Vacuum Ultraviolet Spectroscopy of the Carbon Molecule C 3 in Matrix Isolated State: Experiment and Theory

Transcription

1 J. Phys. Chem. A 2002, 106, Vcuum Ultrviolet Spectroscopy of the Crbon Molecule C 3 in Mtrix Isolted Stte: Experiment nd Theory G. Monninger, M. Fo1rderer, P. Gu1rtler, S. Klhofer, S. Petersen, L. Nemes, P. G. Szly, nd W. Kr1tschmer*, Mx-Plnck-Institut für Kernphysik, Postfch , D Heidelberg, Germny, Hsylb t Desy, Notkestrsse 85, D Hmburg, Germny, Chemicl Reserch Center, Reserch Lbortory for Mterils nd EnVironmentl Chemistry, Hungrin Acdemy of Sciences, Pusztszeri út 59-67, H-1025 Budpest, Hungry, nd Deprtment of Theoreticl Chemistry, EötVös Loránd UniVersity, P. O. Box 32, H-1518 Budpest, Hungry ReceiVed: NoVember 19, 2001; In Finl Form: Mrch 19, 2002 Crbon molecules were produced by evportion of grphite nd mtrix-isolted in solid neon nd rgon. Using synchrotron rdition, the bsorption spectr of the crbon clusters were recorded from 1100 to 5600 Å, nd the fluorescence or phosphorescence spectr were recorded from 1200 to 9000 Å. We observed n intense, brod bsorption bnd system centered t round 1600 Å in neon nd 1700 Å in rgon. By mesuring the excittion spectrum of the 3 Π u f X 1 Σ + g phosphorescence nd checking the intensity correltion with the known 1 Π u r X 1 Σ + g bsorption bnd, we could show tht the observed VUV bnd system is the llowed 1 Σ + u r X 1 Σ + g electronic trnsition of the C 3 molecule. At the blue nd the red side of the bnd system, distinct progressions cn be observed which most likely correspond to symmetric stretch of bout ν cm -1 nd bending mode of bout ν cm -1, respectively. In the bnd center, however, complicted superposition of severl vibrtionl progressions ppers indicting tht besides the 1 Σ + u stte lso other sttes seem to contribute to the bsorption. Quntum chemicl MR-AQCC clcultions suggest tht these contributing sttes re 1 Π g sttes which re close in energy to the 1 Σ + u stte nd cn interct vi vibronic coupling, conjecture supported by preliminry clcultions of synthetic spectr in which such coupling ws included. Furthermore, the clcultions show tht the 1 Σ + u energy decreses upon bending, leding to complex lndscpe of energy surfces which include voided crossing type fetures, nd rendering more detiled spectrl clcultions difficult. 1. Introduction Since the first observtion of the 4050 Å bnd system in the spectr of comets in nd the identifiction of the crrier molecule C 3 seventy yers lter, 2 crbon clusters gined growing experimentl nd theoreticl interest. Reviews on smller crbon species were given by Weltner nd vn Zee 3 nd, for the more recent developments, by vn Orten nd Syklly. 4 During the pst decdes, the role of crbon molecules in strophysics nd combustion chemistry ws recognized to n incresing degree, 5,6 nd recently, fullerenes, i.e., lrge, closed cge crbon molecules, gined considerble ttention. 7,8 The crbon cluster C 3 is the dominnt constituent of crbon vpor generted by heting grphite to bout 3000 K. Under these conditions, the vpor consists pproximtely of tomic C (10%), C 2 (20%), C 3 (70%), nd rther smll frction of lrger species (<1%) Mny groups investigted the spectr of C 3 by the mtrix-isoltion technique, in which crbon molecules re trpped long with n excess of n inert gs (e.g., rgon or neon) on cold (5-15 K) substrte (see, e.g., ref 3, 4, nd 12-16). The dvntges of this technique re tht no moleculr rottions occur in the mtrix nd tht, becuse of the low * To whom correspondence should be ddressed. E-mil: kretschmer@ mpi-hd.mpg.de. Mx-Plnck-Institut für Kernphysik. Hsylb t Desy. Hungrin Acdemy of Sciences. Eötvös Loránd University. temperture, the trnsitions emerge exclusively from the electronic nd vibrtionl ground stte. This simplifies the bsorption spectr; however, the interction of the molecules with the mtrix introduces some rtifcts; tht is, it shifts nd brodens the bsorptions. The ground-stte configurtion of C 3 exhibits completely filled orbitls...(1π u ) 4 (3σ u ) 2 (1π g ) 0. The most prominent bsorption in the ner UV is the cometry bnd t 4050 Å ssigned to 1 Π u r X 1 Σ + g trnsition in which n electron in the 3σ u orbitl is promoted into the empty 1π g orbitl. Becuse of mtrix interctions, the cometry bsorption is shifted towrd 4070 Å in solid neon nd 4100 Å in solid rgon. In emission, strong C 3 phosphorescence t bout 5900 Å ws observed nd tenttively ssigned by Weltner nd co-workers to the lowest triplet trnsition 3 Π u r X 1 Σ + g 13. The ssignment ws lter confirmed by Bondybey nd English 15 nd supported by quntum chemicl clcultions (see, e.g., ref 17). A recent work on the C 3 phosphorescence cn be found in ref 18. Besides the trnsition leding to the 4050 Å bnd, one expects n llowed, i.e., rther strong, 1 Σ + u r 1 Σ + g bsorption system of C 3 to occur t considerbly shorter wvelength. This bnd, lredy predicted by Pitzer nd Clementi 19 in their clssicl rticle of 1959, should originte from the promotion of 1π u electron into the empty 1π g orbitl. Erly quntum chemicl clcultions were crried out by Willims 20 nd Römelt et l. 21 The lter uthors used the MRD CI method to clculte the excittion energy of the 1 Σ + u stte to be bout 8 ev. Experi /jp CCC: $ Americn Chemicl Society Published on Web 05/25/2002

