Rydberg levels 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). 

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. 

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...

A very efficient photoionization process is the excitation of high-lying Rydberg levels above the ionization limit (Fig. 1.36b), which decay by autoionization into lower levels of the ion M+... 

With two visible lasers, levels m) with excitation energies up to 6 eV can be reached. Optical frequency doubling of both lasers allows even the population of levels up to 12 eV. This makes the Rydberg levels of most atoms and molecules accessible to detailed investigations. The population of Rydberg levels of species M can be monitored either by their fluorescence or by detecting the ions or the electrons e that are produced by photoionization, field ionization, collisional ionization, or autoionization of the Rydberg levels. 

Fig. 5.29 Autoionization of molecular Rydberg levels ( , u, / ) above the energy of the lowest level of the molecular ion M+...

A common experimental arrangement for the study of molecular Rydberg states is depicted in Fig. 5.31. The output beams of two pulsed narrow-band dye lasers, pumped by the same excimer laser, are superimposed and cross the molecular beam perpendicularly. The fluorescence emitted from the intermediate level (u, J ) or from the Rydberg levels (u, 7 ) can be monitored by a photomultiplier. The ions produced by autoionization (or for levels slightly below IP by field ionization) are extracted by an electric field and are accelerated onto an ion multiplier or channel plate. This allows the detection of single ions. In order to avoid electric Stark shifts of the Rydberg levels during their excitation, the extraction field is switched on only after the end of the laser pulse. Experimental details and more infor-... 

Often it is possible to tune the ionizing laser L2 to transitions from k) into autoionizing Rydberg levels (Sect. 10.4). For such transitions the probability may be from two to three orders of magnitude larger than for bound-free tran-... 

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