Rotational dynamics of O2 in the electronic ground XΣg−3 state induced by an intense femtosecond laser field

We theoretically and numerically investigate rotational dynamics of ${\mathrm{O}}_{2}$ in the electronic $X^{3}\mathrm{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}$ ground state induced by an intense femtosecond laser field. The rotational dynamics are calculated by two different models. One of the models includes triplet splittings explicitly in rotational energy levels originated from an electronic spin (a spin-dependent model), and the other omits the triplet splittings (a spin-independent model). We find that the final rotational population after the interaction with the laser field does not depend on the models. On the other hand, we find that the rotational dynamics evaluated by alignment degrees and angular distributions of molecular axes depend on the models. Although the rotational dynamics calculated by the spin-independent model is similar to that calculated by the spin-dependent model within 5 ps, the triplet splittings in the rotational energy levels affect the rotational dynamics after 5 ps. This is explained by the inverse of the energy difference between triplet splittings in the rotational levels. We show that the multiplet energy splittings in rotational energies should be included when the rotational dynamics in the multiplet electronic state are considered. We also investigate excitation processes on the basis of time evolutions of state populations and find that there are excitation pathways with double as well as single bifurcations depending on the initial state.