Integral Cross Sections For Collisions Research Articles

Ab initio transport coefficients of Ar⁺ ions in Ar for cold plasma jet modeling.

Collision cross sections and transport coefficients are calculated for Ar{+} ions, in the ground state {2}P_{3/2} and in the metastable state {2}P_{1/2}, colliding with their parent gas. Differential and integral collision cross sections are obtained using a numerical integration of the nuclear Schrödinger equation for several published interaction potentials. The Cohen-Schneider semi-empirical model is used for the inclusion of the spin-orbit interaction. The corresponding differential collision cross sections are then used in an optimized Monte Carlo code to calculate the ion transport coefficients for each initial ion state over a wide range of reduced electric field. Ion swarm data results are then compared with available experimental data for different proportions of ions in each state. This allows us to identify the most reliable interaction potential which reproduces ion transport coefficients falling within the experimental error bars. Such ion transport data will be used in electrohydrodynamic and chemical kinetic models of the low temperature plasma jet to quantify and to tune the active species production for a better use in biomedical applications.

Quantum scattering of NO(X 2Π) with He( 1S): Temperature dependence of rotational (de)-excitation rate coefficients

We report fully-quantum, coupled-state (CS) calculations of inelastic integral cross sections for collisions of NO( 2Π) with He. The scattering calculations, which take into account the fine structure of the NO radical, were done on a grid of collision energies large enough to ensure converged state-to-state rate coefficients for temperatures ranging from 1 K up to 350 K.

Intermolecular interactions of H2S with rare gases from molecular beam scattering in the glory regime and from ab initio calculations

Integral cross sections for collisions of rotationally hot H2S molecules with rare gas atoms (Ne, Ar, and Kr) have been measured, in the collision energy range of 10-60 kJ mol(-1), using a molecular beam apparatus operating under high resolution both in angle and in velocity. A well resolved glory pattern has been measured which permitted the accurate characterization of the intermolecular potentials both at long range (in the attractive region) and at intermediate distances (in the well region). Considering the conditions used in the experiments, the obtained potentials must be considered very close to the spherical averages of the full intermolecular potential energy surfaces. Extensive ab initio calculations have also been carried out in parallel in order to characterize energy minima in the potential energy surfaces and energy barriers associated to the motion of the rare gas atoms around H2S. An assessment of the relative role of the various interaction components has been also attempted: the combined analysis of experimental and theoretical results suggests that H2S-rare gas aggregates are mainly bound by nearly isotropic noncovalent interactions of the van der Waals type.

Steric asymmetry and lambda-doublet propensities in state-to-state rotationally inelastic scattering of NO(2Π1/2) with He

Relative integrated cross sections are measured for rotationally inelastic scattering of NO(2Pi(1/2)),hexapole selected in the upper lambda-doublet level of the ground rotational state (j = 0.5), in collisions with He at a nominal energy of 514 cm(-1). Application of a static electric field E in the scattering region, directed parallel or antiparallel to the relative velocity vector v, allows the state-selected NO molecule to be oriented with either the N end or the O end towards the incoming He atom. Laser-induced fluorescence detection of the final state of the NO molecule is used to determine the experimental steric asymmetry, [formula: see text], which is equal to within a factor of (- 1) to the molecular steric effect, S(i-->f) is identical with (sigma(He-->NO) - (sigma(He-->ON))/(sigma(He-->NO) + sigma(He-->ON)). The dependence of the integral inelastic cross section on the incoming lambda-doublet component is also observed as a function of the final rotational (j'), spin-orbit (omega'), and lambda-doublet (epsilon') state. The measured steric asymmetries are significantly larger than previously observed for NO-Ar scattering, supporting earlier proposals that the repulsive part of the interaction potential is responsible for the steric asymmetry. In contrast to the case of scattering with Ar, the steric asymmetry of NO-He collisions is not very sensitive to the value of omega'. However, the lambda-doublet propensities are very different for [omega=0.5(F1)-->omega'= 1.5(F2)] and [omega=0.5(F1)-->omega'=0.5(F1)] transitions. Spin-orbit manifold conserving collisions exhibit a propensity for parity conservation at low deltaj, but spin-orbit manifold changing collisions do not show this propensity. In conjunction with the experiments, state-to-state cross sections for scattering of oriented NO(2Pi) molecules with He atoms are predicted from close-coupling calculations on restricted coupled-cluster methods including single, double, and noniterated triple excitations [J. Klos, G. Chalasinski, M. T. Berry, R.Bukowski, and S. M. Cybulski, J. Chem. Phys. 112, 2195 (2000)] and correlated electron-pair approximation [M. Yang and M. H. Alexander, J. Chem. Phys. 103, 6973 (1995)] potential energy surfaces. The calculated steric asymmetry S(i-->f) of the inelastic cross sections at Etr= 514 cm(-1) is in reasonable agreement with that derived from the present experimental measurements for both spin-manifold conserving (F1-->Fl) and spin-manifold changing (F1 --F2) collisions, except that the overall sign of the effect is opposite. Additionally, calculated field-free integral cross sections for collisions at Etr = 508 cm(-1) are compared to the experimental data of Joswig et al. [J. Chem. Phys.85, 1904 (1986)]. Finally, the calculated differential cross section for collision energy Etr= 491 cm(-1) is compared to experimental data of Westley et al. [J. Chem. Phys. 114, 2669 (2001)] for the spin-orbit conserving transition F1 (j = 0.5) -F1f (j' = 3.5).

