The pumping of gases desorbed by pulsed ruby laser irradiation of a Si(111) surface

Abstract

The pumpdown process of gases desorbed from a Si(111) surface irradiated by a Q-switched ruby laser was observed in an ultrahigh vacuum chamber made of stainless steel. The chamber was pumped through an orifice by a sputter-ion pump and a Ti getter pump. A Si crystal was mounted on a manipulator installed in the chamber and was irradiated without special cleaning. After laser beam irradiation, ion currents of a quadrupole mass spectrometer were measured for m/e=2, 15, 16, 26, and 28. The currents were composed of two peaks irrespective of mass number. One of the peaks corresponded to the direct time-of-flight distribution of molecules desorbed from the Si surface. The second peak showed exponential decay which corresponded to the pumpdown process of gases released from the Si surface and expanded into the chamber. The time constants of the decrements of partial pressures were smaller than the values calculated. The difference can be explained by a small sticking probability and a large mean sojourn time of adsorption on the chamber surfaces.