Coherence Effects in Infrared Stark Spectroscopy of Molecular Beams of CH₃F

Abstract

We report on the observation of two coherence phenomena which have been observed in a beam of CH₃F molecules produced by a supersonic expansion. A CO₂ laser was used in conjunction with Stark tuning to produce transitions in the v₃ band of ΔH₃F and these transitions were measured by monitoring the energy contents of the molecular beam with a low temperature bolometer located downstream from the laser-beam interaction region. By observing the R(0,0) and P(1,0) transitions with ΔM = ±1, we have recently shown that under high laser power, transitions of this type exhibit coherence between the populations of the degenerate M levels. We have also observed Rabi oscillations for the ΔM = −1, Q(1,1) component of the v₃ band although we have found that the number of oscillations observed is limited because at higher power densities adiabatic rapid passage drives the system to inversion. This inversion has been experimentally confirmed by using two laser beam interaction regions. The molecular beam first interacts with a CW laser beam of variable power and then is probed with a chopped laser beam of a high, but constant power. Under saturation conditions one would expect the chopped signal to go to zero as the CW laser power is increased. We find instead that the signal becomes negative as the power is increased which shows that the population is being inverted and is not going to saturation.