On the propensity rules for inelastic NH3–rare gas collisions

The observed and ab initio calculated propensity rules for collisions of NH3 with rare gas atoms are found to be in reasonable agreement for NH3–Ar, whereas for NH3–He they show large discrepancies. In order to examine these discrepancies we have calculated state-to-state integral cross sections for collisions of NH3 with He using the close coupling method. The NH3–He interaction potential has been obtained from SCF calculations, augmented by a multipole-expanded damped dispersion energy. Our calculations show that the discrepancies can be accounted for if the cross sections are corrected for the imperfect initial state preparation in the experiment. They also clarify why the discrepancies do not occur to the same extent for NH3–Ar. After comparing our new theoretical results with the experimental data we found that for one experimental cross section for NH3–He the earlier assignment must be corrected.

Comparison of cross sections from the quasi-classical trajectory method and the jz-conserving centrifugal sudden approximation with accurate quantum results for an atom-rigid nonlinear polyatomic collision

We report the results of a series of calculations of state-to-state integral cross sections for collisions between O and nonvibrating H2O in the gas phase on a model nonreactive potential energy surface. The dynamical methods used include converged quantum mechanical scattering calculations, the j(z) conserving centrifugal sudden (j(z)-CCS) approximation, and quasi-classical trajectory (QCT) calculations. We consider three total energies 0.001, 0.002, and 0.005 E(h) and the nine initial states with rotational angular momentum less than or equal to 2 (h/2 pi). The j(z)-CCS approximation gives good results, while the QCT method can be quite unreliable for transitions to specific rotational sublevels. However, the QCT cross sections summed over final sublevels and averaged over initial sublevels are in better agreement with the quantum results.

Molecular beam study of the interaction of atomic and molecular oxygen with methane

Absolute integral cross sections for collisions of CH4 with O2 molecules and of O2 molecules and O(3Pj) atoms with CH4 are measured as a function of velocity at thermal energies in a molecular beam apparatus. For the O2–CH4 interaction the glory structure observed allows an analysis in terms of an isotropic potential model and meaningful potential parameter values are obtained. For the O(3Pj)–CH4 interaction the anisotropy due to the O(3Pj) atoms causes a partial quenching of the glory amplitude. Nevertheless a realistic isotropic potential can be extracted from the glory extrema position. The present potential parameters for the O2–CH4 and the O–CH4 interactions can be used to predict realistic parameters for more complicated interactions involving O2 molecules and O atoms with other species of interest also in combustion.

Scattering experiments on the methane-rare-gas interaction

Absolute integral cross sections for collisions of CH 4 with Ne, Ar, Kr and Xe are measured as a function of velocity at thermal energies in a molecular beam apparatus. The presence of fully developed glory oscillations indicates that the anisotropy of methane plays only a minor role in these experiments. The data can therefore be treated in terms of a central force-field assumption. The validity of this assumption is confirmed by comparison with the behaviour of other systems through a correlation between the potential parameters and the polarizabilities.

Scattering experiments on the interaction of atomic and molecular nitrogen with krypton

Absolute integral cross sections for collisions of N 2 molecules and N atoms with Kr are measured as a function of velocity at thermal energies. For the N 2Kr case the glory structure observed in the plot of cross section versus velocity is only slightly affected by the anisotropy of the interaction. An analysis in terms of a spherically averaged potential model can be performed, thus obtaining a reliable potential function. For the NKr system the occurrence of a well developed glory oscillation indicates low anisotropy in the interaction. An analysis in terms of a simple, realistic potential model has been performed to give potential parameters for the NKr system.

Glory undulations in total integral collision cross sections

The first centrifugal sudden (CS) calculations of glory undulations in total integral cross sections have been carried out for Ar-N2 in the velocity range g=501·2-3258·4 m s-1. A model Lennard-Jones (12, 6) interaction potential with P 2(cos ϑ) anisotropy has been used and N2 is assumed to be a rigid rotor. Close coupling (CC) calculations at three velocities have also been performed. The CS and CC calculations are used to assess the reliability of the much simpler infinite order sudden (IOS) approximation. It is found that the IOS method is a reliable technique for analysing glory undulations in the total integral cross section of Ar-N2. The calculations are relevant to recent precise experimental measurements of glory undulations for atom-molecule collision systems.

The interaction of atomic and molecular nitrogen with argon by scattering measurements

Absolute integral cross sections for collisions of N2 molecules and N atoms with Ar are measured as a function of velocity at thermal energies. For the N2–Ar case the glory structure observed in the cross section vs velocity plot does not appear to be affected by the anisotropy of the interaction. An analysis in terms of a spherically averaged potential model can be performed thus obtaining a reliable potential function. The N–Ar cross sections presented here are the first collisional study of N atoms leading to significant information on the interaction potential. Although, as shown by the magnetic behavior, the N atom beam is essentially a mixture of atoms in the two metastable 2DJ and 2PJ states, the results obtained, together with other properties of the excited atoms, indicate the presence of a low anisotropy in the N–Ar interaction. An analysis in terms of a simple and yet realistic potential model has been performed obtaining meaningful potential parameters for the N–Ar system. The results for N–Ar are tentatively inserted in a comparison of trends and regularities of the interactions of some second row elements of the periodic table with Ar.

Determination of state resolved rotationally inelastic cross sections: LiH(j=1) –Ar

Laser induced fluorescence is used to determine rotationally inelastic relative integral cross sections for collisions of state-selected Litt with Ar.(AIP)

Wavelength and relative intensity of the ultraviolet forbidden atomic nitrogen doublet

Absolute integral cross sections for collisions of N2 molecules and N atoms with Kr are measured as a function of velocity at thermal energies. For the N2Kr case the glory structure observed in the plot of cross section versus velocity is only slightly affected by the anisotropy of the interaction. An analysis in terms of a spherically averaged potential model can be performed, thus obtaining a reliable potential function. For the NKr system the occurrence of a well developed glory oscillation indicates low anisotropy in the interaction. An analysis in terms of a simple, realistic potential model has been performed to give potential parameters for the NKr system.