Search

The approach to the equilibrium (Boltzmann) distribution of populations of internal states of a molecule is governed by inelastic collisions in the gas phase and with surfaces. The set of differential equations governing the time evolution of the internal state populations is commonly called the master equation. An analytic solution to the master equation is presented and shows that the equilibrium distribution is the Boltzmann distribution. This solution is applied to the master equation involving collisions of rotational states of a diatomic molecule with a monatomic bath gas. (Contains 1 table and 5 figures.)

The near-equilibrium potential energy surface (PES) of the ã 3 B 1 state of SO2 is developed from explicitly correlated spin-unrestricted coupled cluster calculations with single, double, and perturbative triple excitations with an augmented triple-zeta correlation-consistent basis set. The lowest-lying ro-vibrational energy levels of several sulfur isotopologues have been determined using this PES. It is shown that the new ab initio PES provides a much better description of the low-lying vibrational states than a previous PES determined at the multi-reference configuration interaction level. In particular, the theory-experiment agreement for the three lowest-lying vibrational transitions is within 1–3 cm−1. [ABSTRACT FROM AUTHOR]

State-to-state cross sections and thermal rate constants for vibrational and rotational relaxation of OH((super 2)II) by collision with H atoms are reported. The results revealed a significant propensity toward formation of OH in the II(A') A-doublet level with the collisional excitation from the F1 to the F2 spin-orbit manifold leads to an inverted A-doublet population.

The low-energy wing of the C¹B² ←X¹A1 absorption spectra for SO2 in the ultraviolet region is computed for the 32S,33S,34S and 36S isotopes, using the recently developed ab initio potential energy surfaces (PESs) of the two electronic states and the corresponding transition dipole surface. The stateresolved absorption spectra from various ro-vibrational states of SO2(X ¹A1) are computed. When contributions of these excited ro-vibrational states are included, the thermally averaged spectra are broadened but maintain their key characters. Excellent agreement with experimental absorption spectra is found, validating the accuracy of the PESs. The isotope shifts of the absorption peaks are found to increase linearly with energy, in good agreement with experiment. [ABSTRACT FROM AUTHOR]

We determine from first principles two sets of four-dimensional diabatic potential energy surfaces (PES's) for the interaction of NO(X2Π) with H2, under the assumption of fixed NO and H2 bond distances. The first set of PES'swas computed with the explicitly correlated multi-reference configuration interaction method [MRCISD-F12 + Q(Davidson)], and the second set with an explicitly correlated, coupled-cluster method [RCCSD(T)-F12a] with the geometry scan limited to geometries possessing a plane of symmetry. The calculated PES's are then fit to an analytical form suitable for bound state and scattering calculations. The RCCSD(T)-F12a dissociation energies (D0) of the NO-para-H2(ortho-D2) and the NO-ortho-H2(para-D2) complexes are computed to be 22.7 (31.7) and 23.9 (29.2) cm-1, respectively. The values calculated with the MRCISD-F12 + Q PES's are 21.6 (31.1) and 23.3 (28.4) cm-1, respectively. [ABSTRACT FROM AUTHOR]

A --> X electronic transitions from CN[A(super 2)Pi, epsilon=3, N=60-63) fine-structure lambda-doublet levels induced by collisions with argon and helium are studied. The importance of gateway in facilitating these collisions is examined.

Reactivity of the excited spin-orbit state of Cl with [H.sub.2] to yield ground-state HCl products is forbidden by the Born-Oppenheimer (BO) approximation. We used new ab initio potential energy surfaces and exact quantum scattering calculations to explore the extent of electronic nonadiabaticity in this reaction. In direct contrast to recent experiments, we predict that the BO-allowed reaction of the ground spin-orbit state will be much more efficient than the BO-forbidden reaction of the excited spin-orbit state. Also, Coriolis coupling opens up an electronically nonadiabatic inelastic channel, which competes substantially with reaction., The kinetics of the Cl + [H.sub.2] reaction has been the object of study for more than a century (1, 2). The reaction has played an important role in the [...]

