atom-molecule energy transfer and dissociation processes for nitrogen and oxygen
DESCRIPTION
Atom-molecule energy transfer and dissociation processes for nitrogen and oxygen. Fabrizio Esposito IMIP-CNR, Bari Section (Institute of Inorganic Methodologies and Plasmas). Ro-vibrational excitation-deexcitation and dissociation in heavy particle collisions. M+M 2 (v,j) M+M 2 (v’,j’) - PowerPoint PPT PresentationTRANSCRIPT
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Atom-molecule energy transfer and dissociation processes for
nitrogen and oxygen
Fabrizio EspositoIMIP-CNR, Bari Section
(Institute of Inorganic Methodologies and Plasmas)
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Ro-vibrational excitation-deexcitation and dissociation in heavy particle collisions
M+M2(v,j)M+M2(v’,j’)
M+M2(v,j) 3M M = N (≈10000 states), O (≈6400 states) Quasiclassical method: a good compromise
between global reliability of results and computational resources required
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Method of Calculation: quasiclassical trajectories
Pseudoquantization of reagents and products Classical evolution of the system All the possible outcomes of the collision
process are taken into account (non-reactive, reactive, dissociation, quasibound states)
Perfect for parallelization and distributed calculations
“fast” and modular calculations
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Quasibound states
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States classically trapped by the rotational barrier, but not from a quantum point of view
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Error evaluation and computational time
A trajectory tj(0) is integrated with a time step TSo, then a back-integration tj
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Some Details
In tj calculations, translational energy range is continuous from 10-3 to 3 eV
Discretization of energy axis is made with 500 bins Accuracy of tjs with the step checking (with x=10-10Å) is
of the order of one wrong tj in 105-106 Density of tjs is about 24000 tjs/(Å· eV) for nitrogen,
4000 for oxygen; stratified sampling is applied Over 1200 cpu hours of calculations have been spent for
nitrogen, two years for oxygen up to now LEPS PES of Lagana’ et al. for N+N2; DMBE PES of
Varandas and Pais for O+O2
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Rotationally averaged cross sections
€
Qrot (v) =j
∑ g j exp(−Ev, j / kTrot )
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σ (v,v',Trot ) = σ (v, j,v' )g j exp(−Ev, j / kTrot ) /Qrotj
∑
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Dissociation cross sections for nitrogen
Rotationally averaged cross sections from v=40, Trot= 50,1000,3000K
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Dissociation cross sections for nitrogen
Trot = 3000K, v=40,50,60,65
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Nitrogen dissociation rate coefficients
Rates are obtained at T=300,1000,3000K Lines are interpolations with polynomials of order 3-4
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Comparison of total dissociation rate coefficient for nitrogen
Lines: calculated by usObtained experimentally by Roth and Thielen (1986, stars)and Appleton (1968 “x”)
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Nitrogen vibrational deexcitation rate coefficients at T=1000K
From v to v-1,v-5,v-15,v-25,v-35
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Comparison with Lagana’ and Garcia results (1996)
T = 1000K Lines without points are reactive rates
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Oxygen dissociation rates
T = 300K, 1000K, 3000K
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Oxygen vibrational de-excitation rates at T=1000K
De-excitation from vv-1 as a function of initial v (red)
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Oxygen vibrational de-excitation rates at T=1000K
De-excitation from vv-5 as a function of initial v (green)
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Oxygen vibrational de-excitation rates at T=1000K
De-excitation from vv-15 as a function of initial v (blue)
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Oxygen vibrational de-excitation rates at T=1000K
De-excitation from vv-25 as a function of initial v (magenta)
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Oxygen vibrational de-excitation rates at T=1000K
De-excitation from vv-35 as a function of initial v (light blue)
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Oxygen vibrational de-excitation rates at T=1000K
Comparison of rate coefficients for T=1000K, vv-1 (yellow), vv-5 (black) with Lagana’ and Garcia results on the same PES
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Oxygen rotationally averaged cross sections
Dissociation cross sections for v=30, Trot = 50, 1000, 3000, 10000
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Oxygen rotationally averaged cross sections
Dissociation cross sections for Trot=1000K, v=20, 25, 30, 35, 40
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Comparison of total dissociation rate for oxygen with some experimental fits
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Our rate is similar to that of Shatalov within ±13% over the whole interval 1000-10000K
NF: no correction factor VF: variable factor
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Approximation for excited electronic states
We consider, following Nikitin, an equilibrium among vibrational levels belonging to different electronic states but with approximately the same energy.
Nikitin hypotesis: this equilibrium is not significantly perturbed by molecular dissociation
Dissociation can be calculated as originating concurrently from O2 ground state and electronically excited states of oxygen, counting as many times the process as the sum of the degeneracies of excited states divided by that one of the ground state.
Nikitin proposes for oxygen a global factor 16/3, considering the first six states having a minimum
We propose a variable factor increasing with energy level
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Nikitin approximation
Oxygen electronic states having a minimum
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Conclusions Detailed cross sections database are nowadays fundamental for
kinetic studies In compiling large and detailed sets of cross sections for atom-
molecule collision processes, the application of quasiclassical method is reliable and feasible;
A good compromise between accuracy and computational time is found when step checking is applied
Large sets of detailed dynamical data can be compiled using QCT calculations, substituting then gradually the classical results with semiclassical/quantum ones for more critical processes (tunneling, large energy spacing between initial/final states)
The role of quasibound states in dissociation/recombination processes can now be considered in a detailed approximate way for oxygen and nitrogen in future kinetic studies