Applied Physics B and Optics Springer-Verlag 1996

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Appl. Phys. B 6, 49-53 (1996) Applied Physics B and Optics Springer-Verlag 1996 Temperature and pressure dependences f the laser-induced flurescence f gas-phase acetne and 3-pentanne F. Grssmann, P.

1 Appl. Phys. B 6, (1996) Applied Physics B and Optics Springer-Verlag 1996 Temperature and pressure dependences f the laser-induced flurescence f gas-phase acetne and 3-pentanne F. Grssmann, P. B. Mnkhuse, M. Ridder, V. Sick, J. Wlfrum Physikalisch-Chemisches Institut, Im Neuenheimer Feld 53, D-691 Heidelberg, Germany (Fax: /56-455) Received: 7 December 1994/Accepted: 1 September 1995 Abstract Laser-Induced Flurescence (LIF) frm the Sa state f acetne and 3-pentanne was studied as a functin f temperature and pressure using excitatin at 4 nm. Additinally, LIF f 3-pentanne was investigated using 77 and 31 nm excitatin. Added gases were synthetic air,, and N respectively, in the range -5 bar. At 33 K and fr excitatin at 4 nm, all the chsen cllisin partners gave an initial enhancement in flurescence intensity with added gas pressure. Thereafter, the signal intensity remained cnstant fr N but decreased markedly fr. Fr synthetic air, nly a small decrease ccurred beynd 5 bar. At lnger excitatin wavelengths (77 and 31 nm), the crrespnding initial rise in signal with synthetic air pressure was less than that fr 4 nm. The temperature dependence f the flurescence intensity was determined in the range K at a cnstant pressure f 1 bar synthetic air. Fr 4 nm excitatin, a marked fall in the flurescence signal was bserved, whereas fr 77 nm excitatin the crrespnding decrease was nly half as strng. By cntrast, exciting 3-pentanne at 31nm, the signal intensity increased markedly in the same temperature range. These results are cnsistent with the bservatin f a red shift f the absrptin spectra (~9 nm) ver this temperature range. Essentially, the same temperature dependence was btained at 1 and bar pressure f synthetic air. It is demnstrated that temperatures can be determined frm the relative flurescence intensities fllwing excitatin f 3-pentanne at 4 and 31 nm, respectively. This new apprach culd be f interest as a nn-intrusive thermmetry methd, e.g., fr the cmpressin phase in cmbustin engines. PACS: 33.5; 33.; 34.9 There has been interest in the phtphysics f carbnyl cmpunds fr many years fr several reasns. First, the use f carbnyls as flurescent dpants was recently prpsed fr marking fuels in engine cmbustin [1 3]; the characterizatin f the flurescence f acetne and 3-pentanne fr this particular applicatin was the mtivatin fr this study. Secnd, carbnyls are invlved in the cmbustin f hydrcarbns; their thermal decmpsitin leads largely t free radicals, which serve as chain starters fr chemical reactins. Third, the prductin f free radicals by phtdecmpsitin f carbnyl cmpunds is f cnsiderable atmspheric imprtance fr the same reasn. The absrptin spectrum f acetne and 3-pentanne in the near UV is due t the excitatin f the C =O grup, whereby a nn-bnding electrn f xygen is raised int the antibnding rc~ mlecular rbital. The transitin frm the grund (S, n) state t the first excited ($1, ~*) state is thus spin-allwed but rbital-frbidden, s that the absrptin is relatively weak ( ~ 1 - cm). Absrptin ccurs frm t 33 nm with a maximum near nm, flurescence frm 3 t 55 nm with brad maximum at abut 4 nm. The absrptin and emissin behaviur f a number f ketnes have been studied bth in the bulk gas [4] and islated mlecule regimes [5, 6]. The radiative lifetime fr the first singlet state f acetne (islated mlecule regime) is arund ms. Many investigatins, particularly the earlier nes, were made at ttal pressures f several trr and abve, i.e., in the bulk gas. In this regime, effective lifetimes were fund t increase with excitatin wavelength, varying frm 1.7 ns at nm t.7 ns at 315 nm [4]; the crrespnding quantum yields increase frm 1 x 1-3 t x 1-3. Fr 3-pentanne, the variatin in lifetime given in [4] is smaller, i.e., frm.1 ns at 5 nm t.6 ns at 35 nm and the quantum yield is nearly cnstant in this wavelength range at. x 1-3. In bth cases, the $1 state decays almst entirely (99%) by intersystem crssing t the triplet state. In this wrk, we present results f flurescence intensity measurements f gas-phase acetne and 3-pentanne fr pressures up t 5 bar and in the temperature range 33 65K. The excitatin wavelengths selected were 4,77 and 31 nm, which are n the left, near the centre and n the right f the absrptin maximum, respectively.

