Photon-induced Electron Research Articles

Sub-keV X-Ray Spectroscopy Using a Low-Pressure Electron Counting Detector

A technique for sub-keV x-ray spectroscopy over a large area at room temperature is presented. Photon-induced ionization electron trails deposited in a low-pressure gas are efficiently counted by a multi-stage electron multiplier. The analysis of the number of deposited electrons and the electron trail length, combined with a detailed simulation of the detection process, provide a powerful method for resolving x-ray lines down to 70 eV. Detection efficiencies above 40% and an energy resolution of 55% (FWHM) were measured for the Be K (108.5 eV) x-ray line. It is demonstrated that light elements with an abundance below 1% in composite samples can be quantitatively resolved. The experimental technique, analysis method and x-ray spectroscopic results obtained with the particle-induced x-ray emission technique and at a scanning electron microscope are presented.

Application of the electron-counting method to low-Z elemental analysis

We applied the electron counting method, in low-density gas, for ultra-soft X-ray spectroscopy. The method is based on the spatial expansion of photon-induced ionization electron clusters, and the recording of pulse trails formed by individual electron avalanches in a fast multiplication element. A detector, operating at room temperature with low gas pressure, was characterized with Particle-Induced X-ray Emission (PIXE) and with electron-induced X-ray emission in a Scanning Electron Microscope (SEM). High detection efficiency for soft X-rays is obtained due to the very thin polymer window between the detector and the radiation source. The low-density detector is blind to Bremsstrahlung and charged-particle background. We present the results of the spectral analysis of characteristic X-rays emitted from low-Z elements, in the energy range 100–1500 eV. The recorded X-ray spectra, using both the information of the number of counted electrons and the length of the electron pulse trail, are unfolded with the help of a computer simulation of the detector response to X-ray photons. This detailed simulation of the electron detection and counting process provides an efficient means for a quantitative spectral analysis and permits the precise reconstruction of complex energy spectra.

Photon-induced electron attachment and electron detachment chemistry of p-benzoquinone and its methylated derivatives

Two aspects of the simultaneous interaction of low-energy electrons and light with p-benzoquinone (BQ) and three methyl- substituted BQs are characterized here using an atmospheric pressure ionization mass spectrometer (APIMS) and a photode- tachment-modulated electron capture detector (PDM-ECD). An optically enhanced electron capture reaction by BQ is shown to occur by a mechanism involving the photochemical production of the dimer, (BQ) 2, followed by electron capture by this species. Electron photodetachment cross sections of the benzoquinone negative ions are also reported here for the first time and indicate that the structures of the parent neutrals and molecular anions are significantly different.

Doubly ionized states of hexafluorobenzene studied by high-resolution Auger electron and double-charge-transfer spectroscopies

Double-charge-transfer (DCT) spectroscopy has been used to measure the double-ionization energies of C6F6. OH+ and F+ projectile ions with translational energy of 6 keV, which tend to populate triplet states in the dication, were used. From the position of peaks in the resultant DCT spectra, C6F6 double ionization energies of 26.3 ± 0.5, 28.4 ± 0.5, 29.2 ± 0.5, 32.2 ± 0.5, 35.0 ± 1 and 40.0 ± 1 eV were determined. The photon-induced Auger electron spectra of C6F6 were recorded. The Auger electron spectra are dominated by transitions to final states of singlet symmetry. Ten distinct structures are observed in the C 1s Auger electron spectrum centred at double ionization energies of 26.63 ± 0.1, 30.37 ± 0.1, 34.68 ± 0.1, 36.26 ± 0.1, 37.84 ± 0.1, 39.19 ± 0.2, 42.73 ± 0.3, 46.51 ± 0.1, 49.20 ± 0.1 and 50.40 ± 0.2 eV. Five structures were observed in the F 1s outer–outer spectrum, centred at double ionization energies of 42.20 ± 0.1, 46.14 ± 0.1, 48.61 ± 0.1, 52.53 ± 0.1 and 56.9 ± 0.1. Additional structures are observed in the outer–inner and inner–inner spectrum. A comparison has been made between the results from the present work and those obtained in a recent coincidence study (K. Ibrahim, P. Lablanquie, M-J. Hubin-Franskin, J. Delwiche, M. Furan, I. Nenner, D. Hagan and J. H. D. Eland, J. Chem. Phys., 1992, 96, 1981). Green's function calculations have been performed to assist the interpretation of the spectra.

