Photoabsorption Cross Sections Research Articles

Photoabsorption and photoionization cross sections for formaldehyde in the vacuum-ultraviolet energy range

We report a theoretical-experimental investigation on the interaction of vacuum-ultraviolet radiation with formaldehyde (H2CO) in the gas phase. Experimentally, the absolute photoabsorption cross sections and the photoionization quantum yields were measured in the (11.0–21.5) eV range using the double-ion chamber technique. Also, the absolute photoionization and neutral-decay cross sections were derived from these data. In addition, in the same energy region, the dissociation pattern was obtained with a time-of-flight mass spectrometer using the photoelectron-photoion coincidence technique, and the absolute photoionization cross sections were derived for each ionic fragment observed. Moreover, theoretical photoionization cross sections were calculated for the ionization of the four outermost molecular valence orbitals (2b2, 1b1, 5a1, and 1b2) from the threshold to 35 eV. The calculations were performed using the iterative Schwinger variational method to solve the Lippmann-Schwinger equation in the exact static-exchange level of approximation. In general, there is a good agreement between our experimental and previous data reported in the literature. Our theoretical results show a fair qualitative agreement with the experimental data and with previous theoretical results. Above 20 eV, a better quantitative agreement with the experimental data is also observed.

Photoionization of Ne Atoms and Ne+ Ions Near the K Edge: PrecisionSpectroscopy and Absolute Cross-sections

Abstract Single, double, and triple photoionization of Ne+ ions by single photons have been investigated at the synchrotron radiation source PETRA III in Hamburg, Germany. Absolute cross-sections were measured by employing the photon–ion merged-beams technique. Photon energies were between about 840 and 930 eV, covering the range from the lowest-energy resonances associated with the excitation of one single K-shell electron up to double excitations involving one K- and one L-shell electron, well beyond the K-shell ionization threshold. Also, photoionization of neutral Ne was investigated just below the K edge. The chosen photon energy bandwidths were between 32 and 500 meV, facilitating the determination of natural line widths. The uncertainty of the energy scale is estimated to be 0.2 eV. For comparison with existing theoretical calculations, astrophysically relevant photoabsorption cross-sections were inferred by summing the measured partial ionization channels. Discussion of the observed resonances in the different final ionization channels reveals the presence of complex Auger-decay mechanisms. The ejection of three electrons from the lowest K-shell-excited Ne+( ) level, for example, requires cooperative interaction of at least four electrons.

Photodissociation of FONO: an excited state nonadiabatic dynamics study.

The photo dissociation of nitrosyl fluorite, FONO, a potential source of atmospheric fluorine, underlies its active role in ozone depletion and other activities in the troposphere. In the present work, the electronic structure of FONO is revisited at high level of ab initio and density functional theory (DFT) theoretical levels. Several different post SCF methods were used to compute excited states, vertical excitation energies and intensities, namely configuration interaction with single excitations (CIS), equation of motion coupled cluster with single and double excitations (EOM-CCSD), and symmetry adopted cluster configuration interaction (SAC-CI) methods. The potential energy functions along two internal coordinates, namely the F-ONO bond and the FONO dihedral angle, have been computed on the ground state relaxed potential energy surface (PES) for the ground, 5A' and 5A″ excited states using the EOM-CCSD method. In the gas phase, the decay of the excited states of FONO was examined closely by calculating the UV photoabsorption cross-section spectrum and by nonadiabatic dynamics simulations. Nonadiabatic dynamics were simulated by sampling 300 trajectories in two spectral windows at 3.0 ± 0.25 and 4.5 ± 0.25eV using the surface hopping method. Two different photodissociation reaction pathways with two main products, including multifragmentation (FO+NO) and atomic elimination (F) mechanisms were identified. For the cis-isomer, the main photochemical channel is F+NO2, representing 67% of all processes. For the trans-isomer, however, the main dissociation pathway is (FO+NO). Graphical Abstract Photodisscociation of nitrosyl fluorite (FONO) seems to underlie its active role in ozone depletion and other activities in the troposphere. The present research revisits the electronic structure of FONO at high level of ab initio and DFT theoretical levels. Cis-trans isomerization and dissociation in the ground and low lying excited states were examined. Photoabsorption cross-section spectra were computed and nonadiabatic dynamics were launched for both cis and trans FONO isomers.

Constrained dipole oscillator strength distributions, sum rules, and dispersion coefficients for Br2 and BrCN

Dipole oscillator strength distributions for Br2 and BrCN are constructed from photoabsorption cross-sections combined with constraints provided by the Kuhn-Reiche-Thomas sum rule, the high-energy behavior of the dipole-oscillator-strength density and molar refractivity data when available. The distributions are used to predict dipole sum rules S(k), mean excitation energies I(k), and van der Waals C6 coefficients. Coupled-cluster calculations of the static dipole polarizabilities of Br2 and BrCN are reported for comparison with the values of S(-2) extracted from the distributions.

