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Vibrational energy relaxation in liquid N2-CO mixtures
Volume 18, n u m b e r 1 X8
OPl ICS ( ( ) M M U N I ( A - I I O N S
V I B R A T I O N A L R E L A X A T I O N OF H 2 LIQUID G.M. GALE, C. DELALANDF. and J. DUCUING •
.
ag
Laboratoirc. d Optlque Quanttqu~ , Ecole Polvtechnique 91120 Palaiseau, France Recent experimental advances have opened the way 1o a better understanding of vibrational relaxation in liquids [ 11. However, in the case of polyatomic molecules, the complexity of the structure renders the analysis difficult. It is only through the systematic study of simple, theoretically accessible systems that one can hope to reach a deeper insight into the vibrational relaxation mechanisms involved. In this respect H 2 is especially attractive as extensive m e a s u r e m e n t s in the gas phase [2] at low temperatures ( 4 0 - 4 0 0 K) have permitted direct testing of new, fuRRy quantum-mechanical calculations. We report here relaxation m e a s u r e m e n t s in the liquid (14--33 K) and gas phase (> 33 K) o f normal (room-temperature equilibrated) 1t 2. The H 2 liquid is contained in a small volume, 4 window cell under presst, res of up to 30 bar. Cooling in the range 5 - 3 0 0 K is provided by a continuous flow of helium liquid, with an accuracy of temperature control better t h a n :,: 10-1 K. The normally unpopulated first vibrational level of liquid (or gaseous) ortho H 2 is excited on a nanosecond timcscale by a coherent R a m a n process, employing a r u b y - p u m p e d dye laser in conjunction with the stimulated Stokes radiation generated by this laser in a room temperature, high pressure, H 2 gas cell. The difference A~,, of up to 10 cm - I , in the vibrational transition frequencies of H 2 liquid and gas is compensated by causing the tunable dye laser to emit two l'requcncies separated by A~. This doublet structure is rellected in the Stokes emission, thus enabling resonant excitation conditions in the liquid to be satisfied when the 4, MW peak power, frequencies are recombined at the centre of the t12 cell. The decay o f vibrational energy with time is measured by probing the excitation volume with a high intensity laser pulse and detecting laterally with a photomultiplier the s p o n t a n e o u s anti-Stokes scattering from the excited molecules. The laser probe consists of a stabitised (non-spiking) ruby relaxation oscillator producing a 10 kW peak power, 400 ~ts long pulse, synchronised with the excitation. As the probe intensity is essentially constant on the time scale of the experiment, the detected s p o n t a n e o u s anti-Stokes radiation intensity is linearly proportional to the excited state population and yields directly the vibrational relaxation time. Our m e a s u r e m e n t s in the liquid, between 20 and 32 K (p = 1600 to 1170 amagat) and gaseous phase at 34 K and 740 amagat, yield relaxation times ranging from 14 to 30 ,zs. Within experimental error, these results can be summarised by the formula
r-1 = kloN, where N is the n u m b e r of molecules per cm 3 and k I 0 = 4 X 1 0 - 1 8 c m 3 sec - l . The above relationship would seem to indicate that in the * Laboratoire Propre du Centre National de la Recherche Scientifique, C.N.R.S.
192
h~JV ]')7~k
range or densities investigated, binary collision pr~,cesses are dominant. A similar conclusion was made by t.lx;ing et al. m their study of liquid N 2 [3]. Although the observed value o~ k l0 is identical to that obtained at low lemperattncs m the gas phase of n H2 , further m e a s u r e m e n t s are ncccssar> to vcrif5 this apparent constancy. l.xperiments are in progress to investigate the variations of 1/r with density, at fixed temperature, in the gas and liquid phase in order to determine more accurately what role ternary collisions play in this high density, low temperature environment and to study the phase transition behaviour. References ] 1 ] A. Laubereau, A. Seilmeier and W. Kaiser, (!hem. Phys. Lett. 36 (1975) 232. 121 M.M. Audibert, R. Vilaseca, J. Lukasik and J. I)ucuing, Chem. Phys. Lett. 31 (1975) 232. 131 W.F. Calaway and G.E. Ewing, J. Chem. Phy.~. 63 (1975) 2842.
X9
VIBRATIONAL ENERGY RELAXATION IN LIQUID N 2 - C O MIXTURES* S.R.J. BRUECK and R.M. OSGOOD, Jr.
