Showing papers on "Photoexcitation published in 1991"
TL;DR: Relaxation of a nonequilibrium distribution of electrons and holes in GaAs following femtosecond photoexcitation is investigated via spectrally and time-resolved luminescence, demonstrating carrier-carrier scattering rates higher than predicted by calculations with a statically screened interaction potential.
Abstract: Relaxation of a nonequilibrium distribution of electrons and holes in GaAs following femtosecond photoexcitation is investigated via spectrally and time-resolved luminescence. A rapid onset of luminescence over a broad spectral range shows that both electrons and holes are redistributed over a wide energy range within 100 fs, even at excitation densities as low as ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The data demonstrate carrier-carrier scattering rates higher than predicted by calculations with a statically screened interaction potential. Monte Carlo simulations using dynamical screening account for the experimental results.
214 citations
TL;DR: An alternative method, utilizing the virtual-photon field of a fast electron inelastically scattered at negligible momentum transfer, avoids many of the difficulties associated with the various direct optical techniques that have traditionally been used for absolute optical oscillator strength measurements.
Abstract: An alternative method is described for the measurement of absolute optical oscillator strengths (cross sections) for electronic excitation of free atoms and molecules throughout the discrete region of the valence-shell spectrum at high energy resolution (full width at half maximum of 0.048 eV). The technique, utilizing the virtual-photon field of a fast electron inelastically scattered at negligible momentum transfer, avoids many of the difficulties associated with the various direct optical techniques that have traditionally been used for absolute optical oscillator strength measurements. The method is also free of the bandwidth (line saturation) effects that can seriously limit the accuracy of photoabsorption cross-section measurements for discrete transitions of narrow linewidth obtained using the Beer-Lambert law [${\mathit{I}}_{0}$/I=exp(nl${\mathrm{\ensuremath{\sigma}}}_{\mathit{p}}$)]. Since the line-saturation effects are not widely appreciated and are only usually considered in the context of peak heights, a detailed analysis of this problem is presented, with consideration of the integrated cross section (oscillator strength) over the profile of each discrete peak.The suitability of the high-resolution dipole (e,e) method for general application to atomic and molecular electronic spectra is evaluated by test measurements of the absolute dipole (optical) oscillator strengths for the photoexcitation and photoionization of helium, since for this atom detailed quantum-mechanical calculations using highly correlated wave functions have been reported. The absolute scale is obtained from the Thomas-Reiche-Kuhn sum-rule normalization of the Bethe-Born transformed electron-energy-loss spectrum and does not involve the difficult determinations of photon flux or target density. The measured dipole oscillator strengths for helium excitation (1 $^{1}$S\ensuremath{\rightarrow}n $^{1}$P, n=2--7) are in excellent quantitative agreement with the calculations reported by Schiff and Pekeris [Phys. Rev. 134, A368 (1964)] and by Fernley, Taylor, and Seaton [J. Phys. B 20, 6457 (1987)]. The absolute measurements are also compared with other experimental and theoretical oscillator strength determinations for photoexcitation and photoionization processes in helium up to 180 eV, including the 2snp and 3snp autoionizing resonances in the 59--72-eV energy region.
184 citations
TL;DR: In this paper, the reaction steps occurring immediately after photoexcitation are temporally resolved in pump and probe measurements, where the onset of emission of the reaction product is monitored, at femtosecond time scale.
Abstract: the femtosecond time scale. The reaction steps occurring immediately after photoexcitation are temporally resolved in pump and probe measurements, where the onset of emission of the reaction product is monitored. For HBT in nonpolar solvents, the transfer of the proton occurs with a time constant of 160
121 citations
TL;DR: In this paper, the triplet spectrum with apparent band peaks at 330 and 745 nm covers the whole spectral range of 290-850 nm, and essentially all the photon energy absorbed is released as heat.
99 citations
TL;DR: In this paper, the authors report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared-multiphoton excitation.
Abstract: We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared‐multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (>1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k’sbb≂30 to 100 cm−1 ), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N=6 poly...
95 citations
TL;DR: In this paper, high resolution photoemission studies of solid O 2 in contact with multilayer films of C 60 on solid substrates showed conversion to CO, CO 2 carbonyl-like structures, and a carbon residue upon exposure to low energy and high energy photon sources.
91 citations
TL;DR: In this paper, the rovibrational state distribution of the nascent NH2(A, 2A1) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH3 is determined through an analysis of a major portion (approximately 13,000 cm−1) of the NH 2(A)→X 2B1 near infrared emission spectrum obtained by time-resolved Fourier transform infrared emission spectroscopy.
