The main impetus to the development of strong field physics is measuring and understanding the electronic structure and dynamics of matter on its natural time scale. When measuring the electronic structure and dynamics for molecules, the pre-alignment of the target molecules is required. This can be adiabatically or non-adiabatically achieved by using moderately intense laser pulses. Generally, the non-adiabatic alignment is preferred because it can produce macroscopic ensembles of highly aligned molecules under the field-free condition. During the past decades, the fundamental behavior and dynamics of the field-free alignment have been extensively studied using the Coulomb explosion imaging and polarization spectroscopy. These investigations are motivated by several intriguing applications including the use of alignment revivals to compress laser pulse and the phase control of rotational wave packets. Recently it was demonstrated that the revival structure of non-adiabatic alignment of molecules can also be observed via the high-order harmonic generation (HHG). Therein, the evolution of the alignment parameter is imprinted in the change of the harmonic yield with the delay between the harmonic generating pulse and the alignment pulse. The modulation due to the rotational dynamics, however, is influenced by the symmetry of the molecules and the quantum interference in the HHG. This may complicate the extraction of the information about the rotational dynamics.
The Ultrafast Laser Group directed by Prof. Peixiang Lu of Wuhan National Laboratory for Optoelectronics deeply investigated the probe of rotational wave-packet dynamics via the high-order harmonic generation. In their recent work, they investigated the alignment-dependent spectral minimum in HHG from non-adiabatically aligned molecules around the first half rotational revival. It is found that for the molecular orbitals contributed by two atomic centers, the structural minimum position depends linearly on the inverse of the alignment parameter 1/<cos2 θ>. Furthermore, the linear coefficients are independent on the alignment and probe pulses. As the molecular rotational dynamics is well characterized by the evolution of <cos2θ>, one can trace the molecular rotational dynamics through a simple formula with the measured spectral minimum.
This work was supported by the National Natural Science Foundation of China under Grants No. 11234004 and 61275126, the 973 Program of China under grant 2011CB808103, and the Doctoral fund of Ministry of Education of China under grant 20100142110047. It was published in Opt. Express Vol. 22, No. 6, 6362-6371 (2014).