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Activation of the infrared multiphoton dissociation of CF2HCl
Volume I8 1. number 4
CHEMICAL PHYSICS LETTERS
28 June 1991
Activation of the infrared multiphoton dissociation of CF,HCl C.L. Sigiienza Instituio de optrca Dais de ValdPs. C‘SIC, Serrauo 121, 28006 Madrid, Spain
Received 15 March 199 1
Infrared multiphoton dissociation experiments in difluorochloromethane have been carried out m the presence of different proportions of nitrogen and argon. The influence of homogeneous and heterogeneous collisions in the process is studied and discussed in terms of an intense bottleneck effect which takes place in the low-lying vibrational energy levels of the excited molecule.
1. Introduction The research emphasis in recent years on multiple photon excitation and dissociation [l-6] has been on some collisionless aspects of the process. However, even at low pressure, collisions may play an important role in the absorption of intense infrared laser radiation by polyatomic molecules. Nonetheless, there is some controversy about what such a role could be in molecules which have a rather early bottleneck in the low-lying discrete energy levels. Adding an inert gas to the absorbing medium is a current procedure for studying the unimolecularity and nature of the infrared multiphoton-excitation (MPE) and multiphoton-dissociation (MPD) processes, Under very different experimental conditions (concerning laser wavelength, relative pressure, beam geometry and profile, etc.), Stephenson and coworkers [7-l I], on one hand, and Duperrex and van den Bergh [ 12,13 1, on the other, studied the rather smail molecule CF*HCl, by adding argon as a buffer gas. Whereas, initially, the effect of increasing Ar pressure, PA, enhances the dissociation rate in all cases, above a moderate value of PA the rate becomes saturated according to the results obtained by Stephenson and co-workers, but steadily decreases for those experiments performed by Duperrex and van den Bergh. These authors also found that the tendency of the dissociation rate to decrease inversely depends on the laser fluence. In molecules having a weaker or no discrete-level bottleneck [ 14181, there is no initial increase of the dissociation 0009-26 14/9
rate, at least for moderate laser fluences, but a continuous decrease of the efficiency as PA is being increased. Stone and co-workers [ 19,201 compared their experimental data with a theoretical model which considered the effect of collisions between CFzHCl and Ar in terms of rotational equilibration or “hole filling” in the discrete energy levels. In addition, these authors included collision terms in a generalized Landau-Teller mechanism for V-T collisional relaxation for sufficiently high pressures. They constructed theoretical curves showing a pressure falloff in the CF, production rate due to a deactivation of the excited molecules by collisional V-T energy transfer. This showed that the larger the probability of a collisional transition from the initial level to final level, the stronger the collisional deactivation
[201. McRae et al. [ 2 1 ] have pointed out that in processes involving monoatomic buffer gases, such as Ar or Xe, the probability for R-T energy transfer is larger than for V-T energy transfer, so discounting that the second of these processes occurs. Furthermore, they interpreted their experimental results in C,H,BrF as indicating that even rotational hole filling induced by R-T does not take place. However, their analysis is based on the assumption that the probability for a given energy-transfer process to occur is governed by the quantum energy being transferred, and they used rather high pressures of the molecules where any bottleneck effect might be masked.
I/$ 03.50 0 I99 1 - Elsevier Sctence Publishers B.V. (North-Holland )
351
Volume 181, number4
28 June 1591
CHEMICAL PHYSICS LETTERS
We tentatively suggest here that the differences in all the above controversial results can be simply related to a pressure effect of the absorbing polyatomic molecules. The suggestion is based on a detailed experimental study of the dependence of multiphotondissociation efficiency of CF,HCI on CFzHCl and buffer-gas pressures in the lowest relevant region, and infrared laser fluence, for monoatomic (Ar) and diatomic (N,) buffers. CF>HCI has been widely shown to have a strong discrete-level bottleneck which operates in multiphoton unimolecular infrared excitation or dissociation. We also discuss the effects of adding Ar, by comparing our results with those obtained by using MPD induced by two different laser infrared wavelengths [ 221.
