Resonance-enhanced multiphoton ionization-photoelectron spectra of CO2. I. Photoabsorption above the ionization potential

Abstract

Photoabsorption above the first ionization potential of CO2 was observed at relatively low laser intensity, detected via resonant-enhanced multiphoton ionization-photoelectron spectra through several Rydberg states. This phenomenon can be explained by the presence of accidental resonances with long-lived autoionizing states which make photon absorption within the ionization continuum possible. Laser powers are too low for this to be explained in terms of a ponderomotive potential and conventional above-threshold ionization. This resonance-enhanced above-threshold absorption phenomenon is potentially useful in the study of excited and superexcited states. Photoelectron energies can be assigned to terminations on CO+2 ionic states at both the four- and five-photon levels, allowing measurement of states up to 22 eV. Two unassigned bands may represent terminations on a new state of CO+2, with an ionization potential of 21.4 eV.