Quantum control study of multilevel effect on ultrafast isotope-selective vibrational excitations.

Quantum optimal control calculations have been carried out for isotope-selective vibrational excitations of the cesium iodide (CsI) molecule on the ground-state potential energy curve. Considering a gaseous isotopic mixture of (133)CsI and (135)CsI, the initial state is set to the condition that both (133)CsI and (135)CsI are in the vibrational ground level (v=0) and the target state is that (133)CsI is in the v=0 level while (135)CsI in the first-excited level (v=1). We find that, using the density-matrix formalism, perfect isotope-selective excitations for multilevel systems including more than ten lowest vibrational states can be completed in much shorter time scales than those for two-level systems. It is likely that this multilevel effect comes from the large isotope shifts in the vibrational levels of v>1. To check the reliability of the calculation we also carry out optimal control calculations based on the conventional wave-packet formalism, where the wave-function amplitude is temporally propagated on the grid points in real space, and obtain almost the same results as those with the density-matrix formalism.