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The influence of a magnetic field on the fluorescence and photolysis rate of carbon disulfide vapour
Volume
124. number
6
CHEMICAL
PHYSICS
THE INFLUENCE OF A MAGNETIC FIELD ON THE FLUORESCENCE AND PHOTOLYSIS
LETTERS
14 March 1986
RATE OF CARBON DISULFIDE
VAPOUR
V.I. MAKAROV and N.M. BAZHIN Institute of Chemical Kinetics and Combustion, Novosrbwsk 630090, USSR Received
13 July 1985; in final form 6 January
1986
The effect of a magnetic field on the fluorescence of carbon disulphide vapour under stationary excitation by light with A = 313Ok 30 A has been investigated at 0.014-165.1 Torr. Over the whole range of pressure, the magnetic effect does not depend on the gas pressure. A magnetic field has been found to affect the photolysis rate of carbon disulphide vapour in light with A > 2900 A. The magnetic effect does not vary with pressure from 2.6-90 Torr.
1. Introduction
The magnetic field effect in gas-phase photochemical reactions is a fairly rare phenomenon. It has been observed in the photochemical decomposition of I2 [l], D2C0 [2,3], H2C202 [4], and also in the photochemical reaction of electronically excited sulfur dioxide with n-pentane [3,5]. In the abovementioned systems, the magnetic field effect in luminescence showed up as changed quantum yields and lifetimes of the luminescent state for I2 [6,7], D2C0 [8], SO2 [9] and H2C202 [lO,ll]. Under pulsed excitation the fluorescent signal decay kinetics for CS2 were shown [ 12-181 to obey a biexponential law with the characteristic lifetime of the fast component 7; = 4.1 PS [ 131 and that of the slow component rt = 17-34 /JS [16]. In a magnetic field of about 18 kG the fast-component amplitude reduced by approximately 75% [14,15]. The slow component was noticeably affected in fields above 10 kG. The size of the magnetic effect depended on the gas pressure in the case of the slow component [15] and was pressure-independent for the fast component [12-l 51. Orita, Morita and Nagakura [ 141 believed a magnetic field affected the intramolecular excitation energy redistribution, whilst Silvers, McKeever and Chawla [ 151 assumed the Zeeman splitting reduced the laser radiation absorption. It seems to be interesting to investigate the mag-
netic field effect on the fluorescence and photolysis rate of carbon disulfide under wide-band irradiation. Such investigations would make it possible to verify the hypothesis put forward by Silvers et al. [ 151 and also to determine the magnetic effect mechanism and photochemical activity of particular electronic states. The photochemistry of CS2 was investigated in the near ultraviolet [ 19-241. The primary reaction in CS2 photolysis is considered to be: cs;+cs2+2csts2.
(1)
The origin of photochemically active electronically excited states of CS2 molecules has not been studied [19-211. The present investigation is devoted to magnetic field effects on the fluorescence and photolysis rate of carbon clisulfide vapour under wide-band irradiation.
2. Experimental The apparatus for studying magnetic field effects on fluorescence in gas-phase systems included an electromagnet, with the magnetic field strength in the effective volume adjustable continuously from 0 to 7.6 kG. A 6 cm long cylindrical quartz cavity, 3.0 cm in diameter, was mounted between the electromagnet’s poles. The cavity was connected to a vacuum system.