2 5780 J. Phys. Chem. A, Vol. 106, No. 24, 2002 Monninger et l. Figure 1. Experimentl setup of the sttion HIGITI t Hsylb. Abbrevitions: filter wheel (FW), slit (S), plnr mirror (P), Torroid ellipsoidl mirror (ET), smple holder with MgF 2 substrte (SH), liquid helium cryostt (C), lens (L), fiber optic (FO), photomultiplier (PM), filter holder of the Sey monochromtor (FH). When the synchrotron rdition bem is elongted through the MgF 2 substrte, the trnsmission spectr re recorded behind the smple holder (SH) with photomultiplier (TR). For further detils, see text. mentl evidence for this bnd ws provided for the first time in 1982 by Chng nd Grhm who isolted crbon vpor in n rgon mtrix t 8 K nd ssigned spectrl feture ner 1890 Å (6.6 ev) to this trnsition. 22 The ssignment ws supported by intensity correltions with the known 1 Π u r X 1 Σ + g bsorption bnd system nd from spectr obtined by 13 C substituted species. However, the bnd position ppers to be in conflict with more recent theoreticl predictions, which locte the trnsition between 7.15 nd 8.14 ev. 23,24 The MRD CI clcultions of ref 21 suggest tht the intensity of the 1 Σ + u r 1 Σ + g bsorption of liner C 3 should be distributed over quite lrge energy rnge. Thus, the spectrl feture observed by Chng nd Grhm might be prt of still broder bsorption bnd. The position of the 1 Σ + u r 1 Σ + g bnd system cn lso be estimted from trends of the homologous series of electronic bsorption spectr of liner crbon chins C 2n+1 (n ) 2-7) in neon mtrixes, which were mesured by Mier nd co-workers. An extrpoltion would yield wvelength for the 1 Σ + u r 1 Σ + g trnsition of C 3 ner 1700 Å. 25 To investigte this strong nd thus importnt trnsition of C 3 in more detil, we performed vcuum ultrviolet spectroscopy of the mtrix-isolted crbon species both in rgon nd neon using synchrotron rdition. In ddition, in theoreticl prt of our contribution, we try to interpret the dt nd give n t lest preliminry explntion of the bsic structure of the observed bsorption bnd system. 2. Experimentl Section 2.1. Spectroscopic Instrumenttion. Our mesurements were crried out t the storge ring DORIS III t Desy (HASYLAB) in Hmburg. The setup of the HIGITI sttion is dedicted for opticl spectroscopy of mtrix isolted species using synchrotron rdition in the wvelength rnge from 400 to 5600 Å nd is optimized in rdition intensity nd spectrl resolution between 600 nd 2500 Å (resolution Å). In our experiment, trnsmission spectr re limited to 1150 Å becuse of the bsorption of the MgF 2 substrte, wheres for emission spectr, the bsorption of the rre gs mtrix (neon or rgon) defines the lowest detectble wvelength. This sttion ws described in detil in ref 26 (see Figure 1). In brief, synchrotron rdition is dispersed by primry monochromtor. Severl filters (MgF 2, qurtz, nd BK7) select wvelength rnge nd cut rdition of higher order grting reflections. After pssing the MgF 2 substrte (SH), the rdition hits BK7 viewport which is coted towrd the inside of the chmber with sodium slicylte to convert the rdition λ < 3000 Å into visible light. An externl photomultiplier (TR) detects the converted rdition nd thus indirectly monitors the trnsmitted synchrotron light. Opticl spectr cn be recorded with three different systems. The primry monochromtor which disperses the synchrotron rdition ws used for mesuring trnsmission spectr of the smple in the spectrl rnge of Å with resolution up to 0.3 Å, Sey-Nmiok monochromtor (resolution Å) which covers the vcuum-ultrviolet (VUV) to ultrviolet (UV) wvelength rnge between 1100 nd 3000 Å, nd Spex monochromtor (resolution > 3 Å) for the ultrviolet to visible rnge (2000 nd 9000 Å; Detection: photomultiplier or liquid nitrogen cooled CCD rry). The ltter two instruments were employed for emission spectroscopy UHV Comprtments nd Experimentl Procedures. Crbon vpor molecules were produced in wter-cooled UHV chmber by resistive heting of grphite rods using DC current (typiclly At5-8 V). 18 The MgF 2 substrte ws fcing the rods t distnce of bout 700 mm. To reduce the deposition of rest-gs molecules such s H 2 O, CO, nd CO 2 in the solid noble gs mtrix, ll prts of the source chmber were bked out t 120 C before ech experiment. For the sme reson, the crbon rods were heted to tempertures close to the evportion condition before smple preprtion. A mgneti-

3 Spectroscopy of the Crbon Molecule C 3 J. Phys. Chem. A, Vol. 106, No. 24, clly borne turbo moleculr pump in combintion with membrne pump system provided oil-free conditions nd evcuted the crbon source to pressures smller thn mbr. Between the cluster source nd mtrix deposition-comprtment, nother chmber with liquid-nitrogen-cooled shield ws plced. A smll perture (dimeter 7 mm) in the shield reduced the rdition trnsfer of the heted crbon rods to the cold substrte in the mtrix chmber. A UHV vlve seprted the crbon cluster source nd smple comprtment. The UHV mtrix-deposition comprtment ws equipped with He-flow cryostt, which ws moveble nd rottble under UHV conditions (p < mbr). At the cryostt tip, smple holder with the MgF 2 substrte ws locted. The helium flow through the cryostt nd in ddition resistnce heter mounted on the cryostt just bove the coldfinger were employed for temperture control (Lkeshore Temperture Controller model 330). Commercilly vilble gses with purity of % for rgon nd neon hve been used for mtrix preprtion without further purifiction. Crbon-rgon mtrixes were prepred by depositing rgon t rte of Å/min nd crbon vpor (produced under vrious conditions) onto the low-temperture substrte t 18 K for min. We prepred smples for which the crbon cluster concentrtion ws rther low (i.e., the molr rre-gsto-crbon rtio ws more thn 1000). Under these conditions, mutul cluster-cluster rections were smll, nd the mtrix contined minly the cluster distribution previling in the vpor. The deposition process ws monitored by mesuring bsorption spectr. Argon mtrixes showed reltively low scttering t low deposition rtes nd t n elevted deposition temperture (18 K). These fetures my be relted to the crystlliztion of solid rgon. 27 After prepring the smple, the substrte ws cooled to T ) 10 K, where ll spectr were tken. To investigte possible correlted behvior of the known electronic trnsition 1 Π u r X 1 Σ + g of C 3 nd the new bnd system t round 1700 Å, the smple ws thermlly nneled up to 34 K for severl times. After ech temperture step, the bsorption fetures were recorded gin t 10 K. By heting the smples, diffusion of smller molecules in the mtrix ws possible, leding to rections between crbon molecules nd/or toms. This moleculr growth process forms lrger nd consumes smller crbon species nd thus chnges the intensity of the observed bsorption lines in the mesured spectr. 