Frequency-dependent studies of second-harmonic generation from Ag(110) show resonant enhancement due to a transition between two surface electronic states. An increase in the nonlinear susceptibility, attributed to the coupling of the fundamental light with this transition, is proven by the energy dependence of the second-harmonic signal and by the observed selection rules dictated by the symmetry of the surface states.Second-order nonlinear optical processes such as second-harmonic generation (SHG) are very useful probes for surface and interface structure and phenomena. ' SHG offers excellent time, spatial, and energy resolution, and can be used in a variety of environments, making it a uniquely versatile probe of surface and interface properties. However, very few SHG studies have taken advantage of the energy resolution available from SHG. This work presents frequency-dependent SHG studies, and demonstrates the advantages of such studies in facilitating interpretation of experimental observations that are extremely helpful in furthering our understanding of the SHG process from interfaces. This understanding is necessary in order to take full advantage of the capabilities of SHG. Significant progress toward this goal has been made theoretically and experimentally for simple metals using a "jellium" model for the electrons.In these simple metal systems there are resonant processes occurring in the nonlinear optical process, but they are a result of collective excitations at the surface. In contrast to simple metals, the properties of "real" surfaces and interfaces are usually dominated by single-particle surface or interface states. For these systems a conceptual understanding of the relative importance of various sources contributing to SHG is still lacking.In centrosymmetric media there is no second-order optical response in the dipole approximation, but at a surface or interface, where the inversion symmetry is broken, the nonlinear susceptibility is nonvanishing in this approximation. Thus a strong SHG signal is generated at the interface. ' Because of this surface sensitivity, SHG should be strongly influenced by electronic properties lo-I calized at the interface, i.e. , surface or interface states. On the other hand, quadrupole and other higher-order terms may contribute to a bulk SHG response which may interfere with the surface response. Both the magnitude and phase of the bulk response relative to the surface response are unknown, and the two responses cannot be easily distinguished. ' The surface projected band structure shows the possibility of electronic transitions which are distinctly different, in both energy and symmetry properties, from those possible in the bulk. SHG can be enhanced by resonances with electronic transitions both in the bulk and at the surface. These resonances can be manifested through different physical parameters, namely the nonlinear susceptibility and the dielectric constants.The dielectric response, i.e. , the Fresnel factors, describes the propagation of optical field...

In using pulsed laser excitation of surfaces to induce desorption or reaction of adsorbed molecules, it has generally been assumed that the absorbed energy is rapidly randomized, and a thermal model can be used to calculate the surface-temperature change. In this work, the transient temperature jump on a Ag(llO) surface induced by an 8-nsec laser pulse is directly monitored with a psec probe pulse. The probe is based on a temperature-dependent second-harmonic-generation effect. The experiment provides the first direct evidence that the heat-diffusion model can correctly predict the magnitude and the time evolution of the temperature on the surface. PACS numbers: 79.20.Ds, 42.65.Ky, 68.35.Md The use of lasers to induce and probe chemical and physical processes on surfaces is a subject of much recent interest in surface science. x There are a few unique advantages in using lasers to excite an adsorbate/substrate system to induce surface reactions. The initial excitation is mode specific and may lead to the possibility of nonstatistical reaction channels. In many cases, however, thermalization of the initial excitation may be fast compared to the reaction rate, and all reactions would then proceed under thermal conditions. Even for this latter case of equilibration of the initially absorbed energy, the rapid (>10 10 deg/sec) temperature rise caused by pulsed laser excitation can change the relative yields of products coming from competing chemical pathways. The latter advantage has been demonstrated in studies of surface reactions. 2 In order to correctly interpret the events following pulsed laser excitation of surfaces, it is essential to know the time scale of thermalization from the initially excited mode, and the resultant surface-temperature increase. An accurate account of the temperature evolution on the surface is essential to properly determine the kinetic parameters of the surface process.Many surface studies have used powerful nanosecond laser pulses to excite metal substrates in order to induce surface processes. 2 " 9 In all cases, the transient temperature jump at the surface is not experimentally determined, but is calculated by a classical heat-diffusion model. 10 ' 11 This model assumes that the energy absorbed into a specific mode thermalizes instantaneously and that the transfer of heat from the surface to the bulk can be treated by use of bulk heat-diffusion constants. Furthermore, several working assumptions have been widely used to reduce the complexity of the calculation, such as that higher-order terms in the heat-transfer equation can be neglected.For excitation of metals using light from the near ir to the uv, intraband or interband electronic transitions are involved. Several femtosecond experiments have been performed recently for a variety of metals, where the rates of electron relaxation through electron-phonon collisions where shown to be of the order of several picoseconds. 12 " 15 Therefore, it is reasonable to assume that, on a nanosecond time scale, thermal equilibrium is est...

The azimuthal-angle dependence and the frequency dependence of second-harmonic generation (SHG) measured at the interband transition region of a Ag(111) surface have been examined using a theoretical model developed by Sipe, Moss, and van Driel [Phys. Rev. B 35, 1129 (1987)]. The analyses of the experimental results provide strong evidence that the dramatic variation of the second-harmonic signal with frequency at 2Aco=3. 87 eV is primarily caused by the sharp change of the dielectric function. The dispersion of the dielectric function affects both the azimuthal angle and the frequency dependence through different radiation efficiencies of the isotropic and anisotropic components of the generated second-harmonic field. The observed SHG features at 3.9 eV can be adequately accounted for by the presence of the interband transitions, and it is not necessary to invoke other surface electronic transitions for their assignments.

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Faculty members in family practice residencies are increasingly being asked to help residents develop skills in the use of informatics and evidence-based medicine (EBM). In order to do this successfully the teachers themselves must be skilled in the use of these tools. Recognizing the need for such training, the Maine Medical Center Family Practice Residency Program designed a faculty development project to increase knowledge and skills in the use of information technology. This project, which was carried out in 1999-2001, utilized a multifaceted approach that included improving the residency's technology infrastructure, conducting two instructional workshops, and offering EBM mentoring for preceptors. Faculty members also designed and carried out independent informatics projects. Pre- and post-project assessments of faculty members demonstrated a significant improvement in computer and EBM skills, and informal feedback from residents indicates that these skills have been successfully applied to the faculty members' teaching of residents and their practice of family medicine. This project had a positive impact on the faculty members in the residency program, increasing both their ability to employ information technology in individual and group teaching sessions and their use of EBM in clinical practice. Also, the culture within the residency program has been changed to one of utilizing computers and the Internet as principal resources for up-to-date information.