Volume 124, number 6
CHEMICAL PHYSICS LETTERS
The luminescence was detected with a FEU-79 photoelectromultiplier using the single-photon-counting technique in the direction normal to that of the exciting light beam. A DMR-4 monochromator was employed to select a required range of the fluorescence spectrum. The fluorescence of CS, was detected at a wavelength of 4250 A. The CS, fluorescence was excited by continuous light from a DRShdOO mercury lamp at a wavelength of 3 130 A. This wavelength was selected with the combination of a glassfilter UFS-2 and a fluid filter -water solution of nickel chloride with an optical path of 2.0 cm (saturated solution). Under optimum experimental conditions, the signalto-noise ratio of the system was 6-7. The apparatus for studying magnetic field effects on the photolysis rate of carbon disulfide included an electromagnet, with the magnetic field strength in the effective volume adjustable continuously from 0 to 8.4 kG. A 10.0 cm long cylindrical quartz cavity, 1.5 cm in diameter, connected to a vacuum system was mounted between the electromagnet’s poles. Also, the cavity was connected with a differential bellows pressure pickup with a threshold sensitivity of some 10e4 Torr. To remove temperature drift, the pickup was thermostatted at 25°C with a thermostat U 10. Carbon disulfide vapour was photolyzed by the light of a DRSh-500 mercury lamp with X> 2900 A (the short-wave emission spectrum of the lamp was cut off with a glass filter BS-3). We measured the degree of CS, transformation within a certain time period since for the excited energy range under study, the phototransformation quantum yield was rather low and thus the photolysis rate could not be measured directly by pressure variations in the cavity. Therefore, having been irradiated in the cavity, the gas was kept for a certain time in the dark (to ensure temperature relaxation), thereafter the pressure gradient between the cavity and ballast cylinder was measured [3,5]. We used commercial carbon disulfide of “chemically pure” grade which was distilled in vacuum at -45OC (the CS2 vapour pressure was about 10 Torr [22]). The vacuum distillation was repeated 3-4 times to obtain the average fraction used in our experiments.
500
14 March 1986
3. Experimental results The magnetic field effects in fluorescence were studied with the CS2 pressure varying from 0.014 to 165.1 Torr. Fig. 1 shows the experimental Stern-Vohner dependence obtained in zero and 6.36 kG magnetic fields. In both cases, the Stern-Volmer dependences are seen to deviate from a linear law. Throughout the range of CS2 pressures studied, the magnetic effect is pressure-independent within experimental error which agrees with the data [12-l 51 obtained over a narrower pressure range. The CS, photolysis rate was observed to decrease in a magnetic field. Fig. 2 shows plots of the magnetic effect versus the field strength at a gas pressure of 9.8 Torr. The CS, vapour photolysis rate falls off gradually with increasing field strength. The change of CS2 pressure from 2.6 to 90 Torr does not influence the magnetic effect on the reaction. In experiments with stationary excitation of CS, fluorescence, the spectra for the fast and slow luminescence components cannot be distinguished [12-l 81 since they overlap. It has been shown [IS] that in magnetic fields up to 10 kG, CS, fluorescence is quenched mainly through the fast component, the effect being pressure-independent. Insofar as the magnetic effect is pressure-independent, we can calculate the intensity ratio of the fast and slow fluorescence for our experimental conditions using the data available [ 12- 151.In a field of 6.62 kG the magnetic effect on CS2 fluorescence quenching is IH/Zo = 0.8 1 It:0.03 (&, IH are flUOreSCenCe intenSities in zero and non-zero fields, respectively). The magnetic effect reported [ 12-151 for the same magnetic field is 0.62. As a result, the intensity ratio Ii/Ii = 1.0 f 0.4 where Zi and 1: are short and long fluorescence components of CS2, respectively. With this value it is possible to recalculate the experimental data on magnetic effects in fluorescence for the magnetic effect only in the fast component. Fig. 2 shows the experimental dependence and the curve so recalculated for a CS2 pressure of 10.4 Torr. The experimental dependence is seen to differ from the corresponding curve of magnetic effects, whilst the recalculated dependence fits the curve. Fig. 2 also shows the data obtained [ 13,15 ] for CS2 pressure of 1.0 and 25 mTorr.
(P/I*:
50
0
100
150
P, Torr
Fig. 1. The Stern-Volmer plots of CS2 fluorescence selfquenching in zero magnetic field (0) and in a 6.36 kG field (0). P is the pressure (in Torr) and Zf is the fluorescence intensity in arbitrary units.
I
I
I
295
1
I
I
795
HkG 9
Fig. 2. The field dependence of magnetic field effects in the reaction and fluorescence of CS2. l, P = 10.4 Torr, fluorescence; o, P = 9.8 Torr, reaction; o, calculated for the short fluorescence component; Q an-, P = 1.0 and 25 mTorr, fluorescence [ 13,151. Ve and VH are CS2 photolysis rates in zero magnetic field and in a magnetic f=ld of H kG.
