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Br2 Light emission from cw CO2-laser-induced CF3Br pyrolysis
Voiume 99. number 3
CHEMICAL
PHYSICS LETTERS
Br, LIGHT EMISSION FROM cw CO?-LASER-INDUCED
5 August 1983
CF3Br PYROLYSIS
G. LUIJKS, W. SNIPPERS. G. PETROCELLI. S. STOLTE and J. REUSS ~_SSISCII Lnboratorium. h~afholieX_eUnirersireit. Toemooireld. 6525 ED A’Qmegen. The
Xed~erlands
A bright ~cllow-red slow k observed in a swnplc of C1-xBr g.~s (75 Torr) after cw COz-laser irradiation (G30 I\‘)_The spectrum of rhe clcctronic transition Br2(B - S) is identified. The formation of vibrationally hot Br,(B) molecules via CI‘3Br pj roll sis md 1%recombination credtcs .I strong& atlicr~n~l distribution within the B state.
I. Introduction The visible and infrared luminescence from the recombination of Ur atoms into electronically excited Br, molecules was observed and identified by Gibbs and Ogryzlo [ 1] and Clyne and Coson [2] _For the
production of Br atoms they exposed Br, to microwwc and rf discharges. respectively_ Other sources of Br atoms are excimer-laser photolysis CF,Br, 131 or infrared multiphoton
of CBrl and dissociation of
CF3Br-(e.g. ref. [4])_ The latter technique has been used by Jalenak and Nogar in a qwntitative analysis of the products of CF3 Br dissociation and their subsequent reactions [5]. In this letter we report the obscrvation of a bright visible luminescence when a sample of CF3 Br gas is irradiated by a cw CO, laser (P < 30 W) exciting the v3 mode of CF3Br (1085 ~rn-~). Dissociation of CFjBr into CF; + Br is then accomplished through the process of laser-powered homogeneous pyrolysis (LPIIP) [6]. where the ker is used to heat the gas to wmpera1ures of 700-1000 K. The same lunlinescence is seen in a mhture of SF6 and Cl’, Br if the CO, laser is tuned
into resonance with the y7 1110dc of SF, (94s cn-* ), unambiguously demonstriting the 0ccuTrence of pyrolysis_ In a similar way Karlov et al. [7] have observed luminescence applying LPHP to BCl; _ A spectral analysis of the light (resolution S cm-l fwiun) revealed the vibrational band spectrum of the electronic transition Br?(B + X). This could be concluded from a comparison with the data of Coxon et 232
al._ who rotationally resolved the Br2(B 3 flo”+ + X1 Sf) and Br2(A3111u + X I Zz) spectra in absorption z&d calculated the Fran&-London factors for the vibrational transitions [S-l 11. The measured intensity distribution of the vibrational bands in our spectrum suaests completely athermal vibrational populationsin the electronic B state_ We conclude that BrZ is formed in high vibrational levels near to (and above) the dissociation limit of the B state. For a proper understanding of this feature one must know the reaction typesleading to the formation of Br2, as well as lifetimes and relaxation rates for the excited Br? (B z IIou+). The important reactions are: CF, Br + 22575 Br, + 16130
cmel++
cm -I
CF; -+ Br ~
(1)
ABBri-Br.
(2)
yielding
CF3 + Br, + CF3Br + Br + 6445 cm-l
.
