Nonadiabatic interaction effects on population transfer in H-2 by stimulated Raman transition with partially overlapping laser pulses

We have theoretically investigated the population transfer in a four-level H-2, system by stimulated Raman transition from the ground X (1)Sigma(g)(+) (v(g) = 0,J(g) = 0) level to higher rovibrational levels ( v(f),J(f)) of th e X (1)Sigma(g)(+) State via the excited intermediate B (1)Sigma(u)(+)(v(i) = 14,J(i) = 1) and C (1)Pi(u)(+)(v(i) = 3,J(i) = 1) levels coupled with ea ch other by nonadiabatic interaction, using time-dependent overlapping pump and Stokes laser fields. The density-matrix treatment, which permits the c onvenient inclusion of the spontaneous emissions from the intermediate leve ls, has been employed to describe the dynamics of the two-photon Raman reso nance process. The present study performs the calculations of final populat ions (after both the pulses are over) of the ground and terminal levels for e-branch (J(f) 0) fundamental (v(f) = 1) and first overtone (v(f) = 2) tra nsitions and the S-branch (J(f) 2) fundamental (v(f) 1) transition as a fun ction of time delay between the two pulses for the cases of on-resonance as well as off-resonance excitations in a wide range (2 X 10(5) -2 x 10(7) W/ cm(2)) of peak intensities I-p(0), (I-S(0)) of the pump (Stokes) fields. Bo th fields are assumed to have the same temporal shape, duration, peak inten sities, and linear parallel polarizations. The accurate values of spontaneo us radiative relaxation rates of the intermediate levels to the initial and final levels, taking into account their J and M dependence, are explicitly included in our calculations. The pulse width (full width at half maximum) tau(p), is taken as 170 ns so that total spontaneous decay can occur durin g the pulse duration. The transfer efficiency is found to be very sensitive to the peak intensities of the laser pulses in each case of transition con sidered. Special attention is paid to the effects of the nonadiabatic (NA) interaction between B(14,1) and C(3,1) levels on population transfer effici ency. Calculations are also done in some particular cases using the adiabat ic Born-Oppenheimer (ABO) approximation. The results with ABO approximation are found to differ remarkably from those obtained including NA interactio n. Our calculations for the four-level H2 system reveal that almost complet e population is transferred in counterintuitive pulse order for both on-res onance and off-resonance excitations with intermediate and high values of I -p(0), (I-S(0)). For intuitive pulse sequence also a large population trans fer is achieved for on-resonance excitation at intermediate values of I-p(0 ), (I-S(0)) and for off-resonance excitation at intermediate and high value s of I-p(0), (I-S(0),). [S1050-2947(99)02205-2].