Ultraviolet-laser induced desorption of NO from the Cr2O3(0001) surface: Involvement of a precursor state?

Abstract

NO molecules interact with the Cr2O3(0001) surface to form a chemisorption bond of 1.0 eV. At higher coverages an additional more weakly bound species appears in thermal desorption spectra with a binding energy of 0.35 eV. By infrared spectroscopy the weakly adsorbed species is identified to be an unusually strong bound NO-dimer exhibiting a weak feature at 1857 cm−1 beside the chemisorbate absorption band at 1794 cm−1. Laser induced desorption experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state selectively using resonance enhanced multiphoton ionization. The desorbing molecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibronic states of desorbing NO are bimodal and exhibit a strong coupling of rotation and translation. With increasing coverages an additional channel is observed appearing in the time-of-flight spectra of v″=0 as smoothly increasing intensity at long flight times. The numeric values of these unusually long flight times are indicative for long residence times on the surface rather than small kinetic energies. The desorption efficiencies weakly depend on the concentration and vibrational state ranging from (2.0±0.3)×10−17 cm2 at low coverages to (1.0±0.4)×10−17 cm2 at high coverages for v″=0. The intensity of the desorption signal per laser pulse only increases proportional to the chemisorbate coverage. The data are interpreted assuming the dimers to act as extrinsic precursors within the desorption process.