Transport properties for collisions of oxygen atoms with hydrogen atoms and hydrogen molecules have been computed by means of time-independent quantum scattering calculations. For the O(3P)-H(2S) interaction, potential energy curves for the four OH electronic states emanating from this asymptote were computed by the internally-contracted multi-reference configuration interaction method, and the R-dependent spin-orbit matrix elements were taken from Parlant and Yarkony [J. Chem. Phys. 110, 363 (1999)]. For the O(3P)-H2 interaction, diabatic potential energy surfaces were derived from internally contracted multi-reference configuration interaction calculations. Transport properties were computed for these two collision pairs and compared with those obtained with the conventional approach that employs isotropic Lennard-Jones (12-6) potentials. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

We report new and more accurate adiabatic potential energy surfaces (PESs) for the ground X¹A¹ and electronically excited C¹B2(2¹A') states of the SO2 molecule. Ab initio points are calculated using the explicitly correlated internally contracted multi-reference configuration interaction (icMRCI-F12) method. A second less accurate PES for the ground X state is also calculated using an explicitly correlated single-reference coupled-cluster method with single, double, and non-iterative triple excitations [CCSD(T)-F12]. With these new three-dimensional PESs, we determine energies of the vibrational bound states and compare these values to existing literature data and experiment. [ABSTRACT FROM AUTHOR]

We report new and more accurate adiabatic potential energy surfaces (PESs) for the ground X¹A¹ and electronically excited C¹B2(2¹A') states of the SO2 molecule. Ab initio points are calculated using the explicitly correlated internally contracted multi-reference configuration interaction (icMRCI-F12) method. A second less accurate PES for the ground X state is also calculated using an explicitly correlated single-reference coupled-cluster method with single, double, and non-iterative triple excitations [CCSD(T)-F12]. With these new three-dimensional PESs, we determine energies of the vibrational bound states and compare these values to existing literature data and experiment. [ABSTRACT FROM AUTHOR]

We present the quantum close-coupling treatment of spin-orbit induced transitions between the ¹D and ³P states of an atom in collisions with a closed-shell spherical partner. In the particular case of O colliding with Xe, we used electronic structure calculations to compute the relevant potential energy curves and spin-orbit coupling matrix elements. We then carried out quantum scattering calculations of integral and differential quenching cross sections as functions of the collision energy. The calculated differential cross sections for electronic quenching are in reasonable agreement with measurements [Garofalo et al., J. Chem. Phys. 143, 054307 (2015)]. The differential cross sections exhibit pronounced oscillations as a function of the scattering angle. By a semiclassical analysis, we show that these oscillations result from quantum mechanical interference between two classical paths. [ABSTRACT FROM AUTHOR]

The dynamics of the O(¹D) + Xe electronic quenching reaction was investigated in a crossed beam experiment at four collision energies. Marked large-scale oscillations in the differential cross sections were observed for the inelastic scattering products, O(³P) and Xe. The shape and relative phases of the oscillatory structure depend strongly on collision energy. Comparison of the experimental results with time-independent scattering calculations shows qualitatively that this behavior is caused by Stueckelberg interferences, for which the quantum phases of the multiple reaction pathways accessible during electronic quenching constructively and destructively interfere. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X²π, v = 0, j = 3/2 F1f level, in collisions with molecular hydrogen (H2 and D2). In a crossed beam experiment, the OH radicals were state selected and velocity tuned over the collision energy range 15-155 cm-1 using a Stark decelerator. Relative parity-resolved state-to-state integral cross sections were determined for collisions with normal and para converted H2. These cross sections, as well as previous OH-H2 measurements at 595 cm-1 collision energy by Schreel and ter Meulen [J. Chem. Phys. 105, 4522 (1996)], and OH-D2 measurements for collision energies 100-500 cm-1 by Kirste et al. [Phys. Rev. A 82, 042111 (2010)], were compared with the results of quantum scattering calculations using recently determined ab initio potential energy surfaces [Ma et al., J. Chem. Phys. 141,114309 (2014)]. Good agreement between the experimental and computed relative cross sections was found, although some structure seen in the OH(j = 3/2 F1f → j = 5/2 F1e) + H2(j = 0) cross section is not understood. [ABSTRACT FROM AUTHOR]

