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Study of the emission of the excited acetone vapour at intermediate pressures
Spcctrochimica
Arm. Vol. 46A. No. 4. pp. 617 - 621. 1990.
0584 - 8539/90
$3.00 + 0.00
0 1990 Pergamon
Printed in Great Britain.
STUDY OF THE EMISSION OF THE EXCITED AC&TON& VAPOUR AT INTEmmDIATE
Press plc
PRKSSURKS
M.J.G. BORGE, J.M. FIGUEBA and J. LUQUB Institute Qufmica-Fisica %casoIano’, CSIC, Sermno 119, 28006 bfadrid G+in)
ABSTRACT Wehavedudiedtheflwmwence emissionof the acetone in the presmue range 0.01 - 10 Torr paying special attention to the slow part of the signai (0.6 * after excitation). We have devebped a modeI. that fits the axparimentaIIyobtainsd emissiin curve and psrmit to understandprevbus discrepancissin the quenchingrats constant of emited triplet T1 stats.
INTBGDUCTION The~kinatiaofacetonehave~~been~atlowpresRve[1-3~~~ theram] and low temperatureconditions [4-71 in great detail. sps&mmqy ofacetoneillasasdedsuparsonicmolacularbeamhas mveakd the rasolved fluommence excitation spa&rum of the S1 stats [S]. It has also givsn the onset of the rapid dscqv component (“spike”) at 1200 cm-l abovetheS1stata[7]. Theexistenceofthis”spike”isaxpiainsdasduato the&p~ofthebaat~~~bet~the~tsndthelargenumberoftripletsta~coupledtoit. This fast component is presant in buik conditions for all the acitation emwgies [l]. Inspiteofalltheeeprecisestudiesoftheemission~ottheacetone,noattentionhasbeenpaidtothe very iarge discrepancy,of the order of 103, betwaanthe vaIussfound [2,31for the quenchingrats constant of the “hot” triplet abo calied “the third component” in the iit.eratum Thaas two experiments were dons at different pressure rangesoftheacet4mevapour. Ths ~tslrJrCopekndandCmsiq[2]weredoneatprsssurw up to 0.25 Torr and the ones of Cost& et al. 131in the pressure ~of1-ST~.Wesurpectedthatbothworkswerevalidand thetwodifferentvaIusaofthequenchingmtaamstantawmqond to two diffemnt contributionsto ths total emission. In whole, two components pIus tha phorpholarcsna one. Wethareforedscidedtoexaminethe m (phdsphomacence)emissbn ofthaacetoneinthepressurerange 0.01 - 10 Torr overIappingboth experiments. We have found that the three components co-exist up to a pressure of 1 Torr where the faster one is abeady in the low8 s m. We have deveioped a model that tit this mu.ItiexponentiaI behaviour of the emissionoftheacetoneingasphasaatIowenergies. EKF’EBIMENTALPROCEDURE Acetone vapour was irradiated with a 46 mJ/puIse KeCI Iaser (;\ = 308 run) used at 0.6 IIs repetition rats, F’WHM=6ns. Theoutputbeamofthelaser~focueedPrithablmSpectmsilBlens,oollimatedbyaniris diaphragm(Ealing22-3869)intoa36cmlone46mmdiametethornedfhroHscence cell equippsdwith baffks and with a capacitancemanometer to control the prsasum of the continuosIySowing acetone vapour. Two differsnt capacitance manometerswereusedz atIowprsssum , up to 200 mTorr, a MKS Baratron type 227A (1 TOIT),while the higher ~wencontrolledwithaMKSBaratrwtype221A(lOTorr).Theliqhtemittedatriebt~tothelasar beamrrar~byaVSBK-7lenrontoafiltsrrptem~afanaperhvaoflmmdiameter,twocut-off filtarrat37Oand408nmanda~~~Alter~36-6248. ThetmnubkmofthetIitarsystemwsshigher than 60% between 410 - 460 nm. The !ikared Iight was viewed using an EMI 9816QB phot.omuBipIierwhich signsI warconnectedtoaTektronix2430digital~.Ths~~~aAera~upto258lasarsshots~ storedinaVECTBAHPcomputer. ThescattarsdIightIeveIwasobtainedbyevacuatingtheceIIusingthesame 617