Doubly ionized states of N2O studied by photon-induced Auger electron and double charge transfer spectroscopies

The four lowest triplet state energy levels of N 2O2+ have been measured using a modified form of double-charge-transfer spectroscopy. By studying the energetics of the double-electron-capture reactions of 6 keV OH+ ions in collisions with N2O molecules, the lowest triplet state energy levels have been determined to be 36.0+or-0.5, 38.0+or-0.5, 40.9+or-0.5 and 42.0+or-0.5 eV. The singlet state energy levels of the N2O2+ ion have been investigated by means of photon induced Auger electron spectroscopy. The O and N KLL Auger electron spectra have been recorded where the interpretation of the nitrogen spectrum differs greatly from the earlier assignment. Ten distinct structures are observable in the spectra at energies of 37.4+or-0.1, 39.4+or-0.1, 43.1+or-0.1, 43.8+or-0.2, 44.5+or-0.2, 46.7+or-0.2, 47.9+or-0.5, 49.8+or-0.2, 54.0+or-0.5 and 55.4+or-0.2 eV. All structures in both Auger spectra can be assigned in terms of singlet coupled doubly ionized final states. The experimental results have been compared with recent theoretical and experimental data from other laboratories.

Enhanced IUdR radiosensitization by 241Am photons relative to 226Ra and 125I photons at 0.72 Gy/hr

The dependence of IUdR radiosensitization on photon energy was investigated by irradiating Chinese hamster cells in vitro under aerobic conditions at a dose rate of 0.72 Gy/hr which is typical of temporary brachytherapy implants. It had been observed previously that the IUdR radiosensitization with the 60 keV photons from 241Am is about 1.5 times greater than that with 830 keV (average) photons from 226Ra. It was hypothesized that the enhanced IUdR radiosensitization for 60 keV photons was a result of a larger production of Auger electron cascades from the filling of K-shell vacancies in the iodine atoms, which have a K-shell binding energy of 33.2 keV. Since most of the photons from a 125I source have energies below 33.2 keV, it would be expected that IUdR radiosensitization with 28 keV (average) photons from 125I and 830 keV (average) photons from 226Ra. would both be smaller than the radiosensitization with the 60 keV photons from 241Am. To test this hypothesis we compared IUdR radiosensitization for 226Ra. 241Am, and 251I at 0.72 Gy/hr, using Chinese hamster lung cells in vitro. The measured survival curves led to RBEs of 1.20 ± 0.10 and 1.30 − 0.11 for 241Am and 125I photons relative to 226Ra. to IUdR radiosensitization factors at a 10 −5 M concentration of 1.35 ± 0.11, 1.67 ± 0.09, and 1.47 ± 0.08 for 226Ra. 241Am, and 125I, respectively; and to radiosensitization factors at a 10 −4 M concentration of 1.89 ± 0.16, 3.04 ± 0.13, and 2.48 ± 0.17 for 226Ra. 241Am, and 125I, respectively. These results indicate that IUdR produces significant radiosensitization with all three isotopes 226Ra. 241Am, and 125I) for continuous low dose rate irradiations at 0.72 Gy/hr. Also, we observed greater radiosensitization with 241Am photons compared to 226Ra. on the higher energy side and to 125I on the lower energy side. These findings support the concept that photon-induced Auger electrons produce a significant increase in IUdR radiosensitization when photons with energies just above the K-edge of the iodine atom are employed for continuous low dose rate irradiations. These findings suggest that regimens combining IUdR infusion with temporary brachytherapy implants using low energy photons in relatively quiescent sites such as brain tumors may have clinical potential, and indicate the need for rigorous preclinical evaluation of this approach.