2. Experimental A non-mode locked pulsed TEA CO2 laser (Lumonies 101) tuned at R( 32) (1086 cm-‘) has been used to excite quasiresonantly the CF stretching absorption of the CFzHCl molecule. With use of a typical CO1 gas mixture, pulse shapes consisted of an ~80 ns peak followed by a tail of about I ~.tswhen detected by a photon drag detector (Rofin, model 7415). In all cases, samples were steadily irradiated with 50 laser shots. The experiments were carried out at room temperature in a Pyrex cell 11 cm long and a total volume z 95 cm3 equipped with NaCl end windows and provided with a glass “finger”. A pyroelectric detector (ZOD-Lumonics) was employed to measure the incident pulse energy. A NaCl lens of 2 m focal length was used to obtain a nearly parallel beam in the cell. Attenuation of the incident energy was accomplished by inserting CaF, plates along the optical line. Dissociation-yield measurements were carried out on the final products with infrared absorption spectroscopy by using a Perkin-Elmer FUR spectrometer 1725X. ( CFzHCl) (Matheson Difluorochloromethane 99.9Oio), N, and Ar were used without further purification.
3. Results and discussion In fig. I, we give the curves representing the dependence of the fraction of CF,HCl molecules dissociated per laser pulse,
where k’, and I’, are the cell volume and the irradiated cylindrical volume, respectively, and N, and N, denote concentration of CF2HC1 before and after irradtation with n laser pulses, on the added inert gas pressure for two different CFzHCl initial pressures (0.05 and 0.7 mbar) and two fluence values ( 1.9 and 3.2 J cm-2). In all the curves, it can be observed that f initially increases; the larger the initial CFzHCl pressure, the smoother the initialfincrease. This initial behaviour should be interpreted by considering -In
l!
f=cQ& ‘\ ‘;. F=l9
J.cm-2
’
2.1
L
a.04005mb PCF2HCI .,x-0.7mb
4.:
Fig. I. Deactivation of the bottleneck in CF2HCI as induced by increasing partial pressures, Ps, of two buffer gases (N, and Ar). fis the fraction of molecules dissociated per laser pulse, as given by the formula in the text.
352
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18I, number 4
CHEMICAL PHYSICS LETTERS
that at higher CFzHCl pressures, the intermolecular homogeneous collision effect, giving rise to intermolecular homogeneous rotational hole filling in CF2HCI, dominates over the heterogeneous rotational hole filling induced by collisions with the buffer gas. Given the similarity of the behaviour for Ar and N,, it seems that a V-V energy transfer should not be included among the process giving rise to the heterogeneous rotational hole filling. In addition, the C,F, formed in the CF,HCl decomposition, can be acting as a sensitizer, or a “hot buffer”, during the laser pulse, in transferring energy to the molecules of their environment, and so, a V ( C2F4) +V (CF,HCl) energy-transfer mechanism could be taking place, making the MPD process more efficient by energy pooling. At the lower fluence, we can observe that as we increase the N2 or Ar pressure, the difference in f between the curves at different CF2HCl pressures continuously decreases, reaching practically the same decreasing value for PNI 225 mbar, or giving rise to an inverted regime in which ftakes on larger values for smaller CF?HCl pressures for PAr> 20 mbar. It is worth noting that the inert-gas pressure at which freaches a maximum is slightly shifted towards smaller values as one goes from Ar to N2, and to smaller CFzHCl pressures. A continuous falloff in fis observed in all the curves as the buffer pressure is increased. These features indicate that the poisoning effect has to be attributable, not just to the heterogeneous collisions with the inert gas and even CzF4, but also to the homogeneous collisions between CF,HCl molecules themselves. Since these homogeneous collisions do not produce any deactivation falloff in MPD induced in pure CF,HCI, it seems that they have to be contributing to a more complicated collisional mechanism in which heterogeneous collisions are also simultaneously occurring. In any case, the main conclusion to be drawn from fig. 1 is that CF,HCl opposes a strong anharmonic bottleneck effect to MPD and that such an effect can be circumvented by adding an inert gas. Given that the obtained results seem to be rather independent of the laser-beam geometry, the possible influence of thermal effects in laser multimode experiments [23,24], such as those being discussed in this work, cannot be predominant, though they might still play an important role in other kinds of experiment.
