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Matrix-isolation study of the co2 lowest triplet state
Volume 73, number2
CHEMICAL
MATRIX-ISOLATION
PHYSICS LETTERS
15 July 1980
STUDY OF THE CO2 LOWEST TRIPLET STATE
Hassan H. MOHAMMED, Janine FOURNIER,
Jean DESON and Catherine VERMEIL
Equlpe de Recherche du C N.R.S. (57) assoclh s I’ESPCI. 75231 Paris Cedex OS. France Recewed 31 March 1980.m final form 21 April 1980
We have mvestigated the ennsnon of a CO2 molecule isolated m an Ar matnx at 4.7 K III the spectral region between 350 and 5.50 nm Thus emission 1sa contmuum with a bfetime of 570 ms. We assign It to the transition from the lowest trlplet state (bent) ‘B2 to the fundamental state (lmear) ‘xi_
1. introduction
Electromc spectroscopy of matrix-isolated molecules allows one to study the changes in the propertres of these molecules as compared with the vapor phase. Moreover, the spur-orbrt couplmg induced by the matnx increases the probability of forbidden radiatrve transrtrons [ 1] The emrssroh of CO2 is not well known rn the vapor phase. Thrs molecule has sixteen valence electrons and the ground-state electronic configuration 1s . ..(o.)2(ou)2(o,)2(ou)2(~u)4(rrg)4 leading to a hnear rZ+ state. The linear excited states of this molecule cokespond to the configuration ( 17rg)3(27ru) and the term vaIues are 1s3Ct, 3Au, 1s3Z; and lA,_ Predictrons concemmg the bent exerted states have been made by England et al. [2] and by Wmter et al. 131. The last authors camed out CI calculations for the lowest singlet and triplet states of bent CO, (with L OCO = 120° and R( CO) = 1.374 A) using a minimum
basis set of contracted gausaans. They obtained the following adiabatic excitation energies: 32i(3B2)
= 3.7 eV,
3Au( 3A2) = 4.1 eV,
‘C;(lA2)
= 4.5 eV,
‘Au(‘B2)
= 5.8 eV.
Few experimental observations concerning the emission of this molecule in the vapor phase have been obtained; however, Gaydon [4] detected a contmuum emission from the carbon monoxide flame burning in au and in oxygen at low pressure in the
region between 350 and 450 nm. He proposed that thrs emission is from an excited bent state of CO2 to the linear ground state. Feast [S] confirmed this observation and attributed tt to an unspecified excited state of CO2 rather than from any diatomic emitter. Clyne and Thrush [6] studied the mechanism of chemihrminescent recombination of atomic oxygen and carbon monoxide. They postulated that recombination takes place along an excited state of CO, which may be a triplet state 3B2 with an energy barrier of 16 kJ mole-l. Frad [7] studied the explosion reaction of CO and 02 induced by a pyrolysis laser in the pressure range IO-760 Torr. He detected a continuum emission in the spectral region between 300 and 450 run. He attrrbuted it to unresolved flame emission bands. Strausz and Gunning 181 studied the decomposition of CO2 by exposure to radiation at 253.7 run in the presence of mercury vapor. The proposed mechamsm attnbutes the reaction to the collision of excited Hg(6 3P,) atoms wrth CO2 molecules which have been excited to a metastable state by energy transfer from Hg(6 3Pt) atoms with CO, molecules which have been excited to a metastable state by energy transfer from Hg(6 3P,) or Hg(6 3Po) atoms. CO2 should therefore possess an excited state not higher than 4.88 eV above its ground state and lower than its dissociation energy of 5.49 eV. The threshold energy electron-impact excitation of CO2 has been studred using the SF6 scavenger technique by Hubin-Franskin and Collin 191. They detected
Volume 73, number 2
CHEMICAL
peak at 4 eV and attnbuted it to the autoionisation of the CO; negative Ion in its ground electronic level. They proposed that the tnplet state 3B2 hes at about 6 eV, although the spectrum m thus regron seems qurte diffuse. Dixon [IO] obtained a high-resolution spectrum of the afterglow of CO2 m the wavelength range between 3 10 and 380 nm. He attributes the excited state to a ‘B2 state whrch lies approximately at 5.7 eV above the lowest level of the ground state, and has an OCO angle of 122 f 2” and a CO bond length of 1.246 + 0.008 A. In the field of matnx nolatron, Frosch [ 111 uradtated the CO, molecule in sohd mert gases usmg X-rays. He did not detect any continuum as observed one
W&I the flame bands. Up to now, the 3B2 state has never been rdentrRed, not by optical absorption, emrssron spectroscopy, or electron spectroscopy. The purpose of the present article is to descrrbe the observation of an emrsston spectrum obtamed by VW irradratron of CO2 isolated m an Ar matrix at 4.7 K and to ascertam tts assignment to the lowest tnplet state of CO,.