Abstract: The rovibrational state distribution of the nascent NH2(A 2A1) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH3 is determined through an analysis of a major portion (6000–13 000 cm−1) of the NH2(A 2A1→X 2B1) near infrared emission spectrum obtained by time‐resolved Fourier transform infrared emission spectroscopy. The NH2(A) fragments are observed to be formed predominantly in their zero‐point vibrational level, with substantial rotational excitation about their a‐inertial axis up to the limit of the available energy, ∼3150 cm−1, but with little excitation about the other axes. The pattern of this energy disposal is discussed within the framework of existing knowledge regarding the form of the NH3 A state potential energy surface on which the dissociation occurs. The essential features are entirely consistent with a direct carry over, into the fragment, of the out‐of‐plane bending vibrational motion introduced in the parent molecule by the photoexcitation process.
67 citations
TL;DR: In this paper, the photodissociation of Mo(CO)6 adsorbed on potassium free and potassiumpreadsorbed Cu(111) and Si(111)-7×7 at 85 K has been studied under ultrahigh vacuum conditions.
Abstract: Photodissociation of Mo(CO)6 adsorbed on potassium‐free and potassium‐preadsorbed Cu(111) and Si(111)7×7 at 85 K has been studied under ultrahigh vacuum conditions. The photodissociation yield has been measured as a function of photon power (0.5–30 mW/cm2 ), wavelength (250–800 nm), polarization (s and p), and incident angle (20°–70°). Two surface photoreaction mechanisms are considered: (i) direct electronic excitation of the adsorbate and (ii) attachment of photogenerated hot carriers to the adsorbate. The photodissociation spectra obtained on K‐free Cu(111) and Si(111)7×7 exhibit the same resonant structure as the absorption spectrum of Mo(CO)6. Photodissociation of Mo(CO)6 on K‐free surfaces is thus determined to be dominated by direct electronic excitation of the adsorbate, which proceeds via a single‐photon process. A new photodissociation channel is opened on K‐preadsorbed surfaces. The photoyield increases substantially in the UV and extends to the visible and near IR. By studying the wavelength a...
61 citations
TL;DR: Formation of Frenkel pairs has been found to occur not at the initial stage of relaxation but as a result of the relaxation leading to the lowest STE.
Abstract: Optical-absorption change following photoexcitation of the lowest triplet self-trapped exciton (STE) in NaCl has been measured in the femtosecond time regime. It is found that a short-lived intermediate state is generated within 1 ps and decays accompanied by restoration of the lowest STE. An oscillation in the population of the lowest STE and the intermediate state with a period of 1.1 ps was observed. Formation of Frenkel pairs has been found to occur not at the initial stage of relaxation but as a result of the relaxation leading to the lowest STE.
52 citations
TL;DR: The spin forbidden 3Σ+u ← X 1 Σ+g transition of molecular chlorine was investigated in this paper, where the maximum quantum yield for permanent dissociation was found to be 0.3.
Abstract: A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10−18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge‐transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the ...
48 citations
TL;DR: Using synchrotron radiation as a continuum light source, the photoabsorption and photoionization cross sections of NH3, PH3, H2S, C2H2, and C 2H4 have been measured from their respective ionization thresholds to 1060 A as mentioned in this paper.
Abstract: Using synchrotron radiation as a continuum light source, the photoabsorption and photoionization cross sections of NH3, PH3, H2S, C2H2, and C2H4 have been measured from their respective ionization thresholds to 1060 A. The vibrational constants associated with the nu(2) totally symmetric, out-of-plane bending vibration of the ground electronic state of PH3(+) have been obtained. The cross sections and quantum yields for producing neutral products through photoexcitation of these molecules in the given spectral regions have also been determined. In the present work, autoionization processes were found to be less important than dissociation and predissociation processes in NH3, PH3, and C2H4. Several experimental techniques have been employed in order to examine the various possible systematic errors critically.
TL;DR: In this article, the dependence of the photoionization cross-section on photon energy is calculated for shallow donors in infinite-barrier GaAs/Ga1−xAlx As quantum as well as a function of well width.
TL;DR: In this article, photoionization of small clusters of the refractory elements niobium, tantalum, and tungsten at 308 nm is interpreted in terms of a sequential two-photon excitation mechanism in which a rapid coupling between electronic and vibrational degrees of freedom is proposed.