Lincoln Laboratory, Massachusetts Institute oJ Technology, Lexington, Massachusetts 021 73, USA The m e a s u r e m e n t of v i b r a t i o n - v i b r a t i o n ( V - V ) and vibrat i o n - t r a n s l a t i o n ( V - T ) rates in molecular gases has been instrumental in understanding and developing infrared gas lasers and in increasing the understanding of the coilisional physics of molecular gases in general. Laser induced fluorescence has been the essential m e t h o d o f these measurements [ t 1. Wc report the extension of this technique to liquid systems. Typically, energy relaxation times for complex molecules in the liquid phase are very short ( ~ 1 0 - 1 2 s); however, for simple diatomic liquids, m u c h longer energy relaxation times are possible. Recently Calaway and Ewing [2] have measured energy relaxation times for liquid N 2 of 1.5 +- 0.5 s using the stimulated R a m a n scattering technique first developed by Ducuing [3]. We have developed a m u c h simpler optical p u m p i n g technique which relies on direct excitation of the N 2 vibrational mode at 2350 cm - I with the o u t p u t o f a HBr TEA laser. This mode, which is infrared inactive in low pressure gases, has a collision induced absorption at liquid densities. The decay of the vibrational energy is monitored by doping the nitrogen with small a m o u n t s o f CO and monitoring the fluorescence from the CO v = l vibrational level which is 185 c m - I below that of N 2. The CO dependence of the relaxation rate is shown in fig. 1. For CO densities between 4 X 1017 c m - 3 and 2.5 X 1018 cm - 3 the relaxation rate is linearly dependent on Co concentration. In this low CO density region, the relaxation rate can be shown to be * This work was sponsered by the Department of the Air Force.
LASER APPLICATIONS II
XI0 the energy storage capabilities can be altered by adding Ar. Alternate laser transitions can be investigated by adding molecules such as CO 2 or CH 4 (CD4t. Other cryogenic liquids would make suitable hosts for related liquid laser systems, viz., pumping of liquid H 2 by a HI: chemical laser. Such a system would be the liquid analogue if the recently proposed high pressure H 2 laser [4].
o.~
0.3
@
References [1 ] For a recent review, see E. Weitz and G. Flynn, Ann. Rev. Phys. Chem. 25 (1974) 275. [21 W.F. Calaway and G.E. Ewing, Chem. Phys. Lett. 30 (1975) 485: J. Chem. Phys. 63 (1975) 2892. [ 3 ] J. Ducuing, C. Joffrin, and J.P. Coffinet, Opt. C o m m u n . 2 (1970) 245. 14] W.H. Christiansen and E. Greenfield, Appl. Phys. Lett. 23 (1973) 623.
@ 0.2
O.l 0
[ O2
014061
0!8 XIO
nco( ~" 10-19¢m -3 )
Fig. l. Decay rate of CO fluorescence vs CO density. riCO F = I'N2 + PCO ~ exp(/3Ae),
R O T A T I O N A L R E L A X A T I O N TIMES OF ANISOTROPIC MOLECULES IN MIXED LIQUIDS* R.R. ALI:ANO, P.P. ftO, and W. YU
The City College of New York, Phl,Sics Department, New York, N.Y. 10031, USA (1)
where n c o (nN2) are the respective densities, t3 = 1/kT, Ae is the N 2 - C O vibrational energy difference and FCO(FN2) are vibrational decay constants including b o t h radiative and collisional deactivation processes. F r o m fig. 1, we have FCO = 43 +- 4 s - 1 which is in good agreement with the CO radiative decay rate in liquid N 2 of 48 s - 1 , thus indicating that the CO decay is d o m i n a t e d by radiative processes. The extrapolation of the linear region to zero concentration gives a FN2 of 0.018 -+ 0.004 s - 1 . The N 2 radiative relaxation rate, calculated from integrated band intensity, is 0.014 s - 1 ; thus the N 2 relaxation is also d o m i n a t e d by radiative processes. I:or CO densities above 2.5 X 1018 c m - 3 , the CO absorption length is less t h a n typical sample cell dimensions and radiation trapping decreases the observed relaxation rates. At the lowest CO densities the energy relaxation times are sufficiently long ( ~ 2 5 s - 1 ) that diffusional and/or convective transport of the vibrational energy to the cell walls b e c o m e s the dominant relaxation mechanism. The liquid N 2 - C O system discussed here shows great potential as a high energy storage infrared laser m e d i u m . The long energy storage lifetimes, high density, and fast V - V transfer processes suggest the possibility o f a high energy short pulse laser system. Substantial tunability could be achieved because the broad CO f u n d a m e n t a l emission b a n d w i d t h in the liquid m e d i u m overlaps with the higher level CO emissions. The low translational temperature enhances the population of the CO vibrational ladder so that it is not necessary to obtain a population inversion o f the N 2 vibrational band. The characteristics of the laser m e d i u m can be easily altered by adding additional species to the liquid mixture. Thus, for example,
