Sorption of nitrogen by titanium films evaporated in ultra-high vacuum

Abstract

The sorption rate of Ti films has been measured for nitrogen as a function of time and pressure at 15° C. The films were deposited on the inside of a copper hemisphere (200 cm2) mounted within an ultra-high vacuum chamber. Iodide titanium was degassed and evaporated at pressures below 10-8 torr. Nitrogen was leaked from a reservoir at a pressure PR into the ultra-high vacuum vessel via a conductance (SL) and the pressure above the Ti film (PG) maintained constant during sorption; the mass sorption rate equals SL(PR - PG). Impurities in the sorbate were kept low by thermally degassing the sorption and reservoir vessels and pumping both chambers during evaporation and sorption. The sorption properties of Ti films 23-100 A thick deposited on smooth and grooved substrates were measured at N2 pressures between 2.5 × 10-7 and 10-8 torr. The sorption capacity rose from 1.5 × 10-5 l. torr cm-2 for a 23 A thick film to 4.5 × 10-5 l. torr cm-2 for a 100 A film; the non-linear dependence of the capacity on film mass indicated sorption occurred on interior crystallite surfaces. The mass sorbed as a function of time followed the relation for diffusion into a finite slab with D [similar, equals] 10-18-4 × 10-17 cm2 sec-1. Treating the initial sorption as that for diffusion into a semi-infinite solid the surface gas concentration C0 was found showing that C0 [is proportional to] PG1/2. Values of the initial sorption rate SI were obtained by extrapolating the sorption rate curves to zero time. These results also showed that SI [is proportional to] PG1/2 with SI = 3.75 l. sec-1 at 10-8 torr. SI is usually assumed independent of pressure for an uncovered sorbent at t = 0. However, the dependence of SI and C0 on PG1/2 indicated that gas molecules dissociated on chemisorption giving an equilibrium surface coverage following the Langmuir hyperbolic law. Nitrogen atoms diffused into the film interior along grain boundaries with the outer surface concentration C0 [is proportional to] P1/2. Extrapolating sorption rate curves to zero time would then show an apparent dependence on pressure.