Showing papers on "Photoexcitation published in 1983"

Photoexcitation of an autoionizing resonance in the presence of off-diagonal relaxation

Abstract: We discuss the theory of photoexcitation of an autoionizing resonance. We solve a set of coupled stochastic integrodifferential equations which are based on the Fano model for autoionization, but which include the effects of weak elastic collisions, weak or strong laser excitation, and finite laser bandwidth. We determine the exact photoelectron spectrum and give formulas for spectral peak positions and widths. Redistributive scattering is evident, as are various effects normally associated only with transitions between discrete levels. Under certain conditions symmetries and fixed points of the electron spectrum can be predicted.

Optical Absorption Characteristics of GaAs–AlGaAs Multi-Quantum-Well Heterostructure Waveguides

Abstract: Optical waveguide absorption characteristics of MBE grown GaAs–AlGaAs MQW doubleheterostructure wafers are studied by measuring edge luminescence under photoexcitation, and are compared with those of conventional GaAs DH wafers. The edge luminescence spectra show a double peak structure which is well explained by waveguide self-absorption, rather than by L0-phonon participation as has been suggested by other workers. The optical absorption loss of the unexcited MQW waveguide at its lasing wavelength is found to be much smaller than that for the conventional DH waveguide.

Photoexcitation of the NaKCl Reaction Complex

Abstract: Etude de l'emission des raies D de Na lors d'une experience de faisceaux croises de K et NaCl, irradies par un faisceau laser entre 600 et 700 μm. Attribution a Na * forme par excitation du complexe NaKCl

Crossed-beam measurements of absolute rate coefficients in associative ionisation collisions between Na*(np) and Na(3s) for 5≤n≤15

Abstract: Associative ionisation in slow collisions between Na*(np) levels and the Na(3s) ground state is studied in a crossed-beam experiment. Excited levels are prepared by laser photoexcitation, and ionic species are analysed by time-of-flight mass separation. Rate coefficients for associative ionisation, measured for principal quantum numbers ranging from n=5 to 15, exhibit a pronounced on n with a maximum of approximately 3*10-9 cm3 s-1 near n=11. Comparisons are made with the theoretical model developed by Duman and Shmatov, and Mihajlov and Janev.

Formation kinetics and spectra of aromatic radicals in solution

Abstract: Spectra and kinetics of aromatic radicals have been obtained by means of a two-color laser-induced fluorescence experiment. The rates and dissociation mechanism of 1-(chloromethyl) naphthalene and 1-(bromomethyl)-naphthalene following 266-nm photoexcitation have been established experimentally by the direct observation of the rate of formation of the 1-naphthylmethyl radical product. The radicals have been identified from their emission spectra induced by excitation with a 355-nm picosecond pulse. It is shown that the excited state energy dissipation process of the parent species involves a dual path: (1) direct dissociation from the S/sub 2/(..pi pi..*) state of the naphthalene into the T(sigma*) state of the C-X bond with subsequent fragmentation to a 1-naphthylmethyl radical and a halogen, and (2) internal conversion and vibrational relaxation to the S/sub 1/ with subsequent fluorescence and intersystem crossing to the bound triplet manifold.

Low temperature photoconductivity in a-Si:H films

Abstract: The temperature dependence of the photoluminescence, steady state and transient photoconductivity of a-Si:H has been studied between 4 and 500 K Below 50 K a temperature independent ημτ-product of 10−11cm2/V is observed, which varies only little with the defect density We propose that this mode of photoconduction arises from the drift of photoexcited electrons and holes during their thermalization near the mobility edges It is shown that photoexcitation of deeply trapped holes by IR-radiation leads to an enhancement of the photoconductivity at low temperature Time dependent studies yield information on the mechanism of trap emptying

Shape resonance phenomena in CO following K-shell photoexcitation

Abstract: Electron cross sections and asymmetries following photoexcitation of the CO K shells were measured directly. Energy analysis of ejected electrons yielded cross sections exhibiting sigma shape resonances in the C(1s), C(KVV), and O(1s) channels, and a ..pi.. resonance in the C(KVV) channel, confirming earlier indirect measurements. Asymmetry measurements showed a sigma shape resonance in the C(1s) channel, in good agreement with the predictions of Dehmer and Dill. The C(KVV) ..pi.. resonance showed nearly isotropic behavior.

