Predissociation lifetimes

Huang Z S, Jucks K W and Miller R E 1986 The vibrational predissociation lifetime of the HF dimer upon exciting the free-H stretching vibration J. Cham. Phys. 85 3338-41... 

A nice example of this teclmique is the detennination of vibrational predissociation lifetimes of (HF)2 [55]. The HF dimer has a nonlinear hydrogen bonded structure, with nonequivalent FIF subunits. There is one free FIF stretch (v ), and one bound FIF stretch (V2), which rapidly interconvert. The vibrational predissociation lifetime was measured to be 24 ns when excitmg the free FIF stretch, but only 1 ns when exciting the bound FIF stretch. This makes sense, as one would expect the bound FIF vibration to be most strongly coupled to the weak intenuolecular bond. 

Figure 3-4. Schematic of size and state selection of weakly bound complexes. Weak M-HX intermolecular coupling and the corresponding long predissociation lifetimes enable ro-vibrational levels of the complex to be isolated. Ultraviolet photons at sufficiently long wavelengths act on the excited (but not the unexcited) HX moieties.

It is noteworthy that the photolytic removal of the second hydrogen atom from the complex is quite efficient. Thus, even under ordinary experimental conditions, it will be possible to ensure efficient ultraviolet photoexcitation in the face of subnanosecond predissociation lifetimes. 

Loge, G.W. and Zare, R.N. (1981). Dependence of diatomic photofragment fluorescence polarization on triatomic predissociation lifetime, Mol. Phys., 43, 1419-1428. 

Predissociation Lifetime (in ps) for the Three-Dimensional HeCl2 Molecule"... 

The nature of vibrationally and rotationally predissociating states of atom-diatom Van der Waals molecules and the fundamental considerations governing their predissociation are discussed. Particular attention is focussed on the influence of the potential energy surface and the information about it which might be extracted from accurate measurements of predissociation lifetimes. Most of the results discussed pertain to the molecular hydrogen-inert gas systems, and details of previously unpublished three-dimensional potential energy surfaces for diatomic hydrogen with krypton and xenon are presented. 

Llnewldths and Unlmolecular Decay Rates. Now let us consider the questions posed In the Introduction, In the light cast by these data. The first two questions, having to do with the Interpretation of the lifetimes x and the dynamics of unlmolecular decay, are closely related. One possible Interpretation is that t is.the predissociation lifetime of the vdU molecule. It is Important to realize that a direct measurement of the excited vdU molecule population as a function of time following excitation has not been made in any of the cases cited. These are all frequency-domain experiments. A time-domain measurement is certainly possible in principle, and will probably be done in the near future. In the meantime, we must make do with circumstantial evidence. 

Since the lifetimes t are evidently determined by microscopic dynamics rather than by statistics, one must conclude either that statistical theories do not apply to the dissociation of polyatomic vdW molecules or that the lifetimes t are not the predissociation lifetimes (or, possibly, that both statements are true). The first choice — abandoning a statistical description — should not be too upsetting, for two reasons. First, there have been very few tests of statistical theories on a microscopic level, and It Is easy to Imagine that even If a statistical model were microscopically Incorrect It might be valid (on the average) for macroscopic observations. Second, vdW molecules do represent a special class of systems. In which the dissociation energies are very small, and which contain vibrational frequencies In the vdW coordinates which are unusually low. It Is possible that a statistical description not valid for vdU molecule dissociation might still be valid for dissociation of covalent bound species. 

Time-independent picture. The opposite extreme from short-pulse excitation involves the use of nearly monochromatic radiation. Practically, this means that the interaction between molecule and radiation field is of longer duration than Tnr. In this limit, the quantity measured is the absorption lineshape. It will be shown below that the linewidth observed in an energy-resolved experiment is related in a very simple way to the predissociation lifetime in the time-resolved experiment. 

Table 1 Vibrational Predissociation Lifetime for HFDF and HFHF (ns) ...

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