31 research outputs found
The Effect of Non-equilibrium Kinetics on Oxygen Chemistry in the Interstellar Medium
It has been suggested that in photon-dominated regions, oxygen chemistry is initiated by the O+H2 yields OH+H reaction. The reaction has an energy barrier of about 0.4 eV with ground state reactants and it is slow at low temperatures. There is strong experimental evidence that vibrational excitation of the H2 molecule increases the reactivity significantly. We present extensive quantum calculations of cross sections and rate coefficients for the O+H2(v) reaction for v = 0 - 3 of the H2 molecule and show that the vibrational excitation of the molecule has a significant effect on reactivity, especially at low temperatures
Fundamental studies of hydrogen storage and catalysis
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Quantum calculations of H2-H2 collisions: from ultracold to thermal energies
We present quantum dynamics of collisions between two para-H2 molecules from
low (1 mK) to high collision energies (1 eV). The calculations are carried out
using a quantum scattering code that solves the time-independent Schrodinger
equation in its full dimensionality without any decoupling approximations. The
six-dimensional potential energy surface for the H4 system developed by
Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] is used in the calculations.
Elastic, inelastic and state-to-state cross sections as well as rate
coefficients from T = 1 K to 400 K obtained from our calculations are compared
with available experimental and theoretical results. Overall, good agreement is
obtained with previous studies.Comment: 10 pages, 10 figure
Vibrational energy transfer in ultracold molecule - molecule collisions
We present a rigorous study of vibrational relaxation in p-H2 + p-H2
collisions at cold and ultracold temperatures and identify an efficient
mechanism of ro-vibrational energy transfer. If the colliding molecules are in
different rotational and vibrational levels, the internal energy may be
transferred between the molecules through an extremely state-selective process
involving simultaneous conservation of internal energy and total rotational
angular momentum. The same transition in collisions of distinguishable
molecules corresponds to the rotational energy transfer from one vibrational
state of the colliding molecules to another.Comment: 4 pages, 4 figure
Stereodynamic control of overlapping resonances in cold molecular collisions
Stereodynamic control of resonant molecular collisions has emerged as a new
frontier in cold molecule research. Recent experimental studies have focused on
weakly interacting molecular systems such as HD collisions with H, D
and He. We report here the possibility of such control in strongly interacting
systems taking rotational relaxation in cold collisions of HCl and H. Using
explicit quantum scattering calculations in full six dimensions it is shown
that robust control of the collision dynamics is possible even when multiple
(overlapping) shape-resonances coexist in a narrow energy range, indicating
that cold stereochemistry offers great promise for many molecules beyond simple
systems. We demonstrate a striking case where two prominent peaks in
overlapping resonances are switched-off simultaneously by suitable alignment of
the HCl molecule.Comment: 5 pages, 4 figures, 31 references and Supplemental Material (5 pages,
5 figures
Formation of molecular oxygen in ultracold O + OH reaction
We discuss the formation of molecular oxygen in ultracold collisions between
hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based
on hyperspherical coordinates is employed for the calculations. Elastic,
inelastic and reactive cross sections as well as the vibrational and rotational
populations of the product O2 molecules are reported. A J-shifting
approximation is used to compute the rate coefficients. At temperatures T = 10
- 100 mK for which the OH molecules have been cooled and trapped
experimentally, the elastic and reactive rate coefficients are of comparable
magnitude, while at colder temperatures, T < 1 mK, the formation of molecular
oxygen becomes the dominant pathway. The validity of a classical capture model
to describe cold collisions of OH and O is also discussed. While very good
agreement is found between classical and quantum results at T=0.3 K, at higher
temperatures, the quantum calculations predict a larger rate coefficient than
the classical model, in agreement with experimental data for the O + OH
reaction. The zero-temperature limiting value of the rate coefficient is
predicted to be about 6.10^{-12} cm^3 molecule^{-1} s^{-1}, a value comparable
to that of barrierless alkali-metal atom - dimer systems and about a factor of
five larger than that of the tunneling dominated F + H2 reaction.Comment: 9 pages, 8 figure
Quantum dynamics of the O + OH -> H + O2 reaction at low temperatures
We report quantum dynamics calculations of the O + OH -> H + O2 reaction on
two different representations of the electronic ground state potential energy
surface (PES) using a time-independent quantum formalism based on
hyperspherical coordinates. Calculations show that several excited vibrational
levels of the product O2 molecule are populated in the reaction. Rate
coefficients evaluated using both PESs were found to be very sensitive to the
energy resolution of the reaction probability, especially at temperatures lower
than 100 K. It is found that the rate coefficient remains largely constant in
the temperature range 10-39 K, in agreement with the conclusions of a recent
experimental study [Carty et al., J. Phys. Chem. A 110, 3101 (2006)]. This is
in contrast with the time-independent quantum calculations of Xu et al. [J.
Chem. Phys. 127, 024304 (2007)] which, using the same PES, predicted two orders
of magnitude drop in the rate coefficient value from 39 K to 10 K. Implications
of our findings to oxygen chemistry in the interstellar medium are discussed.Comment: 8 pages, 10 figure