Showing papers on "Photon energy published in 1990"

Above-threshold dissociation of H+2 in intense laser fields.

Abstract: We present nonperturbative, time-independent calculations of the photodissociation rate of H{sup +}{sub 2} in intense laser fields. The energy distribution of the protons consists in a sequence of peaks evenly spaced by half the photon energy, all of equal width but of varying heights. They result from multiphoton absorption above the dissociation threshold, with equal sharing of the excess photon energy between H and H{sup +}. Surprisingly, the distribution of higher-energy peaks decreases with increasing intensity, due to stimulated free-free emission of the dissociating fragments. We also predict a sharp angular distribution of the protons along the electric field vector of a linearly polarized laser.

Cross sections and NO product state distributions resulting from substrate mediated photodissociation of NO2 adsorbed on Pd(111)

Abstract: Ultraviolet irradiation of NO2 adsorbed on top of a NO saturated Pd(111) surface causes the photodissociation of NO2/N2O4 and results in the desorption of NO molecules. This process has been studied using excitation energies between 3.5 and 6.4 eV. At a photon energy of 6.4 eV, a cross section of 3×10−18 cm2 is found. Using laser‐induced fluorescence to detect the desorbed NO molecules, fully state‐resolved data detailing the energy channeling into different degrees of freedom has been obtained. Two desorption channels are found, one characterized by nonthermal state populations, and one showing accommodation to the surface. The yield of the fast channel shows a marked increase above 4 eV photon energy. The slow channel is interpreted as being due to NO molecules which, after formation, undergo a trapping–desorption process. A polarization experiment indicates that the photodissociation is initiated by excitation of metal electrons rather than direct absorption by the adsorbate.

Femtosecond laser study of energy disposal in the solution phase isomerization of stilbene

Abstract: Femtosecond laser studies of cis‐stilbene isomerization show the absorption of the hot trans product decaying in 20 ps. An initial internal temperature of 725±100 K is estimated which decays with a ca. 14 ps time constant. This temperature accounts for less than one‐half the photon energy implying significant energy loss by solvent friction.

Dielectric function of CaF2 between 10 and 35 eV.

Abstract: The dielectric function of ${\mathrm{CaF}}_{2}$ over the photon energy region 10--35 eV has been determined by spectroscopic ellipsometry. Close to the fundamental absorption edge the data agree fairly well with band-structure calculations. A sharp exciton at the Ca 3p threshold (27.9 eV) is immediately followed by structures which can be attributed to variations in the conduction-band density of states. The strong transitions observed at higher energies cannot be explained in the one-electron band-structure model. Detailed theoretical calculations are required.

Absolute x‐ray power measurements with subnanosecond time resolution using type IIa diamond photoconductors

Abstract: Photoconductive devices have been fabricated from type IIa diamonds. The sensitivity of these devices is independent of photon energy from 200 to 2200 eV. The dynamic range is 105. The large band gap of the diamond greatly reduces the sensitivity to photons with an energy less than 5.5 eV which is an attractive feature for many applications. The carrier lifetime in the material is 90 ps and the mobility is 1650 cm2/V/s at 106 V/m.

Optical and magneto‐optical tensor spectra of bismuth‐substituted yttrium‐iron‐garnet films

Abstract: We report on the complete dielectric tensor spectra of Y3−xBixFe5O12 with 0≤x≤1.42 in the photon energy range 1≤ℏω(eV)≤4.8, i.e., 1240–248 nm wavelength. In an attempt to identify the involved optical transitions we have (i) parametrized these spectra in terms of Gaussian (e‘11 spectra) and diamagnetic or paramagnetic (e’12 and e‘12 spectra) line shapes, and (ii) come up with a set of coefficients of a cubic interpolation polynomial for each spectral ‘‘line’’ from which any of the n, α, ΘF, eF, ΘK, eK, e’11, e‘11, e’12, e‘12 spectra can be calculated for any bismuth substitution in the above compositional and spectral range. It is found that such a line‐shape analysis is not unique and requires physical knowledge on the optical excitations involved which, however, is still incomplete for iron garnets.

Observation of resonant photon drag in a two-dimensional electron gas.

Abstract: We observe a photon-drag voltage along the optical path of photons propagating in the plane of an unbiased 2D electron gas in GaAs quantum wells. This voltage is induced by momentum transfer from 10-\ensuremath{\mu}m-wavelength photons to the electrons, resonantly matched in their intersubband energy to the photons by appropriate choice of the 3.2-nm well width. The voltage is reversed when the propagation direction of the photons is reversed. Moreover, the sign of the voltage also changes by sweeping the photon energy through the intersubband resonance condition in agreement with current theory.

