Coherent laser control of bound-to-bound transitions of HCl and CO

We have used the method of Shapiro, Hepburn, and Brumer to control the ionization rates of HCl and CO. This method is based on the principle of quantum mechanical interference between two competing paths. In this case the two paths are one- and three-photon excitation of an intermediate Rydberg state. A vacuum ultraviolet (VUV) (110–115 nm) laser beam was produced by third-harmonic generation from an UV beam in Kr gas. The relative phases of the two beams were controlled by passing them through a chamber containing either Ar or H2. The laser beams were focused into a molecular beam, and the ionization products were measured in a time-of-flight mass spectrometer. As the pressure of the phase-tuning gas was varied, the ionization signal was found to oscillate with a period that is proportional to the difference of the indices of refraction of the tuning gas at the UV and VUV wavelengths. We have applied this technique to transitions involving different intermediate electronic states the j 3Σ−(0+), H 1Σ+(0+), and m 3Π(1) states of HCl and the B 1Σ+ state of CO, different rotational lines of the same electronic transition, and different competing processes (ionization vs dissociation) following excitation of the intermediate state.