Showing papers on "Photon energy published in 1987"

Resonant Raman scattering of hydrogenated amorphous carbon: Evidence for π‐bonded carbon clusters

Abstract: Resonant Raman spectroscopy has been used to study amorphous hydrogenated carbon films. For films containing both sp2 and sp3 bonded carbon a well‐defined high‐frequency shift of the main Raman peak is observed with increasing exciting photon energy. This shift is interpreted in terms of scattering from π‐bonded carbon clusters which is resonantly enhanced for incident photon energies approaching the π–π* resonance.

Soft X-ray emission from subpicosecond laser-produced plasmas

Abstract: Observation of soft x rays from a tungsten plasma produced by intense subpicosecond laser pulses is reported. X radiation of several keV photon energy was observed with just a few millijoules of laser energy. Using two consecutive pulses for the generation of the plasma the x‐ray emission is maximal for a delay of about 30 to 50 ps between the laser pulses.

A Compton scattering spectrometer for determining X-ray photon energy spectra.

Abstract: With the use of more sophisticated diagnostic technologies it is becoming increasingly important to know the energy spectra of the primary photons from clinical X-ray tubes. At the high fluence rates used under working conditions, it is necessary to greatly reduce the number of photons to the detector per unit time in order to avoid pulse pile-up. The Compton scattering method is very suitable for this reduction and hence it has been further developed in this work in the primary-photon energy range 20-200 keV. The movement of the electrons in the scattering target causes an energy broadening of the Compton scattered photons. This broadening results in a decreased energy resolution, which is particularly seen as a smearing out of the characteristic X-ray peaks of the anode material. Comparison between the spectrum obtained using a Compton spectrometer and unfolded with the authors' reconstruction and the spectrum measured directly in the primary beam shows very good agreement even though relatively simple reconstruction algorithms have been used.

Photoelectron dynamics of the valence shells of benzene as a function of photon energy

Abstract: Angle‐resolved photoelectron spectroscopy was carried out on the first nine valence orbitals of benzene using synchrotron radiation as a photon source. From these data cross sections σ and angular distribution parameters β were obtained over a photon energy of 10 to 34 eV. The experimental results are compared with calculations for the same parameters, based on the multiple scattering Xα method. Considering the complexity of the molecule, the comparison is gratifying. A number of shape resonances, which are predicted, have been identified experimentally. The orbital assignments for the photoelectron spectrum of benzene have been reexamined in view of the cross sections and angular distributions and have for the most part been verified.

High-resolution measurements of the 3s satellite spectrum of argon between 77 and 120 eV photon energy.

Abstract: A high-resolution 3s-satellite spectrum of Ar is presented, obtained at five energies between 77 and 120 eV photon energy at the positron-electron storage ring BESSY. The results are in disagreement with recent measurements by Brion et al. obtained at lower resolution but confirm earlier calculations by Smid and Hansen. This means that disagreements between photoelectron and (e,2e) experiments persist for Ar.

Effective atomic numbers for low-energy total photon interactions in human tissues

Abstract: A new method is introduced in which the total photon interaction cross sections per electron of human tissues are used to define effective atomic numbers for blood, bone, brain, fat, heart, kidney, liver, lung, muscle, ovary, pancreas, spleen, and water. These effective atomic numbers are equal within 4% from 10 to 200 keV in each soft tissue, whereas for bones of different chemical compositions the variation ranges from 2.86% to 5.03%. This effective atomic number definition is less energy dependent than a previous definition based on the total photon interaction cross section per atom averaged over all elements in the tissue, from which the computed effective atomic numbers varied by as much as 50% (in bone) as a function of photon energy over the energy range from 10 to 200 keV.

Observation of optical Stark effect in InGaAs/InP multiple quantum wells

Abstract: We report an experimental observation of a blue shift in the n=1 heavy‐hole exciton line of In0.53Ga0.47As/InP multiple quantum wells resulting from a picosecond photoexcitation in the transparent spectral region. The temporal response of this shift follows the excitation and it is attributed to the optical Stark effect. The shift was measured to be 0.19 meV for an incident light with a photon energy 20 meV below the exciton peak and with a 10‐MW/cm2 intensity.

