Threshold-field-ionization photoelectron spectroscopy and delayed forced autoionization of HCl.

Abstract

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 \ensuremath{\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\ensuremath{\ge}100) that converge to successive rotational limits for producing ${\mathrm{HCl}}^{+}$, 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 lower-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 $^{2}\mathrm{\ensuremath{\Pi}}_{1/2}$ threshold and excited rotational limits of the $^{2}\mathrm{\ensuremath{\Pi}}_{3/2}$ ground state of ${\mathrm{HCl}}^{+}$.