28 A distinct intensity correltion of two spectrl bnds is hint to common moleculr origin. Neon mtrixes were prepred by co-depositing neon t rte of 4500 Å/min nd crbon vpor onto the low-temperture substrte t 4.3 K for nerly 60 min. Neon mtrixes showed less scttering in the vcuum UV thn rgon mtrixes. Smple preprtion conditions were chosen so tht the bsorption of C 3 ws the dominnt feture in the recorded spectr. To check for vcuum ultrviolet bsorption fetures of contminting molecules in the mtrix, we inserted fresh grphite electrodes into our evportor nd outgssed the rods up to our stndrd evportion tempertures. The progression of the CO A 1 Π r X 1 Σ trnsition ws the dominnt bnd system. Creful outgssing of the grphite electrodes prior to experiment reduced the CO bsorption to n extent, where monitoring the CO bsorptions becme difficult. Figure 2. Electronic bsorption spectrum of crbon vpor trpped in solid rgon t 10 K in the vcuum ultrviolet-visible wvelength rnge. The bnd systems round 4100 nd 3080 Å re ssigned to the crbon clusters C 3 nd to liner C 9, respectively. The strong feture ner 2470 Å is superposition of n bsorption of C 2 (Mulliken system) nd of liner C 6. The structured nd brod bsorption pttern between 1400 nd 1900 Å origintes from the electronic trnsition 1 Σ + u-x 1 Σ + g of C 3. The spectrum is recorded t resolution of 1ÅintheVUVnd ws bckground-corrected for clrity. Figure 3. Electronic bsorption spectrum of crbon vpor trpped in solid neon t 4.3 K in the vcuum ultrviolet-visible wvelength rnge. Becuse of the high dilution (Ne/C > 2500), the spectrum reflects more closely the crbon cluster distribution of our crbon source. The two electronic trnsitions of C 3 t 1600 nd 4070 Å re the dominnt fetures. 3. Experimentl Results nd Discussion 3.1. Vcuum, Ultrviolet, nd Visible Crbon Cluster Absorptions. Absorption bnds of rgon nd neon mtrixisolted crbon clusters re shown in Figures 2 nd 3, respectively. These figures re n overview of spectr recorded in different wvelength rnges, merged, nd bckground corrected. For wvelengths lrger thn 2000 Å, the bsorption fetures in rgon re in good greement with spectr reported by different other groups Our dt for the first time show the entire 1 Σ + u r 1 Σ + g bsorption feture of C 3 centering t round 1700 Å in rgon nd 1600 Å in neon. The frction of this bnd system between 1700 nd 1890 Å ws lredy observed by Chng nd Grhm in n rgon mtrix t 8 K. 22 However, the whole bnd system extends much further into the vcuum UV, t lest to bout 1400 Å. When smples were produced under different conditions, we noticed tht the dditionl spectrl fetures between 1400 nd 1300 Å correlted in intensity with the 1600/1700 Å bnd system. This observtion suggested tht C 3 might lso be the crrier of the bnds in the Å region, which perhps belong to Rydberg sttes. The excittion energy of the (1π u f 3s) sttes 3 Π u nd 1 Π u were clculted to be t 9.35 (1330) nd 9.51 ev (1303 Å) respectively. 21

4 5782 J. Phys. Chem. A, Vol. 106, No. 24, 2002 Monninger et l. Figure 4. Absorption spectr of the C 3 ( 1 Σ + u-x 1 Σ + g) system () in solid rgon t 10 K nd (b) in solid neon t 4.3 K. The spectr re recorded t resolution of 1 Å, nd no bckground correction ws pplied. Under the deposition conditions chosen, the clusters embedded in rgon show higher degree of moleculr cogultion, i.e., show spectrl fetures belonging to crbon clusters lrger thn C 3. There re wek bnds of the species liner C 9 (t round 3100 Å, see ref 29) nd liner C 6 (the bnd system round 2400 Å, see ref 30) upon which the Mulliken bnd of C 2 t 2380 Å (see ref 34) is superposed. The rgon mtrix spectr exhibit brod structure t 1320 Å which hs no direct counterprt in the neon dt but might be ssigned to the lredy mentioned Rydberg sttes of C 3. In neon mtrixes, the C 3 fetures re most prominent (Figure 3). Known spectrl bnds of other crbon molecules re either wek or could not be observed. Next to the estblished electronic trnsition 1 Π u r X 1 Σ + g of C 3 ner 4070 Å, brod bnd system of liner C 6 t 2300 Å occurs on which the Mulliken bnd of C 2 t 2320 Å is superposed, besides shift in wvelength just like in rgon, however, with much lower intensity. The dominnt bsorption round 1600 Å represents the 1 Σ + u r X 1 Σ + g trnsition of C 3. It is seen gin in both, rgon (Figure 4) nd neon (Figure 4b) mtrixes, t resolution of 1.0 Å nd without bckground correction. The influence of the different mtrix mterils on the spectrl feture of C 3 cn be recognized. For the convenience of the reder, list of wvelengths (wvenumbers) of the bnds is given in Tble 1 in solid rgon nd Tble 1b in solid neon. It is well-known tht the mtrix effects in neon re less severe, nd compred to rgon mtrixes, the spectr in neon re more like the spectr in the gs phse. Thus, we think tht the spectr obtined in neon re superior in qulity to those in rgon. Furthermore, our dt my help to locte specific vibronic trnsitions for closer exmintion by highresolution gs-phse spectroscopy. TABLE 1: Positions of Observed Bnd Mxim of the 1 Σ + u r X 1 Σ + g System in Solid Argon t 10 K nd Solid Neon t 4.3 K λ[å] ν[cm -1 ] λ[å] ν[cm -1 ] λ[å] ν[cm -1 ] A. Peks in the Argon Mtrix # # B. Peks in the Neon Mtrix # # A # indictes the bnd t which the bsorptions (Figure 4) hve mximum Correltion of Excittion nd Absorption Spectr. We serched for new C 3 emission bnds but could only detect the well-known, strong 3 Π u f X 1 Σ + g phosphorescence trnsition of C 3 t 5900 Å. Phosphorescence could be induced resonntly by excittion of the A 1 Π u stte of C 3, nd lso by excittion of the brod VUV bsorption bnd round 1700 Å. This experimentl result suggests tht n excittion of the 1 Σ + u r X 1 Σ + g trnsition results in n intersystem crossing into the triplet mnifold of sttes. Nonrditive electronic relxtion then seems to led to popultion of the vibronic ground level of the triplet sttes including the 3 Π u stte. The detiled mechnism of this lter process remins to be investigted. In ny event, the relxtion of the 3 Π u stte to the X 1 Σ + g ground stte cn be observed s phosphorescence t 5900 Å. 13,18 In mtrix-relxtion studies, the efficient intersystem crossing nd internl conversion were lso observed for vrious other smll molecules, e.g., C 2 or NH. 35,36 The HIGITI sttion llowed us to record the emission of excited molecules simultneously in two different wvelength rnges, while scnning over the 1700 Å feture nd monitoring the trnsmission spectrum. The SEYA spectrometer ws used to record the emission signl of crbon monoxide, which is the

