Photoionization Of He Research Articles

Proton production probability in the dissociative photoionization of H2

We report the proton production probability in the dissociative photoionization of H2 from its threshold to 36 eV. The method depends on differentiating the observed ratios of H+/H+2 with respect to the incident photon energy. The probability presented shows at least three states are involved in this energy region and is attributed to protons formed through the ground state 2Σ+g(1sσg) of H+2, the two-electron excited state 1Σ+u(2pσu2sσg) of H2, and the first excited state 2Σ+u(2pσu) of H+2 by comparing the present results with other observations and theoretical calculations. Further the results indicate that interference between the direct (2Σ+g) and resonant (1Σ+u) processes produces an appreciable amount of protons at photon energies of 20–25 eV.

Transient absorption of electron beam excited helium in the UV and VUV

The transient absorption in helium at 10 bars after excitation by depositing 0.17 J/cm 3 of energy supplied by an electron beam has been investigated. For λ < 260 nm the absorption is caused by photoionisation of He ∗( 3S) and He ∗ 2(a). The experimental cross sections agree with calculated values. For λ > 260 nm several absorption lines appear which are due to transitions from He ∗( 3S) and He ∗ 2(a) to Rydberg levels. Since He ∗( 1S) is quenched rapidly, its absorption is observed only at the 396.5 nm absorption line.

Analysis of the photoionisation of an atom in a1Sestate leaving an ion in a2P0state. Application to helium

It is shown that the photoionisation of an atom in the 1Se ground state leaving the ion in an excited 2P0 state which decays to a 2Se state can be completely described in terms of three independent parameters sigma , beta and xi . Expressions for the polarisation and angular distribution of the emitted radiation are obtained in terms of sigma , beta and xi when the photoelectron and photon decay are observed in coincidence and when the photoelectron is not observed. As an example the photoionisation of He leaving the He+ ion in its 2p state is considered and values for sigma , beta and xi are obtained using a six-state R-matrix calculation. These results are compared with previous theoretical calculations and with experimental measurements.

Photoionization of N2 X 1Σ+g, v″=0 and 1 near threshold. Preionization of the Worley–Jenkins Rydberg series

The high resolution relative photoionization cross section for N2 is reported in the wavelength region from the ionization threshold to 650 Å, with particular attention given to the region between the N+2 X 2Σ+g, v′=0 and 1 ionization limits. Cross sections from both the X 1Σ+g, v″=0 and 1 vibrational levels were determined in this region, and preionized members of the npπu 1Πu Worley–Jenkins Rydberg series converging to N+2 X 2Σ+g, v′=1 are observed in both spectra. In the spectrum excited from v″=1, the Worley–Jenkins series appears prominently as a result of good Franck–Condon overlap between the N2 X 1Σ+g, v″=1 and the N+2 X 2Σ+g, v′=1 levels; the intensities of the series members decrease approximately as 1/n*3, in accord with simple theoretical predictions. However, in the spectrum excited from v″=0, the Worley–Jenkins series converging to N+2 X 2Σ+g, v′=1 is weak as a result of a poor Franck–Condon overlap with the ground vibrational level; the intensities of the series members show large deviations from the simple theory as a result of channel interactions with Rydberg states converging to N+2 A 2Πu. These perturbing Rydberg states have low photoabsorption oscillator strengths for excitation from v″=1 (in contrast to excitation from v″=0) and hence have only a small effect on the cross section from the excited vibrational level. Just as in the case of photoionization of H2, the results demonstrate that channel interaction can redistribute the oscillator strength of a perturbing Rydberg state of low principal quantum number over a number of members of an interacting Rydberg series and that the spectral range affected by the perturber can be much greater than the width of the perturbing level.

Theory of Dissociative Ionization Processes

A quantum mechanical theory for the ℏω+AB→A+B++e- three body breakup is presented. The theory is based on the Continuum Coupled Channels expansion (CCC) in which the three body wave-function is expanded in terms of square integrable and (flux carrying) continuum states. The theory consistently treats the accompanying processes of three body inelastic collisions, dissociative attachment and Penning ionization. Applications to the dissociative photoionization of H2 and HD are presented. The relevant equations are solved exactly using the Artificial Channel method. Excellent agreement is obtained with experiment, concerning the proton kinetic energy distribution.

Dissociative photoionization of H2 at 26.9 and 30.5 eV

A quantal resonance formalism is applied to the calculation of the cross sections for the absorption of photons of energies 26.9 and 30.5 eV into a 1Σ+u resonance Rydberg state of H2. The 1Σ+u state undergoes autoionization into the vibrational continuum of the 2Σ+g state of H+2, producing energetic H+ ions. The calculations reproduce state of H+2, producing energetic H+ ions. The calculations reproduce the maxima in the measured cross sections at H+ energies near 3.2 eV for overestimate the cross sections below 2.5 eV. An arbitrary reduction in the autoionizing widths by a factor of 2 restores agreement over the measured energy range, though more elaborate calculations of the widths suggest that values should be increased.