2. Experimental The experrmental apparatus has already been described by Fourmer et al. [ 12,131. The cryostat IS a 7 Q (1’Air Lrquide) polycryostat whrch was used to cool the gold-coated copper plate by circulation of fiqurd helium. Carbon and platinum resrstors were used to measure the temperature of the deposited sample. Two resonance light sources were used for excitatton of the matrix sample: the Xe resonance lamp emittmg radiation of 147 nm, the Kr resonance lamp emrttmg radiation of 123.6 nm. A LiF window was used with both hunps. Two Interference filters (Matra Divtsion Optrque) centered at 147 MI and 123.6 nm were used to mimmtze scattered low-energy radiatron. These two lamps are pulsed by a microwave generator built m our laboratory from a modef suggested by Sharpless f 14J_ Thrs system operates 1~1a pulsed mode with 2 s pulses at 0.25 Hz maxu-num frequency. The l&t cut-off ttme IS of the order of 500 ~.ls. A 6 cls to 2 s delay Jams the generator to a PAR I I I2 photon counter. Between the lamp and the cryostat, a rotatmg 316
PHYSICS LETTERS
15 July 1980
quartz window is used to remove the far-UV lrght (transparence hnnt of h = 160 nm) so that the excued species emtsston may be drstingmshed from the false lures em&ted by the lamp (X Z= 200 run) m the vrsrble regron and reflected by the sample. The light emitted by the matrix sample IS admitted to a H.R.S. Jobin-Yvon grating spectrometer (CzernyTurner); the holographtc grating (1200 lines/mm) has a large acceptance angle to obviate the need for a lens to focus the hght on the entrance sht. Thrs monochromator 1s equipped wrth a stepping motor (2400 steps/turn) whtch allows either a contmuous wavelength change from 1 to 1000 A/min or a dtscontmuous regtme by Increments of 0.416 to 41.6 A. In these expenments the spectral scan was performed at 200 &mm. A mirror switched on the two extt slits permits the utilisation of two photomultrplier tubes. A R 212 UH Hamamatsu cathode-type photomultrplier was used when the emlssxon IS excited by the Kr resonance Iamp and a R 585 bralkalr cathode m the case of the Xe resonance lamp. The electrical unpulses are sent through an amphfier-drscnmmator to a photon counter and are detected by a recorder (Sefram). The noise is less than 0.5 counts s-l at room temperature A mtxture of 0.1% CO2 (1’Arr Lrqutde N45 99.995%) and Ar (N55,VV 999%) was used without punficatron. Samples were prepared m a two-chamber bulb with specral care taken to avoid contamination by an. The major rmpurities whrch may exist in the matnx sample are trace amounts of an and water vapor. To test our gas handhng device, a mass analysrs wrth a Nrer type mass spectrometer (resolutron 150) has been performed by Dr. Cottm on one sample. The purity of this sample was 0, = 5 X 1O-5, Hz0 = 3 X iO-5 and CO = IO-S, by volume. For hfetlme measurements, the matnx IS photoIysed by the lamp and the decay of the emission IS recorded at dtfferent delays after the extinction of the lamp.
3. Results Figs. la and Lb show the emission spectra of CO2 isolated in an Ar matrix at 4.7 K and exerted at 147 run and 123.6 run, respecttvely. The two spectra are Identical. The continuum ranges from about 350 to
Volume 73, number 2
cl
500
15 July 1980
CHEMICAL PHYSICS LETTERS
460
350
Rg 1 (a) The contmuum emisston of CO2 irradiated by X = 147 nm fsht wrdth = 200 pm). @J)The contmuum emrss?on of CO2 lrtadlated by h = 123 6 run fsht wrdth = 800 pm) (c) With the quartz wmdow.