5 Spectroscopy of the Crbon Molecule C 3 J. Phys. Chem. A, Vol. 106, No. 24, Figure 5. Absorption nd excittion spectr of crbon vpor in n rgon mtrix. Tht the observed VUV bsorption is the ( 1 Σ + u-x 1 Σ + g) trnsition of C 3 molecule is suggested by the greement of the vcuum ultrviolet bsorption (c) nd the excittion spectrum of the known phosphorescence 3 Π u - X 1 Σ + g of C 3 t 5900 Å. In trce, the C 3 phosphorescence ws mesured by CCD rry, nd in b, it ws mesured by photomultiplier. The excittion function of the CO phosphorescence (Cmeron system) round 2150 Å (resolution 150 Å) ws lso monitored to check for possible contribution of the CO bsorption to the C 3 spectrum in the vicinity of 1500 Å (d). The contributions of CO pper to be negligible. min potentil contmintion in our spectr. The progression of the CO A 1 Π r X 1 Σ trnsition extended from 1560 to 1310 Å in rgon nd could interfere with the C 3 bnd system. To detect the fetures belonging to CO, the Sey monochromtor ws set to 2150 Å (with resolution of 150 Å) in order to record the phosphorescence signl of the 3 Π f X 1 Σ progression of CO (Cmeron system), which is excited by the A 1 Π r X 1 Σ trnsition. To monitor the phosphorescence of C 3, the Spex monochromtor ws set to 5900 Å (resolution 18 Å), nd the intensity ws monitored either with CCD rry or photomultiplier (see Figure 1). By this method, the vcuum ultrviolet excittion spectr (which should reproduce the bsorption spectr prt from scling fctor) of both crbon monoxide nd C 3 could be detected simultneously nd from the sme mtrix. Our dt re displyed in Figure 5 nd show definitely tht the strong nd shrp excittion (bsorption) bnds t round 1500 Å, which on first glnce might be ssocited with CO, re in fct originting from C 3. The C 3 excittion spectrum correltes well with the trnsmission spectrum mesured simultneously in the sme wvelength rnge Correltion of Absorptions by Mtrix Anneling Experiments. Figure 6 shows bsorption spectr of crbon vpor in rgon mtrixes tken in two wvelength rnges. The temperture dependence of crbon cluster ggregtion ws tested by nneling. After deposition, the smple ws heted to tempertures of 28, 30, 32, nd 34 K for bout 10 min, while spectr were recorded t 10 K fter ech nneling step. For clrity, only three spectr re shown here verticlly shifted. The lowest spectrum ws mesured fter the deposition; the two others were mesured fter nneling to T ) 30 nd 34 K, respectively. In the wvelength rnge of Å, the chrcteristiclly shped 1 Π u r X 1 Σ + g cometry bsorption bnd of C 3 round 4100 Å decreses in intensity during the therml tretment of the smple. The brod bsorption between 1350 nd 1900 Å shows similr behvior nd supports n ssignment to C 3. To estblish the degree of correltion for the two fetures, the pek heights (or integrted intensities) re plotted ginst ech other. A positive correltion should result in stright line with n intercept t zero. For this purpose, the spectr of smples produced under vrious conditions were Figure 6. Absorption spectr of crbon clusters in rgon t 10 K before nd fter nneling to 30 nd 34 K. All spectr were recorded t 10 K. The two upper curves re shifted verticlly for clrity. The thermlly ctivted diffusion results in cluster-cluster rections nd moleculr growth. The bnd of C 3 t 4100 Å nd the brod VUV structure round 1700 Å show correlted decrese, wheres other bsorption fetures, belonging to lrger crbon molecules, increse. The bnd t 4500 Å probbly origintes from liner C 15 (ref 25). Figure 7. Correltion plot of the integrted bsorptivities of the C 3 bnd system t 4100 Å versus tht of single nrrow pek t 1498 Å nd the entire brod 1700 Å system, respectively. The nrrow pek is mrked by n sterix in Figure 6. studied. Different initil cluster distributions nd concentrtions in the rgon mtrix were obtined, for exmple, by chnging the DC power of the evportion process nd chnging the rgon deposition rte. In totl, there were two nneling runs with nine dt points nd few smples without therml tretment t our disposl. First, we plotted the integrted intensity of the known C 3 bsorption t 4100 Å versus the integrted intensity of the entire VUV C 3 bsorption feture ( 1700 Å bnd in Figure 7) which we plced between 1427 nd 1846 Å, ssuming liner bseline between these points. The experimentl points sctter round the stright line fit. The correltion cn be improved when nonliner bckground is introduced which better ccounts for the scttering of the mtrix. Therefore, we investigted lso nrrow peks, e.g., t 1498 (shown by n sterix in Figure 6) 1522, nd 1547 Å. The dt of the 1498 Å bsorption re displyed in Figure 7. The correltion coefficients were clculted s 0.98 for ll three bnds, nd the intercept ws consistently zero. These dt gin support the ssignment of the bnd s the 1 Σ + u - X 1 Σ + g trnsition of C VUV Absorption Bnd System. Our ssignment of this bnd to the 1 Σ + u r X 1 Σ + g trnsition of C 3 is in good greement with the vilble quntum chemicl clcultions Römelt

