Rydberg levels field ionization

Field ionization is a very efficient method to detect excited atoms or molecules. If the energy of the excited Rydberg level lies above the effective ionization potential IP = Ry/n every Rydberg level is ionized. Field ionization is therefore often used to detect the population of highly excited states. 

Fig. 5.23 Field ionization of atomic Rydberg levels (a) ionization rate of the Na (31 S) level (b) dependence of the threshold field Ec on the effective principal quantum number for NaC ) Rydberg levels [557]...

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. 

Figure 6. Rydberg series in neptunium obtained by field ionization (double arrow) of laser-excited levels. The excitation scheme is shown on the figure. The spectrum contains two series, one converging to the 5f+6d7s7L5 ground state and the other convering to the % level at 24.27 cm 1 in the ion. For the series converging to the 5li level, the Rydberg level positions with n less than 40.7 (short markers) are calculated using a constant fractional defect of 0.3 (4).

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. 5.22 Level scheme for two-step excitation of Rydberg levels of alkali atoms (a) and Rydberg series Na 3 3/2 measured by field ionization of the Rydberg states (b) [557]...

On the other hand, the large dipole moment of Rydberg atoms offers the possibility to use them as sensitive detectors for microwave and submillimeter-wave radiation [566]. For the detection of radiation with frequency to, a Rydberg level n) is selectively excited by stepwise excitation with lasers in an external electric dc field. The field strength is adjusted in such a way that the energy En of the Rydberg level n) is just below the critical value Eq for field ionization, but E -i- hco is just above. Every absorbed microwave photon tuo then produces an ion that can be detected with 100 % efficiency (Fig. 5.26). 

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-... 

The Stark splitting and the field ionization of very high Rydberg levels provide sensitive indicators for measuring weak electric fields. These few examples demonstrate the variety of information obtained from Rydberg state spectroscopy. More examples can be found in Sect. 9.5 and in the literature on laser spectroscopy of Rydberg states [593-597]. 

An alternative to PFI-ZEKE would be mass-analyzed threshold ionization (MATI), where cations rather than ZEKE electrons are detected.In principle, MATI is attractive because of the inherent mass selection. However, MATI experiments are difficult to implement because of ion separation fields affecting the high Rydberg levels required for delayed pulsed field ionizations. The technique has so far only been used for the smallest metal molecules. ... 

In the case of measurements at a cavity temperature of 2K [15], a reduction of the signal could be clearly seen for atomic fluxes as small as 800 atoms/s. An increase in flux caused power broadening and finally an asymmetry and a small shift (Fig. A). This shift is attributed to the ac Stark effect, caused predominantly by virtual transitions to the 6ld level, which is only 50MHz away from the maser transition (Fig. 3). the fact that the field ionization signal at resonance is independent of the particle flux (between 800 and 22 x 10 atoms/s) indicates that the transition is saturated. This, and the observed power broadening show that there is a multiple exchange of photons between Rydberg atoms and the cavity field. 

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