(3)
Eq. (3) describes the production of ‘*hot“ Br atoms recombining into vibrationally excited Br,(B) molecules. A survey concerning the kinetics of the excited states of Br2 has been given by Clyne and McDermid [ I?]_ The mean value for the radiative lifetime of the B state was found to be 7R = 17__4ps [13]_The fastest rate for deactivation of the B state isdue to collisional predissociation, e.g. k,(B)
+ Br2(X) + Br + Br •t Br2(X),
0 009-26 14/S3/0000-0000/S
(4)
03.00 0 1983 North-Holland
For coltisiondi vibrational energy transfer in the B state, the rate rw has only been measured for the Ay = -1 transition, 22‘= 11 -+ 1.3’= 10: Tw,~~=__~P = 1.3 ps Torr [ 143. However, the overall V-V rate (Le. ail Aus included) sboxdd be comparable to rhe case of CLJB) since the ~brat~o~~ splitting is similar.. There, Clyne and ~c~rlnid [26] measured the rate for the initial level u’ = f 17rvv altnuSP= 140 ns Torr, For big&x v3 levels a faster rate’& measured eg. for u’ = 12, 7VVl;tn_hU’ F= 85 ns Torr. For the A state of Br2 a fast co&sionai V-V rate has been found too < 14Q ns Tax. r.171. rwmU8
monochromator
glass windows (WI and W,) and two NaCl.x$ndows (W, and tV,) mou~i~ near the center of the cell in or&r to q&&e the total luminesaxsce ti the sensi~ tive area of the monoc~omator- The unfocused infrared radiation from a home-made ~e~tunab~~ cw CO, laser (P < 30 W) enters the cell throu& Wz and is almost completely absorbed by the CF3Br gas or the SF6--CF3Br mixture, atP = 75 Torr and T= 300 EC_As a result a btigbt yellow-red glow appears afong the laser beam path accompanied with a them& heating of the compfete sample c&i to a tern~~~tn~ of =50aC_ Applying a small CF3Br flow from the top of tt-lf:cell, the Egbt trace becomes r~rn~kab~y bent as if tire gas Bow pushes away the ~~~-ern~tt~g n~ofecu1~~. Regular cleaning oftbe windows is necessary as brown Br2 deposits tend to obscure them, Measuring times up to approximately one fiour art3 passibfe before tile gas samp!e has to be exchanged,
Volume 99. number 3
Through W, and a lens L u= 8 cm) the funtinescence front the center of the cell (detector-sensitive area 5 X 0.5 mm2) is transmitted into a double monochromator (Ramanor HG2S. Jobin-Yvon), where it is spectrally artalysed with a resolution of 8 cm-l fwhm. The photons are detected by a p~to~ornult~p~er (P&fT, . liarttan~atsu R943,GaAs cathode) and a photon-couIttin~ systent (PCI. BrookdeaI 5Cl. first channel). A microprocessor (j.fP. Rockwell AIMfS.32k) controls the IttoItocitro~ttator scans, reads the counter output into its mentory and plots the spectrum on a screen or an X-Y recorder. The total luntinescence is ntonirored Ihrough a glass fiber (GLF) by a second phototnulti~t~er (PhfT,. E~II96j8) and photon counter (PC?. Brookdeal SCI. second channel). This configuration enabies us to compensate the PC1 counting time for fluctuations in the total light intensity during a spectra1 scan. using the preset counts option of the BrookdeJ
counter.