28 June 1991
Although there always is a falloff in J; the maxima in the curves of fig. 1 for lower fluence are apparently much sharper for the larger CFzHCl pressures. On the other hand, the falloff is strongly suppressed for high fluences, as first pointed out by van den Bergh [ 12-141, both for Ar and Nz quite independently of the CFZHCl initial pressure. Although some other likely operating mechanisms, such as a fluenceinduced power broadening and the increase of collisional dissociation, can be invoked to explain the diminution of the falloff at higher fluences, I think that the overwhelming reason which justifies this result is the drastic increase of the energy deposited per molecule which overcomes any poisoning effect. The enhancement offat the inert-gas pressure giving the maximum effectiveness over the value for the neat CF2HCl decreases as the fluence is increased for the two CF>HCl represented in fig. 1, but is always much greater for the smaller pressure, indicating that the involvement of the bottleneck effect in MPD decreases with fluence and initial CF*HCl pressure. If the falloff in the curves of fig. 1 were to be attributed to collisional poisoning effects, in which both homogeneous and heterogeneous collisions would simultaneously take place, then from experiments such as those in fig. 1, one could derive a criterion to show the unimolecular, or rather selective, character of the involved multiphoton process. Since at the higher fluences, the collisional poisoning effect is overcome by the large deposited energy, such a criterion would only be valid when fluences near the minimal dissociation threshold (typically around 1 J cm- ’) were used, and could be stated in terms of the “specific” negative slope (slope per maximum increase off) of the nearly straight falloff line. When the so-defined specific slope would take on very small values, then the process would be essentially unimolecular. This tentative criterion receives some support from the experiments carried out with two infrared wavelengths [ 221. It was shown that the effect of a second nonresonant wavelength could induce a remarkable increase in the selectivity of the process, which was manifested in a significant reduction of the falloff, at least when Ar was used as a buffer. In summary, we have shown that the apparently different behaviour obtained by several authors about the maximum effectiveness of the MPD process in CF,HCl, activated by a buffer gas, is to be attributed 353
Volume 18 I, number 4
CHEMICAL PHYSICS LETTERS
combined effect of the pressures and fluences used, rather than an actual difference. We have also demonstrated that intermolecular V-V transfer does not play any significant role in collisionally enhanced MPD of CF,HCI, induced by an inert gas. This conclusion could be violated nevertheless at higher fluences where the fraction of molecules dissociated per laser pulse is larger for Ar along the inert-gas pressure interval studied after the maximum. to a
Acknowledgement The author wishes to thank P.F. Gonialez Diaz for useful discussions, and M. Santos, J.A. Torresano and A. Pino for their help. This work has been carried out with financial support provided by DGICYT.PGC under Project NPB0252.
References : I ] D.S. King, in: Dynamics of the excited state, ed. K.P. Lawley (Wiley, New York, 1982) p. 105. 121 J.L. Lyman, G.P. Quigley and O.P. Judd, in: Multiplephoton excitation and dissociation, ed. C.D. Cantrell (Springer, Berlin, 1986) p. 9. 131 V.S. Letokhov. Nonlinear laser chemistry. Multiple-photon excitation (Springer, Berlin, 1983). 141 V.S. Letokhov. in: Laser spectroscopy of highly vibrationally excited molecules (Hilger, London, 1989). :51 V.N. Bragatashivili, V.S. Letokhov, A.A. Makarot and E.A. Ryabov, in: Multiple photon infrared laser photophysics and photochemistry (Harwood, New York, 1985).
354
[6] D.W. LupoandM.