Volume124, number6
CHEMICAL PHYSICSLETTERS
4. Discussion The electronic configuration of the lower excited states of a CS, molecule is (rQ3(rru) [23]. CS2 islinear in the ground state and bent in the lower excited states [24] which resultsin the Renner-Teller splitting of the LArrterm of the linear molecule into lA2 and ‘B2 states of the bent molecule. The absorption spectrum for CS, has already been analyzed [23-281. It has been found that the absorption lines at 2900-3500 A correspond to the CS, transitions from the ground LZi state to the electronically excited ‘A2 and ‘B2 ones. The LA, energy has been shown [24] to be lower than the LB, energy. The fluorescence signal decay kinetics were investigated [12-181 under the light of a nitrogen laser (h = 337 1 A) and of a tunable dye laser with doubled frequency (A = 3235-3038 A). Irrespective of the excitation energy, the fluorescence signal amplitude decayed biexponentially in all cases. The characteristic lifetime of the fast and slow components did not depend on the excitation energy; it was the amplitude ratio that varied. As the excitation energy decreased, the fast component amplitude grew as compared to the slow one [14,16]. The energy independence of the characteristic lifetime for the fast and slow components and the increasing fast component amplitude with falling excitation energy, all confirm the fact that under light excitation the LA, and lB2 states are populated simultaneously and independently, the fast component resulting from the ‘A2 luminescence, the slow one from the LB2luminescence. The effect of a magnetic field on the fast component is interpreted [ 151 under the assumption that the light absorption efficiency of CS2 molecules is changed in a magnetic field. The Zeeman level splitting (by the projection of the full angular momentum of a molecule M) is assumed to result in the emergence of some levels from the spectral zone of the laser radiation. As a result, the overall population of the luminescent state reduces. In our experiments, CS, molecules were excited by a wide-band light source (X = 3 130 f 30 A). Nevertheless, the CS, fluorescence was quenched in a mag netic field. This effect is fairly well described by the curve from refs. [12-l 51. So, the model proposed by Silkers et al. [ 151 does not seem to be applicable to 502
14 March1986
the description of magnetoinduced quenching of CS, fluorescence. The assumption of the intramolecular redistribution of excitation energy [12-141 is much more feasible. The comparison of data on the magnetic field effects in the photolysis rate and fluorescence of CS, (see fig. 2) shows the effect in both luminescence and reaction to be induced by the magnetic field influence on the same state (presumably ‘A,). Our experiments on magnetic field effects upon CS2 photolysis rates, showed the generation of a polymeric product which agrees with published results [19-211. Under lowenergy excitation (X = 3130 A, E = 3.96 eV) CS2 molecules react as in refs. [19-211. The S-C bonding energy is 4.46 eV [29] and that of S-S bonding is 4.37 eV [30]. As a result, the process (1) requires 4.55 eV which corresponds to the wavelength of the exciting light 2724 A and hence reaction (1) cannot be the primary photolysis stage. However, our experiments, as well as those described in refs. [20,21], demonstrated CS2 conversion under lowenergy excitation. Therefore, under excitation with a wavelength > 2724 A, the following processes might run, however with a low probability, cs; + cs, + (CS& ,
(2)
(CS&
(3)
+ (W2 + s, 9
(CS& -+ polymer,
(4)
where (CS2)z is an excited collisional complex. Reaction (2) occurs provided there arises a collision complex decomposition to (CS)2 and S2 with a certain probability. Analysis of the Stern-Vohner dependences of CS, fluorescence self-quenching in zero field and in a magnetic field of 6.36 kG demonstrates that at high gas pressures they show increasing rates of collision quenching. Similar deviations were also observed in the case of sulfur dioxide fluorescence quenching by alien gases [3 l] and under the influence of nitrogen oxide on the photolysis rate of S02/n-pentane mixtures [32]. The abovementioned deviations were interpreted either in terms of a kinetic scheme involving new unobservable excited states of SO2 [31], or under the assumption of collision complexing with lifetimes of about 1O-9 s. The latter interpretation seems to be more reliable. Most likely, this is also the case with CS, at high pressures. Thus the available experimen-
Volume 124, number 6
CHEMICAL PHYSICS LETTERS
tal data suggest that electronically excited CS, molecules may form collision complexes with unexcited CS,. A collision complex can decay by the following processes: decomposition to excited and unexcited CS2 molecules (back process), decomposition to (CS)2 and S2 fragments (photochemical transformation), and quenching by a collision with an unexcited CS2.