This is the second in a series of papers detailing a MATLAB based implementation of the finite element method applied to collinear triatomic reactions. Here, we extend our previous work to reactions on coupled potential energy surfaces. The divergence of the probability current density field associated with the two electronically adiabatic states allows us to visualize in a novel way where and how nonadiabaticity occurs. A two-dimensional investigation gives additional insight into nonadiabaticity beyond standard one-dimensional models. We study the F(²P) + HCl and F(²P) + H2 reactions as model applications. Our publicly available code (http://www2.chem.umd.edu/groups/alexander/FEM) is general and easy to use. [ABSTRACT FROM AUTHOR]

We have earlier determined the dependence on the bending angle of the interaction of the methylene radical (CH2) in its X3B1 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 CH2(X). Specifically, we find that cross sections for a loss of one vibrational quantum (vb = 21 and 10) are roughly two orders of magnitude smaller, and those for a loss of two vibrational quanta (vb = 20) four orders of magnitude smaller, than those for pure rotational relaxation. In addition, no clear cut dependence on the energy gap is seen. [ABSTRACT FROM AUTHOR]

For the interaction of OH(X²Π) with H2, under the assumption of fixed OH and H2 bond distances, we have determined two new sets of four-dimensional ab initio potential energy surfaces (PES's). The first set of PES's was computed with the multi-reference configuration interaction method [MRCISD+Q(Davidson)], and the second set with an explicitly correlated coupled cluster method [RCCSD(T)-F12a] sampling the subset of geometries possessing a plane of symmetry. Both sets of PES's are fit to an analytical form suitable for bound state and scattering calculations. The CCSD(T) dissociation energies (D0) of the OH-para-H2 and the OH-ortho-H2 complexes are computed to be 36.1 and 53.7 cm-1. The latter value is in excellent agreement with the experimental value of 54 cm-1. [ABSTRACT FROM AUTHOR]

Collisional energy transfer between the ground (X ³ B1) and first excited (ᾶ ¹ A1) states of CH2 is facilitated by strong mixing of the rare pairs of accidentally degenerate rotational levels in the ground vibrational manifold of the a 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 ᾶ 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 ᾶ vibronic manifolds. Relaxation proceeds through three steps: (a) rapid equilibration of the two mixed-pair levels, (b) fast relaxation within the ᾶ state, and (c) slower relaxation among the levels of the XX state. Collisional transfer between the fine-structure levels of the triplet (X) state is very slow. [ABSTRACT FROM AUTHOR]

We restate the application of the finite element method to collinear triatomic reactive scattering dynamics with a novel treatment of the scattering boundary conditions. The method provides directly the reactive scattering wave function and, subsequently, the probability current density field. Visualizing these quantities provides additional insight into the quantum dynamics of simple chemical reactions beyond simplistic one-dimensional models. Application is made here to a symmetric reaction (H+H2), a heavy-light-light reaction (F+H2), and a heavy-light-heavy reaction (F+HCl). To accompany this article, we have written a MATLAB code which is fast, simple enough to be accessible to a wide audience, as well as generally applicable to any problem that can be mapped onto a collinear atom-diatom reaction. The code and user's manual are available for download from http://www2.chem.umd.edu/groups/alexander/FEM. [ABSTRACT FROM AUTHOR]

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. Tkáˇc, 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. [ABSTRACT FROM AUTHOR]

Transport properties for collisions of water with hydrogen atoms are computed by means of exact quantum scattering calculations. For this purpose, a potential energy surface (PES) was computed for the interaction of rigid H2O, frozen at its equilibrium geometry, with a hydrogen atom, using a coupled-cluster method that includes all singles and doubles excitations, as well as perturbative contributions of connected triple excitations. To investigate the importance of the anisotropy of the PES on transport properties, calculations were performed with the full potential and with the spherical average of the PES. We also explored the determination of the spherical average of the PES from radial cuts in six directions parallel and perpendicular to the C2 axis of the molecule. Finally, the computed transport properties were compared with those computed with a Lennard-Jones 12-6 potential. [ABSTRACT FROM AUTHOR]