M. J. G. BORGEet al.
618
RESULTS AND DISCUSSION Thetime-R4dVedEigZdoftheacetons eminsion exhibiti a multiexponentialbehaviour at room t8mperature. u~thesdequatetimercaleto~componantr~~lifetime~thano.5pwecan~~inthe~~ oftheemission~th!cwtributionof4~~~inthe~~ofO.O1to1Torr. Thetkdeutonewith decay time aborter than 0.5 p coxlvolutel!the amtribution of the kuer ecattering,the “SpW, the singlet fluoIwcence dthe emidoIlfnnnthemixedstate& WehaveMturatedthef~andmoreintenrepartofths~to6~thebehaviourwithp~ oftheother three component&which contribution to the signal amplitudeL, e. g., 6% at 0.5 Torr. Weatudiedthequenchingoftheacetonebyreveralnon-polargasertocheckthesecomponentobebaviourwith the different quencherr. We will call theae componenta“8horV with lifetime around cg; “intedediate” with lifetime of tenth oflap and “long”, with lifetime around hundred8of-. Different presnuw ofacetonewereutxxit.ostlldythe bebaviourwiththequsnchers:1oOmTo~and2oomT~tor~therhortcomponantand1and2Torrto~t~ the intermediateand long components. The quenching rate constant value8 are shown in Table 1 and compared to thaseadculatedfromthekiuetictheoryofgasfs.
Table 1: Quendhq rate constant for the thme triplet componenti in acetone emidon (at 293 IQ.
k (&Tor+Ja
Short
In&mediat8
Ne
0.38 * 0.01
NO 02 xe CF, SF6 Acetone
0.86 f 1.35 f 0.85 f 1.15 f 1.50 f 3.4 f
____ ____ (1.5 f 0.2) ---_ (2.8 f 0.3)10-2 (4.5 f 0.8)10-2 (6.9 f 0.5)10-3
0.08 0.08 0.05 0.02 0.01 0.3
Loag
(3.6 It 0.5)10_6 (5.0 + 3.0)10-4 (0.6 * 0.1) (1.0 + o.l)lo-3 (1.1 f 0.1uo-2 (1.5 f o.l)lo-2 (6.5 f 0.5)10-4
gas kineticb
8.6 11.45 10.83 10.76 10.83 15.22 12.75
a) Aa convemion factor we consider 3.238 Torr-l = lo-l6 cm3molede-1 b) Calculated considering for acetone the Stockmayer potential and the formti for interaction between polar nonpolar moleculeudescribedin Ref. 9. The collision ix&grab for the Studxqyer potential where taken from Appendix 8 of Ref. 10.