References ill W.F. Falkoner and E. Wasserman, J. Chem. Phys. 45 (1966) 1843.
PI N.I. Sorokin, Yu.M. Gusev and N.M. Bazhin, Reaction Kinetics CataI. Letters 14 (1980) 125. 131 N.I. Sorokin, V.I. Makarov, N.L. Lavrik, Yu.M. Gusev, G.I. Skubnevskaya, N.M. Bazhin and Yu.N. Molin, Chem. Phys. Letters 78 (1981) 8. 141 N.I. Sorokin, N.M. Bazhin and G.G. Eremenchuk, Chem. Phys. Letters 99 (1983) 181. 151 V.I. Makarov, N.L. Lavrik, G.I. Skubnevskaya, N.M. Bazhhr and Yu.N. Molin, Dokl. Akad. Nauk SSSR 255 (1980) 1416. 161 S.H. Lin and Y. Fujimura, Excited States 4 (1979) 237. 171 E.O. Degenkolb, J.I. Steinfeld, E. Wasserman and W. Klemperer, J. Chem. Phys. 51 (1969) 615. VI N.I. Sorokin,N.L. Lavrik,G.I. Skubnevskaya,N.M. Bazhin and Yu.N. Molin, Dokl. Akad. Nauk SSSR 245 (1979) 657. [91 V.I. Makarov, N.L. Lavrik, G.I. Skubnevskaya and N.M. Bazhin, Opt. i Spektroskopiya 50 (1981) 290. IlO1 H.G. Ktittner, H.L. SeIzle and E.W. SchIag, Chem. Phys. 28 (1978) 1. v11 A. Matsuzaki and S. Nagakura, 2. Physik. Chem. NF 101 (1976) 283. [W A. Matsuzaki and S. Nagakura,Chem. Letters (1974) 675.
14 March 1986
I131 A. Matsuzaki and S. Nagakura, Bull. Chem. Sot. Japan 49 (1976) 359. iI41 H. Orita, H. Morita and S. Nagakura, Chem. Phys. Letters 81 (1981) 29. [I51 S.J. Silvers, M.R. McKeever and G.K. Chawla, Chem. Phys. 80 (1983) 177. [W H. Orita, H. Morita and S. Nagakura, Chem. Phys. Letters 81 (1981) 33. 1171 L.E. Brus, Chem. Phys. Letters 12 (1971) 116. 1181 S.J. Silvers and M.R. McKeever, Chem. Phys. 18 (1976) 333. 1191 M. de Sorge, A.J. Yarwood, O.P. Strausz and H.E. Gumring, Can. J. Chem. 43 (1965) 1886 WI W.P. Wood and J. Heicklen, J. Phys. Chem. 75 (1971) 854. WI K. Ernst and J.J. Hoffman, Chem. Phys. Letters 68 (1979) 40. WI D. Kei and T. Laby, Spravochnik physlka experimentatora (Moscow, 1949) p. 104. v31 B. Kleman, Can. J. Phys. 41 (1963) 2034. [24] Ch. Jungen, D.N. Malm and A.J. Merer, Can. J. Phys. Sl(l973) 1471. [25] A.E. Douglas and E.R.V. Milton, J. Chem. Phys. 41 (1964) 357. [26] J.T. Hougen, J. Chem. Phys. 41 (1964) 363. [ 271 R.M. Hochstrasser and D.A. Wiersma, J. Chem. Phys. 54 (1971) 4165. [28] J.W. RabaIais, J.M. McDonald, V. Scherr and S.P. McGlymt, Chem. Rev. 71 (1971) 73. [29] G. Herzberg, Molecular spectra and molecular structure, Vol. 3. Electronic spectra and electronic structure of polyatomic molecules (Mir, Moscow, 1969) p. 513. [ 301 H. Okabe, Photochemistry of smaIl molecules (Mir, Moscow, 1981) p. 221. [ 311 L. Strockburger, S. BrasIavsky and J. Heicklen, J. Photothem. 2 (1973/74) 15. [ 321 V.I. Makarov, G.I. Skubnevakayaand N.M. Bazhin, Intern. J. Chem. Kinetics 13 (1981) 231.