2 5 1 Experimental Time-integrated (relative intensity) measurements (Fig. la) were made in the gas phase n acetne and 3-pentanne in a heatable, high-pressure cell, which culd be perated safely up t 65 K and 5 bar. Because the biling pint f 3-pentanne is 375 K, the cell was perated at a minimum temperature f 33 K. The cell was quadratic in uter frm and had a cylindrical prbe vlume f abut 3cm 3. The gemetry and dimensins were selected s as t ensure instantaneus mixing f gases (Fig. lb). The ketne f interest was injected thrugh ~ a 1 I~----k~ Pellin Brca -Raman Shifter ~ Prism I --- I 4'- I Delay... line I [ ~, I I ns, Vacuum Manmeter? Thermcuple Gas-supply -h- F:er' Filter a septum int the cell and nitrgen, xygen r synthetic air was added. Fr mst measurements, a bradband KrF-excimer laser at 4 nm (Lambda Physik, EMG1ii) was used fr excitatin and a phtmultiplier/bxcar system fr detectin (Thrn EMI 973B, Stanfrd Research Systems 5). Additinally, sme measurements were made using 77 and 31nm radiatin, btained by Raman shifting the KrF-excimer wavelength (first and secnd Stkes lines f H, respectively). Laser intensities f < 1 mj/cm z at a repetitin rate f 1 Hz were used. It was verified that the flurescence was in the linear regime; neutral density filters were used as required t avid saturatin f the phtmultiplier. Apprximately 1 laser shts were averaged fr each measurement pint and the cell was then refilled fr the next measurement. Fr the temperature measurements by tw-line-excitatin flurescence, a slightly mdified experimental setup was used. The flurescence intensity was measured fllwing excitatin by tw beams (4 and 31 nm) riginating frm the same laser pulse: As depicted in Fig. la, a Pellin- Brca prism separated the different wavelengths spatially. The 31nm beam was fcused directly int the cell, whereas the remaining laser light at 4 nm was sent thrugh a delay line f apprximately 14 m. The flurescence signals frm the tw excitatins were recrded with the phtmultiplier tube (rise time < 3 ns) and tw gated bxcar-integratrs (gate width 15 ns). Recrded emissin spectra shwed n indicatin f fragments prduced by laser phtlysis even at higher intensities. T check fr thermal and phtlytic decmpsitin f the investigated ketnes, capillary gas chrmatgraphy [WCOT-quartz capillary (51 m*.3 mm)] was perfrmed with an is-thermal prgramme using a Flame Inisatin Detectr (FID). Absrptin spectra f the ketnes were recrded as a functin f temperature using a mdified Perkin Elmer, Lambda 7 UVspectrmeter. b cnnectin t gas, sept~x,,~nd thermcuple / ~~, ~ Suprasil wind Fig. 1. a Experimental setup fr LIF f acetne and 3-pentanne as a functin f pressure and temperature. The dashed lines depict the setup used fr temperature measurements, b Schematic drawing f the heatable high-pressure cell Results and discussin.1 Pressure dependence All measurements were made using a cnstant number f mlecules (. x 117 cm -a fr acetne,.6 x 11~ cm -3 fr 3-pentanne). With an excitatin wavelength f 4 nm, a series f flurescence intensity measurements was made at a cnstant cell temperature f 33 K, adding nitrgen, xygen r synthetic air up t ttal pressures f 3-5 bar. Very similar effects were bserved fr bth acetne and 3-pentanne. As Figs. a and b shw, the flurescence intensity rises initially. In the case f N (nt shwn here), the signal levels ff when relaxatin is cmplete. This ccurred at 5 bar fr 3-pentanne and bar fr acetne. Fr Oz, after the initial increase, a decrease in signal f 17% per 1 bar was measured. Fr synthetic air, n increasing the pressure beynd 5 bar, the signal intensity decreased abut 1% per 1 bar. The first increase f the signal intensity may be attributed t vibratinal relaxatin t levels with higher

