Knowledge Bank

The time dependence of the weak field absorption spectrum of the J = 1 $\to$ 2 transition of OCS, brought about by intensity modulated pumping of the J = 0 $\to$ 1 transition, has been observed with a 100 MHz bandwidth video spectrometer. A computer interfaced multichannel signal averager was used to average together the data of about 50 000 experiments for each set of spectroscopic conditions. The results are interpreted in terms of the time dependence of the density matrix of a three level system with phase randomizing intermolecular collisions. It is found that a time dependent absorption signal is not simply proportional to the corresponding time dependent population difference between the observed levels.