Coadsorbate effects in surface photochemistry: NO and O2 on Pt(111)

Photon-induced desorption (PID) and electron energy loss spectroscopy (EELS) applied to the system NO/O2/Pt(111) yield information on coadsorbate effects in surface photochemistry. A new NO species (denoted γNO) is observed in EELS [νγN−O=236 meV, ν(Pt−γNO)=49 meV], and is found to have a highly enhanced PID cross section compared to that of NO in the top sites of the clean surface [σγNO(O2)(333 nm)=(1.1±0.1)×10−18 cm2]. Photodesorption of O2 is enhanced and photodissociation of O2 is inhibited in the presence of NO resulting in the suppression of NO2 production upon irradiation of NO/O2/Pt(111), in contrast to the photoinduced formation of CO2 from CO/O2/Pt(111). The wavelength dependences of NO and O2 PID cross sections demonstrate that they are independent processes. The final product states created in photoinduced processes with O2 are strongly influenced by coadsorbates.

X-ray photoelectron, Auger electron and ion fragment spectra of O2 and potential curves of O22+

The O22+ ion has been studied by means of electron-impact-induced and photon-induced Auger electron spectroscopy and oxygen ion fragment spectroscopy of O2. The oxygen ion kinetic energy spectrum was recorded by inverting the relevant potentials of an electron spectrometer for the detection of positive particles. The 4 Sigma - and 2 Sigma - O 1s initial core hole states have been studied using monochromatised X-ray photoelectron spectroscopy. Potential energy curves for a number of electronic states of the O22+ dication have been calculated with the complete active space SCF (CASSCF) and multireference contracted CI (MRCCI) methods with a one-particle basis set of medium size ((8s, 6p, 2d)). An analysis of the O2 Auger electron spectrum based on the computed potential curves of O22+ is presented. The autoionisation satellites are analysed and these lines correspond to molecular singly ionised final states. One line at 510.7 eV, however, is associated with an atomic-like transition. Two shake-up Auger satellites are identified by a comparison with a recent O 1s shake-up spectrum from O2.

Effect of exothermicity on electron transfer rates in photosynthetic molecular models

To appreciate fully the significance of the high-resolution crystal structure of the reaction-centre complex from the purple bacterium Rhodopseudomonas viridis, there is increasing impetus to construct models to ascertain how the specific variables such as distance, exothermicity and solvent control the rates of electron transfer through the integral membrane protein complex. Recently, we have determined the effect of a 4 A change in distance (at fixed exothermicity, solvent and temperature) on the rates of photon-induced electron transfer for two porphyrin-quinone assemblies separated by rigid spacers3,4. From picosecond fluorescence lifetime measurements on zinc meso-phenyloctamethylporphyrin coupled to a benzoquinone moiety via one and two bicyclo[2.2.2]octyl spacers (10 and 14 A, edge-to-edge), we deteremined that the rates of electron transfer are 1010 s−1 and ≤107s−1, respectively (the exothermicity, −ΔG = 1.0eV in acetonitrile at 25 °C). This result revealed that the porphyrin-quinone separated by a 10-A spacer would be ideally suited, with regard to picosecond fluorescence techniques for a study of exothermicity effects at fixed distance and solvent. Here we report the fluorescence lifetimes of a homologous series of seven porphyrin-quinone molecules, each with differently substituted ben-zoquinones separated by an identical rigid phenylbicyclo[2.2.2]octane spacer (10 A, edge-to-edge) which vary with respect to driving force for electron transfer from the first excited singlet state of the porphyrin. The key features of this series of molecules lie in the fact that the edge-to-edge distance is fixed and ΔG is tunable. Thus, the effect of exothermicity on electron transfer can be measured while avoiding major perturbations of the electronic structure.

A Comparison of Experiment, CEPXS/ONETRAN, Tigerp, and Tiger Net Electron Emission Coefficients for Various Bremsstrahlung Spectra

This work compares a carefully designed experiment to measure photoemission with the predictions of three different codes (CEPXS/ONETRAN, TIGERP, and TIGER) for the complex bremsstrahlung spectra typical of very intense pulsed power x-ray generators. The Monte Carlo codes TIGER and TIGERP can calculate the net photon-induced electron emission but accurate results may require that statistical error be minimized. CEPXS/ONETRAN is a new deterministic coupled electron/photon transport code that is faster than Monte Carlo and is not subject to statistical error. The comparison of net yields is a sensitive test of the relative accuracy and efficiency of these various codes. We find that all of the codes substantially agree with the experiments for the forward net yields. However, for reverse net yields from high-Z materials, the codes overpredict relative to measurements.

