Rydberg levels molecular

The first approximation to the description of Rydberg levels treats the benzene ion-core as a monopole. This description is known not to be quantitatively accurate. Calculations which include the symmetry of the molecular ion, and the charge delocalization, lead to an energy level spectrum in much better agreement with experiment. Thus, it seems unlikely that the geometric structure of the molecular ion can be completely neglected in the study of photoionization. 

Ground state 02 and the chosen 3drr Rydberg level have the same 3Zg" symmetry but differ in their molecular orbital construction ... 

Sensitive and accurate measurements of atomic and molecular Rydberg levels have been performed [112-114] with thermionic diodes. With a special arrangement... 

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-... 

Fig. 5.31 Experimental arrangement for the measurement of molecular Rydberg levels with two-step excitation in a molecular beam [582]...

From the optically pumped atomic or molecular Rydberg levels neighboring levels can be reached by microwave transitions, as was mentioned above. This triple resonance (two-step laser excitation plus microwave) is a very accurate method to measure quantum defects, fine-structure splitting, and Zeeman and Stark splitting in Rydberg states [592]. 

Of particular importance for the spectroscopy of highly excited states, such as Rydberg levels of atoms and molecules, and for the assignment of complex molecular spectra are various double-resonance techniques where atoms and molecules are exposed simultaneously to two radiation fields resonant with two transitions sharing a common level. In combination with Doppler-free techniques, these double resonance methods are powerful tools for spectroscopy. Some of these methods, representing modem versions of optical pumping techniques of the prelaser era, are introduced in Chap. 10. 

Fig.6.28a-d. Level schemes of ionization spectroscopy (a) photoionization (b) excitation of autoionizing Rydberg levels (c) two-photon ionization of excited molecules (d) experimental arrangement for photoionization spectroscopy in a molecular beam... 

Sensitive and accurate measurements of atomic and molecular Rydberg levels have been performed [6.87-6.89] with thermionic diodes. With a special arrangement of the electrodes, a nearly field-free excitation zone can be realized that allows the measurement of Rydberg states up to the principal quantum numbers n = 300 [6.89] without noticeable Stark shifts. 

The essential point is now that the energy Ep(v, J ) of a molecular Rydberg level with vibrational quantum number v and rotational quantum number J lies above a level E(v ,J ) in the ion ground state, if its vibrational and rotational energy is sufficiently high, i.e.if... 

This auto-ionization process competes with the radiative decay of the Rydberg level, often also with possible predissociation. However, the spontaneous transition probability is approximately proportional to n 3. For large quantum numbers n the radiative decay therefore becomes very slow and even small auto-ionization probabilities may already result in complete auto-ionization of a Rydberg level if predissociation can be neglected. This means that the excitation of these molecular Rydberg levels above the ionization limit can be very effectively monitored through the resulting ion. 

Similar experiments on auto-ionization of molecular Rydberg levels have been performed on Naa and Kg by Martin et al.33 and by Leutwyler et al.34.35. / complete analysis of the auto-ionization spectra of all these alkali molecules is, however, still pending since a full understanding of all perturbations requires very detailed studies of the many molecular states closely below the ionization limit. These will not only be Rydberg states but also doubly excited states which can interact with the Rydberg states. For Lij such investigations are under way in our laboratory. 

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