Formaldehyde and methanol were produced efficiently by the hydrogenation of CO in H2O-CO ice at 10 K in an atomic hydrogen beam experiment. The relative yields to the initial CO were of the order of 10%, 3 orders of magnitude larger than those reported previously. This reveals for the first time experimentally that successive hydrogenation of CO is most likely to produce formaldehyde and methanol on the surface of icy grains in molecular clouds as suggested by theoretical models.

http://iopscience.iop.org/article/10.1086/341412/pdf

Efficient Formation of Formaldehyde and Methanol by the Addition of Hydrogen Atoms to CO in H2O-CO Ice at 10 K

We present the results of optically thick non-LTE radiative transfer calculations of lines and continua of H, C I-IV, and O I-VI and other elements using a new one-dimensional, time-independent model corresponding to the average quiet-Sun chromosphere and transition region. The model is based principally on the Curdt et al. SUMER atlas of the extreme ultraviolet spectrum. Our model of the chromosphere is a semiempirical one, with the temperature distribution adjusted to obtain optimum agreement between calculated and observed continuum intensities, line intensities, and line profiles. Our model of the transition region is determined theoretically from a balance between (a) radiative losses and (b) the downward energy flow from the corona due to thermal conduction and particle diffusion, and using boundary conditions at the base of the transition region established at the top of the chromosphere from the semiempirical model. The quiet-Sun model presented here should be considered as a replacement of the earlier model C of Vernazza et al., since our new model is based on an energy-balance transition region, a better underlying photospheric model, a more extensive set of chromospheric observations, and improved calculations. The photospheric structure of the model given here is the same as in Table 3 of Fontenla, Avrett, Thuiller, & Harder. We show comparisons between calculated and observed continua, and between the calculated and observed profiles of all significant lines of H, C I-IV, and O I-VI in the wavelength range 67-173 nm. While some of the calculated lines are not in emission as observed, we find reasonable general agreement, given the uncertainties in atomic rates and cross sections, and we document the sources of the rates and cross sections used in the calculation. We anticipate that future improvements in the atomic data will give improved agreement with the observations.

https://iopscience.iop.org/article/10.1086/523671/pdf

Models of the Solar Chromosphere and Transition Region from SUMER and HRTS Observations: Formation of the Extreme-Ultraviolet Spectrum of Hydrogen, Carbon, and Oxygen

Total cross sections are calculated for the ionisation of a hydrogen atom by multicharged fully-stripped ions in the 20-1000 keV amu-1 impact energy range. Distortion is accounted for in the entrance channel (via the eikonal approximation) and in the exit channel (via the continuum distorted-wave approximation). The transition amplitude is calculated in the post form so that the electronic nonorthogonal kinetic energy is treated as the perturbation. It is concluded that of the currently available models this theory is the most successful and versatile over a considerable range of energies and charges. Specifically for ionisation of a hydrogen atom by 50 keV protons the authors present doubly differential cross sections for electrons ejected in the forward direction and singly differential cross sections as a function of emission energy. The question of cusps and peaks in the differential cross sections is considered as is the question of charged scaling of the total cross section.

https://iopscience.iop.org/article/10.1088/0022-3700/16/17/015/pdf

Ionisation of atoms by ion impact

[1] In the context of recent observations, microphysical models, and laboratory data, a photochemical model of Titan’s atmosphere, including updated chemistry focusing on rate coefficients and cross sections measured under appropriate conditions, has been developed to increase understanding of these processes and improve upon previous Titan photochemical models. The model employs a two-stream discrete ordinates method to characterize the transfer of solar radiation, and the effects of electron-impact, cosmic-ray deposition, and aerosol opacities from fractal and Mie particles are analyzed. Sensitivity studies demonstrate that an eddy diffusion profile with a homopause level of 850 km and a methane stratospheric mole fraction of 2.2% provides the best fit of stratospheric and upper atmosphere observations and an improved fit over previous Titan photochemical models. Lack of fits for C3H8 ,H C3N, and possibly C2H3CN can be resolved with adjustments in aerosol opacity. The model presents a benzene profile consistent with its detection in Titan’s stratosphere [Coustenis et al., 2003], which may play an important role in the formation of Titan hazes. An electron peak concentration of 4200 cm � 3 is