Monte Carlo simulation of a cobalt-60 beam.

Abstract: We have used the Stanford Electron Gamma Shower (EGS) Monte Carlo code to compute photon spectra from an AECL Theratron 780 cobalt-60 unit. Particular attention has been paid to the careful modeling of the geometry and material construction of the cobalt-60 source capsule, source housing, and collimator assembly. From our simulation, we conclude that the observed increase in output of the machine with increasing field size is caused by scattered photons from the primary definer and the adjustable collimator. We have also used the generated photon spectra as input to a pencil beam model to calculate the tissue-air ratios in water and compared it to a model which uses a monochromatic photon energy of 1.25 MeV.

Optical properties of ZnTe films

Abstract: Optical properties of p-type ZnTe films, deposited by hot-wall vacuum evaporation, were studied extensively in the range of incident photon energy 0.6–2.6 eV. Variations of refractive index, absorption and extinction coefficients with incident photon energy are reported in this communication.

Interface vibrational Raman lines in GaAs/AlxGa1-xAs superlattices

Abstract: We report the observation of interface phonons in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As superlattices using Raman scattering. As predicted by the theory, all the three interface phonons, two in the region of GaAs-like optical phonons and one in the region of AlAs-like optical phonons, are observed. The Raman intensities associated with the interface phonons are enhanced when the incident photon energy is close to the exciton energy in the GaAs quantum wells. The dependence of the interface mode frequency on the incident photon energy is found to be one of the important factors in distinguishing the interface modes from the bulk optical phonons.

Ultraviolet‐assisted growth of GaAs

Abstract: This paper presents results of a comparative study of the UV‐assisted epitaxial growth of GaAs from metal‐organic Ga compounds and AsH3 at 0.1 bar total pressure using a low‐pressure‐Hg lamp (λ=254 nm) or an excimer laser (λ=193 or 351 nm). Since effective stimulation only takes place for reaction‐limited growth conditions these were defined first. It is shown that at 0.1 bar only the Ga compound, no AsH3, is involved in the rate‐limiting step. The rate increase upon illumination becomes larger with increasing stability of the metal‐organic compound, i.e., in the order tri‐isobutyl to triethyl to trimethyl gallium. Experiments with UV of 351 nm, a wavelength at which radiation is not absorbed by the reactants, indicate that in this case growth enhancement is absent. Thus, heating effects do not play a significant role in the stimulation process. The other wavelengths are both absorbed by the metal‐organic compounds and consequently their use leads to larger growth rates. The attenuation of the incident radiation by absorption in the bulk of the gas phase determines the photon density reaching the substrate surface. Since the absorption depends on the wavelength, the photon energy and wavelength are important factors governing the rate of growth. Although the present data show that the photostimulation takes place very close to the surface, it is not possible to determine whether the stimulation process takes place at the surface, in the gas phase near the surface or in both locations.

The temperature and energy distribution of photoexcited hot electrons

Abstract: The concept of electron temperature T e is widely used in the description of photoexcited electrons in semiconductors. Nevertheless the photoexcited hot-electron energy distribution appears to be a Maxwell or Fermi distribution only in the case in which the electron density is very high, n ⪆ 1018 cm-3. Usually the energy distribution may only be adequately described by temperature T e below the threshold of the optical-phonon emission e = ħΩ0 (passive region e ħΩ0) the distribution critically depends on the photon energy and pumping level. In the review we study how the distribution depends on the influence of the lattice and inter-electron scatterings. The electron temperature T e is determined by the balance between the energy imparted to the electron system from the excitation source and the energy lost to the lattice owing to the phonon scattering. As a rule a minority of the electrons are in the active region rather than in the passive one but i...