Doubly-polarised pion photoproduction on the nucleon at MAMI

The A2 Collaboration at MAMI (Mainz) carried out new measurements of the helicity dependence of the total inclusive photo-absorption cross section and of the partial cross sections for several reaction channels on the proton and on the neutron in the photon energy region 200 He targets. Hadronic reaction products were detected by the large acceptance Crystal Ball-TAPS spectrometer, complemented by plastic scintillators and vertex detectors for charged particle tracking and identification.These new, high-quality doubly-polarised pion-photoproduction data sets provide a valuable input to the study of the nucleon structure and excitation spectrum by significantly constraining the electromagnetic multipole evaluation performed by the different available partial wave analysis models. Furthermore, the helicity dependent observables provide the main ingredient for the verification of the well-known Gerasimov-Drell-Hearn (GDH) sum rule, which relates the helicity-dependent photoasborption process to the main static nucleon properties (mass, charge, spin).

Recent results on doubly-polarised pion photoproduction and the GDH sum rule on the nucleon at MAMI

New measurements of the helicity dependence of the total inclusive photoabsorption cross section and of the partial cross sections for several reaction channels on the proton and on the neutron were carried out at MAMI (Mainz) in the energy region 200 He targets.These new doubly-polarised pion-photoproduction data sets give new input to the partial wave analyses and allow to constrain the multipole solution of the different analyses; they also allow to study the double-polarisation observables G and E. Furthermore, the results obtained give information on the well-known Gerasimov-Drell-Hearn sum rule.

Dipole strength in 80Se below the neutron-separation energy for the nuclear transmutation of 79Se

The γ-ray strength function (γSF) in 80 Se is an important parameter to estimate the neutron-capture cross section of 79 Se which is one of the long-lived fission products (LLFPs). Until now, the γSF method was applied for 80 Se only above the neutron-separation energy (S n ) and the evaluated 79 Se(n,γ) cross section has an instability caused by the GSF below S n . We studied the dipole-strength distribution of 80 Se in a photon-scattering experiment using bremsstrahlung produced by an electron beam of an energy of 11.5 MeV at the linear accelerator ELBE at HZDR. The present photoabsorption cross section of 80 Se was combined with results of (γ,n) experiments and are compared with predictions usinmg the TALYS code. We also estimated the 79 Se(n,γ) cross sections and compare them with TALYS predictionms and earlier work by other groups.

E1 and M1 strength functions at low energy

We report photon-scattering experiments using bremsstrahlung at the γELBE facility of Helmholtz-Zentrum Dresden-Rossendorf and using quasi-monoenergetic, polarized γ beams at the HIγS facility of the Triangle Universities Nuclear Laboratory in Durham. To deduce the photoabsorption cross sections at high excitation energy and high level density, unresolved strength in the quasicontinuum of nuclear states has been taken into account. In the analysis of the spectra measured by using bremsstrahlung at γELBE, we perform simulations of statistical γ-ray cascades using the code γDEX to estimate intensities of inelastic transitions to low-lying excited states. Simulated average branching ratios are compared with model-independent branching ratios obtained from spectra measured by using monoenergetic γ beams at HIγS. E1 strength in the energy region of the pygmy dipole resonance is discussed in nuclei around mass 90 and in xenon isotopes. M1 strength in the region of the spin-flip resonance is also considered for xenon isotopes. The dipole strength function of 74 Ge deduced from γELBE experiments is compared with the one obtained from experiments at the Oslo Cyclotron Laboratory. The low-energy upbend seen in the Oslo data is interpreted as M1 strength on the basis of shell-model calculations.

Some Features of Boron Isotopes Separation by the Laser-Assisted Retardation of Condensation Method in Multipass Irradiation Cell Implemented as a Resonator

More cheap production of boron isotopes is needed to provide further progress in nuclear engineering, microelectronics, and nuclear medicine. Separation of boron isotopes by laser-assisted retarded condensation scheme is considered. Continuous irradiation of steady gas flow is considered. In this case, interaction time of gas flow with laser beam (gas flow length) is desirable to make as long as possible in order to increase excitation probability. It can be provided by suitable choice of gas flow expansion conditions, such as small molar fraction of target gas in carrier gas, gas flow, and ambient small gas pressure and temperature. We propose irradiation cell design as a resonant multipass cavity. In order to estimate its length that can be used for efficient isotopes separation, we calculated laser field distribution inside the cavity for different assumed values for mirror wall reflectivities and average photo-absorption cross sections. It has been shown, that significant saving of electricity (4.2-4.6 times less) can be expected.

