Redistribution, collisional

Figure C3.3.10. A schematic energy-level diagram for a molecule capable of undergoing unimolecular reaction above tlie energy depicted as tlie reaction barrier. Arrows to tlie right indicate reaction (collision-free) at a rate kg tliat depends on tlie energy E. Down arrows represent collisional redistribution of tlie hot molecules botli above and below tlie reaction barrier.

Collisional redistribution of radiation. A system A + B of two atoms /molecules may be excited by absorption of an off-resonant photon, in the far wing of the (collisionally) broadened resonance line of species A. One may then study the radiation that has been redistributed into the resonance line - a process that may be considered the inverse of pressure-broadened emission. Interesting polarization studies provide additional insights into the intermolecular interactions [118, 388]. 

P. Thoman, K. Burnett, and J. Cooper. Observation of dynamic correlations in collisional redistribution and depolarization of light. Phys. Rev. Lett., 45 1325, 1980. 

Some of our recent studies of LIF on OH in flames demonstrate the close connection between current work in other areas of physical chemistry—in this case, state-to-state collisional energy transfer—and the development of diagnostic tools for combustion. In these experiments, measurements are made of the collisional redistribution of excited state population following laser excitation of OH to individual levels, in an atmospheric pressure flame. 

K. Burnett and J. Cooper. Collisional redistribution of radiation II. The effects of degeneracy on the equations of motion for the density matrix. Phys. Rev. A, 22 2027-2043... 

J. L. Carlsten and A. Szoke. Collisional redistribution of near-resonant scattered light in Sr vapor. J. Phys. B, 9 L231-L235 (1976). 

J. L. Carlsten, A. Szoke, and M. G. Raymer. Collisional redistribution and saturation of near-resonance scattered light. Phys. Rev. A, 75 1029-1045 (1977). 

With increase in benzene (or added gas) pressure, the rate of collisional redistribution of excitation energy becomes... 

At low gas densities each molecule can only absorb one photon, because the LIF terminates on many lower levels and only a small fraction of all excited molecules reaches the initial ground state from where the absorption started. This is different in liquids or in gases at high pressures. Here fast collisional redistribution between the different levels iv , J") restore thermal equilibrium and refill the level (w", /") depleted by the absorption of laser photons in a time which is short compared to the lifetime r of the upper laser excited state (Fig. 1.57). 

Figure 2.46 illustrates the advantages of this technique. The upper spectrum represents a Lamb peak in the intracavity saturation spectrum of the neon line (l 2p) at A. = 588.2 nm (Sect. 2.3.3). Due to the collisional redistribution of the atomic velocities, a broad and rather intense background appears in addition to the narrow peak. This broad structure is not present in the dichroism and birefrin-gent curves (Fig. 2.46b, c). This improves the signal-to-noise ratio and the spectral resolution. 

Excited n molecules are not discussed here. However, energy transfer in-electronically excited states has been recently studied. In this case a complete preparation of different initial quantum states is possible by laser excitation. The subsequent collisional redistribution can be studied by dispersing the fluorescence or probing the neighboring levels with a second laser. In this way state-to-state data can be obtained. Systems investigated include ZnH, CdH and CaF(A n 2 3/2 gases. 

Collisional Redistribution of Radiation Collision Induced Fluorescence... 

S. Reynaud and C. C. Cohen-Tannoudji, "Dressed atom approach to collisional redistribution," J. Physique (in press). 

Once vibrational population ratios for the reaction product molecule of interest have been determined with one or more of the methods just described, ratios of rate constants for specific vibrational levels can be deduced, provided the effects of collisional redistribution are negligible, or at least not so large that correction for these effects cannot be made with some confidence. An important advantage of the chemical laser techniques is that the ratio of rate constants, kijkoy to the y = 1 and t = 0 vibrational levels can be obtained. This type of information is not directly obtained from chemiluminescence studies. 

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