Nonradiative decay pathways of electronic states of group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation

Abstract: The nonradiative decay channels of the valence electronic states of the gas‐phase tetrahedral ions CF+4, SiF+4, CCl+4, SiCl+4, and GeCl+4 have been studied in the range 35–100 nm by a novel form of photoionization mass spectrometry. Tunable vacuum UV radiation from a synchrotron source ionizes the parent neutral molecule, and electrons and ions are detected by the photoelectron–photoion coincidence technique. The experiment is repeated continuously as a function of photon energy, and a three‐dimensional histogram of photon energy versus ion time of flight versus coincidence count rate is produced. By taking cuts through this histogram, photoionization curves for the different fragment ions can be extracted. The appearance energies of the fragment ions (e.g., CF+2 from CF4, CCl+ from CCl4) occur at the adiabatic ionization potential of an electronic state of the parent ion, and not at the thermodynamic appearance energy of that ion. Attempts to measure the kinetic‐energy releases in the fragmentation pathw...

Photoion anisotropy in dissociative photoionization of CF3I

Abstract: Dissociative photoionization of CF3I is studied in the region of the CF3I+ A state (13–15 eV). We observe a pronounced anisotropy of the I+ fragment ions in the time of flight coincidence spectra obtained at 14.24 and 15.0 eV photon energy, but having the same 13.3 eV parent ion internal energy. It is the result of a strong alignment in the photoionization process at these energies, probably related to a specific autoionization process. A new and lower value of 13.40±0.05 eV is measured for the appearance potential of the CF2I+ fragment.

Variable photon energy photoelectron spectroscopy on FeCl4-. An unusual electronic structure for high-spin d5 complexes

Abstract: Variable energy photoelectron spectroscopy (PES) is used to elucidate the valence band electronic structure and bonding in tetrahedral d{sup 5} FeCl{sub 4}{sup {minus}}. PES spectra obtained over the photon energy range 25-150 eV show intensity changes in the valence band features which indicate that more metal character is present in the deepest bonding levels. This is inverted from the normal electronic structure description of transition-metal complexes. The lack of off-resonance intensity in the deep binding energy satellite, which corresponds to a two-electron transition involving metal ionization plus ligand-to-metal charge transfer, indicates that little relaxation occurs on ionization. This result is confirmed by analysis of the satellite structure in the core level XPS Fe 2p spectra. PES spectra taken at the Fe 3p absorption edge, which provide insight into the bonding description of the ionized final state, show dramatic resonance intensity enhancement of the main band peaks as well as the satellite.

Photon-photon scattering of gamma rays at cosmological distances

Abstract: The total scattering rate, the optical depth of the universe, and the scattered photon spectrum are derived from gamma-ray photons scattering in an isotropic field of blackbody photons. By solving the kinetic equation describing repeated photon-photon scattering for both monoenergetic and power-law injection of photons, it is shown how the photon spectra resulting from pair-photon cascades are further reprocessed by photon-photon scattering. The power of photons injected at the photon energy epsilon(max), where the pair cascades terminate, is reprocessed into a characteristic spectral jump at epsilon(min). Photon-photon scattering thus reduces the spectral cutoff energy from epsilon(max) to epsilon(min). Photon-photon scattering is likely to be important in consideration of the constraints imposed by the primordial abundances on the properties of exotic unstable particles. 41 refs.

A soft X-ray grating monochromator for undulator radiation

Abstract: One of the new insertion devices to be installed in the expansion of the DORIS storage ring consists of three undulators designed to cover with the first harmonic the photon energy range between 70 and 2000 eV. The concepts for a high-resolution and high-throughput monochromator that will take full advantage of this powerful new beam line are discussed. The final design is a modified SX-700 type monochromator in which the focusing elliptical mirror is replaced by a spherical one. The aberrations due to the new mirror are minimized by elongating the exit slit arm of the conventional SX-700. The ultimate resolution expected is 0.03 eV at 152 eV and 0.6 eV at 1086 eV with a photon flux up to 5 × 1011 photons/(s-100 mA) at the sample. The spot size at the sample position is less than 0.4 × 1.0 mm2.

Angular dependence of photofragments from K‐shell excited N2 and O2: Photoabsorption anisotropy and excited state symmetry

Abstract: The symmetry of excited states reached by photoexcitation of K‐shell electrons of N2 and O2 is identified from the fragmentation anisotropy. For each molecule, ion yield excitation spectra were obtained at 90°, 54.7°, and 0° relative to the axis of soft x‐ray polarization. The degree of polarization of the synchrotron radiation was determined experimentally. From this determination and the angular dependence of the ion yield, the photoabsorption anisotropy parameter β is obtained as a function of photon energy. Variations in the β spectra are discussed in terms of previously assigned spectral features and are compared with other experimental results. A comparison also is made with theoretical calculations, which are available for the case of N2.