Subpicosecond ultraviolet multiphoton electron spectroscopy of rare gases

Abstract: The line positions and linewidths of photoelectron spectra arising from multiphoton ionization are analyzed for the first reported time in the subpicosecond (05–10-psec) region Measurements performed with 248-nm radiation at intensity level for which the quiver energy of the free-electron motion is comparable with the photon energy are an reported The short-pulse property causes the photoelectron energy to become a dynamical variable associated with the process of ionization and significantly influences both the line positions and the linewidths of the observed electron spectra

Ultraviolet light stimulated thermal oxidation of silicon

Abstract: The photon‐stimulated oxidation enhancement efficiency is shown to increase by an order of magnitude when the photon energy is raised from the visible range to the UV range just above the SiO2 to Si conduction‐band energy difference. An additional order of magnitude in oxidation enhancement efficiency occurs when the UV photon energy is raised to just above the O2 dissociation energy. A critical UV pulse energy density threshold was found above which major morphological oxidation features developed at the laser beam spot location on the sample surface.

Strong correlation and alignment near the Be 1s photoionization threshold.

Abstract: Photoionization in atomic beryllium has been studied in detail near the 1s threshold. Multiple excitation to the 1s(2s2p /sup 1,3/P) states, i.e., conjugate shakeup, is found to be unusually strong, 40% of single excitation. The /sup 3/P component rapidly loses strength with increasing photon energy, while the /sup 1/P component persists. The 1s2s2p states are highly aligned, as shown by the Auger-electron angular distribution. The A/sub 2/ value of -0.65 implies a d-wave contribution of 0.39. Our results indicate that resonances reported for the solid-state photoabsorption are of atomic origin.

Two‐photon photoelectron spectroscopy of Pd(111)

Abstract: Angle‐resolved two‐photon photoelectron spectroscopy has been used to study the normally unoccupied band structure of Pd(111). Photoemission features are studied as functions of excitation photon energy hν, polarization state, and emission angle. Below hν=4.9 eV, photoemission is dominated by excitation of bulk states near the Fermi level of Λ3 symmetry through Λ1 excited states between the Fermi and vacuum level. The kinetic energy of the corresponding spectral peak is found to increase according to 2 hν−Φ, where Φ is the Pd(111) work function. At hν=4.9 eV, a surface state with 0.65±0.1 eV binding energy is found. The dispersion of this state with k∥ is well‐described by an effective mass m* equal to the free electron mass me. This state is assigned as the n=1 member of the image potential series of states. The relationship between the dispersion of this state and its position in the (111) projected bulk band gap is discussed.

Single and double shake-up processes in the Ne 1s photoelectron spectrum

Abstract: The core electron satellite spectrum accompanying a 1s ionisation in atomic Ne is studied using monochromatised X-ray excited (1487 eV photon energy) photoelectron spectroscopy. Several new 2s-3s satellites are observed and assigned. The shake-off limits for the 2p-np and 2s-ns series are determined and also the continuum strength is measured to 37% of the main 1s line intensity. Fano-type resonances are detected both in the 2p-np and 2s-ns single shake-up regions and among the double shake-up lines. In the double shake-up region 18 lines are observed and a preliminary assignment is made using theoretical MCSCF calculations.

Resonances in the photoemission cross section of a surface state: Results for Ag(111)

Abstract: We present a model to explain the resonances in the photoemission cross section of a surface state as a function of photon energy. The model is based on an extension of the direct-transition model for bulk states, and should be applicable to a surface state with a long decay length. Energy broadening of the final states and momentum broadening of the initial surface state are included in the consideration. Resonances in the surface-state photoemission cross section are expected to occur for photon energies corresponding to direct transitions from the surface state to final Bloch states. The cross section of a surface state in Ag(111) was measured over the photon energy range of 36--65 eV, and a resonance at 54 eV was observed. The data were compared with the theoretical results. The data for a similar surface state in Cu(111), studied previously by Louie et al. (Phys. Rev. Lett. 44, 549 (1980)) were reanalyzed in terms of the present model. The results for these two systems, Ag(111) and Cu(111), will be compared.

A study of valence ionization of ammonia with synchrotron radiation

Abstract: Angular distributions of photoelectrons from the three valence levels of molecular ammonia, 2a1 with a binding energy (BE) of 27.7 eV, 1e (BE = 16.3 eV) and 3a1 (BE = 10.9 eV) have been measured in the 30–130 eV photon energy range. Comparison is made in the case of the 3a1 molecular orbital with the calculated atomic N 2p asymmetry parameters. Photoionization branching ratios have also been obtained in this photon energy range and good agreement has been found with previous quasiphotoionization measurements and with the one-center-expansion calculations of Cacelli et al. The satellite structure in the inner valence (2a1) region has been investigated at 60 eV and compared with a number of energy and intensity calculations.

