Title: Angle-dependent Strong-Field Ionization of Halomethanes

Péter Sándor [1], Adonay Sissay [2], François Mauger [3], Mark W. Gordon [1], Timothy T. Gorman [4], Timothy D. Scarborough [4], Mette B. Gaarde [3], Kenneth Lopata [2,5], Kenneth J. Schafer [3], and Robert R. Jones [1]

[1] Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA

[2] Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA

[3] Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA

[4] Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA

[5] Center for Computation and Technology, Louisiana State University, Baton Rouge, Louisiana 70808, US

Abstract:

We study, experimentally and theoretically, the ionization probability of singly halogenated methane molecules, CH$_3$Cl and CH$_3$Br, in intense linearly-polarized 800 nm laser pulses as a function of the angle between the molecular axis and the laser polarization. Experimentally, the molecules are exposed to two laser pulses with a relative time delay. The first, weaker pulse induces a nuclear rotational wave packet within the molecules, which are then ionized by the second, stronger pulse. The angle-dependent ionization yields are extracted from fits of the measured delay-dependent ionization signal to a superposition of moments of the rotational wave packet's angular distribution. Angle-dependent strong-field ionization (SFI) yields are also calculated using time-dependent density functional theory. Good agreement between measurements and theory is obtained. Interestingly, we find a marked difference between the angle-dependence of the ionization yields for these two halomethane species despite the similar structure of their highest occupied molecular orbitals. Calculations reveal that these differences are a result of multi-channel (CH3Cl) versus single-channel (CH3Br) ionization, and increased hole localization on Br vs Cl. By adding calculations for CH3F we can discern clear trends in the ionization dynamics with increasing halogen mass. These results are illustrative, as chemical functionalization and molecular alignment are likely to be important parameters for initiating and controlling charge migration dynamics via SFI.

Status: Published, J. Chem. Phys. 151, 194308 (2019).