Single- and double-photoionization cross sections of atomic nitrogen from threshold to 31 Å

Abstract: The relative photoionization cross section of atomic nitrogen for the production of singly and doubly charged ions has been measured from 44.3 to 275 A and from 520 to 852 A. The results have been made absolute by normalization to one-half of the molecular nitrogen cross section at short wavelengths. The smoothed atomic nitrogen cross sections sigma can be accurately represented, at short wavelengths, by the equation sigma(Mb) = 36,700 x (E exp-2.3) as a function of the photon energy E (eV), thereby allowing the cross sections to be extrapolated to the nitrogen K edge at 31 A.

One photon double ionization of helium : threshold behaviour

Abstract: In order to study the dynamics of double photoionization of helium, we report new coincidence measurements between low energy electrons and doubly charged ions, from 78 to 95 eV photon energy. We show that the range of validity of the Wannier theory depends upon the observable. For the exponentn of the threshold law, this range amounts to some 3 eV above onset, thus confirming previous published experimental work. In contrast, the energy distribution of the two outgoing electrons is found flat, within 20%, in agreement with the theoretical predictions, but in a 15 eV energy range above threshold.

Measurement of L X-ray production cross sections in elements 57⩾Z⩾92 at 22.6 keV

Abstract: L X-ray fluorescence cross sections have been measured for elements of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Ta, W, Au, Hg, Tl, Pb, Bi, Th and U at a photon energy of 22.6 keV, using K conversion X-rays of Ag. The present results are discussed in the light of the other available experimental and theoretical results.

Room temperature photopumped ZnSe lasers

Abstract: Room-temperature laser oscillation at 469 nm observed in photopumped bulk ZnSe is discussed. The result was obtained by using a pump photon energy very close to the bandgap energy and by enhancing the resonator Q with high-reflectivity coatings. Threshold pump power densities as low as 370 kW/cm/sup 2/ were measured. >

Photon‐induced desorption of CO chemisorbed on the oxidized Ni(111) surface

Abstract: Photon‐induced desorption studies of chemisorbed CO on Ni(111), Ni(111) with chemisorbed oxygen, and oxidized Ni(111) surfaces have been carried out. Only the oxidized Ni(111) surface exhibits measurable CO photodesorption in the energy range from 1.8 to 4.1 eV. Both direct observation of CO photodesorption and post‐irradiation thermal desorption of CO have been performed. The photodesorption process is first order in photon flux and in CO coverage. At a photon energy of 4.1 eV, the cross section for CO photodesorption is measured to be 5×10−18 cm2. The measured threshold energy, 2.7±0.5 eV, is in good agreement with a model involving the O2−2p→Ni2+3d interband transition in NiO as the excitation process leading to photodesorption.

Multiple and partial photoionization cross sections in the Kr 3d ionization region

Abstract: Multiple photoionization cross sections of Kr have been measured in the photon energy region of 90-260 eV. Partial cross sections for valence ionization, 3d ionization and 3d shake (shake-up and shake-off) have been derived from these multiple photoionization cross sections. The experimental results are compared with a recent theoretical calculation.

Polarization-independent semiconductor waveguide

Abstract: A polarization independent guided wave semiconductor device is realized wherein the waveguide region of the device includes one or more strained quantum well layers wherein strain is designed to be tensile in nature so that energy subbands in the quantum well or wells are displaced by a predetermined amount in a direction opposite to that for the quantum size effect. Polarization independence is achieved when, for a lightwave signal having an incident mean photon energy below the absorption bandedge of the strained quantum well layer or layers, the ratio of the oscillator strengths versus the wavelength detuning for heavy and light holes in a first polarization (TE) is substantially equal to a similar ratio computed in a second polarization (TM). Wavelength detuning is understood to be the difference between the wavelength of operation and the wavelength of an exciton resonance peak. Passive and active or actively controllable waveguides and waveguide devices are described.

Reactive‐ion‐ and plasma‐etching‐induced extended defects in silicon studied with photoluminescence

Abstract: Defects introduced by reactive‐ion etching and plasma etching using deuterium have been studied in boron‐doped Si with the photoluminescence (PL) technique. We have observed a set of broad luminescence bands in the below‐band‐gap range between 1.05 and 0.8 eV. These bands change in intensity as well as in photon energy with annealing. This has been studied by isochronal annealing treatments from 75 to 800 °C in steps of 50 °C, each for 30 min. Directly after the plasma treatment we observe overlapping broad bands at liquid‐He temperature, with a peak around 0.9 eV and a half‐width of about 100 meV. There is a large shift of these bands to higher photon energy after the annealing step at 325 °C, peaking at about 0.925 eV with a half‐width of about 60 meV. The intensities of the broad PL bands increase with increasing annealing temperature up to about 375 °C, while they decrease in intensity at higher temperatures. The changes in PL intensity of the broad bands after annealing are shown to be related to the...

