Controlling rotational state distributions using two-pulse stimulated Raman excitation

The femtosecond stimulated Raman process is a versatile technique to excite rotational states in molecules. We demonstrate control over the rotational state population in a sample of NO molecules by varying the time delay between two identical laser pulses. The product of the rotational state distribution is probed by a 1+1 resonance-enhanced multiphoton ionization scheme and simulated quantum mechanically. There is good agreement between theoretical and experimental results. The product in selected quantum states shows an oscillatory dependence on the time delay. Spectral analysis reveals rotational transition energies and the presence of multiple Raman steps. We show that the relative strength of these frequency components can be related to excitation pathways with predominant Delta J=2 transitions toward higher rotational states. The initial step from J=1/2 involves either Delta J=1 or Delta J=2. We find that one can discriminate between two excitation ladders. The results demonstrate the coherent effects of tailoring the shape of an ultrashort excitation pulse.