We present the experimental and simulated (2+1) REMPI spectrum of the C 2Π state of the NO-Ar complex, in the vicinity of the 3p Rydberg state of NO. Two Rydberg states of NO are expected in this energy region: the C2Π (3pπ) and D2Σ+ (3pσ) states, and we concentrate on the former here. When the C2Π (3pπ) state interacts with Ar at nonlinear orientations, the symmetry is lowered to Cs, splitting the degeneracy of the 2Π state to yield C(2A″) and C(2A′) states. For these two states of NO-Ar, we calculate potential energy surfaces using second order Mo\ller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction to describe the excited states, the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectrum, which is in excellent agreement with experiment, providing assignments for the observed spectral lines from the calculated rovibrational wavefunctions. [ABSTRACT FROM AUTHOR]

We report a theoretical investigation of the relaxation of the umbrella vibrational mode (the ν2 mode) of the CH3 molecule in its ground X2A2′′ electronic state in collisions with helium. We have calculated a four-dimensional potential energy surface (PES) for the interaction between CH3 with different umbrella displacements and a helium atom, using a restricted open-shell coupled-cluster method with inclusion of all single, double, and (perturbatively) triple excitations [RCCSD(T)]. With this PES we carried out full close-coupling scattering calculations including all CH3 umbrella-rotational levels with v2 <= 3. To our knowledge, this work represents the first fully quantum calculations of ro-vibrational relaxation of a polyatomic. In more detail, we investigate propensities in the calculated ro-vibrational cross sections and the dependence on initial rotational excitation, as well as determining thermal rate constants. Overall, ro-vibrational relaxation is nearly two orders of magnitude less efficient than pure-rotational relaxation, with a noticeable dependence on the initial rotational level. We predict the room temperature v2 = 1 vibrational relaxation rate constant to be 5.4 × 10-12 cm3 molecule-1 s-1, compared to the rate constants for pure-rotational relaxation of the lower rotational levels (∼2.0 × 10-10 cm3 molecule-1 s-1). [ABSTRACT FROM AUTHOR]

We develop a rigorous quantum mechanical theory for collisions of polyatomic molecular radicals with S-state atoms in the presence of an external magnetic field. The theory is based on a fully uncoupled space-fixed basis set representation of the multichannel scattering wave function. Explicit expressions are presented for the matrix elements of the scattering Hamiltonian for spin-1/2 and spin-1 polyatomic molecular radicals interacting with structureless targets. The theory is applied to calculate the cross sections and thermal rate constants for spin relaxation in low-temperature collisions of the prototypical organic molecule methylene [CH2(X3B1)] with He atoms. To this end, two accurate three-dimensional potential energy surfaces (PESs) of the He-CH2(X3B1) complex are developed using the state-of-the-art coupled-cluster method including single and double excitations along with a perturbative correction for triple excitations and large basis sets. Both PESs exhibit shallow minima and are weakly anisotropic. Our calculations show that spin relaxation in collisions of CH2, CHD, and CD2 molecules with He atoms occurs at a much slower rate than elastic scattering over a large range of temperatures (1 μK-1 K) and magnetic fields (0.01-1 T), suggesting excellent prospects for cryogenic helium buffer-gas cooling of ground-state ortho-CH2(X3B1) molecules in a magnetic trap. Furthermore, we find that ortho-CH2 undergoes collision-induced spin relaxation much more slowly than para-CH2, which indicates that magnetic trapping can be used to separate nuclear spin isomers of open-shell polyatomic molecules. [ABSTRACT FROM AUTHOR]

Following our earlier work on collisions of He with the methylene radical in its excited ã1A1 state [L. Ma, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys. 134, 154307 (2011)], we investigate here the analogous relaxation of CH2 in its ground X3B1 electronic state. The molecule is treated as semi-rigid, with fixed bond lengths but a varying bond angle. We use an ab initio potential energy surface (PES) which is averaged over the CH2 bending angle weighted by the square of the bending wave function. The PES for the interaction of He with CH2 in the X state is considerably less anisotropic than for interaction with the ã state since the two 2p electrons on the C atom are evenly distributed among the bonding and non-bonding molecular orbitals. We report quantum scattering calculations of state-to-state and total removal cross sections as well as total removal rate constants at room temperature. Because of the less pronounced anisotropy, these cross sections and rate constants are considerably smaller than for collisions of CH2(ã) with He. Finally, we investigate the dependence of rotational inelasticity on the bending vibrational quantum number. [ABSTRACT FROM AUTHOR]