Excitation and decay of H4 luminescence in diamond

Abstract: Photoluminescence and cathodoluminescence decay time measurements have been carried out for the H4 (2.499 eV) centre in diamond. The photoluminescence decay time increases progressively from approximately= 19 to 23 ns as the energy of the exciting light is increased from 2.6 to 4.0 eV, and the cathodoluminescence decay curves are non-exponential. Luminescence excitation measurements reveal a previously unreported absorption band peaking at about 3.2 eV in the ultraviolet spectral region; absorption of light in this band produces H4 luminescence. The range of decay times noted above is believed to be due, at least in part, to transitions from the higher excited levels via the 2.499 eV excited state. The decay time of 19+or-1 ns measured with photoexcitation at 2.6 eV shows no significant variation with temperature, and this represents the most likely value of the radiative lifetime of the 2.499 eV centre.

Site determination for palladium on niobium using angle-resolved photoemission

Abstract: We demonstrate the possibility of using angle-resolved photoemission from valence levels to gain overlayer site geometry information. To do this we utilize and discuss polarization effects and photoexcitation cross-section effects (as a function of the photon energy). To illustrate the application of the technique we have studied Pd overlayers on Nb(110) and conclude that the Pd(110) overlayer has a hollow-site geometry.

Study of sulfur‐containing molecules in the EUV region. III. Photoexcitation of CS2

Abstract: Using synchrotron radiation as a continuum background, the absorption cross section of CS2 has been measured using a double ionization chamber. The cross sections range from four to a maximum value of 65 Mb in the 175-760 A region. Two new Rydberg series X and XI have been identified and apparently converge to the D2Pi state of CS2(+) at 16.943 eV.

Nuclear resonance scattering studies on238U using a dense series of γ-ray photons

Abstract: Using the181Ta(n, γ) and141Pr(n, γ) reactions to provide a dense series of intenseγ-rays, photoexcitation of the238U nucleus has been studied in the 4–7 MeV range. It is shown that most of the effective cross sections are in line with an average strength function as given by the GDR Lorentzian line superimposed by Porter-Thomas fluctuations. Resonances of nonstatistical strength are found close to 5.2 MeV.

Intrinsic luminescence of protein as a tool for studying fast structural dynamics

Abstract: The photoexcitation of Trp, Tyr or Phe chromophore in a protein is analogous to the instant (10(-15) to 10(-14) s) introduction of a local probe carrying strong electronic and protonic donor and acceptor moieties and, moreover, having a significant dipole moment (4 to 11 D). Depending on the protein structure mobility during the excited state lifetime, the intra-macromolecular complexes (the exciplexes) with polar groups can be formed and some reversible charge transfer processes, which qaench the fluorescence, can take place with greater or lesser probability. The shifts of Trp fluorescence spectra from 307 to 316 nm (due to the 1:1 exciplex formation) and to 325-330 nm (2:1 exciplex) are typical for many proteins. An additional spectral shift up to 350 nm is caused by the reorientational relaxation of a protein matrix dipoles during the nanosecond excitation lifetimes. The bathochromic fluorescence of several proteins evidences the slow (times much longer than 10 ns) reorientation mobility in these cases. The fluorescence quenching rates by proteic groups and extrinsic small quenchers is linearly related to the diffusion coefficient of a surrounding solvent. Analysis of such relationships allows, in principle, to evaluate the diffusional internal friction inside a protein and parameters of co-operativity and anisotropy of the mobility. Some sources of possible misinterpretations of Trp fluorescence depolarisation as a measure of the rotational mobility of indole.

Selective Photoexcitation of Atoms and Molecules

Abstract: The first stage of any selective laser photoprocess is the selective absorption of one or several laser photons by a desired atom or molecule (in the case of intermolecular selectivity) or by a molecular vibration or functional group (in the case of intramolecular selectivity) In other words, the rate of laser photoexcitation of A (molecules, bonds, etc) must be much higher than that for the rest, designated B It is possible to define the magnitude of the photoexcitation selectivity (sometimes called radiative selectivity) as the ratio of the probability of exciting A to a state with the required energy Eexc to the probability of finding B in a state with the same energy: $$S_{rad} (A/B) = {{P_A (E_{exc} )} \over {P_B (E_{exc} )}}$$ (21)

Correlation of photoluminescence and symmetry studies with photoexcitation and decay processes of infrared active defects in Si

Abstract: Defects introduced in silicon by neutrons and ions which give rise to infrared active electronic defect absorption bands after heat treatment to 500 °C were studied using a dual beam optical method. The measurements were made in the 11–90 K temperature range on four defect bands found in the 900 (11.1)–1100 cm−1 (9.09 μm) region of the infrared spectrum. The studies yield results on the capture cross sections of photoexcited electrons from defects which lead to a model of a defect having three different charge states. These cross sections are much smaller than those reported for the photoionization cross sections of chemical impurities such as Au in silicon. The states found in this study can be identified with photoluminescence peaks reported by others near 1 and 0.76 eV. Additionally, there are two different symmetries assumed by the defects responsible for the defect absorption bands.

