Surface photoabsorption

Whereas SE measures the ratio of reflection coefficients for different polarizations, various reflection difference techniques probe relative differences in reflectivity. Among these techniques one distinguishes surface differential reflectivity (SDR), surface photoabsorption (SPA) and reflection anisotropy spectroscopy (RAS). 

Hinged, K., Aspnes, D., Kamiya, I., and Florez, L. (1993). Relationship among reflectance-difi erence spectroscopy, surface photoabsorption, and spectroeUipsometry. Appl. Phys. Lett., 63 885 - 887. 

Figure B3.4.7. Schematic example of potential energy curves for photo-absorption for a ID problem (i.e. for diatomics). On the lower surface the nuclear wavepacket is in the ground state. Once this wavepacket has been excited to the upper surface, which has a different shape, it will propagate. The photoabsorption cross section is obtained by the Fourier transfonn of the correlation function of the initial wavefimction on tlie excited surface with the propagated wavepacket.

Rhodopsin is a transmembrane protein linked to 11-c/s-retinal, which, on photoabsorption, decomposes to opsin and all-f/a/75-retinal. Rhodopsin has a molecular weight of about 40,000. Its C-terminus is exposed on the cytoplasmic surface of the disk, and its sugar-containing... 

The surface sensitivity is ensured by detecting the decay products of the photoabsorption process instead of the direct optical response of the medium (transmission, reflection). In particular one can measure the photoelectrons, Au r electrons, secondary electrons, fluorescence photons, photodesorbed ions and neutrals which are ejected as a consequence of the relaxation of the system after the photoionization event. No matter which detection mode is chosen, the observable of the experiment is the interference processes of the primary photoelectron with the backscattered amplitude. 

Also, the esterification proceeds only on the film surface and photoabsorption is limited to this region. The result is a constant value for < cv+. 

Schinke, R., Hennig, S., Untch, A., Nonella, M., and Huber, J.R. (1989). Diffuse vibrational structures in photoabsorption spectra A comparison of CH3ONO and CH3SNO using two-dimensional ab initio potential energy surfaces, J. Chem. Phys. 91, 2016-2029. 

The general behavior of the excitation spectra of H- and IF-bands (Fig.lb) reproduces all surface (S) and bulk (transverse = 1,2,3 and longitudinal L) features of photoabsorption. The opposite behavior of the M-band underlines the branching of competing channels of population neutral and charged trapped centers [16],... 

Ab initio and density functional calculations of potential energy surfaces for the ground and excited electronic states of model clusters simulating various point defects, impurities, and their combinations in nanosized silica and germania materials are reported. The accurate geometric and electronic structures of these clusters, calculated photoabsorption and photoluminescence (PL) energies, and predicted absorption and PL spectra are obtained. Our calculations reproduced the experimental excitation energy (1.9-2.0 eV)... 

The photodetachment threshold values for Ga, As, and Pb (0.30(15), 0.81(3), and 1.10(5) eV, respectively) were determined by an older photodetachment technique [30]. The PES values for Ga, 0.43(3), and As, 0.814(8), are more precise, but are supported by the PD values [31, 32]. The PES value for Pb, 0.364(8), is significantly lower than the PD value, 1.10(5) eV. The photodetachment values for the majority of the other main group elements have also been confirmed by photoelectron spectroscopy. The values for the halogens have been determined by most methods photodetachment thresholds, photoabsorption, ion pair photodissociation, relative and absolute surface ionization methods, and the Born Haber cycle. Values for H, Li, C, F, Cl, Cu, Ge, Br, Nb, Ag, Sn, I, W, Re, Au, and Pb have also been determined by relative and absolute surface ionization methods, as shown in Table 8.2. 

Photochemistry on solid surfaces has unveiled the important role of sufaces as reactant media. Solid surfaces work as acids or bases sensitizers or quenchers reaction space for size-specific reactions seed crystals for epitaxial growth etc. Thus, the molecule-surface interaction enhances or reduces photoabsorption, reaction rates, and selectivities. Since there are a lot of parameters for surface reactions, such as adsorption, desorption, diffusion, nucleation etc., it has been difficult to control the photochemistry on solid surfaces. Recently, as it becomes possible to characterize the surface conditions with techniques of ESCA, SIMS, and STM and also to use new light sources, new research field appears as Surface Photochemisty ". 

Figure 1. Photoabsorption between two Born-Oppenheimer potential energy surfaces. (A) The Franck-Condon wavepacket, arising out of = nx L X >s shown on the lower surface, and /j>(t) on the upper], grazes (0) several times on the way to dissociation. The result is an absorption band with some limited vibrational structure. (B) Direct dissociation leading to a broad, featureless absorption band. (Reproduced, with permission, from Ref. 1.)...

Fig. 11.3. Photon stimulated desorption from a surface of Ce02, as a function of photon energy, in the range of the Ad — / giant resonance. The top three curves show the desorption signal for several molecular ions containing H, while the bottom curve shows the photoabsorption cross section for comparison (after B.E. Koel et at. [602]).

The technique which uses the ionic emission is called photon-stimulated desorption (PSD) The ion current due to PSD is proportional to the number of created core holes., i.e., to the photoabsorption cross section of the absorbate. It is a measurement of the surface absorption with a very high surface contrast in comparison with the previously discussed detection methods. A comparison of different techniques is given by Stohr et al. 

---