Collision-induced predissociation

G. W. Hoffman T. J. Chuang, and K. B. Eisenthal, Picosecond smdies of the cage effect and collision induced predissociation of iodine in liquids. Chem. Phys. Lett. 25(2), 201-205 (1974). 

The complete wave function of a molecule will often change in the presence of foreign fields. Foreign fields, either electrostatic or electromagnetic, may under certain conditions change wave functions sufficiently to permit perturbations which would otherwise be unimportant. Perturbations may also be induced by collisions with other molecules, particularly with molecules which are themselves paramagnetic. These effects give rise to what is often called collision-induced predissociation. 

Hagege, J., P. C. Roberge, and C. Vermeil Methanol Photochemistry Collision-Induced Predissociation. Ber. Bunsenges. Physik. Chem. 72, 138 (1968). 

Figure 2.3 Perturbations and predissociations affect absorption and emission line intensities in quite different ways. Two pairs of absorption and emission spectra are shown. The first pair illustrates the disappearance of a weakly predissociated line in emission without any detectable intensity or lineshape alteration in absorption. The second pair shows that emission from upper levels with slow radiative decay rates can be selectively quenched by collision induced energy transfer. The opposite effect, selective collisional enhancement of emission from perturbed, longer-lived levels, is well known in CN B2 +—X2 +(u = 0,v") emission spectra (see Fig. 6.14 and Section 6.5.5). (a) the CO B1S+—X1S+(1,0) band in emission (top) and absorption (bottom). The last strong lines in emission are 11(16) and P(18). Emission from levels with J > 17 is weak because the predissociation rate is larger than the spontaneous emission rate. (Courtesy F. Launay and J. Y. Roncin.) (6) The CO A ll—X1 + (0,0) band in emission (bottom) and absorption (top). The a 3 + —X1 +(8,0) band lines appear in absorption because the A1 FI a 3 + spin-orbit interaction causes a small amount of A1 character to be admixed into the nominal a 3 + levels. These a —X lines are absent from the emission spectrum because collisional quenching and radiative decay into a3II compete more effectively with radiative decay into X1 + from the long-lived a 3 + state than from the short-lived A1 state. In addition, collisions and radiative decay into a3II cause the P(31) extra line (E) (arising from a perturbation by d3A v = 4) to be weakened in emission relative to the main line (M). (Courtesy F. Launay, A. Le Floch, and J. Rostas.)...

Inspection of Table I shows that agreement between one- and two-photon excitation results is reasonable for the A state, and poor for the C and D states. As mentioned above, the C state predissociates on excitation beyond the y = 11/2 level of u = 0. In the absence of a reliable value for the zero-pressure lifetime for the C(u=l) state, only a lower limit for the quenching rate constant can be obtained. It is seen to be much larger than for all other states, possibly indicating a collision-induced predissociation mechanism. 

The model described is in agreement with the major features of the observations and is consistent with the data available for the predissociation of Hel van der Waals complexes. It remains to be seen if detailed close coupling calculations will provide quantitative verification of all of its features. Although there are not now data to support a generalization, it seems plausible that the zero energy orbiting resonance mechanism for efficient collision-induced vibrational relaxation will occur in all systems. It will be particularly interesting to see what selectivity of vibrational pathway exists in the case of relaxation of a polyatomic molecule. 

There should be a correlation between the nature of the vibrational redistribution induced by a zero-energy collision and the vibrational distribution in the polyatomic product partner of a predissociated van der Waals molecule, since the latter can be viewed as a half collision. There are, at present, insufficient data to elucidate that correlation. 

Collisions destroy the coherences associated with the initially prepared state and induce transitions between rotational levels. This results in redistribution of population between all interacting levels but the overall population (the trace of the density matrix) is not affected and decays only radiatively with the rate k Pss + Puu)- Collisions, thus, modify the form of the excited-state decay but the overall fluorescence yield remains equal to one unless the levels s, u, and/or /, m are subjected to additional nonradia-tive decay processes (e.g., in the case of predissociation). 

The investigation of fast processes, such as electron motions in atoms or molecules, radiative or collision-induced decays of excited levels, isomerization of excited molecules, or the relaxation of an optically pumped system toward thermal equilibrium, opens the way to study in detail the dynamic properties of excited atoms and molecules. A thorough knowledge of dynamical processes is of fundamental importance for many branches of physics, chemistry, or biology. Examples are predissociation rates of excited molecules, femtosecond chemistry, or the understanding of the visual process and its different steps from the photoexcitation ofrhodopsin molecules in the retina cells to the arrival of electrical nerve pulses in the brain. 

Processes (3)-(5) represent spontaneous predissociation, collision-induced (self-) quenching, and magnetic predissociation, respectively. 

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