcity and angular distributins f the prducts and transit time thrugh the electrn bmbardment inizer, but d nt crrect fr the effect n the signal intensities frm the relative cracking efficiencies f CH 3 and CH 2 CHO t CH 3 r the ttal inizatin crss sectin f each species. e.g., an rprb fr a given channel may be quite lw if it is derived frm fitting data at a minr daughter f the neutral prduct. We will designate each rprb r apparent crss sectin used t fit the relative signal levels in ur phtdissciatin experiment in the m/e data as. Then, the true rati between the methyl fragments frm the slw dissciatin channel and the surviving mmentum-matched vinxy prduct, 1.0/0.35, is related t the rati f rprb s s determined frm fitting the relative signal intensities lng-dashed and shrt-dashed cntributins, respectively in the m/e TOF spectrum in Fig. 3 by the fllwing equatin: 1.0 slw methyl prducts 0.35 slw stable vinxy prducts slw methyl slw stable vinxy Q vinxy f vinxy. 12 Q methyl f methyl Nte that the apparent crss sectin derived frm fitting the data fr the slw methyl prducts slw methyl is divided by Q methyl because if the inizatin crss sectin fr methyl is large it will give an anmalusly high apparent crss sectin. We can thus use Eq. 12 abve t determine (Q vinxy f vinxy )/(Q methyl f methyl ) frm ur phtdissciatin experiment frm the rati slw methyl / slw stable vinxy determined frm fitting the crrespnding relative signal intensities in ur m/e data. The rati f rprbs r s determined by fitting the signal assigned t slw CH 3 prducts and their mmentum-matched vinxy was 22:0.9 slw CH 3 : their mmentum-matched vinxy in the m/e TOF spectrum Fig. 3. Inserting slw methyl / slw stable vinxy 22/0.9 in Eq. 12 gives a value fr (Q vinxy f vinxy )/(Q methyl f methyl ) frm ur phtdissciatin experiment, btaining (Q vinxy f vinxy )/(Q methyl f methyl ) This sensitivity calibratin factr can nw be used t cnvert apparent crss sectins t real branching ratis fr any experiment which prduces methyl radicals in ne channel and vinxy radicals in anther, such as in the bimlecular reactive scattering data f Schmltner et al. n O( 3 P) ethylene discussed in the next sectin. We use nly

10 J. Chem. Phys., Vl. 1, N. 1, 1 July 2001 Methyl vinyl ether accessibility 213 their m/e data, frm which ne btains the apparent crss sectins f the CH 3 HCO vs the vinxy H prduct channels frm fitting the relative signal intensities f the CH 3 signal at m/e and the vinxy signal at m/e with the usual cm lab frward cnvlutin fitting prcedure. One des nt need t sum up signal at any ther daughter in nr rely n semiempirical estimates f the rati f ttal inizatin crss sectins f methyl t vinxy radicals, as that infrmatin is prvided by the calibratin factr at m/e frm ur phtdissciatin experiment. Nrmally this methd des nt rely n any estimates f inizatin crss sectin r daughter in fragmentatin patterns; we nly had t d s in the methyl vinyl ether calibratin because sme f the vinxy was lst t secndary dissciatin t ketene. See Sec. III B. D. Determining the CH 3 HCOÕH CH 2 CHO prduct branching rati in prir O ethylene reactive scattering experiments by using ur calibrated detectin sensitivity t vinxy and methyl radicals at the mõeä daughter in Schmltner et al. s crssed mlecular beams scattering experiment 1 n the O( 3 P) ethylene reactin at a 6 kcal/ml cllisin energy detected, in qualitative agreement with prir wrk, tw significant primary prduct channels, O 3 P C 2 H 4 CH 3 HCO, 13 H CH 2 CHO. 14 Experimental determinatins f the branching rati fr the O ethylene reactin see the Discussin sectin have varied widely. While the wrk f Schmltner et al. is likely the benchmark fr further wrk n this system, their methd f calculating branching ratis gave a larger branching, 71% 62% 77%, t the CH 3 HCO channel 13 than any f the ther determinatins published at that time. Their methd f extracting branching ratis frm their data relied n cmmnly used but unreliable by virtue f being untested fr plyatmic radicals estimates f the rati f the inizatin crss sectins f the plyatmic radical prducts. They als needed t sum up all the signal frm all the pssible daughter ins, which was dne by estimating by cmparisn with related cmpunds t estimate the cracking t missing masses in their daughter in fragmentatin pattern and cmpare signal frm tw different reactive scattering experiments. Frm ne experiment they used their data at m/e fr vinxy and frm the secnd experiment, dne with istpically labeled reactants, they used their data at m/e 30 (C 18 O ) fr CH 18 O, btaining their branching rati R with R a a Q vinxy f vinxy 30. b b Q CHO f CHO In their equatin abve a is the apparent crss sectin fr the CH 3 HCO channel frm the m/e 30 data and b is the apparent crss sectin fr the vinxy H prduct channel frm their m/e data, and they derived each frm fitting the relative signal intensities in the tw experiments, btaining R The calculatins used the Fitch and Sauter methd fr estimating inizatin crss sectins and estimates f the relevant daughter in cracking patterns. We can circumvent virtually all the surces f errrs in Schmltner et al. s branching rati determinatin by simply using nly their data at m/e, which have cntributins frm bth methyl frm the CH 3 HCO prduct channel a channel 13 abve and vinxy frm the vinxy H prduct channel b channel 14 abve. We crrect the apparent crss sectins frm fitting the methyl and vinxy signal in their m/e data t real prduct branching ratis using the mass-spectrmetric sensitivity calibratin factr fr methyl and vinxy prducts that we derived in Sec. III C frm the phtdissciatin f methyl vinyl ether t methyl vinxy. We prceed as fllws: As Schmltner et al. did nt reprt the apparent crss sectins they used t fit their m/e data, we refit the m/e O ethylene data using their published prduct translatinal energies and angular distributins and varied the rati between the tw apparent crss sectins until we btained a gd fit t their data. We used the standard frward cnvlutin bimlecular scattering prgram f Lee and c-wrkers, GMT, prvided generusly by A. Suits. Mst f the needed input parameters, such as beam velcities, were given in the paper by Schmltner et al., with the exceptin f the interactin regin t chpper flight path, which needed t be set at 17.6 cm fr the published translatinal energy distributin t fit the data. The apparent crss sectins that fit the cntributins frm methyl and frm vinxy prducts in the m/e data f Schmltner et al. were 0.84:0.16 (CH 3 HCO:vinxy H). The branching rati between the tw channels is then trivially calculated using that rati and the mass-spectrmetric sensitivity calibratin factr fr methyl and vinxy prducts at m/e that we derived in Sec. III C frm the phtdissciatin f methyl vinyl ether: R CH3 HCO H CH2 CHO methyl frm CH3 HCO 0 Qvinxy f vinxy Q methyl f methyl vinxy frm H CH2 CHO If nly these tw prduct channels cntribute significantly, this crrespnds t a 38% with estimated errr bars f abut 5% branching t the CH 3 HCO prduct channel a channel 13 abve and 62% branching t vinxy H prduct channel b channel 14 abve. This is cnsiderably different than the 71% branching (R ) t the CH 3 HCO prduct channel that Schmltner et al. derived frm cmparing the istpically labeled experiment fr the HC 18 O daughter signal at m/e 30 with the vinxy signal at m/e. The 38% branching t the CH 3 HCO prduct channel we derive using ur calibratin factr frm the methyl vinyl ether phtdissciatin is in reasnable agreement with the 44 % cntributin reprted by Bley et al., 3