Angle-dependence of strong-field ionization of singly-charged Chloromethane and Bromomethane

We have studied the ionization probability of CH$_3$Cl and CH$_3$Br molecules exposed to intense 780 nm laser pulses as a function of the angle between the molecular axis and the linear laser polarization. Experimentally, the molecules are exposed to two laser pulses. The first induces no ionization but, instead, creates a rotational wave packet within each molecule that exhibits preferential alignment in the laboratory frame at specific time delays. We measure the variation in the single ionization yield as a function of the delay between the two pulses. We obtain the angular dependent ionization probability by fitting the observed delay-dependent yield to moments of the angular distribution of the rotational wavepacket which can be accurately calculated. The experimentally determined angular distributions are compared to results of new Time-Dependent Density Functional Theory (TD-DFT) predictions. Both experiment and theory find that even though the molecules have Highest Occupied Molecular Orbitals (HOMO-s) that are very similar, the angle dependence of their ionization yields differ substantially.