Field-free alignment of triatomic molecules controlled by a slow turn-on and rapid turn-off shaped laser pulse

Abstract

The field-free alignment of XCN (X = F, Cl, Br, I) molecules steered by a slow turn-on and rapid turn-off shaped laser pulse is theoretically investigated by using the density matrix theory. Based on the interactions of permanent dipole, polarizability and hyperpolarizability, the results of relatively accurate molecular alignment are obtained in an non-adiabatic regime, and the alignment differences among the molecules are compared. It is shown that the maximum degree of molecular alignment can be obviously improved when the rising time of the shaped laser pulse is adjusted. It is found that the N = 1 shaped pulse has an advantage over the Gaussian or super-Gaussian shaped pulse in enhancing the maximum degree of field-free alignment. Within a certain range, the optimal alignment can be achieved when the pulse amplitude takes a smaller value for FCN, ClCN and BrCN, while the pulse amplitude must take a larger value for ICN. Finally, we analyse the variation of molecular alignment by changing the rotational temperature and find that the large permanent dipole moment and polarizability, and the small hyperpolarizability for those molecules, are more conducive to create a high degree of molecular alignment at the same temperature. Abbreviations: PACS 33.80.-b: Photon interactions with molecules; PACS 33.15.Vb: Correlation times in molecular dynamics; PACS 33.15.Kr: Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility