Showing papers on "Photoexcitation published in 1973"

Calculation of the Surface Photoelectric Effect

Abstract: A modified form of the Mitchell---Makinson time-dependent perturbative calculation of the surface photoelectric effect has been carried out based on a significantly improved treatment of surface-polarization charge-density variations. The approach taken allows calculation of the excitation current back into the metal as well as into vacuum, and is generally applicable to the surface wave (plasmon) or to the direct optical mode of $p$-polarized-light excitation of the surface effect. Numerical results are presented for aluminum, but the general conclusions of the paper should be applicable to all nearly-free-electron metals. The behavior of the surface charge is shown to have a fundamental effect upon the frequency dependence of the surface effect, causing a marked enhancement in the effect at low energies followed by an almost total suppression of the surface effect at energies near the volume plasma energy. Surface photoexcitation at these lower energies is shown to dominate the photoemission from surface-wave decay as well as from direct optical excitation at high angles of light incidence. The surface-wave (plasmon) mode of excitation is shown to be particularly strong, with surface excitation dominating the decay of the high-$k$ plasmons typically excited on real surfaces. Comparisons of the results of this calculation are made with existing experimental data, and the prediction that surface waves (plasmons) should form a uniquely strong mode of surface-effect excitation is shown to be quantitatively consistent with recent experimental studies of photoemission in surface-plasmon decay. The historical failure to observe direct optical excitation of the surface photoelectric effect in the alkalis is explained, and a suggestion for detecting direct-optical excitation of the effect in other nearly-free-electron metals is made based on the results of this calculation.

The role of highly localized excitons in the photoabsorption spectra of ionic crystals

Abstract: The discrete structure associated with photoexcitation of the core levels of the positive ion in ionic crystals is interpreted in terms of highly localized excitons. These excitons are believed to be confined within the first layer of negative ions owing to a net repulsion between the photoelectron and the occupied levels of the negative ions. By adopting a quasimolecular representation for these final states, expressions are derived for the splittings and relative intensities of the exciton peaks. The relevant parameters involved are the electron-hole interaction, the crystal field interaction between the exciton and the surrounding ions, and the spin-orbit interaction of the hole. This scheme is applied to the photoabsorption by the sodium 2p level in sodium halide crystals. The relation between this quasimolecular point of view and band theory is briefly discussed.

Photoexcitation and photoionization of atoms at arbitrary temperature and matter density

Abstract: Bound-bound and bound-free oscillator strengths are calculated based on a self-consistent Hartree-Fock-Slater model of the atom. The effect of variable matter density is analyzed in detail for cold helium. Data are also presented for the Al13 and Cs55 atoms at different temperatures and matter densities.

Cis-trans isomerization of polymethine dyes in the case of pulsed photoexcitation

Abstract: 1. In the case of pulsed illumination of symmetrical cyanine dyes, there are transitory reversible changes in the absorption spectrum, due to the formation of photoisomers. For carbocyanine molecules with a substituent in the meso-position, as well as for thiamonomethinecyanine, an extremely low value of the quantum yield of photoisomerization was detected. 2. Cis-trans-photoisomerization occurs at the singlet excited level. 3. Measurements of the kinetics of relaxation revealed a first order of the reaction, in accord with the monomolecular process: trans(cis) ⇝ cis(trans).

Comment on M 2 − E 1 Mixing Observed in ( γ , γ ′ ) Reactions

Abstract: It is shown that the effect of overlapping resonances in the photoexcitation process of a ($\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}}$) reaction is very small. The influence of this effect on the angular distributions of the inelastic transitions and hence on the $\frac{M2}{E1}$ mixing ratios is shown to be negligible.