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 + 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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Cosmic-ray ionization of molecular clouds

Cross section data have been compiled for electron collisions with hydrogen molecules based on 71 references. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, emission of radiation, and dissociative attachment. For each process, the recommended values of the cross section are presented for use. The literature has been surveyed through the end of 2006.

Cross Sections for Electron Collisions with Hydrogen Molecules

Cross section data are collected and reviewed for electron collisions with oxygen molecules. Included are the cross sections for total and elastic scatterings, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, electron attachment, and emission of radiations. For each process, the recommended values of the cross sections are presented, when possible. The literature has been surveyed through the end of 2007.

Cross Sections for Electron Collisions with Oxygen Molecules

Accurate values of the electron-impact ionization cross sections for the rare gases are needed in a variety of contexts. However, despite numerous investigations over many decades, uncertainty as to the correct values has persisted. The pioneering total-cross-section measurements of Rapp and Englander-Golden are generally regarded as the most reliable but no comprehensive study has independently verified their correctness. In this paper, measurements of electron-impact ionization cross sections of helium, neon, argon, krypton, and xenon are reported for energies ranging from the first ionization threshold to 1000 eV. These data confirm the essential correctness of Rapp and Englander-Golden's total measurements and at the same time provide a complete set of consistent absolute partial cross sections.

Determination of the absolute partial and total cross sections for electron-impact ionization of the rare gases

Theoretical determination of absolute electron-impact ionization cross sections of molecules

Absolute partial cross sections from threshold to 1000 eV are reported for electron-impact ionization of ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and ${\mathrm{O}}_{2}$. Data are presented for the production of ${\mathrm{H}}_{2}^{\mathrm{}+}$ and ${\mathrm{H}}^{+}$ from ${\mathrm{H}}_{2}$; the production of ${\mathrm{N}}_{2}^{\mathrm{}+}$, ${\mathrm{N}}^{+}$+${\mathrm{N}}_{2}^{\mathrm{}2+}$, and ${\mathrm{N}}^{2+}$ from ${\mathrm{N}}_{2}$; and the production of ${\mathrm{O}}_{2}^{\mathrm{}+}$, ${\mathrm{O}}^{+}$+${\mathrm{O}}_{2}^{\mathrm{}2+}$, and ${\mathrm{O}}^{2+}$ from ${\mathrm{O}}_{2}$. The product ions are mass analyzed with a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that all energetic fragment ions are collected. The overall uncertainty in the absolute cross-section values for singly charged parent ions is \ifmmode\pm\else\textpm\fi{}3.5% and is marginally higher for fragment ions. Previous cross-section measurements are compared to the present results. \textcopyright{} 1996 The American Physical Society.

Absolute partial cross sections for electron-impact ionization of H2, N2, and O2 from threshold to 1000 eV

Absolute partial cross sections from threshold to 1000 eV are reported for the production of ${\mathrm{Ar}}^{\mathit{n}+}$ (n=1--4) by electron-impact ionization of argon. The total cross sections, obtained from an appropriately weighted sum of the partial cross sections, are also reported. These results are obtained with an apparatus incorporating a time-of-flight mass spectrometer with position-sensitive detection of the product ions. The simple apparatus design embodies recent developments in pressure measurement and particle detection and is believed to yield more reliable results than those previously reported. For singly charged ions, the overall uncertainty in the absolute cross section values reported here is \ifmmode\pm\else\textpm\fi{}3.5%. Previous measurements of absolute partial and total cross sections are reviewed and compared with the present results.

Absolute partial and total cross sections for electron-impact ionization of argon from threshold to 1000 eV.

The useful range of a capacitance diaphragm gauge is extended down to 1×10−6 Torr with an uncertainty of ±10−7 Torr using computer‐based averaging. Gauge linearity is measured by observing the deflection of the diaphragm by the force of gravity. Precise pressure increments are generated by tilting the gauge through a range of accurately measured angles. By this procedure the gauge is observed to be linear to within ±1% plus 10−7 Torr between 1×10−6 and 8×10−3 Torr. Additionally, the deflection of the center of the diaphragm per unit pressure is found to be 2×10−5 m Torr−1.

Use of capacitance diaphragm gauges at pressures down to 10 - 6 Torr

Absolute partial cross sections for electron-impact ionization of H_2, N_2, O_2, CO_2, and SO 2 from threshold to 1000 eV

Absolute partial cross sections for electron-impact ionization of H{sub 2}, N{sub 2}, O{sub 2}, CO{sub 2} and SO{sub 2} from threshold to 1000 eV are reported. Data are presented for the production of the ionized parent molecules and various fragment ions. The product ions are mass analyzed with a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that, irrespective of their energy, all fragment ions are collected. The overall uncertainty in the absolute cross section values for singly charged parent ions is 13.5% and is marginally higher for fragment ions. Previous cross section measurements and calculations are compared to the present results.

P. Renault

Papers

Absolute partial cross sections for electron-impact ionization of H2, N2, and O2 from threshold to 1000 eV

Absolute partial and total cross sections for electron-impact ionization of argon from threshold to 1000 eV.

Use of capacitance diaphragm gauges at pressures down to 10 - 6 Torr

Absolute partial cross sections for electron-impact ionization of H_2, N_2, O_2, CO_2, and SO 2 from threshold to 1000 eV

Absolute partial cross sections for electron-impact ionization of H{sub 2}, N{sub 2}, O{sub 2}, CO{sub 2}, and SO{sub 2} from threshold to 1000 eV