Vibration autoionization

Bryant, G. P., Yiang, Y., and Grant, E. R. (1992), Triple-Resonance Spectroscopy of the Higher Excited States of N02. Trends in the Mode Dependence of Vibrational Autoionization via Asymmetric Stretch versus Symmetric Stretch and Bend, J. Chem. Phys. 96,4827. 

The method used in these calculations is based on the theoretical developments of Sakimoto [42] and is also an extension of our previous work on the Stark spectrum of spin-orbit autoionized Rydberg states of argon [43] and of the vibrationally autoionized states of hydrogen [44, 45], Only the homoge-... 

Vibrational mode selectivity can also be used to promote electronic processes. Vibrational autoionization is a process whereby a bound electron acquires sufficient energy to escape by extracting one quantum of vibrational energy from the ionic core of the molecule. For such an energy transfer to occur, the electron must first collide with the core. Scattering of the electron with the core can be promoted if the amplitude of the nuclear motion overlaps the electronic charge density. An example of this process studied by Steven Pratt is vibrational autoionization of the 3d Rydberg electrons of ammonia, which is enhanced... 

Figure 8.12 Relative photoionization cross section of para-H.2, at 78 K, in the region o t e ionization threshold, recorded at a wavelength resolution of 0.016 A. On the right-han si e of the figure, the rotationally autoionized lines of the np2 series appear as emission win °ws-The large peaks on the left are the result of vibrational autoionization (see Section 8.6) [ rom Dehmer and Chupka (1976)].

Figure 8.13 Schematic Illustration of vibrational autoionization of the H2 npnu 1II,4 (v = 1) Rydberg series by the continuum of Hj X2Ej (v+ = 0).

Vibrational autoionization corresponds to an exchange between the kinetic energy of the ejected electron and the vibrational energy of the core ion. This process occurs because the potential curves of the ion and Rydberg states are slightly different. Eq. (8.6.7) can be derived from a more sophisticated model using multichannel quantum defect theory (Raoult and Jungen, 1981). 

In reality, one encounters overlapping multiple resonances interacting with multiple continua. The analytical expressions given here represent an idealized situation. Several examples involving interferences between different autoionization processes [for example, in H2, interference between rotational and vibrational autoionization (Fig. 8.12) in N2, interference between vibrational... 

As discussed in Section 8.2, superexcited states, AB, can decay by both autoionization and dissociation (more specifically, by predissociation). Decay by spontaneous fluorescence can be neglected for superexcited states because, generally, the predissociation or autoionization rates (l/rnr 1012 to 1014s-1) are much faster than the fluorescence rate (l/rr < 108s-1). Only two examples of detected spontaneous fluorescence from superexcited states have been reported (for H2, Glass-Maujean, et ai, 1987, for Li2, Chu and Wu, 1988). The H2 D1 e-symmetry component is predissociated by an L-uncoupling interaction with the B 1B+ state (see Section 7.9 and Fig. 7.27). Since a 4E+ state has no /-symmetry levels, the /-components of the D1 A-doublets cannot interact with the B E+ state and are not predissociated. The v = 8 level of the D1 state, which lies just above the H/ X2E+ v+ = 0 ionization threshold, could in principle be autoionized (both e and / components) by the X2E+ v+ = 0 en continuum. However, the Av = 1 propensity rule for vibrational autoionization implies that the v = 8 level will be only weakly autoionized. Consequently, the nonradiative decay rate, 1 /rnr, is slow only for the /-symmetry component of the D1 v = 8 state. Thus, in the LIF spectrum of the D1] —... 

If we suppose that these Rydberg states have non-negligible lifetimes against autoionization, as could occur via dispersion of the potential energy of the molecule into various vibrational modes, then these states would be very sensitive to electric fields in the measuring apparatus. It is suggested here that this is the reason for the large discrepancies between cross sections measured in the various beam experiments. 

Photoelectron ejection from ArH in nonpolar solvents has been interpreted in terms of autoionizing excited states with x = 10 " sec, which can also be internally converted to the Do states [34,35,136,137]. It is assumed that t-Sf in the vibrational excited state ((t-St ) ) undergoes vibrational relaxation on the time scale of 10 " sec to yield t-St , which has a long enough lifetime v = k/y to be quenched by Bp via ET involving (t-St/Bp )soiv (Scheme 13). = 0.06 0.02 suggests that (t-St ) undergoes photoelectron ejection and... 

Some of the earliest applications of MQDT dealt with vibrational and rotational autoionization in H2 [21-25]. One concept that emerged from these studies is that of complex resonances [26], which are characterized by a broad resonant distribution of photoionization intensity with an associated rather sharp fine structure. These complex resonances cannot be characterized by a single decay width they are the typical result of a multichannel situation where several closed and open channels are mutually coupled. The photoionization spectrum of H2 affords a considerable number of such complex resonances. 

The H2 molecule is a system for which quite recently it has been possible to measure in unprecedented detail state-selected vibrationally and rotation-ally resolved photoionization cross sections in the presence of autoionization [27-29]. The technique employed has been resonantly enhanced multiphoton ionization. The theoretical approach sketched above has been used to calculate these experiments from first principles [30], and it has thus been possible to give a purely theoretical account of a process involving a chemical transformation in a situation where a considerable number of bound levels is embedded in an ensemble of continua that are also coupled to one another. The agreement between experiment and theory is quite good, with regard to both the relative magnitudes of the partial cross sections and the spectral profiles, which are quite different depending on the final vibrational rotational state of the ion. 

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