Autoionizing levels Rydberg

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.

Fig. 1.36 Level schemes of ionization spectroscopy (a) photoionization (b) excitation of autoion-izing Rydberg levels (c) two-photon ionization of excited molecules (d) one-photon ionization of a high lying level, excited by non-resonant two-photon process (e) three-photon excitation of a level which is ionized by a fourth photon (f) non-resonant two-photon ionization...

Rydberg energy levels Even-parity autoionizing levels... 

Ba Rydberg energy levels (—) and continua (///) obtained by adding the second valence electron to these Ba+ levels. The horizontal arrows show the possible interactions between channels associated with different ion levels. Interactions with other bound states lead to series perturbations while interactions with continua lead to autoionization. 

The first and/or second dye lasers were tuned to the specific wavelength(s) to populate the desired level(s). The final laser in the excitation sequence (either the second or third laser) was then continuously scanned to obtain the Rydberg or autoionization spectrum. The spectrum and wavelength calibrations were recorded simultaneously on a two pen recorder. Wavelength calibration was obtained by directing a portion of the scan laser radiation to a monochromator that was preset at known U or Th emission lines from an electrodless lamp. 

From these values, wavelength ranges to search for bound Rydberg series with field or collisional ionization or to search for autoionizing series converging to excited states of the ion were estimated for various parent levels that could be conveniently populated by one or two-step excitation. The threshold determinations reduced the search ranges for Rydberg levels to reasonable values. Scans were made from various parent levels until series were obtained. 

The use of autoionizing Rydberg levels converging to excited states of the ion to determine ionization potentials has been discussed above. If autoionization resonances as narrow as those found in gadolinium exist in the actinides, it should be possible to determine the isotope shifts and hfs of such features. (isotope shifts for actinides range up to 0.4 cm l per mass unit and odd atomic number actinides exhibit hfs with total widths of 4 to 6 cm l and hfs component spacing of 0.2 cm l or more for some transitions). 

These /-levels, in addition to being weakly predissociated, are also very weakly autoionized (see also Fig. 8.23). Another example appears in the spectrum of Li2 (Chu and Wu, 1988). In many other cases, decay by predissociation can be very fast, particularly for electrostatic predissociations (see Tables 7.3 and 7.4). For example, in the spectrum of the NO molecule, most 2Il Rydberg states are predissociated by the vibrational continuum of the B2n valence state so rapidly that autoionization cannot compete (Giusti-Suzor and Jungen, 1984). 

In general, complex features may arise in the absorption spectrum due to competition between the two processes, autoionization and predissociation (see Section 8.12). For example, as will be shown in Section 8.8, the partial width due to electronic autoionization is independent of v for a given nlX Rydberg state, but the partial width due to predissociation can vary considerably from one vibrational level to another because of oscillatory variations of the vibrational part of the interaction with the predissociating state. 

Electronic autoionization occurs between the nth Rydberg state converging to an excited electronic state of the ion in its v vibrational level and the continuum of a lower electronic state of the ion in its v+ vibrational level. In Eq. (8.4.7), the general expression for the width,... 

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. 

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