28 June 1991 Quack, Chem. Rev. 87 (1987) 181.
[ 71 J.C. Stephenson, D.S. King, H.V. Goodman and I. Stone, J. Chem. Phys. 70 (1979) 4496. [8] D.S. King and J.C. Stephenson, Chem. Phys. Letters 66 ( 1979) 33. [ 91 J.C. Stephenson, S.E. Bialkowski and D.S. King, J. Chem. Phys. 72 (1980) 1161. [ IO] J.C. Stephenson, J.A. Blazy, Ch. Li and D.S. King, J. Chem. Phys. 76 (1982) 5989. [ I1 ] J.C. Stephenson and D.S. King, J. Chem. Phys. 78 ( 1983) 1867. [ 121 R. Duperrex and H. van den Bergh. J. Chem. Phys. 71 (1979) 3613. [ 131 R. Duperrex and H. van den Bergh, J. Chem. Phys. 75 (1981) 3371. [ 141 R. Duperrex and H. van den Bergh, Chem. Phys. 40 ( 1979) 275. [IS] G.P. Quigley, Opt. Letters 3 (1978) 106. [ i 61 G.P. Quigley and J.L. Lyman, in: Springer series in chemical physics, Vol. 6. Laser-induced processes in molecules, eds. K.L. Kompaand S.D. Smith (Springer, Berlin, 1979) p. 134. [ 171 V.N. Bragatashivili, V.S. Dolzhikov and VS. Letokhov, Soviet Phys. JETP 49 ( 1979) 8. [ 18 ] C. Sigiienza. M. Santos, J.A. Torresano and P.F. GonzslezDiaz, Spectrochim. Acta 46 A (1990) 1499. [ 191 J. Stone, E. Thiele, M.F. Goodman, J.C. Stephenson and D.S. King, J. Chem. Phys. 73 (1980) 2259. [20] M.F. Goodman, J. Stone and E. Thiele, in: Multiphoton excitation and dissociation of polyatomic molecules, ed. C.D. Cantrell (Springer, Berlin, 1986) p. 159. [21] G.A. McRae, D.K. Evans and J.W. Goodale, J. Chem. Phys. 93 ( 1990) 1689. [22] M. Santos, C. Sigiienza, J.A. Torresano and P.F. GonzllezDiaz, Spectrochim. Acta 46 A (1990) 455. [23] R.T. Bailey, F.R. Cruikshank, D. Pugh, R. Guthrie, W. Johnstone, J. Mayer and K. Middleton, J. Chem. Phys. 77 (1982) 3453. [24] M. Lenzi, E. Molinari, G. Piciacchia, V. Sessa and M.L. Terranova, Spectrochim. Acta 43 A (1987) 129.
CHEMICAL PHYSICS LETTERS
28 June 1991
Activation of the infrared multiphoton dissociation of CF,HCl C.L. Sigiienza Instituio de optrca Dais de ValdPs. C‘SIC, Serrauo 121, 28006 Madrid, Spain
Received 15 March 199 1
Infrared multiphoton dissociation experiments in difluorochloromethane have been carried out m the presence of different proportions of nitrogen and argon. The influence of homogeneous and heterogeneous collisions in the process is studied and discussed in terms of an intense bottleneck effect which takes place in the low-lying vibrational energy levels of the excited molecule.
1. Introduction The research emphasis in recent years on multiple photon excitation and dissociation [l-6] has been on some collisionless aspects of the process. However, even at low pressure, collisions may play an important role in the absorption of intense infrared laser radiation by polyatomic molecules. Nonetheless, there is some controversy about what such a role could be in molecules which have a rather early bottleneck in the low-lying discrete energy levels. Adding an inert gas to the absorbing medium is a current procedure for studying the unimolecularity and nature of the infrared multiphoton-excitation (MPE) and multiphoton-dissociation (MPD) processes, Under very different experimental conditions (concerning laser wavelength, relative pressure, beam geometry and profile, etc.), Stephenson and coworkers [7-l I], on one hand, and Duperrex and van den Bergh [ 12,13 1, on the other, studied the rather smail molecule CF*HCl, by adding argon as a buffer gas. Whereas, initially, the effect of increasing Ar pressure, PA, enhances the dissociation rate in all cases, above a moderate value of PA the rate becomes saturated according to the results obtained by Stephenson and co-workers, but steadily decreases for those experiments performed by Duperrex and van den Bergh. These authors also found that the tendency of the dissociation rate to decrease inversely depends on the laser fluence. In molecules having a weaker or no discrete-level bottleneck [ 14181, there is no initial increase of the dissociation 0009-26 14/9
rate, at least for moderate laser fluences, but a continuous decrease of the efficiency as PA is being increased. Stone and co-workers [ 19,201 compared their experimental data with a theoretical model which considered the effect of collisions between CFzHCl and Ar in terms of rotational equilibration or “hole filling” in the discrete energy levels. In addition, these authors included collision terms in a generalized Landau-Teller mechanism for V-T collisional relaxation for sufficiently high pressures. They constructed theoretical curves showing a pressure falloff in the CF, production rate due to a deactivation of the excited molecules by collisional V-T energy transfer. This showed that the larger the probability of a collisional transition from the initial level to final level, the stronger the collisional deactivation
[201. McRae et al. [ 2 1 ] have pointed out that in processes involving monoatomic buffer gases, such as Ar or Xe, the probability for R-T energy transfer is larger than for V-T energy transfer, so discounting that the second of these processes occurs. Furthermore, they interpreted their experimental results in C,H,BrF as indicating that even rotational hole filling induced by R-T does not take place. However, their analysis is based on the assumption that the probability for a given energy-transfer process to occur is governed by the quantum energy being transferred, and they used rather high pressures of the molecules where any bottleneck effect might be masked.