Inthelightofaur~~rrecanlaythatthe”~~”component~~aperellelbehaviaurtothemixedstate, TS_T,&~didindetdinIkfl. Itirmainlyqusnchedbyacetoneitself,,ntquenchiagiealeoobsarvedbyall chamckx of the depopulatingled Equivalentbehaviour theotherquen&eraddedqeciaUybyO2showingthetriplet is obeerved for the other two componentfswith the additionof 02 On the contrary, veq slow quenching is observed when vibrational quenchen such aa CF4 or SF6 are added. Non appreciablevariation of the in&mediate component
Emission of excited acetone vapour
619
l,itWmeb~whanNe,NzorXaam addad. Fbmthababavhrwlthtbeh84mvyatom xbwecauexpeJctthattbacoupIiugoftllistripIeJt #tat8wlthtbsaingk&statei88mau. oureqelhmltaIcouditioMbndto cm-1 unavoidabb excitatkm of maDy 8tatsr aud 35tbereheonlya~pxqexthcanbe 34-1 obtained. We have deAoped a 6implifled kinetic $3 model (ree Fig. 1) baaed on the qypotbesir _Dacompmitlon 33-f: deEr&edbekwiuordelrto acanmtforthe ThreShold 32- jqwrhiug exphment and the temporaI (32700 f 60 cm-t) 31 behaviaurofthetailoftheacetoneembsionat intermuwe~. 30 Ourh3rphotouaexcitqonthea~, 29thea&ouet.ovibzntional6tate8ofS withan -1 (average 28ex~~of!Z660*425cm energyofacetoneatroomtemperatureb650 N cm-l [4], qectraI width of our laser around 0.5 nm), where the dens@ of VibrationaI s1+ Etatea 1 Thermal iE6maU[7Jl. Eech8ingIet8tatebdegenerated excitation 0 with a bcge number of triplet state8. Initial excitation populate4J the sin&t statea that devehp rapidly (lo-6 a) into mixed statea with . FYgure1.Partialelmlgyleveldia@nmoftheacetoneahowingthe domulant triplet chlmlck. Them mixed date (TS_T in Fig. 1) ark &tie k~n6 life aad berg Ieveb involved in our kinetic model Me text). mMitiveI to we& c0lMoI.u [ll, therehe, they wiIlpopulateTl**bycoIliuio~. ~levelshave*milarpropertiertothe?S_To~~~~muplinetoSl irnot~rtrongftherctnot~~).Thegpoprlatebyco~theTl levebthathavesmaUcouplingtothe &.&et state& These Tl* leveb live kmg enough to decompoee (diseochtion of the acetone trip& at 32700 f 50 cm’ eacitation enemy 161). Faster cohion rates wiU populate &t&s with excitation eneqy kxver than the decompceition hit, named Tl in Fig. 1, that will depopulate by cdlision, photon emissionorintersyatemaut3stoS0. Thetemporal evolutioxl of there hate8 by the foIlowing set of equations:
j4
so
dt
d&+1 =
IN,*‘lkQ,*‘*tQl - [Nl*l*O&
=
IN,*l$$[Ql
+ kQl*.[Ql + kp)
dt
-
[Nll.(kRr
+
k~~*[Ql)
dt l* where INJ, [NJ, and IN11 = the concentrations of the Tl**, Tl+ and Tl statea respecti~, kRl , kRl* and kR1 are their coUision free decay con$anta aud k~l**, k~l* and kQl are their quen&hq rate constanh~ kp ir the coxuxntration of the qwncher. There equations @e the decompo&iouratecouatautoftheTl rtateand[Q]tthe teuqomIbebaviouruptoapswsureof0.9Torr. ++doe4notdhociateandthatatourreference Toreducethnumbwoffiwpammetem,weaemunedthatTl timeitttheonlyonepopulated. ~~thsre~e~~~~~rrsobtainths~~~vaMtionofthelevel
67-O
populations
M. J. G. BORGEet al.
and from them the temporal evolution of the intensity that we compared with the experimental signaL
10'
Acetone
10
1 0
I 40
I
Figure
pressure:
1 Torr
I 120
2. Emission of 1 Torr acetone excited at 308 nm. Theoretical line) and ita comparison with the experimental data (stars).
