Showing papers on "Photoexcitation published in 1979"

Effects on Photoemission of the Spatially Varying Photon Field at a Metal Surface

Abstract: Enhancement of photoionization cross sections due to spatially varying photon fields at a metal surface has been observed in the normal-emission cross sections for the surface state and Fermi level of A1(100) at photon energies between 9 eV and 23 eV. The data for Fermi-level photoexcitation are in excellent agreement with theoretical results for jellium. Below $\ensuremath{\hbar}{\ensuremath{\omega}}_{p}$, the predominant contribution to the photoionization matrix element comes from the spatially varying fields, which provide the momentum required for photoexcitation.

Quaternary structure-induced photoreduction of haem of haemoglobin

Abstract: Spectroscopic studies have provided extensive information on the primary process of visual pigments and photoexcitation of chlorophyll as well as their effects on photoreactivity on the higher-order structures of protein has been observed only rarely. Resonance Raman spectroscopy can reveal the vibrational frequencies of the chromophore in a molecule provided the excitation wavelength is in the absorption band of that molecule. As the visible absorption bands of haemproteins are due to pi pi* transitions of the porphyrin ring, we can selectively observe the vibrational frequencies of iron porphyrin during in situ interactions with immediate amino acid residues of protein when the wavelength of excitation light is close to the Soret or Q band. Correlation of some vibrational frequencies of haem with the oxidation and spin states of the haem iron has been studied in detail and an empirical rules has been established. This method is therefore especially suitable for the study of an effect of higher-order structures of protein on the chromophore. We report here a photoreaction facilitated by a particular quaternary structure of protein--in various haemoglobins resonance Raman spectroscopy showed that reversible photoreduction of haem took place in the T state but not the R state.

Pulsed infrared laser induced visible luminescence

Abstract: : Crystals of saccharin doped with p-toluenesulfonamide, courmarin, NaCl, and KCl are found to emit in the visible, following a 20 nsec pulse of high intensity 1060 nm radiation. The pulses were 0.5 to 4 J/sq.cm. in energy, corresponding to up to 200 MW peak power. The emissions in the first two cases are from molecular excited states, but with differences in detail from those induced by photoexcitation or found in triboluminescence. NaCl and KCl show neither photoemission or relevant triboluminescence, and the laser induced emission is essentially that found thermally or by mechanically stressing x- or gamma-ray irradiated crystals. It is attributed to electron-hole recombination. The mechanism of the laser induced emission may thus involve some combination of mechanical shock wave and of plasma ionization. The trivial explanations of photoexcitation by higher harmonics of the 1060 nm fundamental or through successive multiple photon processes can be ruled out. (Author)

Homogeneous and Inhomogeneous Triplet Exciton Absorption Lines in Molecular Crystals

Abstract: Triplet exciton absorption lines in pure anthracene and pyrene crystals are investigated at very high resolution using photoexcitation in the lower temperature (<90 K) regime. The profiles for the lower Davydov components (zero-phonon) are observed to be most sensitive to crystal quality. Linewidths as small as 0.02 cm−1 are observed for the lowest exciton of anthracene with the upper component about 50 times broader. The T-dependences of the linewidths for the Davydov components of anthracene are shown to be explicable in terms of combined linear and quadratic exciton-photon coupling. The T-dependence of the zero-phonon to phonon sideband intensity ratio for pyrene is well described by the linear exciton-photon interaction in which the exciton is taken to be localized.

Photoinduced charge-transfer process in PLZT ceramics

Abstract: The photoinduced charge‐transfer process in PLZT ceramics is studied by measuring the refractive‐index changes during hologram recording at λ=488 nm. From the measured values of the photoinduced space‐charge fields, it is concluded that the photocarrier drift lengths are comparable with the fringe spacing (a few μm); the photorefractive sensitivity is limited by the low photoexcitation quantum efficiency (10−4) and is not influenced by oxidoreduction treatments.

Photochemie substituierter Cycloheptatriene. III. Sigmatrope Wasserstoffverschiebung an Arylcycloheptatrienen

Abstract: Photochemistry of Substituted Cycloheptatrienes. III. Sigmatropic Hydrogen Shift in Arene Cycloheptatrienes The monochromatic photoexcitation of substituted phenylcycloheptatrienes 1–8 exclusively gives rise to 1, 7-hydrogen shift. The sequences of photoreactions of the isomeric compounds are discussed. The qualities and quantities of the hydrogen shifts of dimethylamino-phenyl-ycloheptatrienes 4–7 depend on the wavelengths of photoexcitations.

