A measurement of the cross section for ionisation of helium by electron impact using a fast crossed beam technique

Montague, R G; Harrison, Mark; Smith, A. C. H.

doi:10.1088/0022-3700/17/16/012

Cited by 150 publications

(73 citation statements)

References 18 publications

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“…For He i, the experimental data of Shah et al (1988) and Montague et al (1984b) have been used together with the more recent measurements of Rejoub et al (2002). These data sets are in very good agreement with the cross-sections presented in Rapp & Englander-Golden (1965), although the value at the peak is 6% lower.…”

Section: He Isoelectronic Sequencesupporting

confidence: 58%

X-ray emission from thin plasmas

Urdampilleta

Kaastra

Mehdipour

2017

A&A

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Every observation of astrophysical objects involving a spectrum requires atomic data for the interpretation of line fluxes, line ratios, and ionization state of the emitting plasma. One of the processes that determines it is collisional ionization. In this study, an update of the direct ionization (DI) and excitation-autoionization (EA) processes is discussed for the H to Zn-like isoelectronic sequences. In recent years, new laboratory measurements and theoretical calculations of ionization cross-sections have become available. We provide an extension and update of previous published reviews in the literature. We include the most recent experimental measurements and fit the cross-sections of all individual shells of all ions from H to Zn. These data are described using an extension of Younger's and Mewe's formula, suitable for integration over a Maxwellian velocity distribution to derive the subshell ionization rate coefficients. These ionization rate coefficients are incorporated in the high-resolution plasma code and spectral fitting tool SPEX V3.0.

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Section: He Isoelectronic Sequencesupporting

confidence: 58%

X-ray emission from thin plasmas

Urdampilleta

Kaastra

Mehdipour

2017

A&A

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“…(Rudd 1991). Experimental data: triangles, Rapp & EnglanderGolden (1965); circles, Montague et al (1984); diamonds, Shah et al (1988); squares, Kossmann et al (1990).…”

Section: Local Interstellar Spectramentioning

confidence: 99%

Cosmic-ray ionization of molecular clouds

Padovani¹,

Galli²,

Glassgold³

2009

A&A

356

475

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Context. Low-energy cosmic rays are a fundamental source of ionization for molecular clouds, influencing their chemical, thermal, and dynamical evolution. Aims. The purpose of this work is to explore the possibility that a low-energy component of cosmic rays, not directly measurable from the Earth, can account for the discrepancy between the ionization rate measured in diffuse and dense interstellar clouds. Methods. We collected the most recent experimental and theoretical data on the cross sections for the production of H + 2 and He + by electron and proton impact and discuss the available constraints on the cosmic-ray fluxes in the local interstellar medium. Starting from different extrapolations at low energies of the demodulated cosmic-ray proton and electron spectra, we computed the propagated spectra in molecular clouds in the continuous slowing-down approximation taking all the relevant energy loss processes into account. Results. The theoretical value of the cosmic-ray ionization rate as a function of the column density of traversed matter agrees with the observational data only if the flux of either cosmic-ray electrons or of protons increases at low energies. The most successful models are characterized by a significant (or even dominant) contribution of the electron component to the ionization rate, in agreement with previous suggestions. However, the large spread of cosmic-ray ionization rates inferred from chemical models of molecular cloud cores remains to be explained. Conclusions. Available data combined with simple propagation models support the existence of a low-energy component (below ∼100 MeV) of cosmic-ray electrons or protons responsible for the ionization of molecular cloud cores and dense protostellar envelopes.

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“…Gryzinski's classical cross section (long-dashed line) overesti- [35]; tron impact. Filled circles, experimental data by Rapp and filled triangles, data by Montague et al [36]; solid line, BED Englander-Golden [39]; open squares, calculations by Schultz cross section (with the t + u + 1 denominator for a neutral et al [40] based on the classical trajectory Monte Carlo target); medium-dashed line, Younger's distorted-wave Born (CTMC) method; solid line, BED cross section; long-dashed cross section [34].…”

Section: Hydrogen Moleculementioning

confidence: 99%

Binary-encounter-dipole model for electron-impact ionization

1994

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A theoretical model, which is free of adjustable or fitted parameters, for calculating electronimpact ionization cross sections for atoms and molecules is presented. This model combines the binary-encounter theory with the dipole interaction of the Bethe theory for fast incident electrons. The ratios of the contributions from distant and close collisions and interference between the direct and exchange terms are determined by using the asymptotic behaviors predicted by the Bethe theory for ionization and for stopping cross sections. Our model prescribes procedures to calculate the singly differential cross section (energy distribution) for each subshell using the binding energy, average kinetic energy, and the differential dipole oscillator strengths for that subshell. Then the singly differential cross section is integrated over the ejected electron energy to obtain the total ionization cross section. The resulting total ionization cross section near the threshold is proportional to the excess energy of the projectile electron. We found that this model yields total ionization cross sections for a variety of atoms and molecules from threshold to several keV which are in good agreement (-10% or better on average) with known experimental results. The energy distributions also exhibit the expected shapes and magnitudes. We offer a simpler version of the model that can be used when differential oscillator strengths are not known. For the ionization of ions with an openshell configuration, we found that a minor modification of our theory greatly improves agreement with experiment. PACS number(s): 34.80. Dp, 34.80.Gs, 34.80.K~

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A measurement of the cross section for ionisation of helium by electron impact using a fast crossed beam technique

Cited by 150 publications

References 18 publications

X-ray emission from thin plasmas

X-ray emission from thin plasmas

Cosmic-ray ionization of molecular clouds

Binary-encounter-dipole model for electron-impact ionization

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