Coherent population transfer in NO with pulsed lasers: the consequences of hyperfine structure, Doppler broadening and electromagnetically induced absorption

Coherent population transfer between vibrational levels of the NO molecule induced by the interaction of two delayed laser pulses, also referred to as stimulated Raman scattering involving adiabatic passage (STIRAP), is studied experimentally in a molecular beam and in the bulk. The consequences of hyperfine splitting and Doppler broadening are discussed in detail. Unlike in previous studies of this kind, transfer occurs simultaneously between more than one group of non degenerate levels. In a molecular beam or in the bulk, the transfer efficiency of STIRAP exceeds that, obtained by Stimulated Emission Pumping (SEP) by a factor of 3.6 or 15, respectively. We estimate the absolute transfer efficiency T in the beam to be T approximate to 60%, while T approximate to 18% is found in the bulk. In both cases: this is 60% of the maximum value expected from numerical studies. Possible reasons for this discrepancy are discussed. Finally we show that the absorption of a pump pulse in a weakly absorbing medium is significantly enhanced by the presence of a copropagating Stokes pulse when the Rabi frequency Omega s of the latter is smaller than the width of the Doppler profile Delta nu(D). The relation of this observation to the phenomenon of Electromagnetically Induced Transparency (EIT), which is observed for Omega s >> Delta nu(D), is also discussed.