Degassing Process of a Vacuum Chamber Observed by Laser-Induced Thermal Desorption

Irradiation of a small spot on the wall of a vacuum chamber with a high power laser pulse causes laser-induced thermal desorption (LITD) of gases which produces a pressure pulse. We measured the peak height and the rate of decay of the pressure pulse of each component gas desorbed at various stages of the degassing process of a vacuum chamber made of stainless steel, A Q-switched YAG laser with a maximum output energy of about 22 mJ/pulse was used for the irradiation. The laser irradiation experiment was started after pumping the chamber for 650 h at room temperature. Subsequent measurements were carried out at room temperature after degassing the chamber for various duration at 100°C, 150°C and 250°C. The sticking probability of gas molecules on the chamber wall can be obtained from the difference between experimental and calculated values of time constants of the decay of pressure pulses. The sticking probability of water molecules increased markedly after degassing the chamber at 250°C.