We report the direct m e a s u r e m e n t of tile relaxation times of the optical Kerr effects of nitrobenzene and m-nitrotoluenc in mixed liquids by measuring the kinetics of the optical Kerr effect using picosecond laser pulses. In these anisotropic molecules, the relaxation of the Kerr effect is primarily due to the molecular reorientational m o t i o n and its conpling with tile collective mode of m o t i o n in the liquid, such as the angular m o m e n t u m correlation and shear modes. The c o n t r i b u t k m to the optical Kerr effect due to electronic cloud distortion or the librational m o t i o n of the molecules is either too small or too fast to be resolved with longer than picosecond pulses. In this experiment the relaxation times are measured as functions of concentration and viscosity of the solution. By varying the concentration of the Kerr-active solute in the solution, we arc able to deduce the effect of pair correlation in liquids. By appropriately choosing the viscosity of the solution, we f o u n d that the relevant parameter to describe the viscosity dependence of the relaxation time in mixed liquids is an effective local viscosiO, which is essentially the bulk viscosity excluding the contribution from solvent molecules. Comparing the results of this experiment and those front light scattering with the data obtained from dielectric relaxation experiment, onr data supports the conclusion that the characteristic angular step o f the rotation by a small molecule in ordinary liquids is in such a size that neither rotation by simple diffusion through small angles nor rotation by a singularly large j u m p is an adequate picture in describing the rotational motion. The observation of a doublet in the depolarized Rayleigh wing scattering in anisotropic molecules confirmed the existence of the coupling of * Supported in part by NSF Grant GH 41812.
193
OPl ICS ( ( ) M M U N I ( A - I I O N S
V I B R A T I O N A L R E L A X A T I O N OF H 2 LIQUID G.M. GALE, C. DELALANDF. and J. DUCUING •
.
ag
Laboratoirc. d Optlque Quanttqu~ , Ecole Polvtechnique 91120 Palaiseau, France Recent experimental advances have opened the way 1o a better understanding of vibrational relaxation in liquids [ 11. However, in the case of polyatomic molecules, the complexity of the structure renders the analysis difficult. It is only through the systematic study of simple, theoretically accessible systems that one can hope to reach a deeper insight into the vibrational relaxation mechanisms involved. In this respect H 2 is especially attractive as extensive m e a s u r e m e n t s in the gas phase [2] at low temperatures ( 4 0 - 4 0 0 K) have permitted direct testing of new, fuRRy quantum-mechanical calculations. We report here relaxation m e a s u r e m e n t s in the liquid (14--33 K) and gas phase (> 33 K) o f normal (room-temperature equilibrated) 1t 2. The H 2 liquid is contained in a small volume, 4 window cell under presst, res of up to 30 bar. Cooling in the range 5 - 3 0 0 K is provided by a continuous flow of helium liquid, with an accuracy of temperature control better t h a n :,: 10-1 K. The normally unpopulated first vibrational level of liquid (or gaseous) ortho H 2 is excited on a nanosecond timcscale by a coherent R a m a n process, employing a r u b y - p u m p e d dye laser in conjunction with the stimulated Stokes radiation generated by this laser in a room temperature, high pressure, H 2 gas cell. The difference A~,, of up to 10 cm - I , in the vibrational transition frequencies of H 2 liquid and gas is compensated by causing the tunable dye laser to emit two l'requcncies separated by A~. This doublet structure is rellected in the Stokes emission, thus enabling resonant excitation conditions in the liquid to be satisfied when the 4, MW peak power, frequencies are recombined at the centre of the t12 cell. The decay o f vibrational energy with time is measured by probing the excitation volume with a high intensity laser pulse and detecting laterally with a photomultiplier the s p o n t a n e o u s anti-Stokes scattering from the excited molecules. The laser probe consists of a stabitised (non-spiking) ruby relaxation oscillator producing a 10 kW peak power, 400 ~ts long pulse, synchronised with the excitation. As the probe intensity is essentially constant on the time scale of the experiment, the detected s p o n t a n e o u s anti-Stokes radiation intensity is linearly proportional to the excited state population and yields directly the vibrational relaxation time. Our m e a s u r e m e n t s in the liquid, between 20 and 32 K (p = 1600 to 1170 amagat) and gaseous phase at 34 K and 740 amagat, yield relaxation times ranging from 14 to 30 ,zs. Within experimental error, these results can be summarised by the formula
r-1 = kloN, where N is the n u m b e r of molecules per cm 3 and k I 0 = 4 X 1 0 - 1 8 c m 3 sec - l . The above relationship would seem to indicate that in the * Laboratoire Propre du Centre National de la Recherche Scientifique, C.N.R.S.
