Paul J Dagdigian

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• Doctor of Philosophy
• Professor Emeritus at Johns Hopkins University

The protonated carbon dioxide (HOCO⁺) ion has been observed in the interstellar medium and can provide an indirect probe of carbon dioxide, which cannot be detected by millimeter spectroscopic observations. Accurate modeling of HOCO⁺ spectra requires accurate radiative and collisional rates. While radiative rates are available, collisional rate coe...

This paper presents the calculation of rate coefficients for transitions between rotational levels of the A-type and E-type levels of methyl mercaptan (CH3SH) resulting from collisions with molecular hydrogen. Radiative transfer modeling requires both radiative and collisional rates to describe the rotational populations under the usual conditions...

An intriguing phenomenon in molecular collisions is the occurrence of scattering resonances, which originate from bound and quasi-bound states supported by the interaction potential at low collision energies. The resonance effects in the scattering behavior are extraordinarily sensitive to the interaction potential, and their observation provides o...

Context . The global context of making numerous data produced by researchers available requires collecting and organising the data, assigning meaningful metadata, and presenting the data in a meaningful and homogeneous way. The BASECOL database, which collects inelastic rate coefficients for application to the interstellar medium and to circumstell...

This paper presents rate coefficients for transitions between rotational levels of the A-type and E-type nuclear spin modifications of methanol induced by collisions with molecular hydrogen. These rate coefficients are required for an accurate determination of methanol abundance in the interstellar medium (ISM), where LTE conditions generally do no...

Ammonia is one of the most widely observed molecules in space, and many observations are able to resolve the hyperfine structure due to the electric quadrupole moment of the nitrogen nucleus. The observed spectra often display anomalies in the satellite components of the lines, which indicate substantial deviations from the local thermodynamic equi...

The potential energy surface (PES) describing the interaction of the methanol molecule with molecular hydrogen has been calculated by the use of the explicitly correlated coupled cluster method, including single, double, and (perturbative) triple excitations [CCSD(T)-F12a] and a correlation-consistent aug-cc-pVTZ basis, with the assumption of fixed...

The determination of physical conditions in interstellar clouds requires reliable estimation of radiative and collisional data for molecules detected in space. In this work, rate coefficients for (de-)excitation of C 2 H and C 2 D induced by collisions with both ortho-and para-H 2 are presented. Calculations have been carried out using a recently p...

Ammonia is one of the most widely observed molecules in space, and many observations are able to resolve the hyperfine structure due to the electric quadrupole moment of the nitrogen nucleus. The observed spectra often display anomalies in the satellite components of the lines, which indicate substantial deviations from the local thermodynamic equi...

The computation of hyperfine resolved cross sections and rate coefficients for open-shell molecules in collision with H 2 is a true methodological and numerical challenge. Such collisional data are however required to interpret astrophysical observations. We report the first hyperfine resolved rate coefficients for (de-)excitation of 13 CCH and C 1...

Among all closed-shell species observed in molecular clouds, molecules with C3v symmetry play a crucial role, as their rotational spectroscopy allows them to behave as a gas thermometer. In the interstellar medium, methyl cyanide (CH3CN) is the second most abundant of those (after ammonia, NH3). Its isomer, methyl isocyanide (CH3NC) is less abundan...

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH3CN) and methyl isocyanide (CH3NC) of interest here, have been observed in various astrophysical environments. The accurate modeling of their abundance requires the calculation of rate coefficients for their collisional excitation with species such as He atoms or H2 mole...

Observations of transitions between the hyperfine levels of the hydroxyl radical (OH) can provide crucial information on the physical conditions in interstellar clouds. Accurate modeling of the spectra requires calculated rate coefficients for the excitation of OH by H atoms, which is often present in molecular clouds in addition to the dominant H2...

State-to-state cross sections and rate coefficients for transitions between rotational/fine- structure levels of OH(X ² Π) induced by collisions with atomic hydrogen are reported in this work. The scattering calculations take into account the full open-shell character of the OH + H system and include the four potential energy surfaces ( ¹ A′, ¹ A′′...

Rate coefficients for transitions between the 14 lowest rotational levels, having energies less than 500 K, of the most abundant isotopolog of carbon monoxide,12C16O, caused by collisions with para-H2 and ortho-H2 are reported for temperatures between 5 and 400 K. These data were computed through time independent close coupled scattering calculatio...

The first set of theoretical rotational cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5–30 K. T...

