223 research outputs found
Insight into CO2 dissociation in plasmas from numerical solution of a vibrational diffusion equation
The dissociation of CO2 molecules in plasmas is a subject of enormous
importance for fundamental studies and the recent interest in carbon capture
and carbon-neutral fuels. The vibrational excitation of the CO2 molecule plays
an important role in the process. The complexity of the present state-to-state
(STS) models makes it difficult to find out the key parameters. In this paper
we propose as an alternative a numerical method based on the diffusion
formalism developed in the past for analytical studies. The non-linear
Fokker-Planck equation is solved by the time-dependent diffusion Monte Carlo
method. Transport quantities are calculated from STS rate coefficients. The
asymmetric stretching mode of CO2 is used as a test case. We show that the
method reproduces the STS results or a Treanor distribution depending on the
choice of the boundary conditions. A positive drift, whose energy onset is
determined by the vibrational to translational temperature ratio, brings
molecules from mid-energy range to dissociation. The high-energy fall of the
distribution is observed even neglecting VT processes which are normally
believed to be its cause. Our study explains several puzzling features of
previous studies, provides new insights into the control of the dissociation
rate and a much sought compression of the required data for modeling
Non-thermal photons and H2 formation in the early Universe
The cosmological recombination of H and He at z \sim 1000 and the formation
of H2 during the dark ages produce a non-thermal photon excess in the Wien tail
of the cosmic microwave background (CMB) blackbody spectrum. Here we compute
the effect of these photons on the H- photodetachment and H2+ photodissociation
processes. We discuss the implications for the chemical evolution of the
Universe in the post-recombination epoch, emphasizing how important a detailed
account of the full vibrational manifold of H2 and H2+ in the chemical network
is. We find that the final abundances of H2, H2+, H3+ and HD are significantly
smaller than in previous calculations that neglected the effect of non-thermal
photons. The suppression is mainly caused by extra hydrogen recombination
photons and could affect the formation rate of first stars. We provide simple
analytical approximations for the relevant rate coefficients and briefly
discuss the additional effect of dark matter annihilation on the considered
reaction rates.Comment: 10 pages, 12 figures, 1 table; accepted for publication in MNRA
Particle propagation and electron transport in gases
In this review, we detail the commonality of mathematical intuitions that
underlie three numerical methods used for the quantitative description of
electron swarms propagating in a gas under the effect of externally applied
electric and/or magnetic fields. These methods can be linked to the integral
transport equation, following a common thread much better known in the theory
of neutron transport than in the theory of electron transport. First, we
discuss the exact solution of the electron transport problem using Monte Carlo
(MC) simulations. In reality we will progress much further, showing the
interpretative role that the diagrams used in quantum theory and quantum field
theory can play in the development of MC. Then, we present two methods, the
Monte Carlo Flux and the Propagator method, which have been developed at this
moment. The first one is based on a modified MC method, while the second shows
the advantage of explicitly applying the mathematical idea of propagator to the
transport problem.Comment: Review paper, 46 pages, 4 figure
The unbiased Diffusion Monte Carlo: a versatile tool for two-electron systems confined in different geometries
Computational codes based on the Diffusion Monte Carlo method can be used to
determine the quantum state of two-electron systems confined by external
potentials of various nature and geometry. In this work, we show how the
application of this technique in its simplest form, that does not employ
complex analytic guess functions, allows to obtain satisfactory results and, at
the same time, to write programs that are readily adaptable from one type of
confinement to another. This adaptability allows an easy exploration of the
many possibilities in terms of both geometry and structure of the system. To
illustrate these results, we present calculations in the case of two-electron
hydrogen-based species (H and H) and two different types of
confinement, nanotube-like and octahedral crystal-field
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