3. Results and discussion
The compIete observed luminescence spcctrunt from 20000 to 11500 c~tt-~ is displayed in fig. 2 and discussed in section 3.2. It has been recorded at a pressure of 75 Torr pure CF; Br i~t the santpie cell. applying 20 W of unfocused COZ-laser power at 9RlS (1077 cm-I). Although the vI resonance of CF;Br lies at 1085 an-l (91~30). the total light yield as detected by PMT, is found to have its maximum around 1070 cnt-I_ i-e- a redshift of 15 cm-’
excitation spectrum_ The anharmonic shifts of the higher vibrational levels of CF3Br (,Y+ see table 2 of ref. [ lS]) introduce red-
shifts of tiie vl transition frequency. leading to the observed e.\citation spectrum. The Sante effect has been c.scitation of SF6 (v;)
in a molec-
l&r-bemt
Raman esperintent (fig. 11 of ref. [ 19])_ In the sme Raman study it is concluded that a complete vibratio1t.d therntalization occurs by V-V transfer if sufficient energy-redistributing collisions take pltlce and if Trol is not tcio low (2 150 K). /vi these
conditions are certainly met in the present case of CF3 Br (P = 75 Torr, T 2 300 K) the gas will be thes-
ntaiIy heated by the CO, laser up to tentperatures 234
around 1000 K, where pyrolysis of CF,Br takes place. The heating and pyrolysis of CF33r has also been achieved using SF6 as absorber, i-e_ exciting the ~3 mode of SF6 (948 cm-l) in a mixture SF6 : CF3Br = 2 : 1. The total Br, luminescence was roughly the same as for the pure CF3Br case_ We have been able to determine the delay time Q between excitation and luminescence using a mechanical chopper to modulate the CO2 laser and measuring the phaseshift of the luminescence signal. We found rD * 1 nts. needed by the CF3Br molecules to reach the pyrolysis temperature and to dissociate into CF, t 3r. More evidence for the existence of this delay time TD is provided by the curved light trace described in section 2, suggesting that the molecules continue to redistribute their absorbed energy and subsequently dissociate while drifting out of the C02-laser beam region. A very steep dependence has been found measuring the total light intensitylon PMT, as a functionof the incident CO,-laser power (pL) at a fired frequency: I a Pi4. This behaviour can be understood considering the Plan& energy distribution at T = 1000 I;; the tttaxintum is found around 2000 cm-l whereas CF- Br dissociation can only take place from 22575 cm-f, i.e. very far in the exponential tail of the distribution,
which is very sensitive for the temperature. Thus, slightly increasing the gas temperature by the CO+ser power strongly enhances the CF3Br dissociation yield and consequently the intensity of the recombination Iuntineseence_
is observed for the very broad (30
CIII-~ fwhnt) and structureless
found for C02-laser
5 August 1983
CHEhilCAL PHYSICS LETTERS
Fig. 3 shows the important potential curves for the Br, molecule. including the vibrational levels. The structures in the luminescence spectrum of fig. 2 have been identified as vibrational bands in the B 3 II,“+ + X 1 ZZl electronic transition using the vibrational constants as determined by Coxon [S] for 79Br7QBr. For transitions B(u’) + X(0”) the frequency of the band origin is then calculated from Av = T, + G(u’) - G(?J”) , where Te 15902 cm-l, C(v’> = 167_55(u’ +:I
- 1_625(u’ -i-i)’
- 0.01 O(u’ + :>3 cm-l
>
CHEMICAL
PHYSICS LETl-ERS
5 August 1983
I-
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I I /
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I
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i.“\; ,
Volume 99. number 3
~(R~/~03crn-’
CHEMICAL
t
,
L
Br(2Pml-
B&P1121
PHYSICS iXJXERS
3r(2P3d*Br(‘P3I2 ----------
- O.O026(u" +$)j cm-~ _ For most
peaks in the spectrum
the vibrational
qusn-
numbers (u’ -+ u”) are indicated. Shifts due to Br, isotopes (79Br*l Br and 81 Brs1 Br) are small and do not influence the low-resolution vibrational band spectrum of fig. 2. At the high-energy end ofthe spectrum (> 17000 cm--1 ) tlte structure disappears due to the increasing density of u’ states near the dissociation threshold of the B state (fig. 3). Luminescence has been observed up to IW = 22500 cm-1 _whicl~ is in agreement with the proposed mechanism for hot formation of Br,(l3) moiccules via cy_ (3). where a maximum energy of 6500 cm-1 per molecule is available, allowing light emission up to iru = Te i 6500 = 22500 cm-l The relative intensities of the structures in the spectrum revea1 a strongly athermal vibrational population distribution in the B state, as determined after correcturn