550 nm, with the maximum mtensrty around 450 nm; m both cases the experunents have been repeated wrth the quartz wmdow Intercalated in the excrtatron beam light to drstinguish tlus continuum from any other contmuum or false lures emitted by the lamp and reflected by the sample. No emission has been detected m those cases (fig. lc). We have also rrradrated the-pure Ar, then we have set the quartz wmdow, no emrssron has been detected in these cases. Thus confirms the presence of a continuum emission that can be attrrbuted to CO,. The beginrung of this contmuum may be estunated in the region between 350 and 390 run. This means that the emitting state ISlocated at about 3 36 + 0.18 eV above the ground state For the hfetune measurements, we have set the exrt sht width at 800 mt fixed the monochromator scan at the wavelength 435 nm of fig. la and we have operated the system in a pulsed mode. Then drfferent delay tunes have been taken III order to obtain the hfetime. The average vahre of several numbers of countsN versus the delay tune is taken to obtain the hfetime. The decay curve can be fitted by the relatron N = IV0 e-‘iT where r is the hfetrme. The naturallogarithm dependence of N versus the time IS shown in fig. 2, The hfetirne of this emission is obtamed from the slope -11~ of the straight hne, and a value 570 f 20 ms 1s obtained. We have repeated the hfetune measurements at two other wavelengths of the
Rg. 2. The natural-loganthmic tune 2.
plot of N versus the delay
fig. lb, namely, 481 run and 418 MI, and the values of 595 +: 38 and 600 +. 40 ms have been obtamed, respectively. The lifetime values are rrot significantly different. The fit measurement is more precise and we recommend a value of 570 C 20 ms for the CO2 tnplet lifetime_
contmuum
4. Discussion Our assignment of the continuum shown in fig. I to the enussion from the to’west excited triplet state 3B2 of the bent CO, molecule to the linear ground state is based on the following: ( 1) The same continuum has been observed [ 151 after VUV rrradiation of a N20 : CO : Ar mixture as a thermoluminescence during the warming up of the sample; then the matrix was vaporised and the gases collected for mass-spectrometric analysis. The results of this analysis indicated that CO2 was formed during the warming. This thermoluminescence has been expfarned by the reactions: co(x
‘i+) +0(3p, -+CC?,f3B2),
C0,(3B2)
* C02(X
‘E*)
+hv.
Accordrng to the correlation rules for the recombinatron reactron (I), the emitter state must be a tnplet; the spm-orbit -oupling induced by the inert3B7
Volume 73, number 2
CHEMICAL
PHYSICS LETTERS
1.5 July 1980
gas matrix should permrt the forbldden radiative transition (2) to occur. (2) The long hfetLme of our tnplet state (570 * 20 ms) 1s good evrdence for the existence of a forbidden transltlon. The oscillator strength of -&IS transition can be calculated from the relation
CO2 at 3.5 + 0.2 eV. The continuum emIssxon indicates that the triplet bent state 3B2 is perturbed by another dissociative state - possibly the singlet bent ‘A2 state, since the gap between these two states pven by Winter et al. [3] IS about I eV.
f = (1 .~9W2Tkulgp
Acknowledgement
where v, the posltlon of the Franck-Condon maxlmum, IS m cm-t and 7 is the hfetime m seconds gu and gp are the degeneracy of the upper and lower electronic levels. Using v = 22222 cm-l, and If we neglect degeneracies, as assumed by Robinson [ 161 for the f value of the 3Blu state of benzene in mertgas matrices,, one calculates& = 5.3 X 1O-p for emisslon. as IS of the right order of magnitude for a spin-forbldden transition. (3) A slmdar continuum m the same spectral reBon has been observed for the CSz molecule, excited by h = 3 10 nm from the xenon arc lamp, m the vapor phase by Lambert and Kimbell [17]. They used two different pressures of CS2, no structure was observed under any of the condltlons used. In our laboratory, we have also observed the same contmuum for this moIecule isolated in a Ne matnx fl8] using the Xe resonance lamp, h = 147 nm, and the hfetime has been measured to be 860 ms. Using the AI14 medmm-pressure mercury arc, X = 260 nm, Meyer et al [ 19 ] observed a contmuum m Ar and N? matrices, the estunated hfetune being 2.6 and 1.8 ms, respectiveIy. This continuum ha5 been attributed to the emission from the lowest triplet state 3B2 of the bent CS2 molecule, at about 3.5 eV above the ground state [20]. As one IS entitled to expect sumlar behavior for the two molecules, the CS2 results give further evidence of our asstgnment of the CO2 triplet state
5. Conclusion We think that the present work presents good evxof the lowest tnpiet state of
dence for the location
318
We wish to thank Dr. S. Leach for reading manuscript and for his valuable comments.
the
References B hIeyer, Sczence 168 (1970) 783. W. England. D. Yeager and A C Wahl, 1. Chem. Phys.