503
124. number
6
CHEMICAL
PHYSICS
THE INFLUENCE OF A MAGNETIC FIELD ON THE FLUORESCENCE AND PHOTOLYSIS
LETTERS
14 March 1986
RATE OF CARBON DISULFIDE
VAPOUR
V.I. MAKAROV and N.M. BAZHIN Institute of Chemical Kinetics and Combustion, Novosrbwsk 630090, USSR Received
13 July 1985; in final form 6 January
1986
The effect of a magnetic field on the fluorescence of carbon disulphide vapour under stationary excitation by light with A = 313Ok 30 A has been investigated at 0.014-165.1 Torr. Over the whole range of pressure, the magnetic effect does not depend on the gas pressure. A magnetic field has been found to affect the photolysis rate of carbon disulphide vapour in light with A > 2900 A. The magnetic effect does not vary with pressure from 2.6-90 Torr.
1. Introduction
The magnetic field effect in gas-phase photochemical reactions is a fairly rare phenomenon. It has been observed in the photochemical decomposition of I2 [l], D2C0 [2,3], H2C202 [4], and also in the photochemical reaction of electronically excited sulfur dioxide with n-pentane [3,5]. In the abovementioned systems, the magnetic field effect in luminescence showed up as changed quantum yields and lifetimes of the luminescent state for I2 [6,7], D2C0 [8], SO2 [9] and H2C202 [lO,ll]. Under pulsed excitation the fluorescent signal decay kinetics for CS2 were shown [ 12-181 to obey a biexponential law with the characteristic lifetime of the fast component 7; = 4.1 PS [ 131 and that of the slow component rt = 17-34 /JS [16]. In a magnetic field of about 18 kG the fast-component amplitude reduced by approximately 75% [14,15]. The slow component was noticeably affected in fields above 10 kG. The size of the magnetic effect depended on the gas pressure in the case of the slow component [15] and was pressure-independent for the fast component [12-l 51. Orita, Morita and Nagakura [ 141 believed a magnetic field affected the intramolecular excitation energy redistribution, whilst Silvers, McKeever and Chawla [ 151 assumed the Zeeman splitting reduced the laser radiation absorption. It seems to be interesting to investigate the mag-
netic field effect on the fluorescence and photolysis rate of carbon disulfide under wide-band irradiation. Such investigations would make it possible to verify the hypothesis put forward by Silvers et al. [ 151 and also to determine the magnetic effect mechanism and photochemical activity of particular electronic states. The photochemistry of CS2 was investigated in the near ultraviolet [ 19-241. The primary reaction in CS2 photolysis is considered to be: cs;+cs2+2csts2.
(1)
The origin of photochemically active electronically excited states of CS2 molecules has not been studied [19-211. The present investigation is devoted to magnetic field effects on the fluorescence and photolysis rate of carbon clisulfide vapour under wide-band irradiation.
2. Experimental The apparatus for studying magnetic field effects on fluorescence in gas-phase systems included an electromagnet, with the magnetic field strength in the effective volume adjustable continuously from 0 to 7.6 kG. A 6 cm long cylindrical quartz cavity, 3.0 cm in diameter, was mounted between the electromagnet’s poles. The cavity was connected to a vacuum system.