5 Au_mst 1983
tions for the spectral ~ans~ion of the monochromator, the spectral sensitivity of the photom~tip~er, the Franck-Condon factors and the p3 lifetime dependence of the states. At high photon energies an additional intensity correction is needed due to self-absorption into the continuum of the A state; emitted photons from the luminescence area near the laser beam can be absorbed by cold Br2(X) molecules surrounding the excitation region. As a result part of the spectrum (IVY> 15500 cm-1 in fig. 3) is obscured, appearing in fig. 2 as a sudden intensity decrease_ Based on our observations and the reactions, lifetimes and rates discussed in the introduction, we have evaluated the following model for the reaction kinetics. As soon as Brz molecules are formed via the process of pyrolysis of CF, Br, eq. (3) describes the production of very hot Br atoms recombining into high vibrational states or the continuum of Br,(i3). During their radiative lifetime TV = 12-4 ~.lsthe-molecules undergo mainly two processes: collisional predissociation or quenching to the ground state and V-V relaxation. Both processes have a comparable rate, r = 1 ns at our pressure (75 Torr). Consequently. the probability for molecules to reach Iow u’ levels via V-V transfer and then emit a photon will be low, since they will be quenched on their way down and thusremain invisible_ The result is a preferred popuIation of high vibrational states within the B state yielding possibly an inversion. From collisionless predissociation to the repulsive i 11lu state no effects are expected because it is a slow process (MOO ns); it would appear in the spectrum as a depletion of specific U*states near the crossing point (u’ =: 3,4), wbicb is not observed_ Diffusion of Br2 molecules out of the sample area has been calculated to last rafl =5 ms at our conditions, and therefore plays no role on the luminescence timescale. An indication of the temperature in the cell is obtained via a computer simulation of the 3-8 vibrational band (i.e. u’ = 3 + U“ = 8) using the detailed rotational data from Coxon (table 1 of ref. f8])_ A convolution was produced of the generated thermal spike spectrum of rotational transitions with the profile of the monochromator (8 cm-1 fwhm)_ The position of the Boltzmann maximum of the 3-g band was moderately sensitive to the rotational temperature, which was used as a simulation parameter. The best fit was obtained for T,,=9OOi 100K.
Volume 99, number 3
CHEMICAL
References
111D-9.
Gibbs and E-A. Ogryzlo, tin. J. Chem. 43 (1965) 1905. PI hfA.A. Clyne and J.A. Coxon, J. Mol. Spectry. 23 (1967) 258. 131 C.L. Sam and J.T. Yardley, Chem. Phys. Letters 61 (1979) 509. 141 V.N. Bagatashvili, V-N. Burimov, LE. Deev, A.V. Zabolotnykh, V.S. Letokhov, G-1. Nazarenko, A.P. Sviridov and V.S. Shaidurov, Soviet 1. Quantum Electron. 12 (1982) 249. 151 W-A. Jalenak and N.S. Nosar, Chem. Phys. 41 (1979) 407. 161 W.M. Shaub and S-H. Bauer, Intern. J. Chem. Kinetics 7 (1975) 509. L71 N.V. Karlov, Yu.N. Petrov, A.M. Prokhorov and O.hI. Stel’makh, JETP Letters 11 (1970) 135. 181 J.A. Coxon, J. Mol. Spectry. 37 (1971) 39.
PHYSICS LETTERS
5 August 1983
PI R.F. Barrow, T.C. Clark, J.A. Coson and RX.
Lee, J. Mol. Spectry. 51 (1974) 428. ilO1 J.A. Coxon, J. Mol. Spectry. 41 (1972) 548. WI J.A. Coson, J. Mol. Spectry. 41 (1972) 566. 1121 M.A.A. Cfyne and 1-S. hfcDermid, Advan. Chem. Phys. 50 (1982) 29. 1131 hfA.A. Clyne, itf.C. Heaven and E. Martinez, J. Chcm. Sot. Faraday Trans. II 76 (1980) 405. HeavenandSJ. Davis, J.Chem.Soc. 1141 M.A.A.Clyne,hI.C. Faraday Trans. 1176 (1980) 961. 1151 hf .A.A. Qyne and M.C. Heaven, J. Chem. Sot. Faraday Trans. II 74 (1978) 1992. l.S.hfcDermid,J.Chem.Soc.Faraday 1161 hf.A.A.Uyneand Trans. 75 (1979) 1313. 1171 M-AA. Clyne. M.C. Heaven and E. Martinez, J. Chem. Sot. Faraday Trans. II 76 (1980) 177. 1181 G. Luijks, S. Stolte and J. Reuss, Chem. Phys. Letters 94 (1983) 48. I191 G. Luijks, J. Timmerman, S. Stolte and J. Reuss. Chcm. Phys. 77 (1983) 169.
