Relaxation rates of ethylene obtained from their effect on coherent transients

Abstract

In a three-level cascade type double resonance experiment on a free molecular jet, under conditions of reduced, but not eliminated collisional relaxation, the upper level population is shown to exhibit an oscillatory behaviour as a function of overlap between pump and probe laser foci. A qualitative interpretation of the results in the framework of the dressed molecule picture is provided. By numerical integration of the optical Bloch equations, allowing for collisional relaxation processes involving all levels, the observed behaviour has been simulated. Numerical results on the rates of depletion of upper and middle level populations as well as on the dephasing rates were obtained in the temperature range between 55 and 30 K, yielding population rates of about 30 MHz/Torr* for the upper level and 5–13 MHz/Torr* for the middle level (1 Torr* is used as a unit of number density corresponding to 3.27 × 10 22 molecules/m 3). The phase relaxation rate was a factor of 1–3.5 larger in the same temperature range, implying the breakdown of the strong collision approximation at low temperatures.