Abstract: Photoionization of small clusters of the refractory elements niobium, tantalum, and tungsten at 308 nm is interpreted in terms of a sequential two‐photon excitation mechanism in which a rapid coupling between electronic and vibrational degrees of freedom is proposed. Ionization is slow and may compete with dissociation. The analogy with thermionic emission is made.
TL;DR: In this paper, photoexcitation of the isomers of 19 nuclides was examined in four accelerators were used as sources of bremsstrahlung to expose the samples and end-point energies covered the range from 0.5 to 11 MeV.
Abstract: Photoexcitation of the isomers of 19 nuclides was examined in this work. Four accelerators were used as sources of bremsstrahlung to expose the samples and end-point energies covered the range from 0.5 to 11 MeV. No evidence was found for nonresonant processes of excitation. However, more than half the cases showed enhanced channels for the resonant photoexcitation of isomers with integrated cross sections approaching ${10}^{\mathrm{\ensuremath{-}}21}$ ${\mathrm{cm}}^{2}$ keV. These results are three to four orders of magnitude larger than values usually characterizing (\ensuremath{\gamma},\ensuremath{\gamma}') reactions.
TL;DR: In this paper, the authors measured yield curves of multiply-charged ions produced by photoexcitation in the vicinity of the Ar K-shell ionization threshold using monochromatized undulator radiation and a time-of-flight mass spectrometer.
Abstract: The authors have measured yield curves of the multiply-charged ions produced by the photoexcitation in the vicinity of the Ar K-shell ionization threshold using monochromatized undulator radiation and a time-of-flight mass spectrometer. The charge distribution of the multiply-charged Ar ions produced by the K-shell excitation suggests that both the electron in the outer shell and the excited Rydberg electron are shaken off with high probability during successive electronic relaxation processes. Post-collision interaction effects are also seen at the photon energies just above the K threshold. The fluorescence yield for the K hole is estimated to be 0.135+or-0.03 from the charge distribution above the K threshold.
TL;DR: In this article, the appearance energy in the F− photodissociation efficiency curve is about 2 eV higher than the thermochemical threshold for the formation of the groundstate ions F−(1Sg) and CF+3(X 1A’1) and peak features observed in the spectrum are interpreted as resulting from transitions to Rydberg states with the 1T2 symmetry.
Abstract: Ion‐pair formation from photoexcitation of CF4 has been studied by negative‐ion mass spectrometry using synchrotron radiation in the 11–31 eV photon energy range. Negative F− ions have been observed. The appearance energy in the F− photodissociation efficiency curve is about 2 eV higher than the thermochemical threshold for the formation of the ground‐state ions F−(1Sg) and CF+3(X 1A’1). The peak features observed in the spectrum are interpreted as resulting from transitions to Rydberg states with the 1T2 symmetry. Assignments of the peaks have been made on the basis of the previous work on photoabsorption cross‐section curves of CF4. Repulsive Rydberg states converging to CF+4(X 2T1, A 2T2) give the strong and broad peaks in the F− efficiency curve. Conversion to the ion‐pair state is considered to occur through the avoided potential surface crossings along the dissociation coordinate of the CF3—F bond. In contrast, the npt2 Rydberg states (n≥4) converging to CF+4(C 2T2) show long vibrational progres...
TL;DR: In this article, an emission band with peak at 3.3 microns was observed from benzene, naphthalene, phenanthrene, anthracene, pyrene, and their methyl derivatives in the gas phase at 193 nm using an ArF laser as a light source.
Abstract: Infrared emissions are observed from photoexcitation of benzene, naphthalene, phenanthrene, anthracene, pyrene, and their methyl derivatives in the gas phase at 193 nm using an ArF laser as a light source. An emission band with peak at 3.3 microns is observed from all the molecules studied. This band is attributed to the aromatic C-H stretching vibrational modes. In addition to the 3.3-microns band, an emission band at 3.4-3.6 microns is observed from photoexcitation of the methyl derivatives, which is attributed to the C-H stretching vibrational modes of the CH3 group. The observed spectra are compared with the IR emission bands observed in many astronomical objects. The current laboratory data lend support to a model in which PAH molecules are responsible for the 'unidentified' interstellar IR emissions. IR emissions from the 193-nm excitation of several interstellar molecules other than PAHs are also observed, but they are quite different from the interstellar IR bands.
TL;DR: In this article, the quantum yield of porphyrin singlet-oxygen luminescence in water, D 2 O, benzene and toluene at photoexcitation by argon laser light has been measured and, based on these data, the emission rate constant of oxygen, K em, has been calculated.
