Experimental study of rotational relaxation processes by pulsed photoacoustic technique

The IR pulsed photoacoustic method was used to investigate rotational relaxation processes in MPA. We have examined the bulk rotational relaxation times and the bulk rotational relaxation cross sections , related to R-R and R-T relaxation mechanisms. The saturation intensities for gas molecules have been measured in the gas mixtures with (buffer) molecules, at constant gas pressure (0.47 mbar) and variable gas pressure (1-140 mbar), using the 10P(16) laser line in the fluences range 0.1-0.7 J . It has been found that for proper investigation of the rotational relaxation processes, the buffer gas pressure range must be divided into two parts: the low pressure range (0-20 mbar) and the high pressure range (20-140 mbar), according to the behaviour of the saturation intensities versus buffer gas pressure. Consequently, two different sets of values for rotational relaxation parameters are obtained. The found step-function behaviour for shows that the rotational hole-filling effect during MPA only dominates at low buffer gas pressure ( mbar). At higher buffer gas pressures (between 20-140 mbar) collisional deactivation takes place. We also derive the enhanced absorption cross sections for absorbing gas molecules from the measured saturation intensities data (at zero buffer gas pressure) confirming the expectation that the enhanced absorbing cross sections are not only a function of laser fluence and buffer gas pressure, but are very strongly dependent on laser beam parameters, too.