Dominance of parent ion formation in the ultrafast, intense-field ionization of the valence isoelectronic molecules CO2 and CS2

Abstract

Abstract We report on the photodynamics of the valence isoelectronic molecules carbon dioxide ( C O 2 ) and carbon disulfide ( C S 2 ) under intense-field, single-molecule conditions in focused 50 fs, 800 nm laser pulses. Ion mass spectra are recorded as a function of intensity in the range from 8 × 10 12 to 9 × 10 13 W c m − 2 . Ion yields are measured in the absence of the focal volume effect without need for data deconvolution. In both targets, formation of the molecular parent ion ( M + ) is by far the dominant process; the yield of other species is only a few percent. We observe that the yield of this parent ion is strictly proportional to an integer power of the intensity, until saturation sets in. We conclude the parent ion is formed through multiphoton single-electron ionization, with the photoelectron originating from the doubly degenerate, non-bonding highest-occupied molecular orbital (HOMO). The non-bonding nature of this HOMO explains the remarkable stability of the parent ion.