Photodissociation as a probe of the H 3 + avoided crossing seam (original) (raw)
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Photodissociation of Triatomic Hydrogen
Physical Review Letters, 1988
We report the first observation of photodissociation of H3. Predissociation of the optically prepared 3s 8 I and 3d E" states by the X E' ground state is detected by monitoring of the production of rovibrationally excited H2 molecules and H atoms. Product excitation is found to be highly dependent on H3 electronic and nuclear configuration. PACS numbers: 33.80.Gj, 34.50.Lf, 34.50.Pi The unstable ground-state potential surface of H3 has served as prototype for the development of bimolecular reaction-rate theory, ' and is still largely territory only of theoretical chemistry. While the ground electronic state of H3 is dissociative, electronically excited states of this molecule are tightly bound.
Excitation of H2in photodissociation regions as seen bySpitzer
Astronomy & Astrophysics, 2011
Aims. We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H 2 physics and gas energetics in photodissociation regions (PDRs) of low to moderate far-ultraviolet (FUV) fields and densities. Methods. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions (χ ∼ 5−10 3), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H 2 rotational lines are complemented with H 2 data, including ro-vibrational line measurements, obtained with ground-based telescopes and ISO, to constrain the relative contributions of ultraviolet pumping and collisions to the H 2 excitation. The data analysis is supported by model calculations with the Meudon PDR code. Results. The observed column densities of rotationally excited H 2 are observed to be much higher than PDR model predictions. In the lowest excitation PDRs, the discrepancy between the model and the data is about one order of magnitude for rotational levels J ≥ 3. We discuss whether an enhancement in the H 2 formation rate or a local increase in photoelectric heating, as proposed for brighter PDRs in former ISO studies, may improve the data-model comparison. We find that an enhancement in the H 2 formation rates reduces the discrepancy, but the models still fall short of the data. Conclusions. This large disagreement suggests that our understanding of the formation and excitation of H 2 and/or of PDRs energetics is still incomplete. We discuss several explanations, which could be further tested using the Herschel Space Telescope.
Mechanism for the destruction of H3 + ions by electron impact [3]
Nature, 2001
The rate at which the simplest triatomic ion (H + 3 ) dissociates following recombination with a low-energy electron has been measured in numerous experiments 1±10 . This process is particularly important for understanding observations of H + 3 in diffuse interstellar clouds 11±13 . But, despite extensive efforts , no theoretical treatment has yet proved capable of predicting the measured dissociative recombination rates at low energy, even to within an order of magnitude. Here we show that the Jahn±Teller symmetry-distortion effect 16±19 Ðalmost universally neglected in the theoretical description of electron±molecule collisionsÐgenerates recombination at a much faster rate than any other known mechanism. Our estimated rate constant overlaps the range of values spanned by experiments. We treat the low-energy collision process as a curve-crossing problem, which was previously thought inapplicable 20 to low-energy recombination in H + 3 . Our calculation reproduces the measured propensity for three-body versus two-body breakup of the neutral fragments 3 , as well as the vibrational distribution 4 of the H 2 product molecules.
The Journal of Physical Chemistry A, 2012
In this work we present a study of the dissociative photoionization of S-methyl thioacetate [CH 3 C-(O)SCH 3 ] by using multicoincidence time-of-flight mass spectrometry and synchrotron radiation in the S 2p, C 1s, and O 1s edges. Total and partial ion yield spectra together with photoelectron−photoion coincidence (PEPICO) and photoelectron−photoion−photoion coincidence (PEPIPICO) spectra were measured. Fragmentation patterns deduced from PEPICO and PEPIPICO spectra at the various excitation energies show a moderate site-specific fragmentation. The dissociation dynamic for the main ion-pair production is discussed. Two-, three-, and four-body dissociation channels have been observed in the PEPIPICO spectra, and the dissociation mechanisms are proposed. The interstellar HCS + and H 3 + ions can be observed during the synchrotron experiments reported in the present work.
Photodissociation of the HeH^{+} ion into excited fragments (n=2,3) by time-dependent methods
Physical Review A, 2009
The total and partial photodissociation cross sections of the molecular ion HeH + are computed by timedependent methods for fragmentation into the excited shells n =1,2,3 up to a photon energy of 40 eV. 1 ⌺ + and 1 ⌸ states are considered for parallel and perpendicular transitions for different initial rotational or vibrational excitations. Nonadiabatic radial and rotational couplings are taken into account. The results from coupledchannel equations are compared with the Born-Oppenheimer approximation. A time-dependent calculation with a femtosecond laser pulse is carried out to simulate a recent crossed beam photodissociation imaging experiment with vacuum ultraviolet free-electron laser ͓H. B. Pedersen et al., Phys. Rev. Lett. 98, 223202 ͑2007͔͒. The dominance of photodissociation perpendicular to the photon polarization is confirmed.
