Vibrationally cold CO2+ probed by intense femtosecond laser pulses

H2 seems the ideal candidate to study ultrashort intense laser-molecule interactions since its simple one-electron structure allows it to be treated accurately by theory. This is aided by the fact that at low intensity (<5×1013 W/cm2) it may be approximated as a two electronic state system. However, as recently pointed out by Posthumus et al. [1], vibrational excitation of H2 , with the population often unknown, can severely complicate its dynamics in an intense laser field. This makes experimental observation, and study, of important laser-induced phenomena such as bond-softening and above-threshold dissociation (ATD) more difficult and confusing. Ion traps are currently used to vibrationally cool H2 [2]. Alternatively, we present novel measurements of intense field dissociation of vibrationally cold CO2+, which is in many respects a similar target to cold H2 . Our method involves production of CO2+ ions, by electron-impact in an ion source, that predissociate during their travel (∼20 μs transport time) to the laser interaction region — leaving a pure target of v=0 ground state CO2+ molecules. Laser-induced fragmentation of CO2+ is measured by 3D momentum imaging of the C+ and O+ fragments detected in coincidence [3]. Specifically, we demonstrate that cold CO2+, like H2 , can be considered as a two-state system at low intensity (involving only its lowest triplet states). This allows some of the important theoretical foundations developed for H2 to be applied and tested on this more complex multielectron system. Work along these lines will be presented in this poster. Moreover, as the CO2+ initial state is well-defined, we observe sharp peaks in the kinetic energy release from dissociation [Fig. 1(a)], attributable to oneand two-photon processes, measured for both 790 nm and 395 nm wavelengths. Interestingly, unlike H2 where dissociation occurs for molecules predominantly aligned to the laser polarization [Fig. 1(c)], CO2+ displays the opposite behavior, showing a preference to be aligned perpendicular to the laser polarization [Fig. 1(b)] due to the nature of the dominant dissociative transition.