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Forthomme D, Hause ML, Yu HG, Dagdigian PJ, Sears TJ, Hall GE. Doppler-Resolved Kinetics of Saturation Recovery. J Phys Chem A 2015; 119:7439-50. [PMID: 25815527 DOI: 10.1021/acs.jpca.5b00628] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Frequency-modulated laser transient absorption has been used to monitor the ground-state rotational energy-transfer rates of CN radicals in a double-resonance, depletion recovery experiment. When a pulsed laser is used to burn a hole in the equilibrium ground-state population of one rotational state without velocity selection, the population recovery rate is found to depend strongly on the Doppler detuning of a narrow-band probe laser. Similar effects should be apparent for any relaxation rate process that competes effectively with velocity randomization. Alternative methods of extracting thermal rate constants in the presence of these non-thermal conditions are evaluated. Total recovery rate constants, analogous to total removal rate constants in an experiment preparing a single initial rotational level, are in good agreement with quantum scattering calculations, but are slower than previously reported experiments and show qualitatively different rotational state dependence between Ar and He collision partners. Quasi-classical trajectory studies confirm that the differing rotational state dependence is primarily a kinematic effect.
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Tkáč O, Saha AK, Loreau J, Ma Q, Dagdigian PJ, Parker DH, van der Avoird A, Orr-Ewing AJ. Rotationally inelastic scattering of ND3with H2as a probe of the intermolecular potential energy surface. Mol Phys 2015. [DOI: 10.1080/00268976.2015.1059958] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Schewe HC, Vanhaecke N, Kłos J, Alexander MH, van de Meerakker SYT, Meijer G, van der Avoird A, Dagdigian PJ. Rotationally inelastic scattering of OH by molecular hydrogen: Theory and experiment. J Chem Phys 2015; 142:204310. [DOI: 10.1063/1.4921562] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Tkáč O, Ma Q, Stei M, Orr-Ewing AJ, Dagdigian PJ. Rotationally inelastic scattering of methyl radicals with Ar and N2. J Chem Phys 2015; 142:014306. [PMID: 25573560 DOI: 10.1063/1.4904901] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
The rotationally inelastic scattering of methyl radical with Ar and N2 is examined at collision energies of 330 ± 25 cm(-1) and 425 ± 50 cm(-1), respectively. Differential cross sections (DCSs) were measured for different final n' rotational levels (up to n' = 5) of the methyl radicals, averaged over k' sub-levels, using a crossed molecular beam machine with velocity map imaging. For Ar as a collision partner, we present a newly constructed ab initio potential energy surface and quantum mechanical scattering calculations of state-resolved DCSs. These computed DCSs agree well with the measurements. The DCSs for both Ar and N2 collision partners are strongly forward peaked for all spectroscopic lines measured. For scattering angles below 60°, the theoretical CD3-Ar DCSs show diffraction oscillations that become less pronounced as n' increases, but these oscillations are not resolved experimentally. Comparisons are drawn with our recently reported DCSs for scattering of methyl radicals with He atoms.
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Ma L, Dagdigian PJ, Alexander MH. Theoretical investigation of the relaxation of the bending mode of CH₂(X̃) by collisions with helium. J Chem Phys 2014; 141:214305. [PMID: 25481142 DOI: 10.1063/1.4902004] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
We have earlier determined the dependence on the bending angle of the interaction of the methylene radical (CH2) in its X̃³B₁ state with He [L. Ma, P. J. Dagdigian, and M. H. Alexander, J. Chem. Phys. 136, 224306 (2012)]. By integration over products of the bending vibrational wave function, in a quantum close-coupled treatment we have calculated cross sections for the ro-vibrational relaxation of CH 2(X̃). Specifically, we find that cross sections for a loss of one vibrational quantum (v(b) = 2 → 1 and 1 → 0) are roughly two orders of magnitude smaller, and those for a loss of two vibrational quanta (v(b) = 2 → 0) four orders of magnitude smaller, than those for pure rotational relaxation. In addition, no clear cut dependence on the energy gap is seen.
