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

Abstract

We theoretically investigate the creation of the rotational wave packet in molecular ions upon the strong-field ionization of diatomic molecules by a femtosecond laser pulse. The rotational excitation of molecular cation is ascribed to the dependence of the strong-field ionization probability on the orientation angle of the molecular axis with respect to the laser polarization direction. By extending the molecular strong-field approximation (MO-SFA) theory, we calculate the rovibrational state distribution of the ground electronic X1Σ+ state of NO+ after the ionization of NO in the electronic ground X2Π1/2 state. We also show that the extent of the rotational excitation of NO+ is enhanced by the stimulated impulsive Raman processes both in neutral NO and in NO+ cation. The resultant time evolution of the rotational wave packet of NO+ is in good agreement with the delay dependent NO2+ yield recorded experimentally in our previous pump-probe study.