I 160
decay curve
(fbll
At higher pressure the apparent lifetime of the ?l** state is a few hundreds nanoeecwds,aointhe~-rangethe signaIcanbede#xGed by the contribution of the Tl and T1 1eveIs. In thir case the intensity only depends on wven value!#areobtaitledby nUmizationoftheleastquarefittothe paramete=pbtheprernua. =m using Marqua& method [II] . Fig. 2 rhowe an BuLmpleof this fitting done for 1 Torr of acetone. eqerime-ntal CU~VBI, To &ther reduce the number of free parametera, we fixed kRl* and kR1 to the vahxi of 0.0066 p-l, value obtained experimenta& for the triplet state [12] and assumiq equivalent intersystem crossing for both, Tl* and Tl states. Fitting the expex&exM curvea from 0.6 to 7 Torr wa found an almost constant value for k$l*= 2*10e3 p-l Torr-l and for key = 1.4*10s3 ~-~Torr-~. It ia noteworthy that these parameter8 obtained independently almost coincide, reinforcing the idea that the main differenca between T1’ and T1 levels is the decompoeition channel. Fi now kR1, kRl*, the new valuea of kQl and kQl* and letting the initial population and the disxsiative rateconetantvaigwefoundforkpalineardependencewiththepraesure (in Torr): kp = 0.012@-1)
+ 0.00943@s-1Torr-1~[Ql
ThtpseemstobecompatiblewithunimolecularreactioIlc~behaviourwhensomeofthemoleculesenterwithan energyaroundthereactionthreshold.P~workdonetorreeinthegrowofbiacetylinasampleofacetone with the number of lnaer shots, and its dependence with the addition of SF6 or CF4 quenchem seen to indicate that thin procetw mainly happen by unimoIecular decompoeition. l were used 88 fued input in the equation written above to Thedecaycon&antaobtainedforthe~JandT1 deduce the decay constanti kRl** and kel of the Tl** triplet state. An example of this tit ir &own in Fig. 3. l * = (0.06 zt 0.02) p-1 and kQl** = (3.7 * 0.3) jm-lTorr-l in good ThevalueaobtainedhwnthiefitarekRl agreement with the vahss previou [21 of (4.06 * 0.36) #Torr-! OthsrLineticmodel~~the~tripbtb~at~dirtinct~~~,haralrobeen~~. The different behaviour bets T1* and T1, was awociated to diffeJrentin~-cmom to 51 and So respecti*. WecouIdnotAndantofparametasthat8uax&My fltourartperimbnhldata. Beriderthat,nrchamodeIwould beinamMentwiththere8uItafoundforthefglen&sxe.
621
Emission of excited acetone vapour
10-l
I 0
I 40
I
I 120
I 160
Figure 3. Experime&al data of the acetone emission at 30 mTorr presswe (starto) and its theoretical fit wllu line).
SUMMARY We have amcentrated
our study in the tail of the emission of the acetone vapour and followed its behaviour in range (0.01 - 10 Torr) and iri tha pmaence of diffemnt quenchw &ases. This has aUowed UB to aIawp=== results [2,3] obtained for the quenching rats constant of the so caBed hot triplet. raconciliat& the former We found in the whole pressure ranga three distinct component+x the hgest life ona identified with the tharmalized triplet Tl, the intermediate one as dua to the contribution of trip& states above tha decomposition *Ievels. Theselevelsshow level, Tl* hIled hot triplet by Co&la et al. [3]) and a shorter ona associated to the Tl l arimilar~~tothemtedstater[llbuttheeoupliaetoSlrtaterLwealer. Thesakwelswerestudiedby ~~andctwley[2landcalledaith~~aamethanin~.3.~diecrepancgintheirvalueeofthe qusnchingrateconrtant~duetothefactthattheywerereferinetotwodifferentcomponentswiththesame nama. AKNOWLEDGMENT: We would Iike to thank Prof. Y. Haas for stimulating and enlighting discuhons. This work was supported by CKX’I’ project number PB87-0048. J.Luque thanks Comunidad de Madrid for studentship.
