Recombinative desorption dynamics: Molecular hydrogen from Cu(110) and Cu(111)

Abstract

The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time-of-flight mass spectrometry. Rotational distributions are found to be non-Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv″=1/Pv″=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv″=1/Pv″=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann-at-Ts ratios would be 0.0009 for H2 and 0.0063 for D2 at T=850 K. Simple models are discussed which attempt to account for the qualitative trends of these results. Detailed balance arguments applied to the vibrational distributions measured in recombinative desorption are unable to predict correctly the dissociative adsorption probability as a function of vibration, indicating that these two processes are dynamically different for this system.