I/$ 03.50 0 I99 1 - Elsevier Sctence Publishers B.V. (North-Holland )
351
Volume 181, number4
28 June 1591
CHEMICAL PHYSICS LETTERS
We tentatively suggest here that the differences in all the above controversial results can be simply related to a pressure effect of the absorbing polyatomic molecules. The suggestion is based on a detailed experimental study of the dependence of multiphotondissociation efficiency of CF,HCI on CFzHCl and buffer-gas pressures in the lowest relevant region, and infrared laser fluence, for monoatomic (Ar) and diatomic (N,) buffers. CF>HCI has been widely shown to have a strong discrete-level bottleneck which operates in multiphoton unimolecular infrared excitation or dissociation. We also discuss the effects of adding Ar, by comparing our results with those obtained by using MPD induced by two different laser infrared wavelengths [ 221.
2. Experimental A non-mode locked pulsed TEA CO2 laser (Lumonies 101) tuned at R( 32) (1086 cm-‘) has been used to excite quasiresonantly the CF stretching absorption of the CFzHCl molecule. With use of a typical CO1 gas mixture, pulse shapes consisted of an ~80 ns peak followed by a tail of about I ~.tswhen detected by a photon drag detector (Rofin, model 7415). In all cases, samples were steadily irradiated with 50 laser shots. The experiments were carried out at room temperature in a Pyrex cell 11 cm long and a total volume z 95 cm3 equipped with NaCl end windows and provided with a glass “finger”. A pyroelectric detector (ZOD-Lumonics) was employed to measure the incident pulse energy. A NaCl lens of 2 m focal length was used to obtain a nearly parallel beam in the cell. Attenuation of the incident energy was accomplished by inserting CaF, plates along the optical line. Dissociation-yield measurements were carried out on the final products with infrared absorption spectroscopy by using a Perkin-Elmer FUR spectrometer 1725X. ( CFzHCl) (Matheson Difluorochloromethane 99.9Oio), N, and Ar were used without further purification.
3. Results and discussion In fig. I, we give the curves representing the dependence of the fraction of CF,HCl molecules dissociated per laser pulse,
where k’, and I’, are the cell volume and the irradiated cylindrical volume, respectively, and N, and N, denote concentration of CF2HC1 before and after irradtation with n laser pulses, on the added inert gas pressure for two different CFzHCl initial pressures (0.05 and 0.7 mbar) and two fluence values ( 1.9 and 3.2 J cm-2). In all the curves, it can be observed that f initially increases; the larger the initial CFzHCl pressure, the smoother the initialfincrease. This initial behaviour should be interpreted by considering -In
l!
f=cQ& ‘\ ‘;. F=l9
J.cm-2
’
2.1
L
a.04005mb PCF2HCI .,x-0.7mb
4.:
Fig. I. Deactivation of the bottleneck in CF2HCI as induced by increasing partial pressures, Ps, of two buffer gases (N, and Ar). fis the fraction of molecules dissociated per laser pulse, as given by the formula in the text.
