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 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 ν“=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)x10−17cm2 at low coverages to (1.0±0.4)x10−17cm2 at high coverages for ν“=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

Adsorption of potassium on Cr₂O₃(0001) at ionic and metallic coverages and uv-laser-induced desorption

Translational energy distributions of neutral potassium atoms are reported as a function of potassium coverage after uv-laser-induced desorption from well-characterized adsorption sites on an epitaxial film of Cr2O3(0001)/Cr(110). Measurements using x-ray photoelectron spectroscopy, low-energy electron diffraction, and work-function measurements revealed that potassium adsorbs in a nonmetallic phase for deposition temperatures around 280–300 K allowing only a maximal saturation coverage to be grown for moderate growth rates. Aggregates are observed after deposition at 90 K; at this temperature any layer thickness is obtainable. The uv-laser-induced desorption for these two different phases was studied using excitation energies of 3.5 eV, 5.0 eV, and 6.4 eV and (1+1)-resonantly enhanced multiphonon ionization via the 6p2P state for detection. Desorption of potassium atoms from the nonmetallic phase proves to be ten times [σ(6.4eV)=(2±1)×10−19cm2] more efficient than desorption from metallic potassium aggregates. The mechanism of desorption from the nonmetallic phase appears to be the inverse harpooning process starting with an ion pair followed by a transfer of hot electrons from the substrate to unoccupied potassium states to neutralize the initially positively charged potassium. The maximum of the translational energy distribution (starting at 0.65 eV for low coverages) decreases with increasing potassium coverage and is by a factor of approximately 4 smaller for desorption from large potassium aggregates (0.16 eV). The decrease of the translational energy with increasing coverages for isolated atoms is ascribed to an increasing lateral interaction between the adsorbates and a concomittant smooth change of the ionicity of the atoms from partially ionic to neutral

Photoinduced Br Desorption from CsBr Thin Films Grown on Cu(100)

Thin films of CsBr deposited onto metals such as copper are potential photocathode materials for light sources and other applications. We investigate desorption dynamics of Br atoms from CsBr films grown on insulator (KBr, LiF) and metal (Cu) substrates induced by sub-bandgap 6.4 eV laser pulses. The experimental results demonstrate that the peak kinetic energy of Br atoms desorbed from CsBr/Cu films is much lower than that for the hyperthermal desorption from CsBr/LiF films. Kelvin probe measurements indicate negative charge at the surface following Br desorption from CsBr/Cu films. Our ab initio calculations of excitons at CsBr surfaces demonstrate that this behavior can be explained by an exciton model of desorption including electron trapping at the CsBr surface. Trapped negative charges reduce the energy of surface excitons available for Br desorption. We examine the electron-trapping characteristics of low-coordinated sites at the surface, in particular, divacancies and kink sites. We also provide a model of cation desorption caused by Franck-Hertz excitation of F centers at the surface in the course of irradiation of CsBr/Cu films. These results provide new insights into the mechanisms of photoinduced structural evolution of alkali halide films on metal substrates and activation of metal photocathodes coated with CsBr

Dynamics of photoinduced reactions at oxide surfaces

This report summarizes our work on UV-laser induced desorption of small molecules and atoms from transition metal oxides. The systems presented serve as examples for a simple photochemical reaction, the fission of the molecule surface bond. State resolved detection methods were used to record the final state distributions of the desorbing neutral molecules. Detailed results on the systems NO/NiO(1 1 1) and CO/Cr2O3 (0 0 0 1) are presented. The experiments include investigations on stereodynamic aspects like the angular distributions of the desorbing molecules and, in the case of CO desorption, the rotational alignment with respect to the surface normal. Large desorption cross sections of (6 ± 1) · 10−17 cm2 for NO and (3.5 ± 1) · 10−17 cm2 for CO have been found for the desorption at 6.4 eV. The wavelength dependence indicates that the primary excitation step is substrate induced. The final state distributions show a high degree of translational, rotational and vibrational excitation and are clearly nonthermal of origin. The results are consistent with the formation of a negative ion intermediate state of the adsorbate. This observation is supported from a comparison to former results on NO/NiO(1 0 0) for which extensive ab initio calculations including electronically excited states exist. A spin state dependence of the vibrational excitation of NO could only be observed for NO/NiO(1 1 1) and is absent for NO/NiO(1 0 0). We attribute this observation to a spin state dependent coupling of the desorbing molecule to the surface in case the spin lattice orientation of the surface shows a preferential orientation. In the (111) plane the spin orientation is parallel within neighbour nickel ions while it is alternating in the (1 0 0) plane. For both systems studied the velocity component parallel to the surface is constant leading to a strong peaking along the surface normal for the fast molecules. The change from a preferred helicopter rotation (angular momentum vector aligned parallel to the surface normal) to a cartwheel motion (angular momentum vector aligned perpendicular to the surface normal) with increasing rotational excitation for desorption of the flat lying CO is consistent with a change of bonding geometry during the desorption process

Laser-stimulated desorption from surfaces

Recent experimental and theoretical results focus on nonthermal laser-induced desorption after UV-laser excitation with the aim of obtaining insights into microscopic processes, including the relevant potential energy surfaces and lifetimes of the excited states that are involved. Results range from measurements and interpretation of quantum state resolved studies to femtochemistry

UV-laser-induced desorption of NO from the pure and modified Cr₂O₃(0001) surfaces: spin effects in surface-state-induced desorption

We shall report the UV-laser induced desorption of NO/Cr2O3(0001) and the coadsorbate system NO/K/Cr2O3(0001). Resonance enhanced multiphoton ionization was used for state selective detection of the desorbing NO after excitation with pulses of nanosecond duration and desorption laser energies between 3.5 eV and 6.4 eV. There are two adsorbate species of NO, a chemisorbed and a physisorbed species. We shall focus on data of the chemisorbed species. The main emphasis within this paper will be put on electron spin effects, particularly the preferential population of a fast translational desorption channel for the 2(Pi) 3/2 state observed specifically in connection with surface state induced processes at desorption energies of 5.0 eV. For those processes changes within the final state distributions of desorbing NO are fond when modifying the electronic surface structure via adsorption of small amounts of potassium