237
CHEMICAL
PHYSICS LETTERS
Br, LIGHT EMISSION FROM cw CO?-LASER-INDUCED
5 August 1983
CF3Br PYROLYSIS
G. LUIJKS, W. SNIPPERS. G. PETROCELLI. S. STOLTE and J. REUSS ~_SSISCII Lnboratorium. h~afholieX_eUnirersireit. Toemooireld. 6525 ED A’Qmegen. The
Xed~erlands
A bright ~cllow-red slow k observed in a swnplc of C1-xBr g.~s (75 Torr) after cw COz-laser irradiation (G30 I\‘)_The spectrum of rhe clcctronic transition Br2(B - S) is identified. The formation of vibrationally hot Br,(B) molecules via CI‘3Br pj roll sis md 1%recombination credtcs .I strong& atlicr~n~l distribution within the B state.
I. Introduction The visible and infrared luminescence from the recombination of Ur atoms into electronically excited Br, molecules was observed and identified by Gibbs and Ogryzlo [ 1] and Clyne and Coson [2] _For the
production of Br atoms they exposed Br, to microwwc and rf discharges. respectively_ Other sources of Br atoms are excimer-laser photolysis CF,Br, 131 or infrared multiphoton
of CBrl and dissociation of
CF3Br-(e.g. ref. [4])_ The latter technique has been used by Jalenak and Nogar in a qwntitative analysis of the products of CF3 Br dissociation and their subsequent reactions [5]. In this letter we report the obscrvation of a bright visible luminescence when a sample of CF3 Br gas is irradiated by a cw CO, laser (P < 30 W) exciting the v3 mode of CF3Br (1085 ~rn-~). Dissociation of CFjBr into CF; + Br is then accomplished through the process of laser-powered homogeneous pyrolysis (LPIIP) [6]. where the ker is used to heat the gas to wmpera1ures of 700-1000 K. The same lunlinescence is seen in a mhture of SF6 and Cl’, Br if the CO, laser is tuned
into resonance with the y7 1110dc of SF, (94s cn-* ), unambiguously demonstriting the 0ccuTrence of pyrolysis_ In a similar way Karlov et al. [7] have observed luminescence applying LPHP to BCl; _ A spectral analysis of the light (resolution S cm-l fwiun) revealed the vibrational band spectrum of the electronic transition Br?(B + X). This could be concluded from a comparison with the data of Coxon et 232
al._ who rotationally resolved the Br2(B 3 flo”+ + X1 Sf) and Br2(A3111u + X I Zz) spectra in absorption z&d calculated the Fran&-London factors for the vibrational transitions [S-l 11. The measured intensity distribution of the vibrational bands in our spectrum suaests completely athermal vibrational populationsin the electronic B state_ We conclude that BrZ is formed in high vibrational levels near to (and above) the dissociation limit of the B state. For a proper understanding of this feature one must know the reaction typesleading to the formation of Br2, as well as lifetimes and relaxation rates for the excited Br? (B z IIou+). The important reactions are: CF, Br + 22575 Br, + 16130
cmel++
cm -I
CF; -+ Br ~
(1)
ABBri-Br.
(2)
yielding
CF3 + Br, + CF3Br + Br + 6445 cm-l
.
