Interference-Induced Phenomena in High-Order Harmonic Generation from Bulk Solids

We consider a quantum mechanical model for the high-order harmonic generation in bulk solids. The bandgap is assumed to be considerably larger than the exciting photon energy. Using dipole approximation, the dynamical equations for different initial Bloch states are decoupled in the velocity gauge. Although there is no quantum mechanical interference between the time evolution of different initial states, the complete harmonic radiation results from the interference of fields emitted by all the initial (valence band) states. In particular, the suppression of the even-order harmonics can also be viewed as a consequence of this interference. The number of the observable harmonics (essentially the cutoff) is also determined by interference phenomena.

Magnetic Resonance in Hydrogenated Nanocrystalline Silicon Thin Films

ABSTRACTWe have investigated the localized electronic states in mixed-phase hydrogenated nanocrystalline silicon thin films (nc-Si:H) with electron-spin-resonance (ESR). The dark ESR signal most likely arises from defects at the grain boundaries or within the crystallites. With illumination with photon energies ranging from 1.2 eV to 2.0 eV, there is no evidence of photo-induced carriers trapped in the bandtail states within the amorphous region. Dependence of the light-induced ESR (LESR) upon the exciting photon energy reveals that, at different excitation photon energies, different regions dominate the optical absorption. This behavior may have potential consequences for understanding the light-induced degradation in nc-Si:H.

ANISOTROPY OF EXCITON RELAXATION IN BeO CRYSTALS

This work presents the results of experimental complex study of electronic excitation dynamics for single BeO crystals. The decay kinetics (1–500 ns) of the photoluminescence, reflectivity (8–35 eV), time-resolved emission (2.5–10.4 eV) and luminescence excitation (6–35 eV) spectra were measured at 10 K for the oriented BeO crystals. The diversity among the channels of the radiative relaxation of the electronic excitations as well as the various multiplicity of the excited states of self-trapped excitons, depending on the crystal orientation and exciting photon energy, are discussed.

PHOTOLUMINESCENCE AND DYNAMICS OF EXCITONS IN Alq3 SINGLE CRYSTALS

The relaxation dynamics of photo-excited states in single crystals of Alq 3, utilized as organic electroluminescent (EL) devices, have been investigated. The photoluminescence (PL) peak of Alq 3 changes depending on the exciting photon energy and the temperature. Nevertheless, a PL peak appears at the same energy position as the EL peak for excitations at energies above the transition edge and a new PL band has been resolved at energies below the EL peak for excitation at the lowest part of the absorption tail. However, for low temperatures and for excitation in the intermediate energy region of the absorption tail, a PL peak appears at the lower energy side. This PL peak shifts into the same energy as the EL peak with increasing temperature. This behavior is explained by a model which takes account of two luminescent states and one non-luminescent intermediate state. In the intermediate energy region, the temperature dependence of the PL for the excitation is also explained by a thermal activation process mediated by the non-luminescent state. The lifetimes of the three states obtained from the temporal profiles of the PL are understood consistently on the basis of this model.

Multivacancy Effects in Atomic and Molecular Spectra

Chemical applications of X-ray spectra are inhibited by the general occurrence of multivacancy processes. These manifest themselves in emission �spectra as extra lines (or satellites) while opening of channels to these initial configurations leads to extra detail in absorption spectra. While there are a few simple cases where this situation has been more or less fully discussed, the more interesting areas are those which are both not simple and not understood. There is, however, a fairly general experimental procedure by which this .complex situation could, in principle, be clarified. This involves carrying out high resolution emission spectroscopy as a function of exciting photon energy in regions containing both single and multiple vacancy thresholds. In earlier work summarised here, it was possible to demonstrate the procedure for a monatomic gas, argon. Subsequently, we have built and now operate a beamline designed specifically for such studies at the National Synchrotron Light Source (NSLS) in Brookhaven. Very recent results from this line and from a few experiments using conventional sources are summarised.

CO2 laser-induced decomposition of methyl iodide sensitized by sulfur hexafluoride

Gaseous mixtures of methyl iodide and sulfur hexafluoride yield upon irradiation by a continuous-wave CO2 laser iodine, methane, ethane, ethylene and acetylene - the products of methyl iodide decomposition. The decomposition rate and the mutual ratio of the products is dependent on the delivered irradiation energy, the methyl iodide/sulfur hexafluoride ratio and the energy of the exciting photon. Methyl iodide-d3 in the mixture with SF6 behaves similarly as methyl iodide yielding perdeuterated methane, ethane, ethylene, acetylene and iodine.

Monte Carlo Simulation of Sample Scattering Effects from Homogeneous Samples Excited by Monoenergetic Photons

The error introduced by sample scattering in EDXRF analysis is evaluated by Monte Carlo simulation. This is accomplished by deriving a Monte Carlo model capable of simulating single Compton and Rayleigh scatters from the exciting photon source and from fluorescent X rays in homogeneous samples. The model also includes primary, secondary, and tertiary fluorescence events. (1) Results are given for Ni-Fe-Cr ternary samples for various exciting energies with and without scattering and indicate that errors as large as 2% can be attributed to this effect.