Photo-double ionization of H 2
Journal of Physics: Conference Series, 2007
The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections (FDCS) are compared with Time Dependent Close Coupling (TDCC) calculations and the first-order helium-like model in the coplanar geometry for equal electron energy sharing. While the helium-like model gives a consistent agreement in shape and magnitude with experimental data, the comparison with the TDCC calculations highlights the limit of this model when the molecular axis' orientation is along the polarization direction. New evidence of changes in the FDCS with internuclear separation is reported for the coplanar geometry.
Photodissociation and Radiative Association of HeH + in the Metastable Triplet State
The Journal of Physical Chemistry A, 2013
We investigate the photodissociation of the metastable triplet state of HeH + as well as its formation through the inverse process, radiative association. In models of astrophysical plasmas, HeH + is assumed to be present only in the ground state, and the influence of the triplet state has not been explored. It may be formed by radiative association during collisions between a proton and metastable helium, which are present in significant concentrations in nebulae. The triplet state can also be formed by association of He + and H, although this process is less likely to occur. We compute the cross sections and rate coefficients corresponding to the photodissociation of the triplet state by UV photons from a central star using a wave packet method. We show that the photodissociation cross sections depend strongly on the initial vibrational state and that the effects of excited electronic states and non-adiabatic couplings cannot be neglected. We then calculate the cross section and rate coefficient for the radiative association of HeH + in the metastable triplet state.
Two-step photoionization of hydrogen atoms in interplanetary space
Planetary and Space Science, 1990
Photoionization is one of the key processes which determine the properties of fluxes of neutral atoms in interplanetary space. A new two-step channel (called indirect) of photoionization of hydrogen atoms is proposed. Hydrogen atoms are at first excited to states with principal quantum number n > 2, then decay to metastable H(2S) states, where they can be photoionized. Competing processes due to the interaction with solar wind plasma and solar radiation are considered and the photoionization rate through the proposed indirect channel is calculated. This rate depends on distance from the Sun as cc l/R4 at large distances (R > 1-2 a.u.) and as cc l/R2 at close approaches, where it is higher than the rate of direct photoionization.
Production of H+3 via photodissociation of organic molecules in interstellar clouds
Monthly Notices of The Royal Astronomical Society, 2007
We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar CH3-X type organic molecules like methanol (CH3OH), methylamine (CH3NH2) and acetonitrile (CH3CN) as alternative route for the production of H+3 in interstellar and star-forming environments. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons with energies between 200 and 310 eV and time-of-flight (TOF) mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross-sections for H+3 production by soft X-rays were determined. We have found that, among the channels leading to molecular dissociation, the H+3 yield could reach values up to 0.7 per cent for single photoionization process and up to 4 per cent for process involving double photoionization. The H+3 photoproduction cross-section due to the dissociation of the studied organic molecules by photons over the C1s edge (200-310 eV) were about 0.2-1.4 × 10-18 cm2. Adopting the typical X-ray luminosity LX >~ 1031 erg s-1 which best fit the observational data for AFGL 2591 we derive an estimative for the H+3 photoproduction rate due to methyl compound dissociation process. The highest value for the H+3 column density from methanol dissociation by soft X-rays, assuming a steady state scenario, was about 1011 cm2, which gives the fraction of the photoproduced H+3 of about 0.05 per cent, as in the case of dense molecular cloud AFGL 2591. Despite the extreme small value, this represent a new and alternative source of H+3 into dense molecular clouds and it is not been considered as yet in interstellar chemistry models.
Formation of H3- by radiative association of H2 and H- in the interstellar medium
Physical Review A, 2011
We develop the theory of radiative association of an atom and a diatomic molecule within a close-coupling framework. We apply it to the formation of H$_3^-$ after the low energy collision (below 0.5 eV) of H$_2$ with H$^-$. Using recently obtained potential energy and permanent dipole moment surfaces of H$_3^-$, we calculate the lowest rovibrational levels of the H$_3^-$ electronic ground state, and the cross section for the formation of H$_3^-$ by radiative association between H$^-$ and ortho- and para-H$_2$. We discuss the possibility for the H$_3^-$ ion to be formed and observed in the cold and dense interstellar medium in an environment with a high ionization rate. Such an observation would be a probe for the presence of H$^-$ in the interstellar medium.