E. Westin

Surface structure of cubic ionic crystals studied by optical second-harmonic generation

Polarization and azimuthal dependencies of optical second-harmonic generation at the surface of cubic ionic crystals have been measured on polished surfaces of BaF2 (111) and (100), CaF2 (111) and NaCl (111), using a fundamental wavelength of 532 nm. The results are interpreted in terms of available theory, which shows that for transparent cubic media only the dipolar surface contribution depends on the crystal orientation. For BaF2 (111) and NaCl (111) the crystallineC3vgeometry could be identified but not for CaF2 (111). Although the nature of the electronic surface structure giving rise to a dipolar polarizability remains undetermined, the experimental result suggests that it may be due to impurity or defect states in the band gap at the surface.

Molecular Cluster Calculations for the Analysis of Laser Induced Emission of Electrons and Ions from the (111) Surface of BaF2

The subject of this presentation is to show how molecular cluster calculations can lead to a deeper understanding of laser induced processes at surfaces of ionic crystals. Molecular cluster calculations were performed for bulk BaF2 and for different clusters modelling the (111) surface of BaF2. The effect of the surrounding crystal ions have been included by an embedding crystal potential. Calculations for a stoichiometric surface give an electronic structure similar to the bulk, while calculations for a local non-stoichiometric surface show the existence of occupied surface states in the upper half of the bandgap, followed by unoccupied states extending above the ionization limit. These calculations give a qualitative understanding of the experimentally observed resonantly enhanced multiphoton processes of electron and ion emission from the (111) surface of BaF2.

Molecular Cluster Calculations of the Electronic Structure of the (111) Surface of CaF2

Molecular cluster calculations within the Local Density Approximation have been performed to analyze the electronic structure of stoichiometric and different non-stoichiometric (111) surfaces of CaF2. The effect of the surrounding crystal ions, i.e. the long range electrostatic potential, have been included by a Fourier summation. Calculations for clusters representing the bulk and stoichiometric surfaces give similar results while calculations for non-stoichiometric surfaces show the existence of occupied surface states in the upper half of the bandgap. Existence of these types of occupied surface states are supported by experimental EELS studies on CaF2 as well as by observation of laser induced emission of ions and electrons from surfaces of BaF2.

Multiphoton Processes at the (111) Surface of BaF2 Crystals

The subject of this contribution is multiphoton interaction of laser light in the blue and green spectral ranges with cleaved and polished (111) surfaces of BaF2 in ultrahigh vacuum and air. We claim that photoemission and laser-induced non-thermal desorption of positive ions are triggered by nonlinear absorption of some order between two and five. Photoemission and ion desorption yields are strongly wavelength dependent and display conspicuous resonance structures, with striking similarities between electron and ion yield spectra. Cluster calculations are performed in an attempt to understand the origin of these resonances. They predict the existence of occupied surface states near the middle of the bandgap together with a high density of resonances in the upper half of the bandgap. Measurements of the surface second harmonic generation in air showed a similar wavelength dependence as the electron yield in vacuum. The transition from pure second harmonic generation to plasma discharge was studied and evidence is presented that the discharge is ignited by resonant multiphoton absorption in adsorbed gas molecules.

Multiphoton-Induced Desorption from BaF2(111)

Wavelength and intensity dependencies of laser-induced electron and positive ion emission from a BaF2 (111) surface were studied in the MW/cm2 intensity range, well below the damage threshold. In the range 500 to 560 nm we find similar shapes for the spectral emission yields of electrons and ions. Electrons are emitted by two- and five-photon absorption below and above 1.5 MW/cm2, respectively. In contrast, ion desorption involves a ten-photon process, suggesting that two holes in the valence band are required. The interpretation of the data is supported by molecular cluster calculations. In the blue spectral range at higher intensities (∼50 MW/cm2) we observe rapid bleaching of the irradiated spots and evidence for layer-by-layer ablation.

Probing surface electronic structure of ionic crystals by second harmonic generation and laser‐induced desorption

Measurements of laser‐induced desorption under ultrahigh vacuum conditions and second harmonic generation on surfaces in air provide evidence for surface electronic states in the band gap of ionic materials. When irradiating BaF2(111) with green laser light narrow resonances of electron and ion emission yields indicate resonance enhancement of multiphoton absorption. Using fundamental light of 532 nm, second harmonic generation behaves different for BaF2(111) and NaCl(111) on one side and CaF2(111) on the other. In the first two cases, its azimuthal dependence proves that the second harmonic yield originates from local dipole interaction at the surface. In contrast, the CaF2(111) surface appears to be more inert and the second harmonic signal originates predominantly from quadrupole interaction in the bulk.

Laser‐induced desorption from the (111) surface of BaF2

We have investigated the wavelength, polarization, and intensity dependence of the electron and ion emission from the (111) surface of BaF2 under irradiation with pulsed tunable laser light in the green and blue spectral ranges. We also carried out cluster calculations to obtain a prediction for the electronic surface structure of BaF2 (111). Based on these results, a qualitative model for the desorption of positive ions from this surface is proposed.

Multiphoton-Stimulated Emission of Electrons and Ions from the (111) Surface of BaF2

The interaction of laser radiation and surfaces of optically transparent ionic materials is an important problem in surface science as well as in applied optics. For the latter, the question of how optical damage occurs, is of fundamental interest. Since the bandgap of ionic crystals is of the order of 10 eV, for visible light only multiphoton absorption can result in energy deposition into the surface. Structural defects, impurities, and adsorbates can generate surface states which are located somewhere in the bandgap, and can resonantly enhance multiphoton photoemission. This, in turn, can lead to desorption of ions and neutrals at comparatively low intensities. In this contribution we present results of photoemission and desorption studies carried out in ultra-high vacuum with cleaved (111) surfaces of BaF2, at laser intensities in the MW/cm2 range.

Second Harmonic Generation on the (111) Surface of BaF2

In this contribution we present experimental results on the angular, polarization, and wavelength dependence of second harmonic generation (SHG) on polished BaF2 (111) surfaces in air. These measurements are part of a program to investigate the electronic surface structure of alkaline-earth fluoride crystals under various conditions. The first results on optical SHG on surfaces of BaF2 were reported as early as 1968 by Wang and Duminski /1/. Meanwhile, guided by the pioneering work of Shen and collaborators /2/, the technique has matured into a powerful analytical tool for the investigation of surface structures, and we find it worthwhile to apply it further to surfaces of transparent ionic materials.

SHG And Laser-Induced Desorption At The Surface Of BaF 2

Laser-induced emission of electrons and positive ions as well as optical second harmonic generation (SHG) at the (111) surface of BaF2 yield insight in both the electronic surface structure and the fundamentals of laser damage in optical materials.