REFERENCES [l] G.D.GREENBLA’lT,S.R and YHAAS, Chem.Phyr.Lett. ll2 ,200 (1984). [2] RACOPELAND and D.RCROSLEY, ChemPhysLett. 116, 382 (1985). [3] A.COSTELA,M.T.CRESPO and J.M.F’IGUERA, J.Photochm. 34,186 (1986). and Y.HAAS#J.Phys.Chem. 89, 1338 (1985). M oANNERJI.zuc~ [51 H.ZUCKERMANN~.SCHMITZ and YHAAS, Chem.PqP.Lett. 161, 323 (1988). [8] H.iXJ~,B.SCHMITZ and Y.HAAS,J.Phgr.Chem. 92, 4836 (1988). [7J H.ZU~,B.SCHM.ITZ and Y.HAAS,J.Phgr.Cham. 93, 4983 (1989). [8] M.BABA and I. HANAZAIU, ChemF’hgr~t. lOS, 93 (1983). ER and C.F.CURTISS, Handbook of Physics, chapter 4, pp. S-41,@dited by Condor [9] R.B.BIRD,J.O.H.IRSCHFELD and Odishaw), McGmw-Hill (1958). [lo] G.I.MAITLAND,M.RIGBY,E.G.SbfITHand WAWAKENHAM, “Intemmlecular forced, Appendix 8, pp. 678-680 Ed. Oxford Schxa PubIications (1981). [ill P.R BEVINGTON,Pata Fhiucth and Error Anal&s for the Physical Scienw’, pp. 236-239, McGraw-Hill (1969). 1121 WA. NOYES Jr., G.B. PORTER and J.E. FOLLEY, Chem. Rev., 59, 49(1969)
Arm. Vol. 46A. No. 4. pp. 617 - 621. 1990.
0584 - 8539/90
$3.00 + 0.00
0 1990 Pergamon
Printed in Great Britain.
STUDY OF THE EMISSION OF THE EXCITED AC&TON& VAPOUR AT INTEmmDIATE
Press plc
PRKSSURKS
M.J.G. BORGE, J.M. FIGUEBA and J. LUQUB Institute Qufmica-Fisica %casoIano’, CSIC, Sermno 119, 28006 bfadrid G+in)
ABSTRACT Wehavedudiedtheflwmwence emissionof the acetone in the presmue range 0.01 - 10 Torr paying special attention to the slow part of the signai (0.6 * after excitation). We have devebped a modeI. that fits the axparimentaIIyobtainsd emissiin curve and psrmit to understandprevbus discrepancissin the quenchingrats constant of emited triplet T1 stats.
INTBGDUCTION The~kinatiaofacetonehave~~been~atlowpresRve[1-3~~~ theram] and low temperatureconditions [4-71 in great detail. sps&mmqy ofacetoneillasasdedsuparsonicmolacularbeamhas mveakd the rasolved fluommence excitation spa&rum of the S1 stats [S]. It has also givsn the onset of the rapid dscqv component (“spike”) at 1200 cm-l abovetheS1stata[7]. Theexistenceofthis”spike”isaxpiainsdasduato the&p~ofthebaat~~~bet~the~tsndthelargenumberoftripletsta~coupledtoit. This fast component is presant in buik conditions for all the acitation emwgies [l]. Inspiteofalltheeeprecisestudiesoftheemission~ottheacetone,noattentionhasbeenpaidtothe very iarge discrepancy,of the order of 103, betwaanthe vaIussfound [2,31for the quenchingrats constant of the “hot” triplet abo calied “the third component” in the iit.eratum Thaas two experiments were dons at different pressure rangesoftheacet4mevapour. Ths ~tslrJrCopekndandCmsiq[2]weredoneatprsssurw up to 0.25 Torr and the ones of Cost& et al. 131in the pressure ~of1-ST~.Wesurpectedthatbothworkswerevalidand thetwodifferentvaIusaofthequenchingmtaamstantawmqond to two diffemnt contributionsto ths total emission. In whole, two components pIus tha phorpholarcsna one. Wethareforedscidedtoexaminethe m (phdsphomacence)emissbn ofthaacetoneinthepressurerange 0.01 - 10 Torr overIappingboth experiments. We have found that the three components co-exist up to a pressure of 1 Torr where the faster one is abeady in the low8 s m. We have deveioped a model that tit this mu.ItiexponentiaI behaviour of the emissionoftheacetoneingasphasaatIowenergies. EKF’EBIMENTALPROCEDURE Acetone vapour was irradiated with a 46 mJ/puIse KeCI Iaser (;\ = 308 run) used at 0.6 IIs repetition rats, F’WHM=6ns. Theoutputbeamofthelaser~focueedPrithablmSpectmsilBlens,oollimatedbyaniris diaphragm(Ealing22-3869)intoa36cmlone46mmdiametethornedfhroHscence cell equippsdwith baffks and with a capacitancemanometer to control the prsasum of the continuosIySowing acetone vapour. Two differsnt capacitance manometerswereusedz atIowprsssum , up to 200 mTorr, a MKS Baratron type 227A (1 TOIT),while the higher ~wencontrolledwithaMKSBaratrwtype221A(lOTorr).Theliqhtemittedatriebt~tothelasar beamrrar~byaVSBK-7lenrontoafiltsrrptem~afanaperhvaoflmmdiameter,twocut-off filtarrat37Oand408nmanda~~~Alter~36-6248. ThetmnubkmofthetIitarsystemwsshigher than 60% between 410 - 460 nm. The !ikared Iight was viewed using an EMI 9816QB phot.omuBipIierwhich signsI warconnectedtoaTektronix2430digital~.Ths~~~aAera~upto258lasarsshots~ storedinaVECTBAHPcomputer. ThescattarsdIightIeveIwasobtainedbyevacuatingtheceIIusingthesame 617