6 5784 J. Phys. Chem. A, Vol. 106, No. 24, 2002 Monninger et l. t l. 21 estimte tht the 1 Σ + u stte is locted bout 8 ev bove the ground stte. This vlue corresponds to 1550 Å which is close to the observed 1 Σ + u r X 1 Σ + g bnd center. These uthors conclude tht the ntibonding chrcter of this excited stte leds to potentil minimum t slightly lrger C-C bond length, i.e., t Å compred to Å in the 1 Σ + g ground-stte. Furthermore, potentil curves for the symmetric nd symmetric C-C stretch nd for the C-C-C bending in the 1 Σ + u stte were clculted in ref 21. The results indicte the presence of bound liner configurtion, i.e., locl minimum t zero bending. The potentil curve s function of bending ngle is M-shped i.e., exhibits locl minimum t 0 in the middle of the M (liner cse) but lso two genuine outer minim t finite bending ngles. These ltter minim originte from n voided crossing of the potentil curve with tht of lower lying Π g stte. This result would imply tht C 3 should be bent in the vibrtionl ground level of the 1 Σ + u stte. The predicted symmetric stretching frequency is quite similr to tht of the ground stte, i.e., ν 1 ) 1235 cm -1. The ntisymmetric stretching frequency ν 3 is estimted to be considerbly lrger, nd no bending mode frequency is given. More recently, Grimme nd Peyerimhoff 37 gin clculted the 1 Σ + u nd djcent excited C 3 sttes, pplying the TD-DFT method. The new dt show tht the potentil curve of the 1 Σ + u stte is more likely W-shped, i.e., shows no minimum t zero bending but rther declines with incresing bending ngle, thereby pproching lower lying 1 Π g nd leding to n voided crossing type of minimum feture in the energy surfce. Our own MR-AQCC clcultion support this finding (see below). A closer exmintion of the bsorption feture (Figure 4, in prticulr 4b) shows reltively cler progressions t the red nd blue portion of the bsorption feture. In the centrl prt, both progressions pper to be superimposed in n irregulr fshion, nd lso dditionl progressions seem to complicte the spectrum. We tried to find further periodicity in the spectrum by pplying severl numericl techniques, but these ttempts remined unsuccessful. The progression t the blue wing of the bsorption bnd extends to 1546 Å in the red nd probbly beyond, but becuse of superposition with other peks, this progression is less cler. The first two peks t the blue side re spced by 985 cm -1, nd the spcing towrd the red increses to 1099 cm -1 probbly becuse of nhrmonicities of the potentil curve. An extrpoltion of the frequency dt to further peks in the red leds to rther tenttive bnd origin t bout 1581 Å in neon (1605 Å in rgon). In view of the complexities of the centrl portion of the bsorption spectrum, we were not ttempting bnd nlysis nd serch for bnd origins which we think would probbly give no meningful results. We insted try to reproduce the bsic pttern of the entire spectrum by quntum chemicl clcultions of synthetic bsorption spectr, s will be reported in the following chpter. From these dt, it ppers tht the bove progression should extend much further into the red, nd in fct, there re wiggles discernible which re reching up to 1780 Å in the observed spectrum. We conclude tht the progression t the blue wing with n verge spcing of bout 1100 cm -1 fits well to the theoreticlly predicted figure 21 of the ν 1 mode; tht is, it very likely belongs to the symmetric stretch. The long progression strting from bout 1859 Å in the red portion nd extending to wvelength smller thn 1635 Å in neon ( Å in rgon) hs spcing of bout 550 cm -1 nd my be ttributed to bending vibrtion. There re vrious irregulrities pprent in this progression, e.g., t round 1820, 1740, nd 1720 Å, which seem to indicte complex bending Figure 8. Electronic energies of the sttes of concern s function of the bending coordinte. The clcultions were performed with fixed C-C bond length of Å. Only the components with B 2 symmetry re shown. Notice the crossing of levels (for detils, see text). potentil. Strongly vrying vibronic couplings between the 1 B 2 components of the 1 Σ u nd the djcent 1 Π g sttes (notice the divergence of the B 2 sttes originting from 1 Σ u nd 3 1 Π g in Figure 8) nd n voided crossing type of energy minimum in the energy surfce my be responsible for these complictions. Compring the progressions t the red nd blue portion of the bnd system in both mtrixes, one finds one-to-one reltion between the bnds in rgon nd neon. However, it is difficult to correlte the bnds observed in both mtrixes through the entire bsorption feture. Apprently, the two mtrixes hve different influence on the spectrl fetures of C 3. We think tht the pek group t 1565, 1602, 1609, nd 1626 Å in neon nd 1594, 1635, 1643, nd 1662 Å in rgon resemble ech other becuse these re similr in shift nd pek shpe. For further discussion, we should consider the low tempertures previling in our mtrixes. Under these conditions, ll electronic trnsitions originte from the lowest vibrtionl level (V ) 0) of the totlly symmetric 1 Σ + g ground-stte. Let us ssume tht C 3 remins liner in the electroniclly excited stte s for exmple ws done in the erly work of Chng nd Grhm. 22 In the 1 Σ + u r X 1 Σ + g bsorption spectrum, one expects only progressions of the symmetric stretch vibrtion to occur. The other possible vibrtions of odd (u) symmetry should be bsent for symmetry resons. However, their even overtones (with V ) 2, 4, nd 6) hve even (g) symmetry nd my occur. Chng nd Grhm ssigned the wek progression t the red wing of the bsorption to the progression of even overtones of the bending vibrtion nd obtined fundmentl frequency of bout 300 cm -1 from their dt in rgon mtrixes. 22 In view of the recent theoreticl results, however, this interprettion hs to be modified. There re electronic sttes with 1 Π g symmetry in close vicinity ( ev, see Figure 8) of the 1 Σ + u level which cn interct with the 1 Σ + u stte vi vibronic coupling nd cn led to the excittion of the bending mode (π u )inthe electroniclly excited 1 Σ + u stte. In other words, becuse of the vibronic interction, C 3 becomes bent (C 2V ) in the electroniclly excited 1 Σ + u (respectively B 2 ) stte which implies tht trnsitions to ll levels of the bending mode (totlly symmetric species 1 ) become electroniclly llowed. The 550 cm -1 progression my thus be directly relted to usul progression in such bending mode, rther thn to progression of only the even overtones of tht mode. A bending frequency of 550 cm -1 is surprisingly lrge. Both, the stbiliztion of C 3 by bending nd the presence