(161 [17] [ 181 1191
1201
66 (1977) 2344; 65 (1976) 684. N.W. Wmter, C E. Bender and W.A Goddard, Chem. Phys Letters 20 (1973) 489. A G Caydon, Proc Roy Sot. A 176 (1940) 505 M.W. Feast, Proc Phys Sot. (London) 63A (1950) 772 hl A A Ciyne and B A. Thrush, Proc Roy Sot. A269 (1962) 404. A Frad, thise 3Gme cycle, Umversltd de Parts XI (1970) p. 34. 0 P Strausz and H E Gunnmg, Can. J Chem 39 (1961) 2224_ hl J Hubm-Fran&m and J.E Coilm, J Electron Spectry 7 (1975) 139. R N Dixon, Proc. Roy Sot. A275 (1963) 431, DISCUSSIOIISFaraday Sot. 35 (1963) 105 R P. Frosch, Ph D Thesis, Umverslty ofcahfornra (1965) p 77. 1. Fourmer, J Deson and C. Vermed, J. Phys E 9 (1976) 879. J Fourmer, II. Deson and C. Vermed, J Chem. Phys. 70 (1979) 5703. R L Sharpless, prwate communication. i Fourmer, 1 Deson. C Vermed and G C. Punentel, J Chem. Phys. 70 (1979) 5726. G W. Robmson, J Mol. Spectry. 6 (1961) 58 C Lambert and GM Klmbeli, Can. J. Chem. 51 (1973) 2601 J. Fournter, J. Deson and C. Vermeti, to be pubhshed. L. BaJema, Icf Gouterman and B. hfeyer, J. Phys. Chem. 75 (1972) 2204. K Hara and D Phfips, J. Chem Sot. Faraday Trans. II 74 (1978) 1441.
CHEMICAL
MATRIX-ISOLATION
PHYSICS LETTERS
15 July 1980
STUDY OF THE CO2 LOWEST TRIPLET STATE
Hassan H. MOHAMMED, Janine FOURNIER,
Jean DESON and Catherine VERMEIL
Equlpe de Recherche du C N.R.S. (57) assoclh s I’ESPCI. 75231 Paris Cedex OS. France Recewed 31 March 1980.m final form 21 April 1980
We have mvestigated the ennsnon of a CO2 molecule isolated m an Ar matnx at 4.7 K III the spectral region between 350 and 5.50 nm Thus emission 1sa contmuum with a bfetime of 570 ms. We assign It to the transition from the lowest trlplet state (bent) ‘B2 to the fundamental state (lmear) ‘xi_
1. introduction
Electromc spectroscopy of matrix-isolated molecules allows one to study the changes in the propertres of these molecules as compared with the vapor phase. Moreover, the spur-orbrt couplmg induced by the matnx increases the probability of forbidden radiatrve transrtrons [ 1] The emrssroh of CO2 is not well known rn the vapor phase. Thrs molecule has sixteen valence electrons and the ground-state electronic configuration 1s . ..(o.)2(ou)2(o,)2(ou)2(~u)4(rrg)4 leading to a hnear rZ+ state. The linear excited states of this molecule cokespond to the configuration ( 17rg)3(27ru) and the term vaIues are 1s3Ct, 3Au, 1s3Z; and lA,_ Predictrons concemmg the bent exerted states have been made by England et al. [2] and by Wmter et al. 131. The last authors camed out CI calculations for the lowest singlet and triplet states of bent CO, (with L OCO = 120° and R( CO) = 1.374 A) using a minimum
basis set of contracted gausaans. They obtained the following adiabatic excitation energies: 32i(3B2)
= 3.7 eV,
3Au( 3A2) = 4.1 eV,
‘C;(lA2)
= 4.5 eV,
‘Au(‘B2)
= 5.8 eV.
