KINETICS OF ADSORPTION, THERMAL DESORPTION AND DISSOCIATION OF NO ON RHODIUM, STUDIED BY PULSED FIELD DESORPTION

Abstract
The interaction of NO with stepped surfaces of a Rh field emitter has been studied by means of pulsed field desorption mass spectrometry (PFDMS). At temperatures 300<T<550 K and pressures p<1.3·10^-3 Pa, the PFD mass spectra of the adsorbed layer were always dominated by NO⁺, Rh_xNⁿ⁺ and RhOⁿ⁺ ions (n,x = 1.2). We conclude that part of the adsorbed NO undergoes dissociation with subsequent deposition of oxygen and nitrogen atoms on the surface. At low coverages, i.e. Θ<0.1 monolayer, NO adsorption was found to be predominantly molecular. Using a probe-hole, ~150 atomic sites close to the (100) pole of the Rh tip surface were analyzed by varying the field-free reaction interval, t_R, between the field pulses (100µs<t_R<1s. The equilibration between NO adsorption and thermal desorption was monitored. The temperature dependence of the mean lifetimes, τ before thermal desorption was evaluated, assuming first order thermal desorption kinetics, yielding an activation energy E_d = 102 kj/mol and a preexponential τ₀ = 4·10^-14 s.