Volume 124, number 6
CHEMICAL PHYSICS LETTERS
The luminescence was detected with a FEU-79 photoelectromultiplier using the single-photon-counting technique in the direction normal to that of the exciting light beam. A DMR-4 monochromator was employed to select a required range of the fluorescence spectrum. The fluorescence of CS, was detected at a wavelength of 4250 A. The CS, fluorescence was excited by continuous light from a DRShdOO mercury lamp at a wavelength of 3 130 A. This wavelength was selected with the combination of a glassfilter UFS-2 and a fluid filter -water solution of nickel chloride with an optical path of 2.0 cm (saturated solution). Under optimum experimental conditions, the signalto-noise ratio of the system was 6-7. The apparatus for studying magnetic field effects on the photolysis rate of carbon disulfide included an electromagnet, with the magnetic field strength in the effective volume adjustable continuously from 0 to 8.4 kG. A 10.0 cm long cylindrical quartz cavity, 1.5 cm in diameter, connected to a vacuum system was mounted between the electromagnet’s poles. Also, the cavity was connected with a differential bellows pressure pickup with a threshold sensitivity of some 10e4 Torr. To remove temperature drift, the pickup was thermostatted at 25°C with a thermostat U 10. Carbon disulfide vapour was photolyzed by the light of a DRSh-500 mercury lamp with X> 2900 A (the short-wave emission spectrum of the lamp was cut off with a glass filter BS-3). We measured the degree of CS, transformation within a certain time period since for the excited energy range under study, the phototransformation quantum yield was rather low and thus the photolysis rate could not be measured directly by pressure variations in the cavity. Therefore, having been irradiated in the cavity, the gas was kept for a certain time in the dark (to ensure temperature relaxation), thereafter the pressure gradient between the cavity and ballast cylinder was measured [3,5]. We used commercial carbon disulfide of “chemically pure” grade which was distilled in vacuum at -45OC (the CS2 vapour pressure was about 10 Torr [22]). The vacuum distillation was repeated 3-4 times to obtain the average fraction used in our experiments.
500
14 March 1986
3. Experimental results The magnetic field effects in fluorescence were studied with the CS2 pressure varying from 0.014 to 165.1 Torr. Fig. 1 shows the experimental Stern-Vohner dependence obtained in zero and 6.36 kG magnetic fields. In both cases, the Stern-Volmer dependences are seen to deviate from a linear law. Throughout the range of CS2 pressures studied, the magnetic effect is pressure-independent within experimental error which agrees with the data [12-l 51 obtained over a narrower pressure range. The CS, photolysis rate was observed to decrease in a magnetic field. Fig. 2 shows plots of the magnetic effect versus the field strength at a gas pressure of 9.8 Torr. The CS, vapour photolysis rate falls off gradually with increasing field strength. The change of CS2 pressure from 2.6 to 90 Torr does not influence the magnetic effect on the reaction. In experiments with stationary excitation of CS, fluorescence, the spectra for the fast and slow luminescence components cannot be distinguished [12-l 81 since they overlap. It has been shown [IS] that in magnetic fields up to 10 kG, CS, fluorescence is quenched mainly through the fast component, the effect being pressure-independent. Insofar as the magnetic effect is pressure-independent, we can calculate the intensity ratio of the fast and slow fluorescence for our experimental conditions using the data available [ 12- 151.In a field of 6.62 kG the magnetic effect on CS2 fluorescence quenching is IH/Zo = 0.8 1 It:0.03 (&, IH are flUOreSCenCe intenSities in zero and non-zero fields, respectively). The magnetic effect reported [ 12-151 for the same magnetic field is 0.62. As a result, the intensity ratio Ii/Ii = 1.0 f 0.4 where Zi and 1: are short and long fluorescence components of CS2, respectively. With this value it is possible to recalculate the experimental data on magnetic effects in fluorescence for the magnetic effect only in the fast component. Fig. 2 shows the experimental dependence and the curve so recalculated for a CS2 pressure of 10.4 Torr. The experimental dependence is seen to differ from the corresponding curve of magnetic effects, whilst the recalculated dependence fits the curve. Fig. 2 also shows the data obtained [ 13,15 ] for CS2 pressure of 1.0 and 25 mTorr.
(P/I*:
50
0
100
150
P, Torr
Fig. 1. The Stern-Volmer plots of CS2 fluorescence selfquenching in zero magnetic field (0) and in a 6.36 kG field (0). P is the pressure (in Torr) and Zf is the fluorescence intensity in arbitrary units.
I
I
I
295
1
I
I
795
HkG 9
Fig. 2. The field dependence of magnetic field effects in the reaction and fluorescence of CS2. l, P = 10.4 Torr, fluorescence; o, P = 9.8 Torr, reaction; o, calculated for the short fluorescence component; Q an-, P = 1.0 and 25 mTorr, fluorescence [ 13,151. Ve and VH are CS2 photolysis rates in zero magnetic field and in a magnetic f=ld of H kG.