M. J. G. BORGEet al.
618
RESULTS AND DISCUSSION Thetime-R4dVedEigZdoftheacetons eminsion exhibiti a multiexponentialbehaviour at room t8mperature. u~thesdequatetimercaleto~componantr~~lifetime~thano.5pwecan~~inthe~~ oftheemission~th!cwtributionof4~~~inthe~~ofO.O1to1Torr. Thetkdeutonewith decay time aborter than 0.5 p coxlvolutel!the amtribution of the kuer ecattering,the “SpW, the singlet fluoIwcence dthe emidoIlfnnnthemixedstate& WehaveMturatedthef~andmoreintenrepartofths~to6~thebehaviourwithp~ oftheother three component&which contribution to the signal amplitudeL, e. g., 6% at 0.5 Torr. Weatudiedthequenchingoftheacetonebyreveralnon-polargasertocheckthesecomponentobebaviourwith the different quencherr. We will call theae componenta“8horV with lifetime around cg; “intedediate” with lifetime of tenth oflap and “long”, with lifetime around hundred8of-. Different presnuw ofacetonewereutxxit.ostlldythe bebaviourwiththequsnchers:1oOmTo~and2oomT~tor~therhortcomponantand1and2Torrto~t~ the intermediateand long components. The quenching rate constant value8 are shown in Table 1 and compared to thaseadculatedfromthekiuetictheoryofgasfs.
Table 1: Quendhq rate constant for the thme triplet componenti in acetone emidon (at 293 IQ.
k (&Tor+Ja
Short
In&mediat8
Ne
0.38 * 0.01
NO 02 xe CF, SF6 Acetone
0.86 f 1.35 f 0.85 f 1.15 f 1.50 f 3.4 f
____ ____ (1.5 f 0.2) ---_ (2.8 f 0.3)10-2 (4.5 f 0.8)10-2 (6.9 f 0.5)10-3
0.08 0.08 0.05 0.02 0.01 0.3
Loag
(3.6 It 0.5)10_6 (5.0 + 3.0)10-4 (0.6 * 0.1) (1.0 + o.l)lo-3 (1.1 f 0.1uo-2 (1.5 f o.l)lo-2 (6.5 f 0.5)10-4
gas kineticb
8.6 11.45 10.83 10.76 10.83 15.22 12.75
a) Aa convemion factor we consider 3.238 Torr-l = lo-l6 cm3molede-1 b) Calculated considering for acetone the Stockmayer potential and the formti for interaction between polar nonpolar moleculeudescribedin Ref. 9. The collision ix&grab for the Studxqyer potential where taken from Appendix 8 of Ref. 10.
