Photoionization definition

A possible role for shape resonances has been postulated in a number of the photoionization studies mentioned above [52, 53, 57, 60], although it has to be noted that except for camphor [57], the evidence for the existence of the shape resonance is not definitive. (It also then remains an open question how any such resonances, inferred from fixed geometry calculations, would manifest themselves in practice in large, and sometimes floppy, molecules, such as these chiral species.)... 

The intramolecular excitation of a sequence of definite electronic levels in the positive ion is the most probable explanation of the appearance of slow electrons. The differences between the electron kinetic energies for the subsequent maxima in the distribution curves are a measure of the discrete values of the excitation energy absorbed by the molecule in the process of photoionization. Table III gives a summary of these values. 

Our observation of the hydrated electron band at a 5 /xsec. delay cannot be attributed to the thermal reaction H + OH - e aq + H20, because the rate constant of 1.8 X 107Af 1 sec. 1 (21) permits only a negligible conversion of H atoms below pH 10. Therefore, the cases indicated as (+) and (+ + ) are taken as definite proof of photoionization. The cases indicated as (f) are less certain, although a photographic density difference of proper lifetime was measured densitometrically because the weak absorptions made the delineation from other transients, such as short-lived triplets, less certain. The absence of the hydrated electron... 

Calculated results for the Ar nondipole Is photoelectron angular-asymmetry parameter yis(< ) for free Ar, Ar C60, Ar C60 (-240 and Ar C6o C240 C540 are shown in Figure 15 (the nondipole parameter Sns vanishes, by definition, and the dipole parameter fins = 2, i.e., is constant, nonrelativistically for the photoionization of ns-states). 

Data on the radical cations are much more definitive because the parent is well established as a fragment formed by the photoionization of benzene and its isomers, and its hetero derivatives such as pyridine and the three diazines. (This ion, its formation and thermochemistry, is reviewed in Ref. 76.) Unequivocally, the most stable C4H4 isomer is the radical cation of methylenecyclopropene. This last species is perhaps more explicitly and sensibly described as the -CH 2 derivative of cyclopropenium cation and so renamed (cyclopropenio)methyl radical (15). From these studies, the heat of formation of this... 

Figure 12. Relative rate constants (left-hand scale, data points) as a function of cluster size for Dj chemisorption onto iron, vanadium, and niobium clusters. Activation energies, derived from cluster photoionization thresholds [Eq. (21)], are indicated by solid line and right-hand scale. See text for definition of parameters.

As is well known, kinetic arguments can be used to disprove mechanistic hypotheses (as in this instance), but never to prove them. The validity of first-order kinetics has been experimentally verified for monoenergetic ions, but the general applicability to ions with a distribution of internal energies has been questioned.Conditions under which first-order kinetics represent a usable approximation have been discussed, and kinetic isotope eflfects (having positive definite values) have been extracted from experimental data for a number of unimolecular decompositions of ions produced by photoionization.Our interpretation of the dissociation in Scheme 6 in terms of positional interchange is treated in the next section. 

Such conflicts indicate the extreme difficulty in using such techniques to investigate the effect of internal energy on the rate constant. It is relevant to report from definitive photoionization studies that the rate constant for formation only decreases slightly with increasing vibrational... 

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