Valence and Rydberg Excitations of 2,4- and 2,6-Difluorotoluene as Studied by Vacuum Ultraviolet Synchrotron Radiation and ab Initio Calculations.

Here we report novel comprehensive investigations on the electronic state spectroscopies of isolated 2,4- and 2,6-difluorotoluene in the gas phase by high-resolution vacuum ultraviolet (VUV) photoabsorption measurements in the 4.4-10.8 eV energy range, with absolute cross-section values derived. We also present the first set of ab initio calculations (vertical energies and oscillator strengths), which we have used in the assignment of valence transitions of the difluorotoluene molecules, together with calculated ionization energies to obtain the Rydberg transitions for both molecules. The measured absolute photoabsorption cross sections have been used to estimate the photolysis lifetimes of 2,4- and 2,6-difluorotoluene in the Earth's atmosphere.

Photoabsorption by the electron subsystem of a semiconductor nanoparticle

The IR photoabsorption cross section of a semiconductor nanoparticle has been calculated. Light is absorbed by conduction electrons and trapped electrons in the volume and surface of the nanoparticle. Electron concentrations have been obtained by minimizing the total free energy of charges in the system. The photoabsorption cross section has two characteristic maxima corresponding to the absorption by conduction electrons and by trapped electrons in the nanoparticle volume. The number of trapped electrons on the surface is relatively small, so that they do not contribute to the total cross section.

Autoionization resonances in the photoabsorption spectra of Fe n+ iron ions

The photoabsorption cross sections of a neutral iron atom, as well as positive Fe+ and Fe2+ ions, are calculated in the relativistic random-phase approximation with exchange in the energy range 20–160 eV. The wavefunctions of the ground and excited states are calculated in the single-configuration Hartree–Fock–Dirac approximation. The resultant photoabsorption spectra are compared with experimental data and with the results of calculations based on the nonrelativistic spin-polarized version of the random-phase approximation with exchange. Series of autoionization resonance peaks, as well as giant autoionization resonance lines corresponding to discrete transitions 3p → 3d, are clearly observed in the photoabsorption cross sections. The conformity of the positions of calculated peaks of giant autoionization resonances with experimental data is substantially improved by taking into account additionally the correlation electron–electron interaction based on the model of the dynamic polarization potential.

Color dipole picture of deep inelastic scattering, revisited

Based upon the color-dipole picture, we provide closed analytic expressions for the longitudinal and the transverse photoabsorption cross-sections at low values of the Bjorken variable of [Formula: see text]. We compare with the experimental data for the longitudinal-to-transverse ratio of the (virtual) photoabsorption cross-section and with our previous fit to the experimental data for the total photoabsorption cross-section. Scaling in terms of the low-[Formula: see text] scaling variable [Formula: see text] is analyzed in terms of the reduced cross-section of deep inelastic scattering.

Absorption of Infrared Radiation by an Electronic Subsystem of Semiconductor Nanoparticles

The frequency dependence of the cross section of photon absorption by semiconductor nanoparticles in the infrared range is considered. Light is absorbed by conduction electrons with plasmon formation and electrons in the traps both within the bulk and on the nanoparticle surface. The regularities of the photoabsorption cross section largely depend on the size of the nanoparticles. For sufficiently large nanoparticles, there appear only two characteristic peaks in the cross section distribution corresponding to light absorption by two types of electrons—conduction electrons and electrons in the volume traps of nanoparticles. The number of electrons trapped on the surface is relatively small, and the electrons do not contribute to the total cross section of photoabsorption. In the case of photon absorption by quantum dots, the contribution of these surface electrons as well as a third peak is evident. This result occurs due to a complex dependence of the number of electrons in the traps on their depth and s...

Dipole strength inSe80forsprocess and nuclear transmutation ofSe79

The dipole strength distribution of $^{80}\mathrm{Se}$ was studied in a photon-scattering experiment by using bremsstrahlung produced with an electron beam of energy 11.5 MeV at the linear accelerator ELBE. We identified $180\phantom{\rule{0.28em}{0ex}}\ensuremath{\gamma}$ transitions up to an energy of 9.6 MeV, and analyzed the strength in the quasicontinuum of the spectrum. Simulations of statistical $\ensuremath{\gamma}$-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section below the neutron-separation energy derived in this way was combined with the photoabsorption cross section obtained from an earlier $(\ensuremath{\gamma},n)$ experiment and used as an input for the calculation of $^{79}\mathrm{Se}(n,\ensuremath{\gamma})$ reaction rates on the basis of the statistical reaction model.

