Modulation of non-adiabatic alignment of N₂ molecule by femtosecond laser pulses

Abstract

The alignment of molecules in the femtosecond laser field induces the refractive index of the propagation medium to change, resulting in the spectral modulation effect. In this paper, the time evolution of the wavelength offset of a probe laser pulse under the influence of the non-adiabatic molecular alignment and Kerr effect is measured experimentally by the pump-probe method in nitrogen medium, and the alignment degree of N₂ molecules is obtained. By solving the time-dependent Schrödinger equation theoretically, the expression of the degree of molecular non-adiabatic alignment is obtained, and the time evolution of non-adiabatic alignment of N₂ molecules is calculated. Taking into account the combined influence of the non-adiabatic molecular alignment and Kerr effect on the change of refractive index of the propagation medium, the modulation effect of birefringence on the spectrum of the probe pulse is achieved. The experimental result accords well with the theoretical calculation, demonstrating that the spectral modulation results obtained by experimental measurement can be used to characterize the alignment degree. Further, the double pulse pump method is used to control the degree of molecular alignment. It is found that the degree of molecular alignment can be enhanced by the double pulse pump method. Moreover, by adjusting the delay time between the two pump laser pulses, that is, adding the second pump laser pulse at one rotational period and half rotational period, respectively, the enhancement and loss of the alignment of N₂ molecules can be achieved, which is named the “alignment switch” effect. The molecular alignment control induced by the double pulse pump method can also be applied to the other molecular systems with different alignment and anti-alignment times, such as CO₂ molecules and O₂ molecules, indicating that the double pulse pump method can be used universally.