Rotation, autoionization

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. 

In rotational autoionization, the coupling matrix element is J-dependent and has the same origin as the coupling that occurs for heterogeneous rotational interactions where the selection rule is AO = 1 (cf. Section 3.5.3). Rotational autoionization has been studied, especially in the H2 molecule (Herzberg and Jungen, 1972 Jungen and Dill, 1980). 

Figure 8.11 Schematic illustration of rotational autoionization in para-H2 showing the transition from case (b) to case (d). The electronic continua associated with X2Ej (u+ = 0, N+ = 0 and 2) are depicted by light horizontal lines. The stable and autoionizing levels of the H2 np Rydberg series with v = 0 and J = 1 are shown as heavy horizontal lines.

Thus, it is possible to view rotational autoionization as arising from the departure of the molecular-ion core from spherical symmetry (an a ), which causes the np complex to split into II and E levels. Figure 8.11 illustrates rotational autoionization in H2. The right side of Fig. 8.12 displays lines of the v = 0, J = 0 series converging to the N+ = 2 limit of the ion (called np2) that are autoionized by the continuum of X2E+(u+ = 0,iV+ = 0) resulting in the appearance of emission windows (see Section 8.9). 

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 8.21 ZEKE-PFI spectrum resulting from ionization via the P(3) rotational transitions associated with the F1 A2(u" = 0, J" = 2e) state of HC1. Upper panel observed (de Beer, et al., 1993) middle panel calculated for direct ionization lower panel calculated taking into account the population decay by spin-orbit and rotational autoionization after a delay of 200 ns (from Lefebvre-Brion, 1996)...

Those Rydberg states converging to the J+ =3/2 first rotational level of 2n3/2 cannot decay because all autoionization channels are closed. Those states converging to the other rotational levels of 2n3/2 can decay in the lower J+ levels of 2n3/2 only by what has improperly been called rotational autoionization (improper because it is due in part to spin-orbit interaction), with lifetimes for the d complex (n = 600) between 100 ns and 10-6 s. However, the high-n Rydberg levels converging to 2n1/2 rotational levels can decay into the rotational levels of both 2n3/2 and 2Hi/2 and their lifetimes are shorter than 10 6 s. 

Many experimental and theoretical studies have been published on this subject, for example the decay kinetics of the ZEKE intensity due to rotational autoionization in NO (Remacle and Vrakking, 1998). 

Simons J 1989 Modified rotationally adiabatic model for rotational autoionization of dipole-bound molecular anion J. 

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