Role of Multi-Electron Effects in the Asymmetry of Strong-Field Ionization and Fragmentation of Polar Molecules: The Methyl Halide Series

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 119; no. 49; pp. 11772 - 11782
• Main Authors: Walt, Samuel G, Bhargava Ram, N, von Conta, Aaron, Tolstikhin, Oleg I, Madsen, Lars Bojer, Jensen, Frank, Wörner, Hans Jakob
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10.12.2015
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.5b07331

We report angle- and momentum-resolved measurements of the dissociative ionization and Coulomb explosion of methyl halides (CH3F, CH3Cl, CH3Br, and CH3I) in intense phase-controlled two-color laser fields. At moderate laser intensities, we find that the emission asymmetry of low-energy CH3 + fragments from the CH3 + + X+ (X = F, Cl, Br, or I) channel reflects the asymmetry of the highest occupied molecular orbital of the neutral molecule with important contributions from the Stark effect. This asymmetry is correctly predicted by the weak-field asymptotic theory, provided that the Stark effect on the ionization potentials is calculated using a nonperturbative multielectron approach. In the case of high laser intensities, we observe a reversal of the emission asymmetries for high-energy CH3 + fragments, originating from the dissociation of CH3X q+ with q ≥ 2. We propose ionization to electronically excited states to be at the origin of the reversed asymmetries. We also report the measurements of the emission asymmetry of H3 +, which is found to be identical to that of the low-energy CH3 + fragments measured at moderate laser intensities. All observed fragmentation channels are assigned with the help of CCSD­(T) calculations. Our results provide a benchmark for theories of strong-field processes and demonstrate the importance of multielectron effects in new aspects of the molecular response to intense laser fields.