Measured Cross Sections for Photoionization of Ground-State He toHe+(n=2)

Cross sections for simultaneous photoionization of He and excitation to ${\mathrm{He}}^{+}$ ($n=2$) have been measured from threshold to 160 eV. The data show clear evidence of autoionization from levels converging on ${\mathrm{He}}^{+}$ ($n=3$). Away from such structure, they are in good agreement with previous experimental values. None of the theoretical predictions gives the correct magnitude at low energies.

Diabatic and Resonance Molecular States

The concept of diabatic and resonance molecular states is discussed. A recent definition of diabatic states is presented. Current investigations of resonance states of molecular hydrogen are summarized and detailed calculations of the dissociative photoionization of H2 are reported.

Dissociative photoionisation of H2: proton kinetic energy spectra

The kinetic energy spectra of the protons produced in the dissociative photoionisation of H2 and directed predominantly along the electric vector of a photon beam have been obtained at certain wavelengths between 19.8 and 40.8 eV. The time-of-flight method described employed a specially constructed wide aperture analyser with multichannel analysis to extract the very weak signals available. The system has been used to measure the energy distributions of the protons formed through the ground state of H2+ and at higher energies, where two-electron excited states of H2 and other states of H2+ also contribute to proton production. Protons identified with the lowest 1 Sigma u+ state (predominantly 2p sigma u2s sigma g) exhibit a peak in their kinetic energy distribution which appears to move to higher energies as the photon energy is increased. The contributions from direct and autoionising processes are assessed and related to other published results including electron impact data.

Resonant dissociative photoionization of H2

The absorption of photons with an energy of 26.9 eV by H2 produces H+ ions with kinetic energies ranging up to 4.4 eV. We attribute the energetic H+ ions to absorption into a 1Σu+ resonance state of H2 which autoionizes into a continuum corresponding to a free electron moving in the field of the 1sσg core of H2+. We present calculations of the real and imaginary parts of the complex potential energy surface of the resonance state and of the transition dipole moment between the ground state of H2 and the resonance state. The resulting cross section for the production of energetic H+ ions and the calculated ratio of H+ to H2+ ions produced are in agreement with measurements. The energy distribution of the H+ ions has a maximum at about 3.2 eV. The maximum cannot be reproduced by a classical description of the nuclear motion in the autoionizing resonance state, but it does arise when a quantal treatment is used, in which the nuclear motion is governed by the real part of the complex potential.

The Balmer 9 and Balmer 11 lines of He ii in the Sun

We identify the Balmer 9 and 11 lines of He ii at 959 A and 942 A in solar spectra. These lines are produced mainly by recombination following photoionization of He ii by coronal XUV radiation. From analysis of the line intensities, we confirm the theoretical model of Avrett et al. (1976), who found that an appreciable amount of He++ is present at temperatures of 1–2 × 104 K and that the anomalously strong He ii λ304 line is produced primarily by collisional excitation. We also confirm the suggestion of Kohl (1977) that the photoionization-recombination process is more important in active regions than in the quiet Sun, and we find that the λ304 line is produced largely by recombination in solar flares.

Theoretical studies in photoelectron spectroscopy: Extraction of dynamical and structural information from the angular distributions for oriented or rotationally resolved unoriented molecular samples

We analyze the angular distributions of molecular photoelectrons for three different experimental situations: (i) photoionization in unoriented samples for rotationally unresolved transitions, (ii) photoionization in unoriented samples for rotationally resolved transitions, and (iii) photoionization in oriented samples. It is known that (iii) provides a means of determining the molecular orientation relative to the direction of polarization of the incident light and a probe of the anisotropy of the initial molecular orbital greater than that of (i). We show that (ii) also provides a probe of molecular orbital anisotropy greater than that of (i) and a probe of dynamical effects such as ’’l spoiling’’ and the penetration of waves of higher l into the inner regions of the molecular field greater than that of (i) or (iii). Calculations are carried out for the photoionization of H2, for which experimental results exist for processes (i) and (ii). Since molecular orbital anisotropy and the above mentioned dynamical effects are small in H2, it is a good example for the study of enhanced molecular effects available in the angular distribution for process (ii). However, the anisotropy of H2 (and in the field of H+2) is sufficiently small that the information in the angular distribution for process (iii) is overwhelmingly geometrical rather than dynamical or structural.

Photoionization of He and N2 by uv radiation from a copper plasma produced by a CO2 laser

Measurements of the attenuation in a background gas, He or N2, of the uv radiation produced by a 1.7-ns 4-J CO2 laser irradiation of a copper target are reported. Photoionization cross sections of 0.4 Mb in He and 0.9 Mb in N2 are obtained. The photoionizing radiation is attributed to the 110.6-Å M1 transition in copper.

