THRESHOLD-FIELD-IONIZATION PHOTOELECTRON-SPECTROSCOPY AND DELAYED FORCED AUTOIONIZATION OF HCL

A time-of-flight photoelectron spectrometer is oriented coaxially with a pulsed molecular beam to detect high Rydberg states of HCl with lifetimes as long as 7-mu-s. Two classes of such states are found. Delayed extraction following double-resonant excitation field-ionizes Rydberg states of very high principal quantum number (n greater-than-or-equal-to 100) that converge to successive rotational limits for producing HCl+, effectively yielding a rotationally resolved threshold photoelectron spectrum. Surprisingly, excitation under more rigorously field-free conditions (< 100 mV/cm) produces additional resonances which can be assigned to series of long-lived Rydberg states extending to principal quantum numbers as low as 10. The low-principal-quantum-number resonances isolated by their long field-free lifetimes appear to constitute a high-angular-momentum nonpenetrating subset of the autoionizing Rydberg series that converge both to accessible rotational levels of the upper 2-PI-1/2 threshold and excited rotational limits of the 2-PI-3/2 ground state of HCl+.