Strong-field ionization of the triplet ground state of O2

Abstract

Using strong-field ionization as a probe, we observe highly nonperiodic evolution of the spin-rotation wave packet launched by a nonionizing femtosecond pulse in oxygen. The nonperiodicity is readily apparent only in rotationally cold molecules that are pumped with a weak alignment pulse. We show that this behavior is a consequence of the spin-rotation and the spin-spin couplings in the triplet ground state of the neutral molecule. A model that includes these couplings in the field-free Hamiltonian but in neither the alignment nor the ionization step explains most of the observed dynamics, suggesting that neither process depends explicitly on the electronic spin. We also show that the angle dependence of strong-field ionization can be retrieved from the delay-dependent signal even when coupling to spin complicates the rotational dynamics.

Rotational wave packet of NO+ created upon strong-field ionization of NO

“…Although the electronic ground state of neutral species, such as H 2 , N 2 and CO, is singlet and its rotational energy structure is simple, the electronic ground state of the cation is doublet and its rotational energy structure becomes complex due to the coupling between rotational and electronic orbital/spin angular momenta. In the case of O 2 , the electronic ground state of the neutral is X 3 Σ − g and its rotational energy structure is expressed by the Hund's case (b) [48,49]. On the other hand, the electronic ground state of O + 2 is X 2 Π g and its rotational energy structure is similar to the neutral NO case.…”