Inthelightofaur~~rrecanlaythatthe”~~”component~~aperellelbehaviaurtothemixedstate, TS_T,&~didindetdinIkfl. Itirmainlyqusnchedbyacetoneitself,,ntquenchiagiealeoobsarvedbyall chamckx of the depopulatingled Equivalentbehaviour theotherquen&eraddedqeciaUybyO2showingthetriplet is obeerved for the other two componentfswith the additionof 02 On the contrary, veq slow quenching is observed when vibrational quenchen such aa CF4 or SF6 are added. Non appreciablevariation of the in&mediate component
Emission of excited acetone vapour
619
l,itWmeb~whanNe,NzorXaam addad. Fbmthababavhrwlthtbeh84mvyatom xbwecauexpeJctthattbacoupIiugoftllistripIeJt #tat8wlthtbsaingk&statei88mau. oureqelhmltaIcouditioMbndto cm-1 unavoidabb excitatkm of maDy 8tatsr aud 35tbereheonlya~pxqexthcanbe 34-1 obtained. We have deAoped a 6implifled kinetic $3 model (ree Fig. 1) baaed on the qypotbesir _Dacompmitlon 33-f: deEr&edbekwiuordelrto acanmtforthe ThreShold 32- jqwrhiug exphment and the temporaI (32700 f 60 cm-t) 31 behaviaurofthetailoftheacetoneembsionat intermuwe~. 30 Ourh3rphotouaexcitqonthea~, 29thea&ouet.ovibzntional6tate8ofS withan -1 (average 28ex~~of!Z660*425cm energyofacetoneatroomtemperatureb650 N cm-l [4], qectraI width of our laser around 0.5 nm), where the dens@ of VibrationaI s1+ Etatea 1 Thermal iE6maU[7Jl. Eech8ingIet8tatebdegenerated excitation 0 with a bcge number of triplet state8. Initial excitation populate4J the sin&t statea that devehp rapidly (lo-6 a) into mixed statea with . FYgure1.Partialelmlgyleveldia@nmoftheacetoneahowingthe domulant triplet chlmlck. Them mixed date (TS_T in Fig. 1) ark &tie k~n6 life aad berg Ieveb involved in our kinetic model Me text). mMitiveI to we& c0lMoI.u [ll, therehe, they wiIlpopulateTl**bycoIliuio~. ~levelshave*milarpropertiertothe?S_To~~~~muplinetoSl irnot~rtrongftherctnot~~).Thegpoprlatebyco~theTl levebthathavesmaUcouplingtothe &.&et state& These Tl* leveb live kmg enough to decompoee (diseochtion of the acetone trip& at 32700 f 50 cm’ eacitation enemy 161). Faster cohion rates wiU populate &t&s with excitation eneqy kxver than the decompceition hit, named Tl in Fig. 1, that will depopulate by cdlision, photon emissionorintersyatemaut3stoS0. Thetemporal evolutioxl of there hate8 by the foIlowing set of equations:
j4
so
dt
d&+1 =
IN,*‘lkQ,*‘*tQl - [Nl*l*O&
=
IN,*l$$[Ql
+ kQl*.[Ql + kp)
dt
-
[Nll.(kRr
+
k~~*[Ql)
dt l* where INJ, [NJ, and IN11 = the concentrations of the Tl**, Tl+ and Tl statea respecti~, kRl , kRl* and kR1 are their coUision free decay con$anta aud k~l**, k~l* and kQl are their quen&hq rate constanh~ kp ir the coxuxntration of the qwncher. There equations @e the decompo&iouratecouatautoftheTl rtateand[Q]tthe teuqomIbebaviouruptoapswsureof0.9Torr. ++doe4notdhociateandthatatourreference Toreducethnumbwoffiwpammetem,weaemunedthatTl timeitttheonlyonepopulated. ~~thsre~e~~~~~rrsobtainths~~~vaMtionofthelevel
67-O
populations
M. J. G. BORGEet al.
and from them the temporal evolution of the intensity that we compared with the experimental signaL
10'
Acetone
10
1 0
I 40
I
Figure
pressure:
1 Torr
I 120
2. Emission of 1 Torr acetone excited at 308 nm. Theoretical line) and ita comparison with the experimental data (stars).
