Momentum resolved electron stimulated desorption ion angular distribution,a new technique, probing the low frequency motion of adsorbed molecules onsingle crystal surfaces

A new technique, momentum resolved electron stimulated desorption ion angul ar distribution (ESDIAD), provides a method for taking snapshots of the zer o-point position and lateral momentum of particles adsorbed on crystalline surfaces. By employing state-of-the-art electronics and computer technology it is possible to record for each desorbing particle the desorption direct ion together with the flight time. High momentum and directional resolved i mages are obtained, with time-of-flight resolution in the picosecond range and data acquisition rates up to 100 kHz. This enables us to deconvolute sp atial and momentum contributions to the ESDIAD pattern and to map the low f requency motion of the adsorbed particles. These maps reflect the adsorbate interactions with the substrate and with neighboring species on the substr ate. For selected examples it is demonstrated that by measuring the three d imensional momentum vector for each desorbing particle it is possible to pr obe the lowest energy states of adsorbed species, as well as to measure the momentum distribution when the adsorbed species gains thermal energy. Such information can be used as a basis for thinking about anisotropies in late ral motion of particles on surfaces. One major opportunity involves the stu dy of dissimilar chemisorbed species which, when imaged together in momentu m and real space, give new insights into the first stages of interaction be tween the species, leading ultimately to a chemical reaction. (C) 1999 Amer ican Vacuum Society. [S0734-2101(99)18704-9].