A compton scattering spectrometer for determining X-ray photon energy spectra

Abstract: The importance of knowing energy spectra of the primary photons from clinical X-ray tubes under working conditions increases with the use of more sophisticated diagnostic technologies. At the high fluence rates of primary photons from diagnostic X-ray tubes it is nessesary to strongly reduce the number of photons to the detector per unit time in order to avoid pulse pileup. The compton scattering method is very suitable for this reduction. This method, applied in the diagnostic energy range by Yaffe et al. [1] is further developed in this work. The electrons moving in the target cause an energy broadening (Compton broadening) of the scattered photons. This broadening smears out the characteristic peaks of the anode material and results in a decreased energy resolution. Comparison of the spectrum obtained with the Compton spectrometer, and unfolded with our reconstruction, to the spectrum measured directly in the primary beam shows a very good agreement, although relatively simple reconstruction algorithms are used.

Charge distribution of Xe ions as a result of multiple photoionisation of Xe atoms between 4.1 and 8.0 keV

Abstract: The charge distributions of Xe ions as a result of photoionisation have been studied experimentally and theoretically. The initial vacancies are produced by 4.1-8.0 keV synchrotron orbital radiation and the charge distribution of ions was measured with a time-of-flight mass spectrometer at various photon energies. The theoretical estimation of the charge distribution was made by the use of the Monte Carlo simulation. The calculated charge distributions as a function of incident photon energy are in good agreement with the measured results.

Optical admittance spectroscopy: A new method for deep level characterization

Abstract: A new method for the optical cross‐section measurement of deep levels in junctions is presented. It is based on the measurement of the capacitance and conductance of the junction under illumination, as a function of the photon energy hν and the frequency of the measuring signal. This technique has been applied to the measurement of the optical capture cross section σon of the Au acceptor level corresponding to the emission of electrons to the conduction band.

Characterization of phosphors in the soft X-ray region

Abstract: The phosphor detector has unique properties for soft X-ray spectroscopy. As a calorimetric detector, it is insensitive to surface contamination, it is rather rugged, and reproducible. We examined the response functions of the phosphors P31, P15, P46-Y1, P46-Y2, and P47 to photons in the energy range of 320–1000 eV and in the flux range of 107–1010 photons/s mm2. All phosphors showed nearly linear response with X-ray intensity, but only P31 showed nearly linear response with the photon energy. The luminescence yield from other phosphors increases less rapidly with photon energy. The energy response function is also found to depend slightly on the way the phosphor is applied.

Angle‐resolved photoelectron spectroscopic study of the outer‐ and inner‐valence shells of NH3 in the 20–80 eV photon energy range

Abstract: We report here highly resolved photoelectron spectra of the outer‐ and inner‐valence shells of NH3, together with the corresponding angular distribution parameter. The measurements were made in the 20–80 eV photon energy range, using synchrotron radiation as an ionizing source. In the inner‐valence region, three spectral features were evidenced, and namely a main component at 27.4 eV and two weaker features at 31.4 and 35.4 eV. The spectral feature at 35.4 eV was not evident in the previous (e, 2e) measurements. Our experimental findings are compared with three different sets of calculations performed by the Green’s function method. The best agreement is found with one of them which predicts three main lines and some low‐intensity components. From the angular distribution measurements, we conclude that the nature of the spectral feature at 31.4 eV is different from the one of the main component at 27.4 eV.

Partial photoionization cross sections of Kr 3d, 4s, and 4p levels in the photon energy range 37-160 eV.

Abstract: The relative photoionization cross sections of Kr 3d, 4s, and 4p levels have been determined from photoelectron measurements using synchrotron radiation in the photon energy range 37\char21{}160 eV. By using the total absorption cross sections from absorption measurements (Marr and West) the absolute partial cross sections have also been determined. The experimental results are compared with the theoretical calculations of Huang et al.