Few experimental observations concerning the emission of this molecule in the vapor phase have been obtained; however, Gaydon [4] detected a contmuum emission from the carbon monoxide flame burning in au and in oxygen at low pressure in the
region between 350 and 450 nm. He proposed that thrs emission is from an excited bent state of CO2 to the linear ground state. Feast [S] confirmed this observation and attributed tt to an unspecified excited state of CO2 rather than from any diatomic emitter. Clyne and Thrush [6] studied the mechanism of chemihrminescent recombination of atomic oxygen and carbon monoxide. They postulated that recombination takes place along an excited state of CO, which may be a triplet state 3B2 with an energy barrier of 16 kJ mole-l. Frad [7] studied the explosion reaction of CO and 02 induced by a pyrolysis laser in the pressure range IO-760 Torr. He detected a continuum emission in the spectral region between 300 and 450 run. He attrrbuted it to unresolved flame emission bands. Strausz and Gunning 181 studied the decomposition of CO2 by exposure to radiation at 253.7 run in the presence of mercury vapor. The proposed mechamsm attnbutes the reaction to the collision of excited Hg(6 3P,) atoms wrth CO2 molecules which have been excited to a metastable state by energy transfer from Hg(6 3Pt) atoms with CO, molecules which have been excited to a metastable state by energy transfer from Hg(6 3P,) or Hg(6 3Po) atoms. CO2 should therefore possess an excited state not higher than 4.88 eV above its ground state and lower than its dissociation energy of 5.49 eV. The threshold energy electron-impact excitation of CO2 has been studred using the SF6 scavenger technique by Hubin-Franskin and Collin 191. They detected
Volume 73, number 2
CHEMICAL
peak at 4 eV and attnbuted it to the autoionisation of the CO; negative Ion in its ground electronic level. They proposed that the tnplet state 3B2 hes at about 6 eV, although the spectrum m thus regron seems qurte diffuse. Dixon [IO] obtained a high-resolution spectrum of the afterglow of CO2 m the wavelength range between 3 10 and 380 nm. He attributes the excited state to a ‘B2 state whrch lies approximately at 5.7 eV above the lowest level of the ground state, and has an OCO angle of 122 f 2” and a CO bond length of 1.246 + 0.008 A. In the field of matnx nolatron, Frosch [ 111 uradtated the CO, molecule in sohd mert gases usmg X-rays. He did not detect any continuum as observed one
W&I the flame bands. Up to now, the 3B2 state has never been rdentrRed, not by optical absorption, emrssron spectroscopy, or electron spectroscopy. The purpose of the present article is to descrrbe the observation of an emrsston spectrum obtamed by VW irradratron of CO2 isolated m an Ar matrix at 4.7 K and to ascertam tts assignment to the lowest tnplet state of CO,.
2. Experimental The experrmental apparatus has already been described by Fourmer et al. [ 12,131. The cryostat IS a 7 Q (1’Air Lrquide) polycryostat whrch was used to cool the gold-coated copper plate by circulation of fiqurd helium. Carbon and platinum resrstors were used to measure the temperature of the deposited sample. Two resonance light sources were used for excitatton of the matrix sample: the Xe resonance lamp emittmg radiation of 147 nm, the Kr resonance lamp emrttmg radiation of 123.6 nm. A LiF window was used with both hunps. Two Interference filters (Matra Divtsion Optrque) centered at 147 MI and 123.6 nm were used to mimmtze scattered low-energy radiatron. These two lamps are pulsed by a microwave generator built m our laboratory from a modef suggested by Sharpless f 14J_ Thrs system operates 1~1a pulsed mode with 2 s pulses at 0.25 Hz maxu-num frequency. The l&t cut-off ttme IS of the order of 500 ~.ls. A 6 cls to 2 s delay Jams the generator to a PAR I I I2 photon counter. Between the lamp and the cryostat, a rotatmg 316
PHYSICS LETTERS
15 July 1980