(3)
Eq. (3) describes the production of ‘*hot“ Br atoms recombining into vibrationally excited Br,(B) molecules. A survey concerning the kinetics of the excited states of Br2 has been given by Clyne and McDermid [ I?]_ The mean value for the radiative lifetime of the B state was found to be 7R = 17__4ps [13]_The fastest rate for deactivation of the B state isdue to collisional predissociation, e.g. k,(B)
+ Br2(X) + Br + Br •t Br2(X),
0 009-26 14/S3/0000-0000/S
(4)
03.00 0 1983 North-Holland
For coltisiondi vibrational energy transfer in the B state, the rate rw has only been measured for the Ay = -1 transition, 22‘= 11 -+ 1.3’= 10: Tw,~~=__~P = 1.3 ps Torr [ 143. However, the overall V-V rate (Le. ail Aus included) sboxdd be comparable to rhe case of CLJB) since the ~brat~o~~ splitting is similar.. There, Clyne and ~c~rlnid [26] measured the rate for the initial level u’ = f 17rvv altnuSP= 140 ns Torr, For big&x v3 levels a faster rate’& measured eg. for u’ = 12, 7VVl;tn_hU’ F= 85 ns Torr. For the A state of Br2 a fast co&sionai V-V rate has been found too < 14Q ns Tax. r.171. rwmU8
monochromator
glass windows (WI and W,) and two NaCl.x$ndows (W, and tV,) mou~i~ near the center of the cell in or&r to q&&e the total luminesaxsce ti the sensi~ tive area of the monoc~omator- The unfocused infrared radiation from a home-made ~e~tunab~~ cw CO, laser (P < 30 W) enters the cell throu& Wz and is almost completely absorbed by the CF3Br gas or the SF6--CF3Br mixture, atP = 75 Torr and T= 300 EC_As a result a btigbt yellow-red glow appears afong the laser beam path accompanied with a them& heating of the compfete sample c&i to a tern~~~tn~ of =50aC_ Applying a small CF3Br flow from the top of tt-lf:cell, the Egbt trace becomes r~rn~kab~y bent as if tire gas Bow pushes away the ~~~-ern~tt~g n~ofecu1~~. Regular cleaning oftbe windows is necessary as brown Br2 deposits tend to obscure them, Measuring times up to approximately one fiour art3 passibfe before tile gas samp!e has to be exchanged,
Volume 99. number 3
Through W, and a lens L u= 8 cm) the funtinescence front the center of the cell (detector-sensitive area 5 X 0.5 mm2) is transmitted into a double monochromator (Ramanor HG2S. Jobin-Yvon), where it is spectrally artalysed with a resolution of 8 cm-l fwhm. The photons are detected by a p~to~ornult~p~er (P&fT, . liarttan~atsu R943,GaAs cathode) and a photon-couIttin~ systent (PCI. BrookdeaI 5Cl. first channel). A microprocessor (j.fP. Rockwell AIMfS.32k) controls the IttoItocitro~ttator scans, reads the counter output into its mentory and plots the spectrum on a screen or an X-Y recorder. The total luntinescence is ntonirored Ihrough a glass fiber (GLF) by a second phototnulti~t~er (PhfT,. E~II96j8) and photon counter (PC?. Brookdeal SCI. second channel). This configuration enabies us to compensate the PC1 counting time for fluctuations in the total light intensity during a spectra1 scan. using the preset counts option of the BrookdeJ
counter.