51 ~1.5 t~ -a.5 m.~,-,' '~ -b g _~.5 [3 DD Drn D U DD D~ D D[3 D DDD E]D E]DD ~u OO I F I * I I I I I I F I i 4 6 11 1416146 Pressure [ bar ] --~ 1.5. D E] ~3 )~E] D CJ D Q r~ D D D D D D D C]r~ D D D 1I f~ ~ O OOOO I r I I I I I I q I E E T,,, 4 6 111416146 Pressure [ bar ]]--~ Fig.

3 51 ~1.5 t~ -a.5 m.~,-,' '~ -b g _~.5 [3 DD Drn D U DD D~ D D[3 D DDD E]D E]DD ~u OO I F I * I I I I I I F I i Pressure [ bar ] --~ 1.5. D E] ~3 )~E] D CJ D Q r~ D D D D D D D C]r~ D D D 1I f~ ~ O OOOO I r I I I I I I q I E E T,,, Pressure [ bar ]]--~ Fig. a,b. Pressure dependence f the relative LIF signal fr excitatin at 4 nm; cell temperature 33 K: a acetne, b 3-pentanne; (O) xygen, (E]) synthetic air flurescence quantum yields. Hwever, quenching f the $1 state by is able t cmpete effectively with this vibratinal relaxatin. Quenching f excited rganic mlecules by grund-state (triplet) xygen is well knwn [7, ] and in this case may be due t the frmatin f a chargetransfer cmplex with the ketne, invlving the electrnegative xygen atm f the excited carbnyl bnd and electrns f the duble bnd. Frm this mdel ne expects n change in the lifetime f the $1 state in the presence f air r xygen as ppsed t nitrgen, as cnfirmed by Ossler and Alden [9], wh btained the same lifetime (.6 _+.5 ns) fr 3-pentanne in 9. bar nitrgen and air at 53 K. A similar result was btained with aldehydes [1]. Tw further series f experiments were cnducted fr excitatin f 3-pentanne at 77 and 31 nm. At 77nm, the flurescence intensity increased slightly with synthetic air pressure up t bar and then decreased apprximately 4% at 5bar. Fr 31nm excitatin, almst n initial increase in signal intensity is measured. Similar results were btained at 473 and 533 K. Figure 3 shws the difference f the relative flurescence intensity at 473 K using excitatin at 4 and 31 nm. The lnger wavelength at 31 nm is very clse t the lwest vibratinal level f the $1 state f the ketne. Then, n r little vibratinal relaxatin takes place and, therefre, the quenching effect is bservable at much lwer pressures. Because f the interest in ketnes fr engine diagnstics, the pressure dependence f 3-pentanne flurescence n synthetic air pressure was als investigated using a mixture f is-ctane and 3-pentanne (:%) and 4 nm excitatin. Again, an initial increase in signal intensity was bserved up t 5 bar, fllwed by an almst cnstant intensity up t 5 bar. >,.5,I-,._ 1.5 C =.5 ~DD~D D [] ' ' i'6'1 3 r --J Pressure [ bar ] -4~ Fig. 3. Pressure dependence f the relative LIF signal f 3-pentanne. ex: 4 nm (O) and 31 nm ([~) at 473 K. Temperature dependence Relative intensity measurements were made fr bth acetne and 3-pentanne at a cnstant pressure f 1 bar synthetic air in the temperature range K. Using 4 nm as the excitatin wavelength, the signal decreased as shwn in Fig. 4 (apprximately 35% per 1 K). T establish whether this effect was due t the decmpsitin f the flurescing mlecule, gas chrmatgraphic measurements were made by sampling frm the cell via a septum. Bth the ketne and its decmpsitin prducts (ethylene, butenes and a small amunt f prpinaldehyde) were mnitred. The results shwed that even after 1min residence time in the cell at 575 K, nly 1% f the injected cmpund had dissciated. Thus, the bserved fall in flurescence intensity (measured immediately after injectin) cannt be due t decmpsitin. Fr further clarificatin, absrptin spectra f acetne and 3-pentanne were recrded as a functin f temperature in the range K; Fig. 5 shws the results fr 3-pentanne. The absrptin peak at 75 nm shifts t lnger wavelengths with increasing temperature, while the halfwidth remains almst cnstant (apprximately 3 nm). This means that the absrptin strength will decrease at 4 nm (n the left f the absrptin maximum) and a decrease in flurescence signal intensity is t be expected using this excitatin wavelength, as bserved. A smaller temperature dependence at 77 nm is expected since this wavelength is clse t the absrptin maximum. Accrdingly, if this ketne is excited n the right-hand side f the absrptin maximum, a psitive temperature dependence shuld be measured, as bserved by Greenhalgh and Tait [11] fr 3 nm. The temperature dependence f 3-pentanne was als determined using 77 and 31 nm excitatin (Fig. 4). In the temperature range K, the flurescence intensity decreased by 15% per 1 K fr 77 nm excitatin, but increased abut % fr 31nm excitatin. Fr 77 nm excitatin the same measurements were perfrmed at 1, 1 and bar (synthetic air), the bserved temperature dependence was essentially the same at all three pressures (Fig. 6). Tait and Greenhalgh [11] investigated the temperature dependence f several carbnyl cmpunds at atmspheric pressure in a flw system. The