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Kłos J, Alexander MH, van der Avoird A, Dagdigian PJ. The interaction of OH(X2Π) with H2:Ab initiopotential energy surfaces and bound states. J Chem Phys 2014; 141:174309. [DOI: 10.1063/1.4900478] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Ma L, Alexander MH, Dagdigian PJ. Theoretical investigation of intersystem crossing between the ã¹A₁ and X³B₁ states of CH₂ induced by collisions with helium. J Chem Phys 2014; 141:064312. [PMID: 25134576 DOI: 10.1063/1.4892377] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Collisional energy transfer between the ground (X³B₁) and first excited (ã¹A₁) states of CH2 is facilitated by strong mixing of the rare pairs of accidentally degenerate rotational levels in the ground vibrational manifold of the [Formula: see text] state and the (020) and (030) excited bending vibrational manifolds of the X state. The simplest model for this process involves coherent mixing of the scattering T-matrix elements associated with collisional transitions within the unmixed ã and X states. From previous calculations in our group, we have determined cross sections and room-temperature rate constants for intersystem crossing of CH2 by collision with He. These are used in simulations of the time dependence of the energy flow, both within and between the X and ã vibronic manifolds. Relaxation proceeds through three steps: (a) rapid equilibration of the two mixed-pair levels, (b) fast relaxation within the ã state, and (c) slower relaxation among the levels of the X state. Collisional transfer between the fine-structure levels of the triplet (X) state is very slow.
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Dagdigian PJ, Alexander MH. Transport Properties for Systems with Deep Potential Wells: H + O2. J Phys Chem A 2014; 118:11935-42. [DOI: 10.1021/jp505769h] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Tkáč O, Rusher CA, Greaves SJ, Orr-Ewing AJ, Dagdigian PJ. Differential and integral cross sections for the rotationally inelastic scattering of methyl radicals with H2and D2. J Chem Phys 2014; 140:204318. [DOI: 10.1063/1.4879618] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Tkáč O, Orr-Ewing AJ, Dagdigian PJ, Alexander MH, Onvlee J, van der Avoird A. Collision dynamics of symmetric top molecules: a comparison of the rotationally inelastic scattering of CD3 and ND3 with He. J Chem Phys 2014; 140:134308. [PMID: 24712794 DOI: 10.1063/1.4869596] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We compare rotationally inelastic scattering of deuterated methyl radicals (CD3) and ammonia (ND3) in collisions with helium using close-coupling quantum-mechanical scattering calculations performed with ab initio potential energy surfaces (PESs). The theoretical methods have been rigorously tested against angle-resolved experimental measurements obtained using crossed molecular beam apparatuses in combination with velocity map imaging [O. Tkáč, A. G. Sage, S. J. Greaves, A. J. Orr-Ewing, P. J. Dagdigian, Q. Ma, and M. H. Alexander, Chem. Sci. 4, 4199 (2013); O. Tkáč, A. K. Saha, J. Onvlee, C.-H. Yang, G. Sarma, C. K. Bishwakarma, S. Y. T. van de Meerakker, A. van der Avoird, D. H. Parker, and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. 16, 477 (2014)]. Common features of the scattering dynamics of these two symmetric top molecules, one closed-shell and the other an open-shell radical, are identified and discussed. Two types of anisotropies in the PES influence the interaction of an atom with a nonlinear polyatomic molecule. The effects of these anisotropies can be clearly seen in the state-to-state integral cross sections out of the lowest CD3 rotational levels of each nuclear spin symmetry at a collision energy of 440 cm(-1). Similarities and differences in the differential cross sections for the ND3-He and CD3-He systems can be linked to the coupling terms derived from the PESs which govern particular initial to final rotational level transitions.