Resonance Raman and luminescence spectra from colloidal semiconductor surfaces

Abstract: Spectra are reported for the surface-adsorbed free radical anion of methlviologen (MV+˙) formed from MV2+ by electron transfer from colloidal CdS particles following photoexcitation by continuous wave Raman lasers.

Phase transition and chaos in electrical avalenche breakdown caused by weak photoexcitation at 4.2 K in n-GaAs

Abstract: Periodic oscillations and turbulence of the electrical avalanche breakdown have been observed in high-purity n-GaAs at 4.2 K, under the pulse voltage ( ∼ 4 V/cm) and with the weak photoexcitation around band-gap energy. The oscillatory behaviors follow the chaotic regime in the fluid flow system, and give a first example of the onset of chaos which originates in a new type of phase transition in semiconductors.

De-Excitation Pathways of Highly-Excited Self-Trapped Exciton and Electron Plus Self-Trapped Hole

Abstract: The yield of F - H pairs in KBr upon photo-excitation of the electron of the self-trapped excitons to n ≥3 orbitals is found to be much lower than that upon photoexcitation to the n =2 orbitals and than the overall yield of the F centers under ionizing radiation. The results imply that the de-excitation pathways of excited self-trapped excitons and the electron trapped by the self-trapped holes are different.

Photoexcitation of levels at 6605 keV in 48Ti and 7362 keV in 68Zn

Abstract: Levels at 6605 keV in 48Ti and 7362 keV in 68Zn were photoexcited using neutron-capture gamma rays emitted by V(n, gamma ) and Cr(N, gamma ) reactions respectively. The widths and other spectroscopic properties of these levels were measured. The possibility of utilising these resonance scattering events for solid-state studies are discussed in detail.

Mechanism for formation of the XeI(B) excimer by photoexcitation of an Xe–I2 mixture

Abstract: An investigation was made of the mechanism of formation of the XeI(B) excimer by photoexcitation of an Xe–I2 mixture in the λ =110–195 nm range. Two bands, B→X (λ ~250 nm) and B→A (λ ~320 nm) were recorded in the luminescence spectrum of the XeI excimer. It is shown that when XeI luminescence is quenched by xenon, an Xe2 excimer is formed, its luminescence being emitted in the λ ~300–400 nm range. It is shown that the efficiency of the reaction between an excited iodine molecule and a xenon atom resulting in the formation of an XeI(B) excimer depends on the excitation energy of the iodine molecule. The maximum luminescence quantum efficiency of XeI(B→X) is achieved by optical pumping in the λ =183 nm range.

Energy-distribution studies of photoexcitation in photofield emission

Abstract: Total energy distributions of the photocurrent in photofield emission from tungsten have been measured for $s$- and $p$-polarized light The results show that for photon energies below 35 eV the dominant mechanisms of photoexcitation are the surface photoeffect and the indirect bulk photoeffect In the photofield-emission configuration the surface and indirect bulk distributions can be separated by suitably choosing the polarization and angle of incidence of the illumination

Photoelectric Investigation of Recombination Centre Parameters in InP(Fe)

Abstract: Stationary and kinetic properties of photoconductivity in Fe-compensated semi-insulating InP are investigated, including stationary photoconductivity and optical quenching spectra of the intrinsic photocurrent in the range of 0.45 to 1.5 eV; temperature quenching and stimulation, as well as lux–ampere characteristics (LAC) of the stationary photocurrent, and kinetics of photo-current decay in the intrinsic photoexcitation region. In the interpretation of the results obtained the rapid (r) and slow (s) recombination channels are considered. From the analysis of mentioned relations a number of main (s) and (r)-recombination centre parameters are found, including energy position of s-recombination centres (Evs), capture cross-section of electrons on rapid S,lr-and slow Sns,-recombination centres, and initial occupation of s-centres by holes –P. The intrinsic photoconductivity-temperature stimulation is interpreted as the result of reverse optical recharge of s-centres and electron deep trapping levels. [Russian Text Ignored]