Pump saturation in molecular far-infrared lasers

Precise absorption measurements on vibration-rotation transitions pumping far-infrared (FIR) laser transitions in HCOOH, CH
3
OH, CH
3
I and CH
3
F were carried out using a frequency-stabilized and intensity-controlled CO
2
laser. Linear absorption coefficients and saturation intensities as a function of pressure are obtained. With the help of a rate equation model it is concluded that within the pressure range relevant for CW FIR lasers the rate of deexcitation of pumped molecules is limited by rotational relaxation. For CH
3
F, limitation by diffusion of excited molecules to the cell wall is observed.