Effects of alternative transition sequences on coherent photoexcitation

Abstract: The photoexcitation part of laser isotope separation can lead to a situation more complicated than a simple chain of transitions between successive pairs of energy levels In the rotating wave approximation, coherent photoexcitation for these situations leads to graphs with side branches or cycles For many cases a simple change of basis removes the time dependence of the interaction Hamiltonian and reduces the calculation of level populations and interlevel flows to an eigenvalue problem Analytical and numerical examples are shown It is shown that a side branch with a large matrix element strongly inhibits the flow of population along the path to which it is attached The dependence of populations and flows on phase differences when the radiation consists of several phase-locked fields with different frequencies is studied For acyclic graphs, there is no phase dependence For a cycle, the phase dependence is on one parameter which is a linear combination of the phases associated with the cycle In a simple example photoexcitation was changed from that of complete population inversion to that of no population inversion by varying that parameter

Inhomogeneous electric fields in oscillatory photoconductivity

Abstract: Monochromatic photoexcitation of a semiconductor can produce a photoconductive response which is a periodic function of the optical excitation energy. This 'oscillatory photoconductivity' effect is caused by the interaction of the photoexcited carriers with optical phonons. Under conditions in which this effect exists, it is theoretically possible for a spatially uniform negative drift velocity state to occur for certain ranges of excitation energy. This type of state has the interesting property that a fraction of the optical excitation energy is continuously transferred directly to the electric field. For this type of excitation energy, the drift velocity as a function of electric field exhibits, in addition to the region of negative velocity, a region of negative differential mobility which is intrinsically unstable against space-charge formation. The results of a simple numerical model are presented here in order to classify the conditions under which a stable negative resistance or periodic instabilities can occur in the bulk of such a sample connected in a resistive circuit. The nature of these propagating instabilities is described.

Use of autoionization transitions in isotopically selective photoexcitation

Abstract: Apparatus and process for isotopically selective multistep photoionization in which the final or ionizing step is tuned to produce a specific transition to an excited state above ionization for which the ionization cross-section is substantially greater than for ionization transitions in general. The autoionization transition to an ionized state is typically made from a highly excited bound state which is reached in one or two isotopically selective energy jumps from the ground state or other low-lying levels. The isotope shift for the ionization transition is typically small compared to the bandwidth of the ionization transition and relatively broad band photoionization radiation covering the entire absorption line can be employed. Broad band radiation is more economic and is preferable for use whereever possible. A technique is also shown for identifying the ionization transitions of augmented cross-section.

Hot photoexcited electrons: Time evolution of the electron temperature in high magnetic fields. Application to InSb

Abstract: We study the time evolution of the temperature of electrons photoexcited by a laser pulse in the conduction band of a semiconductor in extreme quantum limit conditions We analyze the influence of several parameters such as magnetic field, photoexcitation intensity, photoexcitation energy, recombination time, lattice temperature and we discuss the role of the various types of electron-phonon scattering mechanisms in the particular case of InSb

Photoexcitation of the 7632 keV Level in 207Pb

Abstract: A neutron-emitting level at 7632 keV in 207Pb was photoexcited by n-capture γ rays of iron. The spin and parity of this level was established (Jπ=3/2+) by angular distribution measurements. The (γ,n) cross section was found to be σ(γ,n)=370 ± 30 mb.

Anomalous thermal quenching of luminescence in ZnS:Cu phosphor crystals

Abstract: We studied the thermal quenching of luminescence (TQL) in the case of photoexcitation (PE, λe=313 and 365 nm) and cathodoexcitation (CE) with a high excitation density (∼1018 cm−3· sec−1) in ZnS:Cu single crystals with various degrees of copper supersaturation, EL-455 and K-83 copper-activated powders, and ZnS∶Cu epitaxial thin films. Our study of TQL in the blue and green bands showed that two types of quenching anomalies are observed in the specimens used. An anomaly of the first type manifests itself in the fact that, with band-to-band excitation (λe = 313 nm and CE). the green band is quenched earlier than the blue band and small steps are observed on the quenching front. This anomaly is explained with the aid of a dislocation model of the blue center. An anomaly of the second type is observed in specimens not supersaturated with copper and consists of the quenching of luminescence beginning earlier at a high excitation (CE) density than at a low excitation (PE) density and proceeds in the same way in the blue and green bands. The thermal depths of the acceptors level are estimated from TQL for the three fundamental luminescence bands 440, 468, and 525 nm which are equal to 0.38 ±0.05, 0.7–0.8, and 0.9–1 eV, respectively.

Giant Resonance Excitation in Various Processes. Example: 13C

Abstract: Excitation functions for radiative capture of stopped pions, electro- and photoexcitation through the giant resonance region were calculated for 1p shell nuclei, using shell model functions1 which span the full space of 0 ℏω and 1ℏω excitations.