352
Volume
18I, number 4
CHEMICAL PHYSICS LETTERS
that at higher CFzHCl pressures, the intermolecular homogeneous collision effect, giving rise to intermolecular homogeneous rotational hole filling in CF2HCI, dominates over the heterogeneous rotational hole filling induced by collisions with the buffer gas. Given the similarity of the behaviour for Ar and N,, it seems that a V-V energy transfer should not be included among the process giving rise to the heterogeneous rotational hole filling. In addition, the C,F, formed in the CF,HCl decomposition, can be acting as a sensitizer, or a “hot buffer”, during the laser pulse, in transferring energy to the molecules of their environment, and so, a V ( C2F4) +V (CF,HCl) energy-transfer mechanism could be taking place, making the MPD process more efficient by energy pooling. At the lower fluence, we can observe that as we increase the N2 or Ar pressure, the difference in f between the curves at different CF2HCl pressures continuously decreases, reaching practically the same decreasing value for PNI 225 mbar, or giving rise to an inverted regime in which ftakes on larger values for smaller CF?HCl pressures for PAr> 20 mbar. It is worth noting that the inert-gas pressure at which freaches a maximum is slightly shifted towards smaller values as one goes from Ar to N2, and to smaller CFzHCl pressures. A continuous falloff in fis observed in all the curves as the buffer pressure is increased. These features indicate that the poisoning effect has to be attributable, not just to the heterogeneous collisions with the inert gas and even CzF4, but also to the homogeneous collisions between CF,HCl molecules themselves. Since these homogeneous collisions do not produce any deactivation falloff in MPD induced in pure CF,HCI, it seems that they have to be contributing to a more complicated collisional mechanism in which heterogeneous collisions are also simultaneously occurring. In any case, the main conclusion to be drawn from fig. 1 is that CF,HCl opposes a strong anharmonic bottleneck effect to MPD and that such an effect can be circumvented by adding an inert gas. Given that the obtained results seem to be rather independent of the laser-beam geometry, the possible influence of thermal effects in laser multimode experiments [23,24], such as those being discussed in this work, cannot be predominant, though they might still play an important role in other kinds of experiment.
28 June 1991
Although there always is a falloff in J; the maxima in the curves of fig. 1 for lower fluence are apparently much sharper for the larger CFzHCl pressures. On the other hand, the falloff is strongly suppressed for high fluences, as first pointed out by van den Bergh [ 12-141, both for Ar and Nz quite independently of the CFZHCl initial pressure. Although some other likely operating mechanisms, such as a fluenceinduced power broadening and the increase of collisional dissociation, can be invoked to explain the diminution of the falloff at higher fluences, I think that the overwhelming reason which justifies this result is the drastic increase of the energy deposited per molecule which overcomes any poisoning effect. The enhancement offat the inert-gas pressure giving the maximum effectiveness over the value for the neat CF2HCl decreases as the fluence is increased for the two CF>HCl represented in fig. 1, but is always much greater for the smaller pressure, indicating that the involvement of the bottleneck effect in MPD decreases with fluence and initial CF*HCl pressure. If the falloff in the curves of fig. 1 were to be attributed to collisional poisoning effects, in which both homogeneous and heterogeneous collisions would simultaneously take place, then from experiments such as those in fig. 1, one could derive a criterion to show the unimolecular, or rather selective, character of the involved multiphoton process. Since at the higher fluences, the collisional poisoning effect is overcome by the large deposited energy, such a criterion would only be valid when fluences near the minimal dissociation threshold (typically around 1 J cm- ’) were used, and could be stated in terms of the “specific” negative slope (slope per maximum increase off) of the nearly straight falloff line. When the so-defined specific slope would take on very small values, then the process would be essentially unimolecular. This tentative criterion receives some support from the experiments carried out with two infrared wavelengths [ 221. It was shown that the effect of a second nonresonant wavelength could induce a remarkable increase in the selectivity of the process, which was manifested in a significant reduction of the falloff, at least when Ar was used as a buffer. In summary, we have shown that the apparently different behaviour obtained by several authors about the maximum effectiveness of the MPD process in CF,HCl, activated by a buffer gas, is to be attributed 353
Volume 18 I, number 4
CHEMICAL PHYSICS LETTERS
combined effect of the pressures and fluences used, rather than an actual difference. We have also demonstrated that intermolecular V-V transfer does not play any significant role in collisionally enhanced MPD of CF,HCI, induced by an inert gas. This conclusion could be violated nevertheless at higher fluences where the fraction of molecules dissociated per laser pulse is larger for Ar along the inert-gas pressure interval studied after the maximum. to a
Acknowledgement The author wishes to thank P.F. Gonialez Diaz for useful discussions, and M. Santos, J.A. Torresano and A. Pino for their help. This work has been carried out with financial support provided by DGICYT.PGC under Project NPB0252.