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Chen J, Dagdigian PJ. Radiative and Nonradiative Decay of Electronically Excited HNF and DNF. Isr J Chem 2013. [DOI: 10.1002/ijch.199400006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Melchior A, Mark Lambert H, Dagdigian PJ, Bar I, Rosenwaks S. The Photodissociation of Ground and Vibrationally Excited Halogenated Alkanes. Isr J Chem 2013. [DOI: 10.1002/ijch.199700051] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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Dagdigian PJ, Alexander MH. Exact quantum scattering calculations of transport properties for the H2O–H system. J Chem Phys 2013; 139:194309. [DOI: 10.1063/1.4829681] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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McGurk SJ, McKendrick KG, Costen ML, Alexander MH, Dagdigian PJ. Parity-dependent oscillations in collisional polarization transfer: CN(A²Π, v = 4) + Ar. J Chem Phys 2013; 139:124304. [PMID: 24089764 DOI: 10.1063/1.4821602] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We report the first systematic experimental and theoretical study of the state-to-state transfer of rotational angular momentum orientation in a (2)Π-rare gas system. CN(X(2)Σ(+)) was produced by pulsed 266 nm photolysis of ICN in a thermal bath (296 K) of Ar collider gas. A pulsed circularly polarized tunable dye laser prepared CN(A(2)Π, v = 4) in two fully state-selected initial levels, j = 6.5 F1e and j = 10.5 F2f, with a known laboratory-frame orientation. Both the prepared levels and a range of product levels, j' F1e and j' F2f, were monitored using the circular polarized output of a tunable diode laser via cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM Doppler lineshapes for co-rotating and counter-rotating pump-and-probe geometries reveal the time-dependence of the populations and orientations. Kinetic fitting was used to extract the state-to-state population transfer rate constants and orientation multipole transfer efficiencies (MTEs), which quantify the degree of conservation of initially prepared orientation in the product level. Complementary full quantum scattering (QS) calculations were carried out on recently computed ab initio potential energy surfaces. Collision-energy-dependent tensor cross sections for ranks K = 0 and 1 were computed for transitions from both initial levels to all final levels. These quantities were integrated over the thermal collision energy distribution to yield predictions of the experimentally observed state-to-state population transfer rate constants and MTEs. Excellent agreement between experiment and theory is observed for both measured quantities. Dramatic oscillations in the MTEs are observed, up to and including changes in the sign of the orientation, as a function of even/odd Δj within a particular spin-orbit and e/f manifold. These oscillations, along with those also observed in the state-to-state rate constants, reflect the rotational parity of the final level. In general, parity-conserving collisions conserve rotational orientation, while parity-changing collisions result in large changes in the orientation. The QS calculations show that the dynamics of the collisions leading to these different outcomes are fundamentally different. We propose that the origin of this behavior lies in interferences between collisions that sample the even and odd-λ terms in the angular expansions of the PESs.
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Lehman JH, Lester MI, Kłos J, Alexander MH, Dagdigian PJ, Herráez-Aguilar D, Aoiz FJ, Brouard M, Chadwick H, Perkins T, Seamons SA. Electronic Quenching of OH A 2Σ+ Induced by Collisions with Kr Atoms. J Phys Chem A 2013; 117:13481-90. [DOI: 10.1021/jp407035p] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Dagdigian PJ. Theoretical investigation of collisional energy transfer in polyatomic intermediates. INT REV PHYS CHEM 2013. [DOI: 10.1080/0144235x.2012.758543] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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Lique F, Werfelli G, Halvick P, Stoecklin T, Faure A, Wiesenfeld L, Dagdigian PJ. Spin-orbit quenching of the C+(2P) ion by collisions with para- and ortho-H2. J Chem Phys 2013; 138:204314. [PMID: 23742482 DOI: 10.1063/1.4807311] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Spin-orbit (de-)excitation of C(+)((2)P) by collisions with H2, a key process for astrochemistry, is investigated. Quantum-mechanical calculations of collisions between C(+) ions and para- and ortho-H2 have been performed in order to determine the cross section for the C(+) (2)P3∕2 → (2)P1∕2 fine-structure transition at low and intermediate energies. The calculation are based on new ab initio potential energy surfaces obtained using the multireference configuration interaction method. Corresponding rate coefficients were obtained for temperatures ranging from 5 to 500 K. These rate coefficients are compared to previous estimations, and their impact is assessed through radiative transfer computation. They are found to increase the flux of the (2)P3∕2 → (2)P1∕2 line at 158 μm by up to 30% for typical diffuse interstellar cloud conditions.