Light-induced giant dipoles in simple model compounds for photosynthesis

The primary steps in photosynthesis involve very rapid (sub-nanosecond) electron transfer between molecular entities that are rigidly embedded within a lipid membrane and separated from each other by well-defined distances on the order of 10 A. In an attempt to simulate such systems we have studied photon-induced electron transfer within specially synthesized molecular assemblies in which a donor moiety is separated from an electron acceptor by a rigid, saturated hydrocarbon framework of variable length, from 5 to 13 A. We find charge separation to occur on a sub-nanosecond timescale with close to unit quantum efficiency in all cases. The lifetimes of the resulting charge-transfer states, with dipole moments approaching 70 debye units, can extend to several hundred nanoseconds. Non-conjugated hydrocarbon bridges may be important in determining the rate and direction of electron transfer in photo-excited natural or artificial molecular systems.

Experimental investigation and Monte Carlo calculation of photon-induced electron emission from solids

A new algorithm is developed for the calculation of secondary electron characteristics which considerably reduces the expenditure of computing time. The validity of the calculation scheme is confirmed by comparing the calculated results with the experimental data on yields and the differential energy spectra of secondary electrons induced by 0.0363–2.75 MeV photons in different targets, measured in a unified experimental setup.

Formation kinetics and quantum yield of photon-induced electron ejection from NADH in aqueous solution

Transient absorption spectra are obtained after photoexcitation of the reduced form of nicotinamide adenine dinucleotide (NADH) and are identified as due to a solvated electron (e aq −) based on comparison with known spectra and the observation of the same transient upon photolysis of N-propyl-1,4-dihydronicotinamide. The risetime of the photogenerated transient is 40 ps, with no perceptible decay within 2 ns following excitation. The quantum yield of solvated electron formation is estimated as 0.46 ± 0.20.

Photon-induced electron capture

De R\'ujula has noted that if $K$ or $L$ capture is kinematically forbidden, enhancement of internal bremsstrahlung electron capture (IBEC) can occur. This enhancement may make IBEC useful in measurements of the neutrino mass. It is shown that a similar enhancement occurs for photon-induced electron capture. The cross section for the most promising case, $^{163}\mathrm{Ho}$, is calculated. Unfortunately, one finds that the cross section is too small to be of use.

Abstract: Ultraviolet photon-induced secondary electron emission from TiO2: Theory and experiment

Abstract: Ultraviolet photon-induced secondary electron emission from TiO2: Theory and experiment

Photon-induced electron transfer transitions of the solvated electrons

The role of electron transfer transitions in the absorption spectrum of the solvated electron in liquid water is studied in terms of the generalized line shape function. When the density of trapping sites is greater than one third of the density of solvent molecules, the transition-dipole moment for photon-induced electron transfers was found to be greater than that of the single-site 1s→2p transition. The solvated electron is assumed to be coupled with low-frequency solvent modes and a high-frequency molecular mode. The latter mode is responsible for a small isotope shift in the peak energy. The spectrum consists of contributions from short-distance transfers (’’bound–bound’’) and long-distance (’’bound–free’’) transfers, the latter being the minor component of the total spectrum.

Electrical and Photon Tests of a Resonant Satellite Shape

A resonant body has been tested and analyzed to determine its SGEMP response. Both electrical and photon excitation were used and their results compare favorably with predictions. A modal analysis of the experimental data using Prony's algorithm yielded the dominant frequencies, damping and excitation amplitudes. Within the accuracy of the measurements, the presence of the photon-induced electron cloud had no effect on the period or damping times of the RESMOD modes. The proximity of an electrical pulser and its 200-Ohm terminating resistor markedly decreased the damping time during electrical excitation.

Photon-Induced Electron Pairing

The earlier work on the possibility of interband electron pairing in the presence of a strong radiation field has been further extended. Some additional terms, neglected earlier, have been taken into account and generalized to a situation where the electron-phonon coupling coefficients for the two conduction bands (valleys) are different. It is found that the pairing interaction is attractive and the strength depends on the photon density.

Direct observation of light-induced bloch wall pinning

Individual Bloch walls moving under the influence of an alternating magnetic field in Yttrium Iron Garnet have been observed to be blocked by irradiation of the sample with white light. This blocking can be described as a light induced increase of domain wall coercive force. The centres pinning the walls are assumed to be ferrous ions. These ions are redistributed between inequivalent lattice sites by photon-induced electron transitions.