4 5 #._c ~ " 3 [] ~ A I I i Temperature [ K ] -= A O 5; Fig. 4. Temperature dependence f the relative LIF signal fr 3- pentanne fr excitatin at 4 nm (O), 77 nm (A) and 31 nm (D). All data are nrmalized with respect t the signal intensities at 33K 1, -4 3,-._ =,6- g,4-,- z, O 5 ad Wavelength [nm ] Fig. 5. Absrptin spectrum f 3-pentanne at different temperatures and atmspheric pressure. 9 K (full line) and 573 K (dtted line) == = ~ ~3 I I r Temperature [ K ] Fig. 6. Temperature dependence f the relative LIF signal fr 3- pentanne. ex = 77 nm, at 1 bar (O), 1 bar (A) and bar ([~) synthetic air pressure excitatin wavelength there was 3 nm (XeCl-excimer) and the temperature range studied was 3- K. Fr acetne and 3-pentanne, a steady increase in signal f 16% per 1 K was bserved..3 Temperature measurement using tw-line-excitatin technique f 3-pentanne The temperature dependence f the absrptin strength - and hence the flurescence intensity f ketnes ffers the pssibility t determine the temperature. T [] 35 7 v 6 LL 5- == 4 3 3OO i J r Thermcuple Temp. [ K ] J 7d Fig. 7. Temperature measurement using tw-line flurescence f 3-pentanne. The calculated temperature frm these measurements is shwn as a functin f the crrespnding thermcuple readings demnstrate the pssibilities f this new apprach, we measured the rati f the flurescence intensity fllwing 4 and 31 nm excitatins. Measurements were perfrmed at atmspheric pressure (synthetic air) in the temperature range 35-64K. Again, the flurescence intensities were measured as a functin f temperature and all data were nrmalized with respect t the signal strengths at 33 K. Fr each pair f excitatin pulses the rati 131 nm(t) /14- nm(t) was then calculated. Frm the results f the measurements mentined befre (Fig. 4), we determined the relative flurescence intensity as a functin f temperature and used this calibratin curve t calculate temperature frm the measured rati. Figure 7 shws the cmparisn f the calculated temperature and the thermcuple readings. Each measurement pint crrespnds t the mean value f 15 successive laser pulses. Within the whle temperature range investigated here, there is gd agreement between the calculated and the measured temperatures. Hwever, at the highest temperatures, the calculated values tend t underestimate the thermcuple readings by 4-5 K. The mean scatter within a series fr a given temperature was +_ 1 K. 3 Cnclusins Over a wide pressure range (ca. 5 5 bar), the flurescence intensity f bth acetne and 3-pentanne are cnstant. The temperature dependence f the flurescence is dependent n the excitatin wavelength and can be minimized near the absrptin maximum. These results shw that these ketnes will be suitable flurescent dpants fr marking fuel distributins in cmbustrs and fast-flw devices. 3-pentanne can be used with cmmn mdel fuels, such as is-ctane [, 3], because f similar biling pints and mass diffusin cefficients. These ketnes are chemically relatively stable and nly dissciate at higher temperatures (i.e., in the flame frnt). It is demnstrated that tw-line flurescence f 3-pentanne is a new ptical technique fr nn-intrusive thermmetry. Since there is nly little influence f pressure n