TL;DR: Close-coupling calculations for photoionization and recombination using the R -matrix method is described, and various approaches are described to overcome practical difficulties arise.
Abstract: We describe close-coupling calculations for photoionization and recombination using the R -matrix method. Photoionization of a large number of bound states of each atom or ion is considered, and cross sections are computed at a fine mesh of energies to delineate the extensive resonance structures. Photo ionization of ground and excited configurations with n ~ 10 and 1~ 3 is considered. Partial photoioniza tion cross sections into excited states of the residual ion are also obtained for the ground states of CJ, N+, and 0 2 + . An important type of autoionizing resonances due to the photoexcitation of the core ion is discussed. As the photoiomzation cross sections incorporate autoionization in detail, it is possible in principle to employ the Milne relation to obtain total, radiative plus dielectronic, recombination cross sections and rate coefficients. However, practical difficulties arise, and various approaches are described to overcome these. Comparison is made with the earlier results for photoionization cross sections and recombination coefficients. This work forms an extension of the Opacity Project.
TL;DR: In this article, the photorefractive bismuth germanium oxide (BGO) at low temperatures to 2.5-3.3 eV light produces photochromic absorption bands.
Abstract: Exposing the photorefractive bismuth germanium oxide (BGO) at low temperatures to 2.5–3.3 eV light produces photochromic absorption bands. In both undoped and Fe‐doped material the absorption consists of overlapping bands at 1.5, 2.2, 2.7, and 3.1 eV. The 1.5‐ and 2.2‐eV components are relatively weaker in Fe‐doped BGO. In undoped BGO the photochromic bands anneal together above 200 K. Doping with iron adds an anneal stage in the 110–200 K range that matches the recovery of Fe3+. Optical bleaching with 1.5‐eV light was much less efficient than with visible band light. Thus, the 1.5‐eV photochromic band may be an internal transition of the center responsible for the 2.2‐eV band. In BGO:Al the aluminum electronically compensates the deep donor center responsible for the yellow coloration observed in undoped crystals. Photoexcitation at 9 K produces overlapping absorption bands at 1, 1.38, and 2.45 eV. All three bands have major anneal stage in the 80–100 K range. They also bleach out together when the sampl...
TL;DR: In this article, the authors measured the magnitude and transient response of the third-order nonlinear optical susceptibility χ(3) (ω;ω,−ω,ω) at 620 nm in nonoriented conducting polymers.
Abstract: Degenerate four‐wave mixing with femtosecond time resolution is used to measure the magnitude and transient response of the third‐order nonlinear optical susceptibility χ(3) (ω;ω,−ω,ω) at 620 nm in nonoriented conducting polymers including polydiacetylene, polyacetylene, polyaniline, polydiethynylsilane, polythiophene, and polythiophene derivatives. Resonant and nonresonant excitations influence the magnitude and transient response of χ(3). The electronic response is instantaneous for preresonant excitation, but for resonant excitation it has ultrafast and slow components which illustrate photoexcitation dynamics. The magnitude of χ(3) for all of the polymers is in the range from 10−10 to 5×10−8 esu depending on the energy difference between the laser excitation and the polymer absorption maximum.
TL;DR: In this article, the photoluminescence properties of polyoxomolybdoeuropate solid (NH4)12H2[Eu4(MoO4)(H2O)16(Mo7O24)4]·13H 2O are studied.
TL;DR: In this paper, the photoexcitation of the electron acceptor naphthalenedicarbonitrile (DCN) is required for cycloadditions of azirines with imines, which afford N-substituted imidazoles.
Abstract: Photoinduced electron-transfer conditions (the photoexcitation of the electron acceptor naphthalenedicarbonitrile (DCN)) are required for [3 + 2] cycloadditions of azirines (1) with imines, which afford N-substituted imidazoles (3). Probable intermediates are the radical cations 2, which were detected by the trapping reaction with trifluoroethanol.
TL;DR: In this paper, the aluminum-magnesium photoresonant x-ray laser scheme was investigated using detailed atomic models for both elements and it was found that monochromatic pump powers of 100 GW/eV are obtainable within the 48-A aluminum pump line over a broad range (125-400 eV) of pump plasma temperatures.
