Effects of polarizability and hyperpolarizability on molecular spatial alignment in intense light fields

Considering the influences of polarizability and hyperpolarizability on molecular spatial alignment, we newly deduce the classical equation of molecular motion for the internuclear distance r and for the angle θ between molecular axis and laser polarization direction in intense light fields. Numerical simulations of molecular dynamic alignment are performed and discussed at intensities of 1015-1017 W/cm2 and pulse durations of 100 fs and shorter than 100 fs. In our calculation, N2 and Cl2 molecules are selected as sample because they can represent the light molecule and heavy molecule, respectively. Our computational results show that the contributions of the molecular polarizability and hyperpolarizability to the degree of dynamic alignment all become strong when the laser intensity increases and the pulse duration narrows. And the role of hyperpolarizability on molecular dynamic alignment becomes comparable to that of linear polarizability term, even exceeds it in the conditions of high laser intensity and short pulse length. The characteristics of molecular angular distribution with only considering the effect of polarizability, second hyperpolarizability, and higher-order correction term of the hyperpolarizability, respectively, are very different. Their contributions to molecular dynamic alignment are systematically calculated and discussed in different laser intensities and pulse durations.