Laser-initiated Coulomb explosion imaging of small molecules

Abstract

Momentum vectors of fragment ions produced by the Coulomb explosion of CO2z+ (z = 3 - 6) and CS2z+ (z = 3 - 13) in an intense laser field (~50 fs, 1 x 1015 W/cm2) are determined by the triple coincidence imaging technique. The molecular structure from symmetric and asymmetric explosion channels is reconstructed from the measured momentum vectors using a novel simplex algorithm that can be extended to study larger molecules. Physical parameters such as bend angle and bond lengths are extracted from the data and are qualitatively described using an enhanced ionization model that predicts the laser intensity required for ionization as a function of bond length using classical, over the barrier arguments.

As a way of going beyond the classical model, molecular ionization is examined using a quantum-mechanical, wave function modified ADK method. The ADK model is used to calculate the ionization rates of H2, N2, and CO2 as a function of initial vibrational level of the molecules. A strong increase in the ionization rate, with vibrational level, is found for H2, while N2 and CO2 show a lesser increase. The prospects for using ionization rates as a diagnostic for vibrational level population are assessed.