quartz window is used to remove the far-UV lrght (transparence hnnt of h = 160 nm) so that the excued species emtsston may be drstingmshed from the false lures em&ted by the lamp (X Z= 200 run) m the vrsrble regron and reflected by the sample. The light emitted by the matrix sample IS admitted to a H.R.S. Jobin-Yvon grating spectrometer (CzernyTurner); the holographtc grating (1200 lines/mm) has a large acceptance angle to obviate the need for a lens to focus the hght on the entrance sht. Thrs monochromator 1s equipped wrth a stepping motor (2400 steps/turn) whtch allows either a contmuous wavelength change from 1 to 1000 A/min or a dtscontmuous regtme by Increments of 0.416 to 41.6 A. In these expenments the spectral scan was performed at 200 &mm. A mirror switched on the two extt slits permits the utilisation of two photomultrplier tubes. A R 212 UH Hamamatsu cathode-type photomultrplier was used when the emlssxon IS excited by the Kr resonance Iamp and a R 585 bralkalr cathode m the case of the Xe resonance lamp. The electrical unpulses are sent through an amphfier-drscnmmator to a photon counter and are detected by a recorder (Sefram). The noise is less than 0.5 counts s-l at room temperature A mtxture of 0.1% CO2 (1’Arr Lrqutde N45 99.995%) and Ar (N55,VV 999%) was used without punficatron. Samples were prepared m a two-chamber bulb with specral care taken to avoid contamination by an. The major rmpurities whrch may exist in the matnx sample are trace amounts of an and water vapor. To test our gas handhng device, a mass analysrs wrth a Nrer type mass spectrometer (resolutron 150) has been performed by Dr. Cottm on one sample. The purity of this sample was 0, = 5 X 1O-5, Hz0 = 3 X iO-5 and CO = IO-S, by volume. For hfetlme measurements, the matnx IS photoIysed by the lamp and the decay of the emission IS recorded at dtfferent delays after the extinction of the lamp.
3. Results Figs. la and Lb show the emission spectra of CO2 isolated in an Ar matrix at 4.7 K and exerted at 147 run and 123.6 run, respecttvely. The two spectra are Identical. The continuum ranges from about 350 to
Volume 73, number 2
cl
500
15 July 1980
CHEMICAL PHYSICS LETTERS
460
350
Rg 1 (a) The contmuum emisston of CO2 irradiated by X = 147 nm fsht wrdth = 200 pm). @J)The contmuum emrss?on of CO2 lrtadlated by h = 123 6 run fsht wrdth = 800 pm) (c) With the quartz wmdow.
550 nm, with the maximum mtensrty around 450 nm; m both cases the experunents have been repeated wrth the quartz wmdow Intercalated in the excrtatron beam light to drstinguish tlus continuum from any other contmuum or false lures emitted by the lamp and reflected by the sample. No emission has been detected m those cases (fig. lc). We have also rrradrated the-pure Ar, then we have set the quartz wmdow, no emrssron has been detected in these cases. Thus confirms the presence of a continuum emission that can be attrrbuted to CO,. The beginrung of this contmuum may be estunated in the region between 350 and 390 run. This means that the emitting state ISlocated at about 3 36 + 0.18 eV above the ground state For the hfetune measurements, we have set the exrt sht width at 800 mt fixed the monochromator scan at the wavelength 435 nm of fig. la and we have operated the system in a pulsed mode. Then drfferent delay tunes have been taken III order to obtain the hfetime. The average vahre of several numbers of countsN versus the delay tune is taken to obtain the hfetime. The decay curve can be fitted by the relatron N = IV0 e-‘iT where r is the hfetrme. The naturallogarithm dependence of N versus the time IS shown in fig. 2, The hfetirne of this emission is obtamed from the slope -11~ of the straight hne, and a value 570 f 20 ms 1s obtained. We have repeated the hfetune measurements at two other wavelengths of the
Rg. 2. The natural-loganthmic tune 2.
plot of N versus the delay
fig. lb, namely, 481 run and 418 MI, and the values of 595 +: 38 and 600 +. 40 ms have been obtamed, respectively. The lifetime values are rrot significantly different. The fit measurement is more precise and we recommend a value of 570 C 20 ms for the CO2 tnplet lifetime_
contmuum
4. Discussion Our assignment of the continuum shown in fig. I to the enussion from the to’west excited triplet state 3B2 of the bent CO, molecule to the linear ground state is based on the following: ( 1) The same continuum has been observed [ 151 after VUV rrradiation of a N20 : CO : Ar mixture as a thermoluminescence during the warming up of the sample; then the matrix was vaporised and the gases collected for mass-spectrometric analysis. The results of this analysis indicated that CO2 was formed during the warming. This thermoluminescence has been expfarned by the reactions: co(x
‘i+) +0(3p, -+CC?,f3B2),
C0,(3B2)
* C02(X
‘E*)
+hv.
