Coherent control of vibrational population transfer in theLi2molecule with femtosecond laser pulses in the presence of manifolds of rotational levels

Abstract

We have theoretically investigated the vibrational population transfer in the presence of manifolds of rotational levels in a model 15-level ${\text{Li}}_{2}$ molecular system by stimulated Raman adiabatic passage with intense unchirped femtosecond laser pulses. We have considered five rotational levels ${J}_{g}=0,2,\dots{},8$ associated with the initial vibrational level ${v}_{g}=0$ of the ground electronic state $X\text{ }{^{1}\ensuremath{\Sigma}}_{g}^{+}$, five rotational levels ${J}_{i}=1,3,\dots{},9$ associated with the vibrational level ${v}_{i}=1$ of the intermediate excited electronic state $A\text{ }{^{1}\ensuremath{\Sigma}}_{u}^{+}$, and five rotational levels ${J}_{f}=0,2,\dots{},8$ associated with the final target vibrational levels ${v}_{f}=1,2$ of the $X\text{ }{^{1}\ensuremath{\Sigma}}_{g}^{+}$ state of the ${\text{Li}}_{2}$ alkali dimer. The pump and Stokes laser pulses are taken to have the same temporal (Gaussian) shapes, pulse widths, and linear parallel polarizations. The laser wavelengths are in the (visible) range of 700--740 nm. The (peak) intensities of the pulses are of the order of $1\ifmmode\times\else\texttimes\fi{}{10}^{12}--2.5\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{W}/{\text{cm}}^{2}$ and the pulse widths are of the order of 20--50 fs. We have solved the 15 coupled time-dependent Schr\"odinger equations to study the population transfer to the target-excited vibrational levels. We have observed that, even with ``mushrooming'' of closely spaced rotational levels, efficient and selective population transfer to excited vibrational levels in ${\text{Li}}_{2}$ alkali dimer can be achieved for counterintuitive pulse sequence by controlling the pump and Stokes laser parameters. We have endeavored to explain the results within the framework of adiabatic picture.