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Dagdigian PJ, Alexander MH. Exact quantum scattering calculations of transport properties: CH2(X̃3B1, ã1A1)–helium. J Chem Phys 2013; 138:164305. [DOI: 10.1063/1.4801789] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Dagdigian PJ, Alexander MH. Theoretical study of the vibrational relaxation of the methyl radical in collisions with helium. J Chem Phys 2013; 138:104317. [DOI: 10.1063/1.4794167] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Tkáč O, Sage AG, Greaves SJ, Orr-Ewing AJ, Dagdigian PJ, Ma Q, Alexander MH. Rotationally inelastic scattering of CD3 and CH3 with He: comparison of velocity map-imaging data with quantum scattering calculations. Chem Sci 2013. [DOI: 10.1039/c3sc52002a] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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Dagdigian PJ, Alexander MH. Exact quantum scattering calculation of transport properties for free radicals: OH(X2Π)–helium. J Chem Phys 2012; 137:094306. [DOI: 10.1063/1.4748141] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
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47
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Gubbels KB, Ma Q, Alexander MH, Dagdigian PJ, Tanis D, Groenenboom GC, van der Avoird A, van de Meerakker SYT. Resonances in rotationally inelastic scattering of OH(X2Π) with helium and neon. J Chem Phys 2012; 136:144308. [PMID: 22502519 DOI: 10.1063/1.3697816] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We present detailed calculations on resonances in rotationally and spin-orbit inelastic scattering of OH (X(2)Π, j = 3/2, F(1), f) radicals with He and Ne atoms. We calculate new ab initio potential energy surfaces for OH-He, and the cross sections derived from these surfaces compare well with the recent crossed beam scattering experiment of Kirste et al. [Phys. Rev. A 82, 042717 (2010)]. We identify both shape and Feshbach resonances in the integral and differential state-to-state scattering cross sections, and we discuss the prospects for experimentally observing scattering resonances using Stark decelerated beams of OH radicals.
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Dagdigian PJ, Alexander MH. Theoretical investigation of rotationally inelastic collisions of CH2(X̃) with helium. J Chem Phys 2012; 136:224306. [DOI: 10.1063/1.4729050] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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McGurk SJ, McKendrick KG, Costen ML, Bennett DIG, Kłos J, Alexander MH, Dagdigian PJ. Depolarization of rotational angular momentum in CN(A2Π, v = 4) + Ar collisions. J Chem Phys 2012; 136:164306. [PMID: 22559481 DOI: 10.1063/1.4705118] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Angular momentum depolarization and population transfer in CN(A(2)Π, v = 4, j, F(1)e) + Ar collisions have been investigated both experimentally and theoretically. Ground-state CN(X(2)Σ(+)) molecules were generated by pulsed 266-nm laser photolysis of ICN in a thermal (nominally 298 K) bath of the Ar collision partner at a range of pressures. The translationally thermalized CN(X) radicals were optically pumped to selected unique CN(A(2)Π, v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F(1)e) levels on the A-X (4,0) band by a pulsed tunable dye laser. The prepared level was monitored in a collinear geometry by cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM lineshapes for co- and counter-rotating circular pump and probe polarizations were analyzed to extract the time dependence of the population and (to a good approximation) orientation (tensor rank K = 1 polarization). The corresponding parallel and perpendicular linear polarizations yielded population and alignment (K = 2). The combined population and polarization measurements at each Ar pressure were fitted to a 3-level kinetic model, the minimum complexity necessary to reproduce the qualitative features of the data. Rate constants were extracted for the total loss of population and of elastic depolarization of ranks K = 1 and 2. Elastic depolarization is concluded to be a relatively minor process in this system. Complementary full quantum scattering (QS) calculations were carried out on the best previous and a new set of ab initio potential energy surfaces for CN(A)-Ar. Collision-energy-dependent elastic tensor and depolarization cross sections for ranks K = 1 and 2 were computed for CN(A(2)Π, v = 4, j = 1.5-10.5, F(1)e) rotational/fine-structure levels. In addition, integral cross sections for rotationally inelastic transitions out of these levels were computed and summed to yield total population transfer cross sections. These quantities were integrated over a thermal collision-energy distribution to yield the corresponding rate constants. A complete master-equation simulation using the QS results for the selected initial level j = 6.5 gave close, but not perfect, agreement with the near-exponential experimental population decays, and successfully reproduced the observed multimodal character of the polarization decays. On average, the QS population removal rate constants were consistently 10%-15% higher than those derived from the 3-level fit to the experimental data. The QS and experimental depolarization rate constants agree within the experimental uncertainties at low j, but the QS predictions decline more rapidly with j than the observations. In addition to providing a sensitive test of the achievable level of agreement between state-of-the art experiment and theory, these results highlight the importance of multiple collisions in contributing to phenomenological depolarization using any method sensitive to both polarized and unpolarized molecules in the observed level.
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Dagdigian PJ. EXPERIMENTAL STUDIES OF ROTATIONALLY INELASTIC STATE-RESOLVED COLLISIONS OF SMALL MOLECULAR FREE RADICALS. ACTA ACUST UNITED AC 2012. [DOI: 10.1142/9789814317184_0008] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/07/2023]
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