A study of the valence shell absolute photoabsorption, photoionisation and photodissociation cross sections and the photoionisation quantum efficiency of carbonyl sulphide

The absolute photoabsorption, photoionisation and photodissociation cross sections and the photoionisation quantum efficiency of carbonyl sulphide have been measured using a double ion chamber and synchrotron radiation in the energy range from the ionisation threshold to 24eV. In addition to the absorption bands associated with well established Rydberg series, some previously unassigned features have been tentatively attributed to an f-type Rydberg series converging onto the B̃2Σ+ state ionisation threshold. Structure appearing in the photodissociation spectrum has been correlated with that observed in earlier fluorescence yields for emission, due mainly to the CS photofragment, occurring between 160 and 300nm. The photoionisation quantum efficiency reaches a plateau value close to unity for energies above ∼16eV. The predissociation of Rydberg states into neutral fragments does not appear to affect the photoionisation quantum efficiency of carbonyl sulphide to the extent that is commonly observed in other small molecules. A sum rule analysis has been carried out by combining the present absolute photoabsorption measurements with similar data covering the remaining energy regions.

Nitrogen isotopic fractionations in the low temperature (80 K) vacuum ultraviolet photodissociation of N2

N2 is a diatomic molecule with complex electronic structure. Interstate crossings are prominent in the high energy domain, introducing significant perturbations to the system. Nitrogen mainly photodissociates in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum through both direct and indirect predissociation. Due to the complexity introduced by these perturbations, the nitrogen isotopic fractionation in N2 photodissociation is extremely hard to calculate, and an experimental approach is required. Here we present new data of N-isotopic fractionation in N2 photodissociation at low temperature (80 K), which shows a distinctly different 15N enrichment profile compared to that at relatively higher temperatures (200 and 300 K). The new data, important to understanding the N-isotopic compositions measured in meteorites and other planetary bodies, are discussed in light of the knowledge of N2 photochemistry and calculated photoabsorption cross sections in the VUV.

Evaluation of the forward Compton scattering off protons. II. Spin-dependent amplitude and observables

The forward Compton scattering off the proton is determined by substituting the empirical total photoabsorption cross sections into dispersive sum rules. In addition to the spin-independent amplitude evaluated previously [Phys. Rev. D 92, 074031 (2015)], we obtain the spin-dependent amplitude over a broad energy range. The two amplitudes contain all the information about this process, and we, hence, can reconstruct the nonvanishing observables of the proton Compton scattering in the forward kinematics. The results are compared with predictions of chiral perturbation theory where available. The low-energy expansion of the spin-dependent Compton scattering amplitude yields the Gerasimov-Drell-Hearn (GDH) sum rule and relations for the forward spin polarizabilities (FSPs) of the proton. Our evaluation provides an empirical verification of the GDH sum rule for the proton, and yields empirical values of the proton FSPs. For the GDH integral, we obtain $204.5(21.4)$ $\mu$b, in agreement with the sum rule prediction: $204.784481(4)$ $\mu$b. For the FSPs, we obtain: $\gamma_0=-92.9(10.5) \times 10^{-6}$ fm$^4$, and $\bar{\gamma_0}=48.4(8.2) \times 10^{-6}$ fm$^6$, improving on the accuracy of previous evaluations.

A study of the valence shell electronic states of s-triazine by photoabsorption spectroscopy and ab initio calculations

The absolute photoabsorption cross section of s-triazine has been measured between 4 and 40eV, and is dominated by bands associated with valence states. Structure due to Rydberg excitations is both weak and irregular. Jahn-Teller interactions affect the vibronic structure observed in the Rydberg absorption bands due to excitation from the 1e″ or 6e′ orbitals. The interpretation of the experimental spectrum has been guided by transition energies and oscillator strengths, for Rydberg and valence states, calculated with the time-dependent version of density functional theory and with the coupled cluster linear response approach. The theoretical studies indicate that Rydberg/Rydberg and Rydberg/valence mixing is important.

Valence and lowest Rydberg electronic states of phenol investigated by synchrotron radiation and theoretical methods.

We present the experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectra of phenol covering for the first time the full 4.3-10.8 eV energy-range, with absolute cross sections determined. Theoretical calculations on the vertical excitation energies and oscillator strengths were performed using time-dependent density functional theory and the equation-of-motion coupled cluster method restricted to single and double excitations level. These have been used in the assignment of valence and Rydberg transitions of the phenol molecule. The VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the 6.401 eV transition, which is here assigned to the 3sσ/σ(∗)(OH)←3π(3a″) transition. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of phenol in the earth's atmosphere (0-50 km).