References : I ] D.S. King, in: Dynamics of the excited state, ed. K.P. Lawley (Wiley, New York, 1982) p. 105. 121 J.L. Lyman, G.P. Quigley and O.P. Judd, in: Multiplephoton excitation and dissociation, ed. C.D. Cantrell (Springer, Berlin, 1986) p. 9. 131 V.S. Letokhov. Nonlinear laser chemistry. Multiple-photon excitation (Springer, Berlin, 1983). 141 V.S. Letokhov. in: Laser spectroscopy of highly vibrationally excited molecules (Hilger, London, 1989). :51 V.N. Bragatashivili, V.S. Letokhov, A.A. Makarot and E.A. Ryabov, in: Multiple photon infrared laser photophysics and photochemistry (Harwood, New York, 1985).
354
[6] D.W. LupoandM.
28 June 1991 Quack, Chem. Rev. 87 (1987) 181.
[ 71 J.C. Stephenson, D.S. King, H.V. Goodman and I. Stone, J. Chem. Phys. 70 (1979) 4496. [8] D.S. King and J.C. Stephenson, Chem. Phys. Letters 66 ( 1979) 33. [ 91 J.C. Stephenson, S.E. Bialkowski and D.S. King, J. Chem. Phys. 72 (1980) 1161. [ IO] J.C. Stephenson, J.A. Blazy, Ch. Li and D.S. King, J. Chem. Phys. 76 (1982) 5989. [ I1 ] J.C. Stephenson and D.S. King, J. Chem. Phys. 78 ( 1983) 1867. [ 121 R. Duperrex and H. van den Bergh. J. Chem. Phys. 71 (1979) 3613. [ 131 R. Duperrex and H. van den Bergh, J. Chem. Phys. 75 (1981) 3371. [ 141 R. Duperrex and H. van den Bergh, Chem. Phys. 40 ( 1979) 275. [IS] G.P. Quigley, Opt. Letters 3 (1978) 106. [ i 61 G.P. Quigley and J.L. Lyman, in: Springer series in chemical physics, Vol. 6. Laser-induced processes in molecules, eds. K.L. Kompaand S.D. Smith (Springer, Berlin, 1979) p. 134. [ 171 V.N. Bragatashivili, V.S. Dolzhikov and VS. Letokhov, Soviet Phys. JETP 49 ( 1979) 8. [ 18 ] C. Sigiienza. M. Santos, J.A. Torresano and P.F. GonzslezDiaz, Spectrochim. Acta 46 A (1990) 1499. [ 191 J. Stone, E. Thiele, M.F. Goodman, J.C. Stephenson and D.S. King, J. Chem. Phys. 73 (1980) 2259. [20] M.F. Goodman, J. Stone and E. Thiele, in: Multiphoton excitation and dissociation of polyatomic molecules, ed. C.D. Cantrell (Springer, Berlin, 1986) p. 159. [21] G.A. McRae, D.K. Evans and J.W. Goodale, J. Chem. Phys. 93 ( 1990) 1689. [22] M. Santos, C. Sigiienza, J.A. Torresano and P.F. GonzllezDiaz, Spectrochim. Acta 46 A (1990) 455. [23] R.T. Bailey, F.R. Cruikshank, D. Pugh, R. Guthrie, W. Johnstone, J. Mayer and K. Middleton, J. Chem. Phys. 77 (1982) 3453. [24] M. Lenzi, E. Molinari, G. Piciacchia, V. Sessa and M.L. Terranova, Spectrochim. Acta 43 A (1987) 129.