Abstract: The aluminum‐magnesium photoresonant x‐ray laser scheme is investigated using detailed atomic models for both elements. The aluminum pump source is assumed to be a Z pinch driven by a current of several MA. It is found that monochromatic pump powers of 100 GW/eV are obtainable within the 48‐A aluminum pump line over a broad range (125–400 eV) of pump plasma temperatures. The gain in Mg ix is calculated assuming separations of 1 or 2 cm from the pump plasma. The highest gain appears in the 2s3d 1D2‐2s4f 1F3 line at 228 A. However, photoionization as well as resonant photoexcitation appears to be crucial to the success of this scheme. If photoionization by lines other than the pump line is neglected, Mg ix does not exist below∼60 eV. In the 60–100‐eV Mg plasma temperature range, collisional excitation from low‐lying states to the lower laser level severely degrades the gain. However, when photoionization of Mg by all of the strong Al xi lines is taken into account, Mg ix is found to exist at much lower temp...
TL;DR: These studies permit the direct observation of a unique process, that of the recombination of surface electrons with bulk holes mediated by phonons at the surface, which is found to be the primary mechanism involved in the ultrafast decay of the transient surface electron population.
Abstract: Using angle-resolved laser photoemission we have studied the ultrafast electron relaxation processes on the Ge(111) π-bonded 2×1 surface with subpicosecond time resolution. Photoexcitation of the bulk bands and subsequent bulk-surface coupling results in a transient population of the normally unoccupied π * -antibonding surface band. Our studies permit the direct observation of a unique process, that of the recombination of surface electrons with bulk holes mediated by phonons at the surface, which we find to be the primary mechanism involved in the ultrafast decay of the transient surface electron population
TL;DR: In this article, large integrated cross sections for photoexcitation of 115Inm have been detected around Ex=3MeV and it is shown that the g.s. transition strength is largely confined to the same energy region and the intermediate excited states are identified.
TL;DR: In this article, a band transport model that uses a single donor level with only electrons as free charge carriers, taking the nonlinear recombination of free charge carrier (owing to variation of the trap density) and the time dependence of the free-charge-carrier density into account is presented.
Abstract: Photorefractive and photoconductivity steady-state as well as time-resolved measurements have been performed in strongly reduced KNbO3. The recorded photorefractive gratings show a nonexponential dark decay and an intensity-dependent two-wave mixing gain. The measured photoconductivity depends sublinearly on light intensity. The observed behavior is interpreted in terms of a band transport model that uses a single donor level with only electrons as free charge carriers, taking the nonlinear recombination of free charge carriers (owing to variation of the trap density) and the time dependence of the free-charge-carrier density into account. An analytical solution for the nonexponential dark decay of the photorefractive grating is derived. As an application of this model the results of the photorefractive and the stationary and transient photoconductivity experiments are used to determine the model parameters (thermal excitation rate, photoexcitation cross section, recombination coefficient, acceptor and donor densities, and electron mobility).
TL;DR: In this article, an optical-optical double resonance technique is used to study the ion-pair states of Br 2 correlating to Br − ( 1 S)+Br + ( 3 P 2 ).
Abstract: An optical-optical double resonance technique is used to study the ion-pair states of Br 2 correlating to Br − ( 1 S)+Br + ( 3 P 2 ). In the (1+2) photoexcitation sequence, the pump laser excites the A 3 II(1 u )−X 1 ∑ g + transition at a fixed frequency for the state selection while the probe laser is scanned to execute the second step by a coherent two-photon transition
TL;DR: The photophysics of silylene (SiH2), formed during the IRMPD of organosilanes, was investigated using photofragmentation excitation spectroscopy (PHOFEX) as mentioned in this paper.
Abstract: The photophysics of silylene (SiH2), formed during the infrared multiphoton dissociation (IRMPD) of organosilanes, is investigated using photofragmentation excitation spectroscopy (PHOFEX). Silylene molecules are formed in the X 1A1(000) ground state via IRMPD of n‐butylsilane. Laser induced fluorescence (LIF) is used to detect ground state (3p2 3P0) Si atoms following rovibronically resolved photoexcitation of SiH2 to the A 1B1(0v20) state. Variations in Si atom production are measured simultaneously with the SiH2 excitation spectrum, allowing comparisons to be made between Si yield and the rovibronic structure in the SiH2 1B1 manifold. We have examined the correlation between the widely varying fluorescence lifetimes of the individual rovibronic states of SiH2 and the relative yields of Si production. The presence of additional Si precursors in the primary dissociation process is suggested. Mechanisms for Si release following IRMPD of n‐butylsilane and electronic excitation of SiH2 are developed and d...