7 Spectroscopy of the Crbon Molecule C 3 J. Phys. Chem. A, Vol. 106, No. 24, TABLE 2: Clculted Verticl Excittion Energies (ev) (Oscilltor Strength in Prentheses) of the Low-Lying Vlence Sttes of C 3 stte chrcter MCSCF MR-CI MR-AQCC 1 1 Π u σ u f π g (0.02) 1 1 Σ - u π u f π g u π u f π g Π g σ g f π g Φ g σ u, π u f π g, π g Π g σ u, π u f π g, π g Π g σ u f π u Σ u+ π u f π g (0.94) 2 1 Π u σ g, π u f π g, π g (0.00) 1 1 Φ u σ g, π u f π g, π g Σ u+ σ u, π u, π u f π g, π g, π g Clcultions hve been performed t the experimentl equilibrium geometry ( Å). 47 The 8 8 CAS reference spce with orbitls 4σ g,3σ u,1π u,1π g, nd 2π u hs been used. The bsis set ws cc-pvtz. of electrons in the higher π orbitls my be responsible for the stiff bending behvior of the excited molecule. The ltter feture ws lredy noticed in the well studied 1 Π u stte of C 3 (see, e.g., ref 3). As further consequence of vibronic coupling, excittion to the 1 Π g sttes cn borrow intensity nd thus contribute to the bsorptions in the observed spectrum. We think tht such borrowing effects my, to some extent, be responsible for the complex pttern of bnds one cn notice in the centrl portion of the bsorption feture. Our mesurements showed no dditionl fluorescence emerging from the excited 1 Σ + u stte. Apprently, efficient rditionless relxtion nd n intersystem crossing into the triplet mnifold of C 3 tkes plce. The theoreticl results presented in the following suggest t lest in prt n explntion for this observtion. Clcultions of the energy surfces predict funnel like feture, i.e., close pproch (voided crossing or conicl intersection) of the sttes originting from 1 Σ + u nd 2 1 Π g t bout 163 bending ngle (see Figure 8). As consequence, the Born-Oppenheimer pproximtion must brek down t round this configurtion, nd photoexcited C 3 will, vi nucler vibrtions, efficiently rech lower levels, from which further rditionless relxtion seems to occur. 4. Theoreticl Anlysis of the VUV Bnd As described in the experimentl prt, ll doubts concerning the origin of the observed bsorption hve been removed, nd the bnd is ttributed entirely to the C 3 moiety. In this section, we present theoreticl results which intend to help in the ssignment nd understnding of this bnd. For comprison with the experiment, we concentrte on the C 3 spectrum observed in neon. To obtin ccurte theoreticl dt, the method used to describe the electronic structure hs been chosen very crefully. First, preliminry model clcultions were performed which show tht there re severl excited sttes overlpping in energy with the mesured spectrum. A qulittive chrcteriztion of these sttes is given in Tble 2. It is seen from the tble tht some of these sttes re doubly excited, so multireference type methods need to be used. To include lso dynmicl correltion, which is necessry if ccurte excittion energies re required, MR-CI-type methods seem to be the best choice. The ccurcy of MR-CI-type methods for excited sttes hs been investigted recently by Müller et l. 38 on ditomic molecules including C 2. It hs been concluded tht the error of the MR-CI method becuse of the well-known size extensivity error is substntil, nd the use of MR-CI with Dvidson correction 39,40 or pproximtely extensive versions such s MR-AQCC (multireference verged qudrtic coupled cluster) 41,42 is necessry to obtin relible results. These uthors concluded tht the error of T e obtined by the MR-AQCC method using vlence CAS (complete ctive spce) reference spce is bout cm -1 only if the bsis set error is eliminted by extrpoltion. Relevnt for this study is tht the error ws only bout cm -1 with the moderte triple-ζ correltion consistent (cc-pvtz) bsis by Dunning et l. 43,44 when only vlence sttes were considered. For C 3, the use of the full vlence CAS reference spce is not prcticl. Insted, the reference spce hs been selected to rech blnced description of the sttes of interest, i.e., the 1 1 Σ u,2 1 Π g,1 1 Φ g, nd 3 1 Π g sttes. In this process, severl reference spces hve been compred, nd the following hs been selected for further clcultions: eight electron six orbitl CAS including the 4σ g,1π u,1π g, nd 1σ u orbitls ugmented by single excittion to the 5σ g nd 2π u orbitls. All single nd double excittions out of these reference functions hve been considered in the subsequent MR-AQCC clcultions except tht the three MOs corresponding to the core orbitls hve been kept doubly occupied. This type of wve function included bout 6-7 million configurtions depending on the symmetry of the sttes. We expect somewht lrger error thn with full vlence CAS. The error in the excittion energies cn be estimted by compring results obtined by different reference spces nd by the MR-CI nd the MR-AQCC methods to be bout cm -1. We used the cc-pvtz tomic bsis set in ll clcultions. We did not include diffuse functions becuse Rydberg sttes re higher in energy thn the ones investigted in this study. The orbitls were obtined from stte-verged MCSCF clcultion bsed on eight electron nd nine orbitl CAS wve function including the 4σ g,1π u,1π g,1σ u,5σ g nd 2π u orbitls, s mentioned bove. The verging included the 1 Σ g + ground stte, the two lowest 1 Σ u + sttes, the four lowest 1 Π g sttes, nd 1 Φ g stte. The Columbus progrm system ws used in ll clcultions. 