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

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 u nusually strong bound NO-dimer exhibiting a weak feature at 1857 cm(-1) bes ide the chemisorbate absorption band at 1794 cm(-1). Laser induced desorpti on experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state se lectively using resonance enhanced multiphoton ionization. The desorbing mo lecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibro nic states of desorbing NO are bimodal and exhibit a strong coupling of rot ation and translation. With increasing coverages an additional channel is o bserved appearing in the time-of-flight spectra of v(')=0 as smoothly incre asing intensity at long flight times. The numeric values of these unusually long flight times are indicative for long residence times on the surface r ather than small kinetic energies. The desorption efficiencies weakly depen d on the concentration and vibrational state ranging from (2.0+/-0.3)x10(-1 7) cm(2) at low coverages to (1.0+/-0.4)x10(-17) cm(2) at high coverages fo r v(')=0. The intensity of the desorption signal per laser pulse only incre ases proportional to the chemisorbate coverage. The data are interpreted as suming the dimers to act as extrinsic precursors within the desorption proc ess. (C) 1999 American Institute of Physics. [S0021-9606(99)72027-1].