Photon‐induced field desorption of NO from Rh

Abstract

The effect of an electric field on the NO photo-desorption from Rh was investigated using a pulsed laser atom probe. Experiments were carried out at 78 K by irradiating a [111] oriented Rh field-emitter tip and a Rh single crystal of 1.4 mm diameter at 6.4 eV and at 3.2 eV photon energy. The energies and masses of the emitted ions were evaluated from time-of-flight (ToF) measurements. For 6.4 eV irradiation of the Rh tip, a quadratic yield dependence on the laser fluence has been observed for NO. This is interpreted as a two-photon ionization of field-adsorbed NO; this is supported by the same behavior of O 2 and Xe but at a comparatively lower yield. In cases where the ionization energy of the probed species (Ar, N 2 , CO) is higher than 12.8 eV no such effect is observed. Laser irradiation with 2.3 eV photons initiates thermally assisted field desorption of the species at a comparatively higher fluence. Ion energy analysis suggests re-neutralization of NO + in the forbidden zone, but the translational energy gained appears to be sufficient to overcome the desorption barrier and field ionizion at the critical distance becomes probable. At low laser fluence a weak linear photo-desorption process has been detected, presumably caused by substratemediated electronic excitation as a first step. In this model, the photo-desorbed NO will subsequently be field-ionized, and accelerated away by the field. For the Rh single crystal, NO photo-desorption, with two-photon ionization in a second step, is deduced as the most probable mechanism.

Study of the Ionization in a Field Ion Microscope Using Pulsed-Laser

“…However, if the ions of T2 were not noises, one possible explanation for the ionization of these ions is the photon-induced field desorption. In the photon-induced field desorption, the adsorbed gas on the specimen surface is excited by the pulsed laser and is ionized [8][9][10]. Furthermore, an emitted ion is accelerated by a local electric field.…”