45 From the sttes in Tble 2, excittions to the 1 1 Σ u +,1 1 Π u, nd 2 1 Π u sttes re opticlly llowed. The clculted trnsition moment for the lter is, however, very smll, nd therefore, one expects negligible contribution from the 2 1 Π u stte to the spectrum. This cn esily be understood qulittively lso, becuse this stte is dominted by double excittion from the ground stte. Therefore, the observed VUV bnd cn be sfely identified with trnsition from the ground stte to the 1 1 Σ u + stte. This ssignment is in greement with the previous suggestions of Chng nd Grhm 22 nd the theoreticl predictions of Römelt et l. 21 According to group-theoreticl-bsed selection rules, the bsorption bnd corresponding to the excittion 1 1 Σ u + r 1 1 Σ g + should show progressions corresponding to vibrtions of g (even) symmetry, e.g., the symmetric stretching vibrtion, or possibly even overtones of the other u (odd) modes in the excited stte. A spectrum bsed on the theoreticl dt (see below) nd using Frnck-Condon-type nlysis is seen in Figure 9 (the cse Λ ) 0). The synthetic spectrum shows vibrtionl progression similr to tht observed on the high energy side of the experimentl bsorption feture. However, the somewht irregulr progressions observed on the low energy side re not reproduced, indicting tht some other interctions must influence the spectrum. As hs been pointed out lredy in ref 21, such interction is possible with sttes of Π g symmetry vi the C-C-C bending vibrtion. The group theoreticl restriction

8 5786 J. Phys. Chem. A, Vol. 106, No. 24, 2002 Monninger et l. Figure 9. Simulted VUV C 3 spectr compred with the experimentl dt (neon mtrix, see Figure 3). Clcultions were crried out for vrious interstte coupling prmeters Λ (Λ ) 0 mens no coupling) under the simplifying ssumption tht only the djcent 3Π g stte (see Figure 8) couples. Notice tht (prt from the bsolute position of the bnd system) the synthetic spectrum cn qulittively reproduce some of the bsic ptterns of the experimentl bnd system. for these interctions is given by Γ elec,1 X Γ vib Γ elec,2 where Γ elec,1 nd Γ elec,2 re the species of the sttes coupled by the vibrtion of species Γ vib. Using the species of the liner bending mode (Π u ), the eqution is stisfied for the excited Π g but not for the Φ g sttes. There re two Π g sttes close to the 1 1 Σ + u stte, so closer investigtion of the vibronic interction is wrrnted. Considering the sttes in Tble 2, we do not see ny other possibility for vibronic coupling. In first step, we hve clculted prts of the potentil energy surfce long the bending nd stretching coordinte. Qulittively, we reched similr conclusions s the uthors of ref 37. With respect to the stretching coordinte, the surfce shows distinct minimum t the liner configurtion. With respect to the bending coordinte, the results re shown in Figure 8. Clerly, the energy of the 1 1 Σ + u stte decreses with bending; feture which implies tht the 1 1 Σ + u energy surfce exhibits sddle point t the liner geometry. The lowering of the energy upon bending cn be explined by the interction of the 1 1 Σ + u stte with one of the components of the 3 1 Π g stte which hs the sme B 2 symmetry (in the bent form) s the 1 1 Σ + u. The picture is even more complicted by the presence of the 2 1 Π g stte which ppers to cross the 1 1 Σ + u curve t bending ngle of bout 163. To gin more insight, we pplied the simple liner vibronic interction model developed by Köppel nd co-workers, 46 nd it ppers tht qulittive understnding of the observed complicted vibrtionl structure cn be obtined when vibronic coupling is tken into ccount. In the liner vibronic coupling model 46 the vibronic Hmiltonin is expnded round the symmetric (liner) geometry using the vibrtionl norml coordintes of the ground stte. The intrstte coupling prmeter (κ i ) of the individul sttes is essentilly the derivtive of the energy of the excited stte long the symmetric norml coordinte, wheres the interstte coupling prmeter for the vibroniclly coupled excited sttes λ ij describes the interction of the sttes long the ntisymmetric (in our cse bending) coordintes. The ground-stte properties entering the vibronic coupling model re given in Tble 3, wheres the prmeters of the excited sttes re listed in Tble 4. The simultion of the spectrum hs been performed with the xsim progrm of Sttelmyer nd Stnton 48 which hs been modified for the tretment of liner molecules. TABLE 3: Ground Stte Properties of C 3 MR-AQCC expt (ref 47) r e (Å) ω e (stretching) cm ω e (bending) cm b totl energy (hrtree) Clcultions hve been performed t the MR-AQCC level (see text for detils) using the cc-pvtz bsis. b Anhrmonic vlue. TABLE 4: Clculted Prmeters for the Liner Vibronic Model stte 1Σ + u 2 1 Π g 1 1 Φ g 3 1 Π g verticl excittion energy (ev) E/ q stre (ev) b Clculted t the theoreticl ground-stte equilibrium geometry ( Å). Clcultions hve been performed t the MR-AQCC level (see text for detils) using the cc-pvtz bsis. b Derivtive of the energy ccording to the symmetric stretching norml coordinte (dimensionless) of the ground stte. The ground-stte bond length nd the symmetric stretching hrmonic frequency re in good greement with the experiment nd other predictions from theory. 47 The ccurcy is in line with tht obtined for C 2 using the cc-pvtz bsis. 38 The bending frequency, however, is significntly lrger thn the one determined from the experiment. As shown by Mldenovic et l., 47 this quntity is very sensitive to the bsis functions, nd diffuse functions re bsolutely necessry to get relible results. Becuse our gol is the description of the excittion spectrum rther thn tht of the ground stte, we hve not pushed the clcultion ny further, especilly becuse this bending mode enters only in the expnsion nd its ctul vlue should not influence the results. The κ i prmeters hve been clculted using the nlytic derivtive of the excited-stte energy which then hs been trnsformed to the norml coordintes. The λ ij coupling for the 1 1 Σ u + nd 3 1 Π g sttes hs been clculted from the potentil energy curve of the two sttes long the bending coordinte ssuming tht their divergence is due to their vibronic interction. To obtin the λ ij coupling for the 1 1 Σ u + nd 2 1 Π g sttes, we hve clculted the 3D potentil energy surfce of the two sttes round their ssumed crossing using bending nd symmetric stretching coordintes. It turned out tht these surfces do not cross exctly but come very close to ech other: the energy difference is only bout 50 cm -1 t nd 1.419