53 the rati f the flurescence intensities f tw different excitatin wavelengths, this new apprach culd be f special interest t applicatins perating at elevated pressures, e.g., in the cmpressin strke f Diesel engines.

5 53 the rati f the flurescence intensities f tw different excitatin wavelengths, this new apprach culd be f special interest t applicatins perating at elevated pressures, e.g., in the cmpressin strke f Diesel engines. T imprve the signal-t-nise rati, i.e., t cmpensate fr laser fluctuatins r any ther tempral drift effects, the laser pwer shuld be measured nline befre as well after the pressure vessel. Whereas the precisin f this apprach is quite satisfactry up t 55 K, the deviatin at higher temperatures may be due t uncertainties f the calibratin curve abve 6 K; this will be investigated in further wrk. Acknwledgements. This wrk was supprted by the Bundesministerium fiir Frschung und Technlgie (Arbeitsgemeinschaft TEC- FLAM) and the Cmmissin f the Eurpean Unin (Prject MID- COM II) References 1. A. Arnld, H. Becker, R. Suntz, P. Mnkhuse, J. Wlfrum, R. Maly, W. Pfister: Opt. Lett. 15, 31 (199). A. Arnld, A. Buschmann, B. Cusyn, M. Decker, F. Vannbel, V. Sick, J. Wlfrum: SAE Paper (1993) 3. H. Neij, M. Ald6n: Cmbust. Flame 99, 449 (1994) 4. D.A. Hansen, E.K.C. Lee: J. Chem. Phys. 6, 13 (1975) 5. A. Lzan, B. Yip, R.K. Hansn: Exp. Fluids 13, 369 (199) 6. Y. Haas: Spectrchim. Acta A 46, 541 (199) 7. O.L. Gijzmann: Faraday Trans. II 7, 7 (1973). P.B. Merkel, D.J. Kearns: J. Chem. Phys. 5, 39 (1973) 9. F. Ossler, M. Ald6n: Appl. Spectrsc. (1996) (submitted) 1. A. Arnld, A. Br~iumer, F. Dinkelacker, P. Mnkhnse, K. Witte, M. Sch~ifer, M. K611ner, W. Ketterle, J. Wlfrum: Final Reprt, CEC-Prject JOULE1-UK/JC, Eurpean Cmmissin (199) 11. R. Tait, D. Greenhalgh: Ber Bunsenges. Phys. Chem. 97, 1619 (1993)

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