Numerical Simulations of femtosecond-laser-induced dynamic alignment of molecules in the high-frequency off-resonance regime

Abstract

The motion equation for ϑ between the molecular axis and laser polarization direction in a high-frequency off-resonance femtosecond laser field is deduced while simultaneously examining the effects of a permanent dipole moment and field-induced polarizability and hyperpolarizability to molecular rotation. Femtosecond-laser-induced dynamic alignment of CO, N2, and Br2 molecules are investigated by numerically solving the obtained rotation equation for the angle ϑ. The effects of the molecular permanent dipole moment and the field-induced polarizability and hyperpolarizability on the degree of alignment are presented at different intensities. Our computational results show that the dynamic alignment of molecules is primarily determined by field-induced polarizability and the second hyperpolarizability for the laser intensity range from 5 × 1014 to 5 × 1016 W/cm2. The contributions of higher order correction terms to molecular alignment can usually be neglected. The polarizability-field interaction makes the angular distributions of a molecule have a maximum along the polarization axis and a minimum perpendicular to it. The role of the second hyperpolarizability keeps the molecular counts maximum along the laser polarization direction but minimum at an angle of 45° between the molecular axis and the polarization direction. There is also a second maximum of molecular counts perpendicular to the polarization axis. For CO, N2, and Br2 molecules, the dependences of laser-induced dynamic alignment on laser intensity exhibit completely different characteristics.