High-resolution pulsed field ionization study of high Rydberg states of benzene

The influence of dc electric fields and ion concentrations on the long-term stability of high Rydberg states of benzene has been studied by delayed pulsed field ionization and millimeter wave spectroscopy following resonance-enhanced two-photon excitation via selected rotational levels of the \S(1)6(1)> intermediate state. The long-term stability, which is probed by applying pulsed electric fields at delay times of more than 1 mus after photoexcitation, originates from Stark mixing of weakly penetrating series, presumably g series, with quantum defect of approximately 0.003 with nonpenetrating high-l states. The pulsed field ionization yield is strongly influenced by the experimental conditions and is characterized by a marked dependence on the principal quantum number n. At n values beyond 100, the weak stray electric fields in the apparatus are sufficient to stabilize an important fraction of the optically accessible Rydberg states by Stark mixing. In the range n = 40-80, long-term stability beyond 1 mus cannot be achieved by Stark mixing induced by homogeneous dc electric fields but can be attained by generating a sufficient concentration of ions in the experimental volume. A simple rule (c/(ions/cm(3)) greater than or equal to (9.14 x 10(15)\mu(i)\/n(5))(3/2)) is derived that relates the minimum ion concentration required to induce Ion,--term stability to the principal quantum number and the quantum defect of the optically accessible Rydberg states. Below n = 40 long-term stability cannot be induced unless very large ion concentrations beyond 10(8) ions/cm(3) are generated. Spectra of Rydberg states in the range n = 27-40 can nevertheless be recorded at high laser fluence by monitoring the fragmentation products that result from the absorption of one or more photons by the Rydberg molecules. The results enable a quantitative discussion of important features of the new technique of cross-correlation ionization energy spectroscopy (CRIES) recently developed by Neuhauser et al., (J. Chem. Phys. 106 (1997) 896). In particular, we show that (1) the technique only works because of Stark mixing of at least one of the optically accessible series with nonpenetrating high-l states induced by weak electric fields, (2) this series must possess a quantum defect significantly smaller than 0.1 and an orbital angular momentum quantum number l greater than or equal to 3, (3) significant n-dependent energy shifts in the position of the Rydberg states induced by electric fields can introduce systematic errors in the extrapolation of the series limits. (C) 2001 Elsevier Science B.V. All rights reserved.