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 μs. Two classes of such states are found. Delayed extraction following double-resonant excitation field-ionizes Rydberg states of very high principal quantum number (n≥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 (${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{1/2}^{}{}_{}{}^{}$ threshold and excited rotational limits of the ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{3/2}^{}{}_{}{}^{}$ ground state of ${\mathrm{HCl}}^{+}$.