title: Role of Multi-Electron Effects in the Asymmetry of Strong-Field Ionization and Fragmentation of Polar Molecules: The Methyl Halide Series.
abstract: 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.
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: Samuel G
authors.initials: SG
authors.lastname: Walt
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: N
authors.initials: N
authors.lastname: Bhargava Ram
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: Aaron
authors.initials: A
authors.lastname: von Conta
authors.affiliation: Moscow Institute of Physics and Technology , Dolgoprudny 141700, Russia.
authors.forename: Oleg I
authors.initials: OI
authors.lastname: Tolstikhin
authors.affiliation: Department of Physics and Astronomy, Aarhus University , DK-8000 Aarhus C, Denmark.
authors.forename: Lars Bojer
authors.initials: LB
authors.lastname: Madsen
authors.affiliation: Department of Chemistry, Aarhus University , DK-8000 Aarhus C, Denmark.
authors.forename: Frank
authors.initials: F
authors.lastname: Jensen
authors.affiliation: Laboratorium für Physikalische Chemie, ETH Zürich , 8093 Zürich, Switzerland.
authors.forename: Hans Jakob
authors.initials: HJ
authors.lastname: Wörner