Application of the relativistic random-phase approximation to the photoionisation of atoms

Exploratory calculations of atomic photoionisation cross sections including both relativistic and correlation effects are undertaken using the relativistic random-phase approximation (RRPA). In these preliminary calculations the authors examine the photoionisation of He and Be atoms. The cross sections and asymmetry parameters for these light elements agree precisely with non-relativistic RPA values. Elastic phaseshifts for the e-+He+ and e-+Be+ systems are obtained and agree with non-relativistic calculations in the 1P eigenchannel; the 3P phaseshifts have no counterpart in non-relativistic theory.

Dissociative photoionization of H2at 26.9 eV

Protons with kinetic energies up to 4.4 eV have been observed in the dissociative photoionization of H2 using Ne II 26.9 eV line radiation. The most energetic protons must result from the dissociative autoionization of doubly excited H2, and it seems most probable that the lowest state of 1 Sigma u+ symmetry is involved. It is estimated from the integrated kinetic energy spectrum that at 26.9 eV the photoionization cross section for the production of 2 eV to 4.4 eV protons from H2 is 0.012-0.004+0.007 Mb.

The photoionization and dissociative photoionization of H2, HD, and D2

view Abstract Citations (64) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The photoionization and dissociative photoionization of H2, HD, and D2. Ford, A. L. ; Docken, K. K. ; Dalgarno, A. Abstract The cross sections for photoionization of H2 to the bound and continuum vibrational levels of the 1s sigma g state of H2(+) have been calculated in the range 804-425 A ionizing radiation. The dipole matrix elements have been computed as a function of ejected electron energy and internuclear distance using a simple Coulomb approximation to the final-state scattering wave function. The relative population of the V' levels of H2(+) at 584 A, the ratio of production of H(+) to H2(+) as a function of photon energy, and the total photoionization cross section are in good agreement with experimental measurements. Publication: The Astrophysical Journal Pub Date: February 1975 DOI: 10.1086/153387 Bibcode: 1975ApJ...195..819F Keywords: Deuterium; Gas Ionization; Hydrogen Ions; Ionization Cross Sections; Photoionization; Vibrational Spectra; Gas Dissociation; Interstellar Gas; Molecular Energy Levels; Photodissociation; Planetary Atmospheres; Ultraviolet Radiation; Wave Functions; Atomic and Molecular Physics full text sources ADS |

Theoretical studies in photoelectron spectroscopy: Use of angular distribution data to deduce molecular orbital parameters

We present formulas for the differential cross section for ejection of energetic molecular photoelectrons. The cross section has been factorized into a radial and angular part. The angular part has been evaluated by use of standard angular momentum algebra, while the radial part requires explicit expressions for each partial wave component of the initial molecular orbital (which must be in the form of a single-center partial wave expansion) and for each partial wave of the photoelectron. We assume a parametrized form for the initial molecular orbital, and we calculate the photoelectron wavefunction by consecutive use of the distorted-wave-Born and Coulomb-Born approximations. This is a perturbative form, valid to O(k−2) in the asymptotic region of the wavefunction, where in atomic units k is the velocity of the photoelectron. The theory provides the means to invert angular distribution data, if available, to determine the parameters of the initial molecular orbital. Or, if this orbital is known, the theory provides an accurate high energy limit to test low energy theories or to normalize relative cross section measurements. We calculate the cross section for photoionization of H2+ from states of different symmetries as a function of internuclear distance. We indicate how the method can be extended to more complicated molecules.

Photoionization of Helium Metastable Atoms near Threshold

We have experimentally determined the absolute cross sections for the photoionization of He($2^{1}S$) and He($2^{3}S$) metastable atoms from threshold to 2400 \AA{}. The results are compared with recent theoretical calculations.

Dissociative photoionization of H2and D2through the 1sσgionic state

The dissociative photoionization of H2 and D2 has been investigated experimentally and theoretically from just below threshold to 30.5 eV (405 AA). Experimentally the ratios of the number of dissociated to undissociated photoions H+/H2+, D+/D2+ have been determined using a mass spectrometric system essentially free from discrimination against ion species. These ratios are found to rise to plateau values of 0.0208+or-0.0017 and 0.0082+or-0.0008 respectively. Over most of the energy range investigated, the photoions are formed solely through the 1s sigma g ionic state, permitting approximate calculations to be made. A comparison between the calculated and experimental results indicates the need for improved calculations. Significant differences are found on comparing the photoionization data reported with the corresponding values obtained by particle impact.

Comparison of photoelectron intensities and Franck-Condon factors in the photoionization of H2, HD and D2

Abstract The relative intensities of vibrational bands corresponding to the photoionization reaction X 1 Σ g + (υ″ = 0) + hv → X 2 Σ g + (υ′ = 0, 1, 2 …) + e − have been measured for H 2 , HD and D 2 , using He I radiation and a cylindrical mirror analyzer. These relative intensities differ significantly from squared overlap integrals (Franck-Condon factors) based on accurate potential curves for X 1 Σ g + and X 2 Σ g + , but are in good agreement with calculations performed by Itikawa which include the variation of transition moment with internuclear distance and the kinetic energy of the departing electron.