Accordrng to the correlation rules for the recombinatron reactron (I), the emitter state must be a tnplet; the spm-orbit -oupling induced by the inert3B7
Volume 73, number 2
CHEMICAL
PHYSICS LETTERS
1.5 July 1980
gas matrix should permrt the forbldden radiative transition (2) to occur. (2) The long hfetLme of our tnplet state (570 * 20 ms) 1s good evrdence for the existence of a forbidden transltlon. The oscillator strength of -&IS transition can be calculated from the relation
CO2 at 3.5 + 0.2 eV. The continuum emIssxon indicates that the triplet bent state 3B2 is perturbed by another dissociative state - possibly the singlet bent ‘A2 state, since the gap between these two states pven by Winter et al. [3] IS about I eV.
f = (1 .~9W2Tkulgp
Acknowledgement
where v, the posltlon of the Franck-Condon maxlmum, IS m cm-t and 7 is the hfetime m seconds gu and gp are the degeneracy of the upper and lower electronic levels. Using v = 22222 cm-l, and If we neglect degeneracies, as assumed by Robinson [ 161 for the f value of the 3Blu state of benzene in mertgas matrices,, one calculates& = 5.3 X 1O-p for emisslon. as IS of the right order of magnitude for a spin-forbldden transition. (3) A slmdar continuum m the same spectral reBon has been observed for the CSz molecule, excited by h = 3 10 nm from the xenon arc lamp, m the vapor phase by Lambert and Kimbell [17]. They used two different pressures of CS2, no structure was observed under any of the condltlons used. In our laboratory, we have also observed the same contmuum for this moIecule isolated in a Ne matnx fl8] using the Xe resonance lamp, h = 147 nm, and the hfetime has been measured to be 860 ms. Using the AI14 medmm-pressure mercury arc, X = 260 nm, Meyer et al [ 19 ] observed a contmuum m Ar and N? matrices, the estunated hfetune being 2.6 and 1.8 ms, respectiveIy. This continuum ha5 been attributed to the emission from the lowest triplet state 3B2 of the bent CS2 molecule, at about 3.5 eV above the ground state [20]. As one IS entitled to expect sumlar behavior for the two molecules, the CS2 results give further evidence of our asstgnment of the CO2 triplet state
5. Conclusion We think that the present work presents good evxof the lowest tnpiet state of
dence for the location
318
We wish to thank Dr. S. Leach for reading manuscript and for his valuable comments.
the
References B hIeyer, Sczence 168 (1970) 783. W. England. D. Yeager and A C Wahl, 1. Chem. Phys.
(161 [17] [ 181 1191
1201
66 (1977) 2344; 65 (1976) 684. N.W. Wmter, C E. Bender and W.A Goddard, Chem. Phys Letters 20 (1973) 489. A G Caydon, Proc Roy Sot. A 176 (1940) 505 M.W. Feast, Proc Phys Sot. (London) 63A (1950) 772 hl A A Ciyne and B A. Thrush, Proc Roy Sot. A269 (1962) 404. A Frad, thise 3Gme cycle, Umversltd de Parts XI (1970) p. 34. 0 P Strausz and H E Gunnmg, Can. J Chem 39 (1961) 2224_ hl J Hubm-Fran&m and J.E Coilm, J Electron Spectry 7 (1975) 139. R N Dixon, Proc. Roy Sot. A275 (1963) 431, DISCUSSIOIISFaraday Sot. 35 (1963) 105 R P. Frosch, Ph D Thesis, Umverslty ofcahfornra (1965) p 77. 1. Fourmer, J Deson and C. Vermed, J. Phys E 9 (1976) 879. J Fourmer, II. Deson and C. Vermed, J Chem. Phys. 70 (1979) 5703. R L Sharpless, prwate communication. i Fourmer, 1 Deson. C Vermed and G C. Punentel, J Chem. Phys. 70 (1979) 5726. G W. Robmson, J Mol. Spectry. 6 (1961) 58 C Lambert and GM Klmbeli, Can. J. Chem. 51 (1973) 2601 J. Fournter, J. Deson and C. Vermeti, to be pubhshed. L. BaJema, Icf Gouterman and B. hfeyer, J. Phys. Chem. 75 (1972) 2204. K Hara and D Phfips, J. Chem Sot. Faraday Trans. II 74 (1978) 1441.