/pdf/current-state-of-modeling-the-photochemistry-of-titan-s-49aa9vsqnk.pdf

Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere

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

/pdf/cosmic-ray-ionization-of-molecular-clouds-66bxscyz4l.pdf

Cosmic-ray ionization of molecular clouds

A pulsed crossed-beam technique incorporating time of flight spectroscopy has been successfully developed and applied to measurements of the electron impact ionisation cross sections of atomic hydrogen in the range 14.6-4000 eV. The method has been derived from a crossed-beam coincidence technique. By substituting a pulsed proton beam for the pulsed electron beam in the present work, measured electron impact ionisation cross sections have been normalised by reference to known equivelocity proton impact cross sections for both ionisation and charge transfer. The results resolve the discrepancy between earlier less accurate cross sections measured using the modulated cross-beam technique. They provide a reliable check on the range of validity of theoretical predictions over a wide energy range.

https://iopscience.iop.org/article/10.1088/0022-3700/20/14/022/pdf

Pulsed crossed-beam study of the ionisation of atomic hydrogen by electron impact

Cross sections for the ionisation of ground state hydrogen atoms by 38-1500 keV H+ and 125-2200 keV He2+ ions have been determined using a crossed-beam technique. Collision products produced in the intersection of the primary ion beam with a highly dissociated thermal energy beam of hydrogen are identified by time-of-flight spectroscopy and counted using a coincidence technique. Cross sections sigma (H+) for proton impact, which involve much smaller uncertainties and cover a wider energy range than previous measurements, now permit an accurate assessment of the validity of the various theoretical predictions. Cross sections sigma (He2+) for He2+ impact, the first such measurements, are also compared with theory and measured cross section ratios sigma (He2+)/ sigma (H+) are considered in terms of the Z2 scaling predicted by the Born approximation.

https://iopscience.iop.org/article/10.1088/0022-3700/14/14/009/pdf

Experimental study of the ionisation of atomic hydrogen by fast H+ and He2+ ions

A pulsed crossed-beam technique incorporating time-of-flight spectroscopy which was recently developed in this laboratory has been applied to measurements of the cross sections for single and double ionisation of helium. Measurements over the unusually wide energy range from near threshold to 10000 eV provide valuable checks on previous measurements based on different experimental approaches and an assessment of the range of validity of a number of theoretical predictions.

http://iopscience.iop.org/article/10.1088/0953-4075/21/15/019/pdf

Single and double ionisation of helium by electron impact

A crossed-beam technique previously developed and used in this laboratory for studies of ionisation and electron capture in H+-He and He2+-He collisions down to 64 keV amu-1 and 50 keV amu-1 has been adapted to extend measurements down to 9 keV amu-1 and 6 keV amu-1 respectively. The method, which incorporates time-of-flight spectroscopy together with electron-ion and ion-ion coincidence counting of the collision products, provides high precision cross sections for individual collision channels. Some serious discrepancies are obtained with previous cross sections for single ionisation measured by Afrosimov et al. (1969, 75) and the important role of transfer ionisation at low energies is confirmed. The need for better theoretical descriptions of low-energy ionisation is demonstrated.

Electron capture and ionisation in collisions of slow H+ and He2+ ions with helium

A crossed beam method incorporating time-of-flight analysis and coincidence counting of the collision products was used previously in this laboratory to study H+-H ionisation in the range 38-1500 keV The same approach has now been used to extend measurements down to 94 keV The authors measured cross sections involve only small uncertainties and, for the first time, permit a detailed assessment of a number of theoretical descriptions of ionisation in slow collisions Comparison is made with the modulated cross beam measurements of Fite et al(1960), the only previous measurements which extend down to keV energies While the agreement is good at 40 keV, the author's measured cross section at 94 keV is only about 27% of the value based on the data of Fite et al This large discrepancy is consistent with an error arising from neglect of secondary electron emission in their experiment

H B Gilbody

Papers

Pulsed crossed-beam study of the ionisation of atomic hydrogen by electron impact

Experimental study of the ionisation of atomic hydrogen by fast H+ and He2+ ions

Single and double ionisation of helium by electron impact

Electron capture and ionisation in collisions of slow H+ and He2+ ions with helium

Ionisation of atomic hydrogen by 9-75 keV protons