UV laser desorption of nitric oxide from semiconducting C₆₀/Cu(111)

Abstract

The desorption of NO from a well-characterized, epitaxially grown semiconducting C₆₀ surface is reported. Two different channels are identified in the laser desorption. Both channels yield a comparably high desorption cross section of σ₁=7.0×10^-17 cm² and σ₂=5.5×10^-17 cm² for the first and second channel, respectively. The laser desorbed NO molecules are detected with rovibrational state selectivity by (1+1) REMPI in the $A^2\Sigma^+ \to{X}^2 \Pi\,\gamma$-bands. In the first channel the desorbing molecules are highly excited with an average kinetic energy of 〈E_kin〉=174 meV. The rotational population distribution can be fitted by a rotational temperature of T_rot=800 K. A rotational–translational coupling is observed, with velocities ranging from 1000 m/s for low to 1300 m/s for high rotational states. The vibrationally excited population is estimated to be less than 1% of the ground state. The second channel yields less excited molecules and an almost Boltzmann distributed rotational population with a temperature of T_rot=280 K. The apparent velocity distribution derived from the pump-probe delay yields molecules much too slow to be explained by even a thermal desorption. This desorption is probably caused by a long-lived electronic excitation in the substrate for which a lifetime of τ≈160 μs is estimated.