192
h~JV ]')7~k
range or densities investigated, binary collision pr~,cesses are dominant. A similar conclusion was made by t.lx;ing et al. m their study of liquid N 2 [3]. Although the observed value o~ k l0 is identical to that obtained at low lemperattncs m the gas phase of n H2 , further m e a s u r e m e n t s are ncccssar> to vcrif5 this apparent constancy. l.xperiments are in progress to investigate the variations of 1/r with density, at fixed temperature, in the gas and liquid phase in order to determine more accurately what role ternary collisions play in this high density, low temperature environment and to study the phase transition behaviour. References ] 1 ] A. Laubereau, A. Seilmeier and W. Kaiser, (!hem. Phys. Lett. 36 (1975) 232. 121 M.M. Audibert, R. Vilaseca, J. Lukasik and J. I)ucuing, Chem. Phys. Lett. 31 (1975) 232. 131 W.F. Calaway and G.E. Ewing, J. Chem. Phy.~. 63 (1975) 2842.
X9
VIBRATIONAL ENERGY RELAXATION IN LIQUID N 2 - C O MIXTURES* S.R.J. BRUECK and R.M. OSGOOD, Jr.
Lincoln Laboratory, Massachusetts Institute oJ Technology, Lexington, Massachusetts 021 73, USA The m e a s u r e m e n t of v i b r a t i o n - v i b r a t i o n ( V - V ) and vibrat i o n - t r a n s l a t i o n ( V - T ) rates in molecular gases has been instrumental in understanding and developing infrared gas lasers and in increasing the understanding of the coilisional physics of molecular gases in general. Laser induced fluorescence has been the essential m e t h o d o f these measurements [ t 1. Wc report the extension of this technique to liquid systems. Typically, energy relaxation times for complex molecules in the liquid phase are very short ( ~ 1 0 - 1 2 s); however, for simple diatomic liquids, m u c h longer energy relaxation times are possible. Recently Calaway and Ewing [2] have measured energy relaxation times for liquid N 2 of 1.5 +- 0.5 s using the stimulated R a m a n scattering technique first developed by Ducuing [3]. We have developed a m u c h simpler optical p u m p i n g technique which relies on direct excitation of the N 2 vibrational mode at 2350 cm - I with the o u t p u t o f a HBr TEA laser. This mode, which is infrared inactive in low pressure gases, has a collision induced absorption at liquid densities. The decay of the vibrational energy is monitored by doping the nitrogen with small a m o u n t s o f CO and monitoring the fluorescence from the CO v = l vibrational level which is 185 c m - I below that of N 2. The CO dependence of the relaxation rate is shown in fig. 1. For CO densities between 4 X 1017 c m - 3 and 2.5 X 1018 cm - 3 the relaxation rate is linearly dependent on Co concentration. In this low CO density region, the relaxation rate can be shown to be * This work was sponsered by the Department of the Air Force.
LASER APPLICATIONS II
XI0 the energy storage capabilities can be altered by adding Ar. Alternate laser transitions can be investigated by adding molecules such as CO 2 or CH 4 (CD4t. Other cryogenic liquids would make suitable hosts for related liquid laser systems, viz., pumping of liquid H 2 by a HI: chemical laser. Such a system would be the liquid analogue if the recently proposed high pressure H 2 laser [4].
o.~
0.3
@
References [1 ] For a recent review, see E. Weitz and G. Flynn, Ann. Rev. Phys. Chem. 25 (1974) 275. [21 W.F. Calaway and G.E. Ewing, Chem. Phys. Lett. 30 (1975) 485: J. Chem. Phys. 63 (1975) 2892. [ 3 ] J. Ducuing, C. Joffrin, and J.P. Coffinet, Opt. C o m m u n . 2 (1970) 245. 14] W.H. Christiansen and E. Greenfield, Appl. Phys. Lett. 23 (1973) 623.