Photoelectron spectroscopy of the freon molecules CF3Cl, CF2Cl2 and CFCl3 using synchrotron radiation from 41 to 160 eV

Abstract: Photoelectron spectra of the outer and inner valence regions of the freon molecules CF 3 Cl, CF 2 Cl 2 and CFCl 3 were obtained using synchrotron radiation in the photon energy range 41–160 eV. The spectra were analyzed using a curve fitting procedure to yield photoelectron branching ratios as a function of photon energy. These were then combined with recently measured total photoabsorption oscillator strengths to give electronic state partial photoionization oscillator strengths (cross sections). The results for CF 3 Cl and CF 2 Cl 2 are in generally good agreement with previously published photoelectron branching ratios. In addition the CF 3 Cl data is quite successfully predicted by MS-Xα calculations. The gross features of the branching ratios and oscillator strengths with changing photon energy may be accounted for in terms of the constituent atomic orbital characters of the ionized molecular orbitals. The inner valence molecular orbitals of F 2s character are found to make large contributions to the total photoionization oscillator strength (10–20%) at photon energies above ≈ 70 eV.

New detection method of hot-carrier degradation using photon spectrum analysis of weak luminescence on CMOS VLSI

Abstract: A method developed to find the weakest transistor against hot-carrier-induced degradation by counting photon emission of various wavelengths in an operating VLSI circuit is presented. The method's underlying principle is that high-energy photons emitted from the transistors are caused by hot-carrier effects. The spectral distribution of photon energy emitted from n-channel MOSFETs is studied, and is found to follow the Maxwell-Boltzmann distribution. Photon emission with about 200 nm wavelength strongly correlates with hot-carrier-induced degradation. This method was applied to the static random access memory in a microprocessor. Transistors which are estimated to be seriously degraded by hot-carrier effect were detected. This method improves the reliability of VLSI circuits without long-term testing. >

Laser effects in photoionization. II. Numerical solution of coupled equations for atomic hydrogen

Abstract: The time-dependent Schr\"odinger equation was solved numerically, as a set of coupled equations, for an electron moving in three spatial dimensions, bound initially in the ground state of a hydrogen atom, and acted upon by a linearly polarized, single-frequency, classical electromagnetic field, turned on abruptly at t=0. Characteristic photoionization times ${\mathrm{\ensuremath{\tau}}}_{\mathrm{PI}}$ were computed for a wide range of laser intensities, at a photon energy of 5 eV. Comparison was made with the predictions of two widely used analytic expressions for ${\mathrm{\ensuremath{\tau}}}_{\mathrm{PI}}$. Emitted electron energy spectra and angular distributions were computed. The spectrum of radiation emitted during ionization was also determined.

Ionic fragmentation of SiH4 following the K-shell excitation

Abstract: Ionic fragmentation of SiH4 has been studied in the vicinity of the Si: K-edge in the photon energy range 1800-1900eV. Ionic fragments observed at photon energies above the Si: ls → σ* excitation are mostly atomic ions Siq+ (q = 1-3) and H+. Ratios of the abundances for Si+, and Si2+ and Si3+ are roughly 4:4:1, and the abundance of H+ is much higher than the sum of the Si+, Si2+ and Si3+ abundances. The averaged kinetic energy release given to H+ is estimated to be about 12eV. This complete decomposition of SiH4 with the ejection of energetic H+ is interpreted as Coulomb explosion of the multiply-charged (mainly +4 or +5) parent ion produced via the vacancy cascade (successive Auger and/or Coster-Kronig transitions).

Resonant tunneling photodetector for long wavelength applications

Abstract: A long wavelength detector is formed by coupling a highly doped cathode to an anode through an undoped quantum well or superlattice filter structure. The absorption mechanism is free-carrier absorption in a heavily doped direct bandgap semiconductor (the cathode). The cathode material is preferably chosen such that the conduction band edge is lower than the conduction band edge of the material forming the well of the resonant-tunneling filter. The cathode material should also be semiconductor material of relatively narrow direct bandgap with low effective electron mass. The detector device is biased so that electrons pass through the filter structure by resonant tunneling. To stimulate conduction, incident radiation must have a frequency (i.e., photon energy) sufficient to boost the energy of the cathode electrons from the Fermi energy level of the cathode to the resonance energy level of the quantum-well filter. Thus, photons of incident radiation are absorbed in the cathode which may also be referred to as the photon absorber. In a second, voltage-tunable, embodiment a single quantum well filter having an undoped subband is disposed between a doped cathode and an anode structure. The single quantum well filter structure includes a quantum well with one or more subbands surrounded by two barrier regions. The detection frequency is changed by increasing or decreasing the bias slope of the conduction band which, in turn, moves the quantum well subband energy level relative to the cathode Fermi energy level.