I 160
decay curve
(fbll
At higher pressure the apparent lifetime of the ?l** state is a few hundreds nanoeecwds,aointhe~-rangethe signaIcanbede#xGed by the contribution of the Tl and T1 1eveIs. In thir case the intensity only depends on wven value!#areobtaitledby nUmizationoftheleastquarefittothe paramete=pbtheprernua. =m using Marqua& method [II] . Fig. 2 rhowe an BuLmpleof this fitting done for 1 Torr of acetone. eqerime-ntal CU~VBI, To &ther reduce the number of free parametera, we fixed kRl* and kR1 to the vahxi of 0.0066 p-l, value obtained experimenta& for the triplet state [12] and assumiq equivalent intersystem crossing for both, Tl* and Tl states. Fitting the expex&exM curvea from 0.6 to 7 Torr wa found an almost constant value for k$l*= 2*10e3 p-l Torr-l and for key = 1.4*10s3 ~-~Torr-~. It ia noteworthy that these parameter8 obtained independently almost coincide, reinforcing the idea that the main differenca between T1’ and T1 levels is the decompoeition channel. Fi now kR1, kRl*, the new valuea of kQl and kQl* and letting the initial population and the disxsiative rateconetantvaigwefoundforkpalineardependencewiththepraesure (in Torr): kp = 0.012@-1)
+ 0.00943@s-1Torr-1~[Ql
ThtpseemstobecompatiblewithunimolecularreactioIlc~behaviourwhensomeofthemoleculesenterwithan energyaroundthereactionthreshold.P~workdonetorreeinthegrowofbiacetylinasampleofacetone with the number of lnaer shots, and its dependence with the addition of SF6 or CF4 quenchem seen to indicate that thin procetw mainly happen by unimoIecular decompoeition. l were used 88 fued input in the equation written above to Thedecaycon&antaobtainedforthe~JandT1 deduce the decay constanti kRl** and kel of the Tl** triplet state. An example of this tit ir &own in Fig. 3. l * = (0.06 zt 0.02) p-1 and kQl** = (3.7 * 0.3) jm-lTorr-l in good ThevalueaobtainedhwnthiefitarekRl agreement with the vahss previou [21 of (4.06 * 0.36) #Torr-! OthsrLineticmodel~~the~tripbtb~at~dirtinct~~~,haralrobeen~~. The different behaviour bets T1* and T1, was awociated to diffeJrentin~-cmom to 51 and So respecti*. WecouIdnotAndantofparametasthat8uax&My fltourartperimbnhldata. Beriderthat,nrchamodeIwould beinamMentwiththere8uItafoundforthefglen&sxe.
621
Emission of excited acetone vapour
10-l
I 0
I 40
I
I 120
I 160
Figure 3. Experime&al data of the acetone emission at 30 mTorr presswe (starto) and its theoretical fit wllu line).
SUMMARY We have amcentrated
our study in the tail of the emission of the acetone vapour and followed its behaviour in range (0.01 - 10 Torr) and iri tha pmaence of diffemnt quenchw &ases. This has aUowed UB to aIawp=== results [2,3] obtained for the quenching rats constant of the so caBed hot triplet. raconciliat& the former We found in the whole pressure ranga three distinct component+x the hgest life ona identified with the tharmalized triplet Tl, the intermediate one as dua to the contribution of trip& states above tha decomposition *Ievels. Theselevelsshow level, Tl* hIled hot triplet by Co&la et al. [3]) and a shorter ona associated to the Tl l arimilar~~tothemtedstater[llbuttheeoupliaetoSlrtaterLwealer. Thesakwelswerestudiedby ~~andctwley[2landcalledaith~~aamethanin~.3.~diecrepancgintheirvalueeofthe qusnchingrateconrtant~duetothefactthattheywerereferinetotwodifferentcomponentswiththesame nama. AKNOWLEDGMENT: We would Iike to thank Prof. Y. Haas for stimulating and enlighting discuhons. This work was supported by CKX’I’ project number PB87-0048. J.Luque thanks Comunidad de Madrid for studentship.
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