Volume124, number6
CHEMICAL PHYSICSLETTERS
4. Discussion The electronic configuration of the lower excited states of a CS, molecule is (rQ3(rru) [23]. CS2 islinear in the ground state and bent in the lower excited states [24] which resultsin the Renner-Teller splitting of the LArrterm of the linear molecule into lA2 and ‘B2 states of the bent molecule. The absorption spectrum for CS, has already been analyzed [23-281. It has been found that the absorption lines at 2900-3500 A correspond to the CS, transitions from the ground LZi state to the electronically excited ‘A2 and ‘B2 ones. The LA, energy has been shown [24] to be lower than the LB, energy. The fluorescence signal decay kinetics were investigated [12-181 under the light of a nitrogen laser (h = 337 1 A) and of a tunable dye laser with doubled frequency (A = 3235-3038 A). Irrespective of the excitation energy, the fluorescence signal amplitude decayed biexponentially in all cases. The characteristic lifetime of the fast and slow components did not depend on the excitation energy; it was the amplitude ratio that varied. As the excitation energy decreased, the fast component amplitude grew as compared to the slow one [14,16]. The energy independence of the characteristic lifetime for the fast and slow components and the increasing fast component amplitude with falling excitation energy, all confirm the fact that under light excitation the LA, and lB2 states are populated simultaneously and independently, the fast component resulting from the ‘A2 luminescence, the slow one from the LB2luminescence. The effect of a magnetic field on the fast component is interpreted [ 151 under the assumption that the light absorption efficiency of CS2 molecules is changed in a magnetic field. The Zeeman level splitting (by the projection of the full angular momentum of a molecule M) is assumed to result in the emergence of some levels from the spectral zone of the laser radiation. As a result, the overall population of the luminescent state reduces. In our experiments, CS, molecules were excited by a wide-band light source (X = 3 130 f 30 A). Nevertheless, the CS, fluorescence was quenched in a mag netic field. This effect is fairly well described by the curve from refs. [12-l 51. So, the model proposed by Silkers et al. [ 151 does not seem to be applicable to 502
14 March1986
the description of magnetoinduced quenching of CS, fluorescence. The assumption of the intramolecular redistribution of excitation energy [12-141 is much more feasible. The comparison of data on the magnetic field effects in the photolysis rate and fluorescence of CS, (see fig. 2) shows the effect in both luminescence and reaction to be induced by the magnetic field influence on the same state (presumably ‘A,). Our experiments on magnetic field effects upon CS2 photolysis rates, showed the generation of a polymeric product which agrees with published results [19-211. Under lowenergy excitation (X = 3130 A, E = 3.96 eV) CS2 molecules react as in refs. [19-211. The S-C bonding energy is 4.46 eV [29] and that of S-S bonding is 4.37 eV [30]. As a result, the process (1) requires 4.55 eV which corresponds to the wavelength of the exciting light 2724 A and hence reaction (1) cannot be the primary photolysis stage. However, our experiments, as well as those described in refs. [20,21], demonstrated CS2 conversion under lowenergy excitation. Therefore, under excitation with a wavelength > 2724 A, the following processes might run, however with a low probability, cs; + cs, + (CS& ,
(2)
(CS&
(3)
+ (W2 + s, 9
(CS& -+ polymer,
(4)
where (CS2)z is an excited collisional complex. Reaction (2) occurs provided there arises a collision complex decomposition to (CS)2 and S2 with a certain probability. Analysis of the Stern-Vohner dependences of CS, fluorescence self-quenching in zero field and in a magnetic field of 6.36 kG demonstrates that at high gas pressures they show increasing rates of collision quenching. Similar deviations were also observed in the case of sulfur dioxide fluorescence quenching by alien gases [3 l] and under the influence of nitrogen oxide on the photolysis rate of S02/n-pentane mixtures [32]. The abovementioned deviations were interpreted either in terms of a kinetic scheme involving new unobservable excited states of SO2 [31], or under the assumption of collision complexing with lifetimes of about 1O-9 s. The latter interpretation seems to be more reliable. Most likely, this is also the case with CS, at high pressures. Thus the available experimen-
Volume 124, number 6
CHEMICAL PHYSICS LETTERS
tal data suggest that electronically excited CS, molecules may form collision complexes with unexcited CS,. A collision complex can decay by the following processes: decomposition to excited and unexcited CS2 molecules (back process), decomposition to (CS)2 and S2 fragments (photochemical transformation), and quenching by a collision with an unexcited CS2.
