Control of population transfer in a multilevel Li₂ molecule by stimulated hyper-Raman nonadiabatic passage with chirped laser pulses

We have theoretically studied the population transfer in a five-level Li₂ dimer by (2+2)-photon stimulated hyper-Raman nonadiabatic passage (STIHRNAP) from the initial ground level v_g=0, J_g=0 to the final rovibrational levels v_f=1 (2), J_f=0,2, of the ground electronic state X ¹Σ_g⁺ via the resonant intermediate levels v_i=1, J_i=0,2, of the excited electronic state 2 ¹Σ_g⁺. Linearly chirped pump and Stokes laser pulses with different chirp rates and without any initial detuning are applied simultaneously. Both the pulses are taken to have the same temporal shape, pulse width, and linear parallel polarizations. The density matrix method has been used to compute the populations of the levels. We have investigated in detail the population transfer for laser wavelengths (at time t=0) in the range of 992–1028 nm and (peak) intensities in the range of 1.5×10¹⁰–3.0×10¹¹ W∕cm². The required pulse widths are of the order of 35–275 ps for maximum population inversion. We have controlled rotational branching in population transfer to the final rotational levels J_f=0 (Q branch) and J_f=2 (S branch) of the fundamental (v_f=1) and first overtone (v_f=2) transitions by judicious choice of laser parameters. We have applied a (2+2)-photon STIHRNAP process to a model multilevel molecular system (Li₂) and achieved almost complete population transfer from the initial ground to the final target rovibrational levels with chirped ir laser pulses.