@ 0.2
O.l 0
[ O2
014061
0!8 XIO
nco( ~" 10-19¢m -3 )
Fig. l. Decay rate of CO fluorescence vs CO density. riCO F = I'N2 + PCO ~ exp(/3Ae),
R O T A T I O N A L R E L A X A T I O N TIMES OF ANISOTROPIC MOLECULES IN MIXED LIQUIDS* R.R. ALI:ANO, P.P. ftO, and W. YU
The City College of New York, Phl,Sics Department, New York, N.Y. 10031, USA (1)
where n c o (nN2) are the respective densities, t3 = 1/kT, Ae is the N 2 - C O vibrational energy difference and FCO(FN2) are vibrational decay constants including b o t h radiative and collisional deactivation processes. F r o m fig. 1, we have FCO = 43 +- 4 s - 1 which is in good agreement with the CO radiative decay rate in liquid N 2 of 48 s - 1 , thus indicating that the CO decay is d o m i n a t e d by radiative processes. The extrapolation of the linear region to zero concentration gives a FN2 of 0.018 -+ 0.004 s - 1 . The N 2 radiative relaxation rate, calculated from integrated band intensity, is 0.014 s - 1 ; thus the N 2 relaxation is also d o m i n a t e d by radiative processes. I:or CO densities above 2.5 X 1018 c m - 3 , the CO absorption length is less t h a n typical sample cell dimensions and radiation trapping decreases the observed relaxation rates. At the lowest CO densities the energy relaxation times are sufficiently long ( ~ 2 5 s - 1 ) that diffusional and/or convective transport of the vibrational energy to the cell walls b e c o m e s the dominant relaxation mechanism. The liquid N 2 - C O system discussed here shows great potential as a high energy storage infrared laser m e d i u m . The long energy storage lifetimes, high density, and fast V - V transfer processes suggest the possibility o f a high energy short pulse laser system. Substantial tunability could be achieved because the broad CO f u n d a m e n t a l emission b a n d w i d t h in the liquid m e d i u m overlaps with the higher level CO emissions. The low translational temperature enhances the population of the CO vibrational ladder so that it is not necessary to obtain a population inversion o f the N 2 vibrational band. The characteristics of the laser m e d i u m can be easily altered by adding additional species to the liquid mixture. Thus, for example,
We report the direct m e a s u r e m e n t of tile relaxation times of the optical Kerr effects of nitrobenzene and m-nitrotoluenc in mixed liquids by measuring the kinetics of the optical Kerr effect using picosecond laser pulses. In these anisotropic molecules, the relaxation of the Kerr effect is primarily due to the molecular reorientational m o t i o n and its conpling with tile collective mode of m o t i o n in the liquid, such as the angular m o m e n t u m correlation and shear modes. The c o n t r i b u t k m to the optical Kerr effect due to electronic cloud distortion or the librational m o t i o n of the molecules is either too small or too fast to be resolved with longer than picosecond pulses. In this experiment the relaxation times are measured as functions of concentration and viscosity of the solution. By varying the concentration of the Kerr-active solute in the solution, we arc able to deduce the effect of pair correlation in liquids. By appropriately choosing the viscosity of the solution, we f o u n d that the relevant parameter to describe the viscosity dependence of the relaxation time in mixed liquids is an effective local viscosiO, which is essentially the bulk viscosity excluding the contribution from solvent molecules. Comparing the results of this experiment and those front light scattering with the data obtained from dielectric relaxation experiment, onr data supports the conclusion that the characteristic angular step o f the rotation by a small molecule in ordinary liquids is in such a size that neither rotation by simple diffusion through small angles nor rotation by a singularly large j u m p is an adequate picture in describing the rotational motion. The observation of a doublet in the depolarized Rayleigh wing scattering in anisotropic molecules confirmed the existence of the coupling of * Supported in part by NSF Grant GH 41812.
193