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In this paper, we investigate UV picosecond laser-induced effects on undoped chemical vapor deposited (CVD) diamond surfaces as a function of irradiation time. Diamond films are negative electron affinity (NEA) emitters and contain defects (non-diamond phases). Surface analysis is performed before and after laser irradiation, in order to clearly demonstrate surface structural transformations. A diamond peak shift after pulsed laser irradiation is determined by micro-Raman analysis and correlated to stress variations in diamond. The post-irradiated diamond surface reactivity with ambient air is studied by X-ray photoelectron spectroscopy (XPS). XPS results show surface bonding modifications due to carbon–oxygen interaction enhanced by UV pulsed laser irradiation.

In situ reflectivity measurements were carried out at room temperature and atmospheric pressure to follow dynamic structural variations occurring during pulsed laser irradiation of diamond-like carbon (DLC) thin films. Laser irradiation consisted of second harmonic (λ = 532 nm) picosecond pulses of a Nd-YAG laser, leading to a modification of the carbon layer structure. Irradiation at this wavelength usually leads to graphitization, i.e. an increase of the number and/or size of sp 2 clusters dispersed in the amorphous carbon structure of DLC films. One of the consequences of such a transformation is a pronounced change in the optical properties. Upon graphitization, the optical absorption strongly increases in the near-infrared region. In order to monitor such changes we used a probe beam tunable in this wavelength region. An optical parametric generator (OPG) and amplifier (OPG/OPA) with low energy pulses tunable in the near-infrared range between 700 nm and 2 μm was used to record reflectivity changes at several wavelengths. Such an in situ characterization reveals how the structural transformations proceed in the early phase of graphitization.

Picosecond pulsed laser irradiation of several metallic photocathodes (W, Ta, Al and Au) was carried out along with work function measurements during the irradiation process, work function measurements were performed before irradiation and after different irradiation periods. Metals used in this study behaved differently according to their affinity towards oxygen. Most of these metal surfaces were covered with a native oxide layer, except gold. Photolythic or thermal processes could occur depending on the laser wavelength, in the former case, the native oxide layer was removed pulse by pulse while in the latter, surface oxidation was enhanced. For the tungsten and tantalum samples, the kinetics of oxide removal occurred in distinct stages relative to the native oxide multilayer structure. In the case of the Au sample, no native oxide layer formed on its surface (noble metal) and the laser irradiation induced a surface smoothing only. A linear increase of the photoelectric signal was observed during laser irradiation of the aluminium sample probably due to the partial surface oxide removal and the sub-surface oxide incorporation. The presence of various solid phases of the oxide layer could be determined from pulsed laser irradiation measurements.

In this study, we investigate the photoelectric emission from CVD diamond films. These diamond samples present NEA properties due to their as-grown surfaces terminated with hydrogen atoms. Photocathodes are characterised by UV pulsed laser-induced photoelectric measurements and photoelectric threshold measurements because the photoelectric emission is strongly dependant on the electron affinity of the diamond surface. Photoelectric threshold measurements show the existence of a sub-bandgap signal associated to a defect-band level for both samples, with the lowest value obtained for the highest defect-density diamond film. Moreover, the quantum efficiency of undoped diamond is measured at 213 nm as a function of the CH4 concentration. The highest quantum efficiency value is measured for the highest defect-density diamond film. Surface bonds modifications occurring during a prolonged laser irradiation are responsible for the decrease in the photoemissive performances of diamond films. # 2000 Elsevier Science B.V. All rights reserved.

K+-implanted W samples with various implantation depths were investigated. Generally, implantation of alkali ions gives rise to two competitive effects: it lowers the surface work function, however it enhances surface oxidation too which in turn leads to a slight work function increase. In opposite to alkali overlayers, implanted species confined within a `metallic cage' resist the applied laser irradiation and alkali removal occurs only to a small extent. Measurements of laser-induced photoelectric charge pulses indicate a variation of the photoemission yield as a function of surface oxide thickness. Moreover, following the charge pulse evolution over a longer period (2–3 h), one finds that the laser-induced oxide removal characteristics depends on the implantation parameters as well. Results are compared to those obtained for pure, non-implanted W and the mechanisms responsible for the work function lowering are discussed.

Photocurrent measurements yielded new data, which were used to determine the surface work function of a native oxide-covered tungsten photocathode. The photoemission was generated by a continuous UV source. The surface work function has been measured at various stages of a pulsed UV laser-induced oxide removal in order to characterize the process. Since the measured surface work function can be correlated with the surface coverage, along with the measurements of charge of laser-induced photoelectrons a description of the laser–oxide interaction is presented. A fully computer-controlled experimental setup is demonstrated, achieving photoelectric measurements at the spot of laser illumination. This apparatus enables us to follow the work function temporal dependence during laser illumination, thus giving a detailed description for the surface coverage dynamics.

A native oxide film on a metallic surface corresponds generally to a multilayer. We describe a method using a number of picosecond laser pulses at 213 nm to achieve partial removal of the oxide overlayer on a polycrystalline W substrate under UHV conditions. The process of oxide removal was monitored by measuring the charge of emitted photoelectrons by using an appropriate anode (collector)–cathode (target) setup, and simultaneously, the surface reflectivity for each pulse. The temporal evolution of the photoelectric charge shows – as expected – an increasing tendency due to the lowering of the surface work function, but moreover – under certain experimental conditions – it shows the presence of several kinks, which are interpreted as a sign of changes in the surface morphology. The results are discussed within the frame of a model describing mono/multilayer desorption phenomena.