References ill W.F. Falkoner and E. Wasserman, J. Chem. Phys. 45 (1966) 1843.
PI N.I. Sorokin, Yu.M. Gusev and N.M. Bazhin, Reaction Kinetics CataI. Letters 14 (1980) 125. 131 N.I. Sorokin, V.I. Makarov, N.L. Lavrik, Yu.M. Gusev, G.I. Skubnevskaya, N.M. Bazhin and Yu.N. Molin, Chem. Phys. Letters 78 (1981) 8. 141 N.I. Sorokin, N.M. Bazhin and G.G. Eremenchuk, Chem. Phys. Letters 99 (1983) 181. 151 V.I. Makarov, N.L. Lavrik, G.I. Skubnevskaya, N.M. Bazhhr and Yu.N. Molin, Dokl. Akad. Nauk SSSR 255 (1980) 1416. 161 S.H. Lin and Y. Fujimura, Excited States 4 (1979) 237. 171 E.O. Degenkolb, J.I. Steinfeld, E. Wasserman and W. Klemperer, J. Chem. Phys. 51 (1969) 615. VI N.I. Sorokin,N.L. Lavrik,G.I. Skubnevskaya,N.M. Bazhin and Yu.N. Molin, Dokl. Akad. Nauk SSSR 245 (1979) 657. [91 V.I. Makarov, N.L. Lavrik, G.I. Skubnevskaya and N.M. Bazhin, Opt. i Spektroskopiya 50 (1981) 290. IlO1 H.G. Ktittner, H.L. SeIzle and E.W. SchIag, Chem. Phys. 28 (1978) 1. v11 A. Matsuzaki and S. Nagakura, 2. Physik. Chem. NF 101 (1976) 283. [W A. Matsuzaki and S. Nagakura,Chem. Letters (1974) 675.
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I131 A. Matsuzaki and S. Nagakura, Bull. Chem. Sot. Japan 49 (1976) 359. iI41 H. Orita, H. Morita and S. Nagakura, Chem. Phys. Letters 81 (1981) 29. [I51 S.J. Silvers, M.R. McKeever and G.K. Chawla, Chem. Phys. 80 (1983) 177. [W H. Orita, H. Morita and S. Nagakura, Chem. Phys. Letters 81 (1981) 33. 1171 L.E. Brus, Chem. Phys. Letters 12 (1971) 116. 1181 S.J. Silvers and M.R. McKeever, Chem. Phys. 18 (1976) 333. 1191 M. de Sorge, A.J. Yarwood, O.P. Strausz and H.E. Gumring, Can. J. Chem. 43 (1965) 1886 WI W.P. Wood and J. Heicklen, J. Phys. Chem. 75 (1971) 854. WI K. Ernst and J.J. Hoffman, Chem. Phys. Letters 68 (1979) 40. WI D. Kei and T. Laby, Spravochnik physlka experimentatora (Moscow, 1949) p. 104. v31 B. Kleman, Can. J. Phys. 41 (1963) 2034. [24] Ch. Jungen, D.N. Malm and A.J. Merer, Can. J. Phys. Sl(l973) 1471. [25] A.E. Douglas and E.R.V. Milton, J. Chem. Phys. 41 (1964) 357. [26] J.T. Hougen, J. Chem. Phys. 41 (1964) 363. [ 271 R.M. Hochstrasser and D.A. Wiersma, J. Chem. Phys. 54 (1971) 4165. [28] J.W. RabaIais, J.M. McDonald, V. Scherr and S.P. McGlymt, Chem. Rev. 71 (1971) 73. [29] G. Herzberg, Molecular spectra and molecular structure, Vol. 3. Electronic spectra and electronic structure of polyatomic molecules (Mir, Moscow, 1969) p. 513. [ 301 H. Okabe, Photochemistry of smaIl molecules (Mir, Moscow, 1981) p. 221. [ 311 L. Strockburger, S. BrasIavsky and J. Heicklen, J. Photothem. 2 (1973/74) 15. [ 321 V.I. Makarov, G.I. Skubnevakayaand N.M. Bazhin, Intern. J. Chem. Kinetics 13 (1981) 231.
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