3. Results and discussion
The compIete observed luminescence spcctrunt from 20000 to 11500 c~tt-~ is displayed in fig. 2 and discussed in section 3.2. It has been recorded at a pressure of 75 Torr pure CF; Br i~t the santpie cell. applying 20 W of unfocused COZ-laser power at 9RlS (1077 cm-I). Although the vI resonance of CF;Br lies at 1085 an-l (91~30). the total light yield as detected by PMT, is found to have its maximum around 1070 cnt-I_ i-e- a redshift of 15 cm-’
excitation spectrum_ The anharmonic shifts of the higher vibrational levels of CF3Br (,Y+ see table 2 of ref. [ lS]) introduce red-
shifts of tiie vl transition frequency. leading to the observed e.\citation spectrum. The Sante effect has been c.scitation of SF6 (v;)
in a molec-
l&r-bemt
Raman esperintent (fig. 11 of ref. [ 19])_ In the sme Raman study it is concluded that a complete vibratio1t.d therntalization occurs by V-V transfer if sufficient energy-redistributing collisions take pltlce and if Trol is not tcio low (2 150 K). /vi these
conditions are certainly met in the present case of CF3 Br (P = 75 Torr, T 2 300 K) the gas will be thes-
ntaiIy heated by the CO, laser up to tentperatures 234
around 1000 K, where pyrolysis of CF,Br takes place. The heating and pyrolysis of CF33r has also been achieved using SF6 as absorber, i-e_ exciting the ~3 mode of SF6 (948 cm-l) in a mixture SF6 : CF3Br = 2 : 1. The total Br, luminescence was roughly the same as for the pure CF3Br case_ We have been able to determine the delay time Q between excitation and luminescence using a mechanical chopper to modulate the CO2 laser and measuring the phaseshift of the luminescence signal. We found rD * 1 nts. needed by the CF3Br molecules to reach the pyrolysis temperature and to dissociate into CF, t 3r. More evidence for the existence of this delay time TD is provided by the curved light trace described in section 2, suggesting that the molecules continue to redistribute their absorbed energy and subsequently dissociate while drifting out of the C02-laser beam region. A very steep dependence has been found measuring the total light intensitylon PMT, as a functionof the incident CO,-laser power (pL) at a fired frequency: I a Pi4. This behaviour can be understood considering the Plan& energy distribution at T = 1000 I;; the tttaxintum is found around 2000 cm-l whereas CF- Br dissociation can only take place from 22575 cm-f, i.e. very far in the exponential tail of the distribution,
which is very sensitive for the temperature. Thus, slightly increasing the gas temperature by the CO+ser power strongly enhances the CF3Br dissociation yield and consequently the intensity of the recombination Iuntineseence_
is observed for the very broad (30
CIII-~ fwhnt) and structureless
found for C02-laser
5 August 1983
CHEhilCAL PHYSICS LETTERS
Fig. 3 shows the important potential curves for the Br, molecule. including the vibrational levels. The structures in the luminescence spectrum of fig. 2 have been identified as vibrational bands in the B 3 II,“+ + X 1 ZZl electronic transition using the vibrational constants as determined by Coxon [S] for 79Br7QBr. For transitions B(u’) + X(0”) the frequency of the band origin is then calculated from Av = T, + G(u’) - G(?J”) , where Te 15902 cm-l, C(v’> = 167_55(u’ +:I
- 1_625(u’ -i-i)’
- 0.01 O(u’ + :>3 cm-l
>
CHEMICAL
PHYSICS LETl-ERS
5 August 1983
I-
/) I( I’
/ I / / I
I I /
/ff I
I
f’
,”
//’
/
/ I’
/
/ ,/
/
//
,/
//
f’
E*ZtZ--01z--11Z-vi -
i.“\; ,
Volume 99. number 3
~(R~/~03crn-’
CHEMICAL
t
,
L
Br(2Pml-
B&P1121
PHYSICS iXJXERS
3r(2P3d*Br(‘P3I2 ----------
- O.O026(u" +$)j cm-~ _ For most
peaks in the spectrum
the vibrational
qusn-