9 Spectroscopy of the Crbon Molecule C 3 J. Phys. Chem. A, Vol. 106, No. 24, Å. We think we hve here the sitution of n voided crossing. In two stte model like the liner vibronic coupling model, the distnce of the two surfces is proportionl to the interstte coupling prmeter; in cse of no coupling, the two sttes would be degenerte. In the sitution considered here, the distnce in energy is so smll tht the λ ij vlue should be negligible. Therefore, we consider only the 1 Σ + u nd the 3 1 Π g sttes in the simultions. Figure 9 shows the experimentl spectrum compred to the obtined simulted spectr, () without interstte coupling, cse we lredy hve discussed, nd (b) with interstte coupling. Λ is the coupling prmeter λ ik between the considered sttes. Although there re quite significnt differences, the bsic chrcteristics of the VUV bnd re clerly discernible in the simulted spectrum (b). On the high energy side of the synthetic spectrum where the coupling is smll, there is n lmost regulr progression with n verge spcing of 1200 cm -1.Inthe experimentl dt, one cn recognize similr progression nd thus identify these fetures with the V progression of the symmetric stretching mode in the 1 Σ + u stte. The other significnt observtion is the loss of regulrity on the low energy side, both in the experimentl nd in the simulted spectrum. In this region, ll vibrtionl trnsitions will contin some symmetric stretching nd bending chrcter becuse of the vrible composition of the wve functions in the intercting sttes. Thus, the interprettion of the vibrtionl progressionlike series on the high energy side s V symmetric vibrtion progression should lso be hndled with reservtions. The other fct which prevents the usul ssignment of the different lines to given progressions is tht on the potentil energy surfce of the 1 1 Σ + u stte there is no minimum which could be interpreted s the equilibrium structure for this stte. The minimum in the vicinity of the voided crossing between the 1 1 Σ + u nd 2 1 Π g sttes cnnot be considered s such becuse of the brekdown of the Born-Oppenheimer pproximtion becuse of the very smll seprtion of the two sttes. The strting edge for the bnd systems on the low energy side, however, grees well between theory nd experiment. When considering the differences of the clculted nd observed spectrum, one should, however, keep in mind the pproximtions of the model used in the clcultions. The liner vibronic coupling model ssumes n pproximte form of the underlying (Born-Oppenheimer) surfces obtined form the b initio clcultions, nd therefore, the clculted vibrtionl levels re lso bised by this pproximtion. We lso expect the model to brek down ner the voided crossing between the 1 1 Σ + u nd the 2 1 Π g surfces. Becuse this point is low energy point on the 1 1 Σ + u surfce, the effect of this brekdown influences the low energy prt of the spectrum. Unfortuntely, we re unble to predict the mgnitude of this effect, lthough we ssume tht it is smll becuse the voided crossing is lredy fr from the Frnck-Condon region. A more relible simultion of the spectrum would require going beyond the liner coupling model nd perform dynmicl clcultions with three surfces corresponding to the 1 1 Σ + u,2 1 Π g, nd 3 1 Π g sttes. This would be rther big undertking, however, nd therefore out of the scope of the present pper. Such n extended study is in progress. We hope tht this will provide better greement with the experimentl VUV spectrum nd will llow us to explin ll relevnt fetures of the observed spectrum in more detil. Conclusions Using synchrotron rdition, the spectrum of the crbon molecule C 3 ws investigted in the vcuum ultrviolet-visible wvelength rnge t resolution up to 0.3 Å. In prticulr, we mesured bsorption spectr of the 1 Σ + u r X 1 Σ + g trnsition of C 3 round 1600 Å in neon t 4.3 K nd 1700 Å in rgon t 10 K. Intensity correltions with the known electronic trnsition nd the excittion spectrum of the 3 Π u f X 1 Σ + g phosphorescence emission of C 3 show tht the VUV bnd system origintes from the C 3 molecule. Our own dt nd the vilble quntum chemicl dt support the bove ssignment for the observed bsorption. At the red nd the blue portion of the observed bsorption bnd system, simple progressions cn be recognized, probbly originting from bending nd symmetric stretching vibrtions, respectively, wheres the middle prt of the bnd system shows complex superposition of progressions. Our quntum chemicl clcultions of synthetic spectr show tht the complexities in the observed spectrum t lest in prt come from interstte vibronic coupling between the 1 Σ + u stte nd the djcent Π g sttes. Furthermore, theoreticl clcultions suggest tht the 1 Σ + u stte is unstble upon bending, leding to n voided crossing type of feture with the lower lying Π g energy surfces. The intersection of the energy surfces nd their effect on the resulting spectr remins to be investigted by more detiled future clcultions. Acknowledgment. The rther generous finncil support by the Deutsche Forschungsgemeinschft is grtefully cknowledged. The uthors from Heidelberg thnk I. Čermák (Chemnitz) for discussions nd pprecite the help of S. Grimme (Münster) nd S. Peyerimhoff (Bonn) for crrying out exploring quntum clcultion. W.K. thnks A. L. Ong (Heidelberg) for his help in prepring the mnuscript nd T. Wkbyshi (Kyoto) for his mny vluble comments. P.G.Sz. pprecites the stimulting discussions with H. Köppel. 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