Abstract This paper studies the molecular rotational excitation and field-free spatial alignment in a nonresonant intense laser field numerically and analytically by using the time-dependent Schr?dinger equation. The broad rotational wave packets excited by the femtosecond pulse are defined in conjugate angle space, and their coefficients are obtained by solving a set of coupled linear equations. Both single molecule orientation angles and an ensemble of O2 and CO molecule angular distributions are calculated in detail. The numerical results show that, for single molecule highest occupied molecular orbital (HOMO) symmetry $\sigma$ tends to have a molecular orientation along the laser polarization direction and the permanent dipole moment diminishes the mean of the orientation angles; for an ensemble of molecules, angular distributions provide more complex and additional information at times where there are no revivals in the single molecule plot. In particular, at the revival peak instant, with the increase of temperature of the molecular ensemble, the anisotropic angular distributions with respect to the laser polarization direction of the $\pi$g orbital gradually transform to the symmetrical distributions regarding the laser polarization vector and for two HOMO configurations angular distributions of all directions are confined within a smaller angle when the temperature of the molecular ensemble is higher.
Received: 12 November 2008
Revised: 23 December 2008
Accepted manuscript online:
(Ultrafast processes; optical pulse generation and pulse compression)
Fund: Project supported by
the Program of Excellent Team in Harbin Institute of Technology, the
Program for New Century Excellent Talents in University (NCET), and
National Natural Science
Foundation of China (Grant Nos 10774033, 60878018 and 10674036).
Cite this article:
Chen De-Ying(陈德应), Wang Yu-Quan(王玉铨), Xia Yuan-Qin(夏元钦), Fan Rong-Wei(樊荣伟), and Zhang Sheng(张盛) Role of highest occupied molecular orbitals in molecular field-free alignment by a femtosecond pulse 2009 Chin. Phys. B 18 3850
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