numbers (u’ -+ u”) are indicated. Shifts due to Br, isotopes (79Br*l Br and 81 Brs1 Br) are small and do not influence the low-resolution vibrational band spectrum of fig. 2. At the high-energy end ofthe spectrum (> 17000 cm--1 ) tlte structure disappears due to the increasing density of u’ states near the dissociation threshold of the B state (fig. 3). Luminescence has been observed up to IW = 22500 cm-1 _whicl~ is in agreement with the proposed mechanism for hot formation of Br,(l3) moiccules via cy_ (3). where a maximum energy of 6500 cm-1 per molecule is available, allowing light emission up to iru = Te i 6500 = 22500 cm-l The relative intensities of the structures in the spectrum revea1 a strongly athermal vibrational population distribution in the B state, as determined after correcturn
5 Au_mst 1983
tions for the spectral ~ans~ion of the monochromator, the spectral sensitivity of the photom~tip~er, the Franck-Condon factors and the p3 lifetime dependence of the states. At high photon energies an additional intensity correction is needed due to self-absorption into the continuum of the A state; emitted photons from the luminescence area near the laser beam can be absorbed by cold Br2(X) molecules surrounding the excitation region. As a result part of the spectrum (IVY> 15500 cm-1 in fig. 3) is obscured, appearing in fig. 2 as a sudden intensity decrease_ Based on our observations and the reactions, lifetimes and rates discussed in the introduction, we have evaluated the following model for the reaction kinetics. As soon as Brz molecules are formed via the process of pyrolysis of CF, Br, eq. (3) describes the production of very hot Br atoms recombining into high vibrational states or the continuum of Br,(i3). During their radiative lifetime TV = 12-4 ~.lsthe-molecules undergo mainly two processes: collisional predissociation or quenching to the ground state and V-V relaxation. Both processes have a comparable rate, r = 1 ns at our pressure (75 Torr). Consequently. the probability for molecules to reach Iow u’ levels via V-V transfer and then emit a photon will be low, since they will be quenched on their way down and thusremain invisible_ The result is a preferred popuIation of high vibrational states within the B state yielding possibly an inversion. From collisionless predissociation to the repulsive i 11lu state no effects are expected because it is a slow process (MOO ns); it would appear in the spectrum as a depletion of specific U*states near the crossing point (u’ =: 3,4), wbicb is not observed_ Diffusion of Br2 molecules out of the sample area has been calculated to last rafl =5 ms at our conditions, and therefore plays no role on the luminescence timescale. An indication of the temperature in the cell is obtained via a computer simulation of the 3-8 vibrational band (i.e. u’ = 3 + U“ = 8) using the detailed rotational data from Coxon (table 1 of ref. f8])_ A convolution was produced of the generated thermal spike spectrum of rotational transitions with the profile of the monochromator (8 cm-1 fwhm)_ The position of the Boltzmann maximum of the 3-g band was moderately sensitive to the rotational temperature, which was used as a simulation parameter. The best fit was obtained for T,,=9OOi 100K.
Volume 99, number 3
CHEMICAL
References
111D-9.
Gibbs and E-A. Ogryzlo, tin. J. Chem. 43 (1965) 1905. PI hfA.A. Clyne and J.A. Coxon, J. Mol. Spectry. 23 (1967) 258. 131 C.L. Sam and J.T. Yardley, Chem. Phys. Letters 61 (1979) 509. 141 V.N. Bagatashvili, V-N. Burimov, LE. Deev, A.V. Zabolotnykh, V.S. Letokhov, G-1. Nazarenko, A.P. Sviridov and V.S. Shaidurov, Soviet 1. Quantum Electron. 12 (1982) 249. 151 W-A. Jalenak and N.S. Nosar, Chem. Phys. 41 (1979) 407. 161 W.M. Shaub and S-H. Bauer, Intern. J. Chem. Kinetics 7 (1975) 509. L71 N.V. Karlov, Yu.N. Petrov, A.M. Prokhorov and O.hI. Stel’makh, JETP Letters 11 (1970) 135. 181 J.A. Coxon, J. Mol. Spectry. 37 (1971) 39.
PHYSICS LETTERS
5 August 1983
PI R.F. Barrow, T.C. Clark, J.A. Coson and RX.
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