Rate coefficients for transitions between the lower rotational levels of two isotopologs of hydrogen sulfide, specifically HDS and D2S, induced by collisions with para-H2 and ortho-H2 are presented in this work. The availability of these rate coefficients will allow accurate estimates to be made of the abundance of these species in the interstellar...

The first set of theoretical cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5-30 K. These collis...

Collisional data for the excitation of NH by H2 are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH–H2 van der Waals complex. The ab initio calculations of the PES were carried out using the explicitly correlated partially spin-restricted coupled cluster m...

Accurate estimates of the abundance of CH2 in the interstellar medium require knowledge of both the radiative and collisional rate coefficients for the transfer of population between rotational levels. In this work, time independent quantum close coupling calculations have been carried out to compute rate coefficients for the (de-)excitation of ort...

The potential energy surface describing the interaction of methylene (CH2) in its ground X~3B1 electronic state with molecular hydrogen has been calculated through explicitly correlated coupled cluster theory with inclusion of single, double, and (perturbatively) triple excitations [RCCSD(T)-F12a] and a correlation-consistent aug-cc-pVTZ basis. The...

Cross sections and rate coefficients for transitions between hyperfine levels associated with the 14 lowest rotational/fine-structure levels of OD induced by collisions with ortho-H2 and para-H2 are presented. These collisional parameters have been computed in time-independent close-coupling quantum-scattering calculations with a potential energy s...

Relaxation of the spin-orbit excited C+(2P3/2) ion by collisions with H2 is an important process in the interstellar medium. Previous calculations of rate coefficients for this process employed potential energies computed for only collinear and perpendicular approach of H2 to the ion. To capture the full angular dependence of the C+–H2 interaction,...

This paper addresses the need for accurate rate coefficients for transitions between fine- and hyperfine-structure resolved rotational transitions in the formyl (HCO) radical induced by collisions with the two nuclear spin modifications of H2, the dominant molecule in the interstellar medium (ISM). These rate coefficients, as well as radiative tran...

The potential energy surface describing the interaction of the HCO radical with molecular hydrogen has been computed through explicitly correlated coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)-F12a], with the assumption of fixed molecular geometries. The computed points were fit to an analyti...

Accurate estimates of the abundance of H2S, and inferences about the unmeasured H2 density, require accurate knowledge of radiative and collisional rate coefficients. Time-independent close-coupling quantum scattering calculations have been employed to compute rate coefficients for (de-)excitation of para- and ortho-H2S in collisions with para- and...

Observations of hyperfine resolved transitions of the hydroxyl radical (OH) are unique probes of the physical conditions in molecular clouds. In particular, hyperfine intensities can be used as an effective thermometer over a wide range of molecular densities. Accurate modelling of the OH emission spectra requires the calculation of collisional rat...

Context. Solar-like protostars are known to be chemically rich, but it is not yet clear how much their chemical composition can vary and why. So far, two chemically distinct types of Solar-like protostars have been identified: hot corinos, which are enriched in interstellar Complex Organic Molecules, such as methanol (CH 3 OH) or dimethyl ether (CH...

Context: Solar-like protostars are known to be chemically rich, but it is not yet clear how much their chemical composition can vary and why. So far, two chemically distinct types of Solar-like protostars have been identified: hot corinos, which are enriched in interstellar Complex Organic Molecules (iCOMs), such as methanol (CH$_3$OH) or dimethyl...

The potential energy surface describing the interaction of H2S with molecular hydrogen has been computed through explicitly correlated coupled cluster calculations including single, double, and (perturbative) triple excitations [CCSD(T)-F12a], with the assumption of fixed molecular geometries. The computed points were fit to an analytical form suit...

Time-independent close-coupling quantum scattering calculations are employed to compute hyperfine-resolved rate coefficients for (de-)excitation of SH+ in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of SH+(X3Σ−) with H2 recently computed by the explicitly correlated RCCSD(T)-F12a co...

The C⁺ + OH reaction is an important process in the interstellar synthesis of the CO molecule and the CO⁺ ion. This reaction has two possible chemical outcomes, namely, formation of CO + H⁺ and CO⁺ + H products. Formation of CO + H⁺ occurs on the ground 1¹A′ potential energy surface (PES), while CO⁺ + H can be formed on the 1³A′ and 2¹A′ PESs. The...

The first chiral interstellar organic molecule, propylene oxide (CH3CHCH2O), was detected recently toward the galactic center. Accurate determination of its abundance relies on the knowledge of collisional cross sections. We investigate here the rotational excitation of propylene oxide induced by collisions with helium. The calculations are based o...

The potential energy surface describing the interaction of the SH+ ion in its ground X3Σ- electronic state with molecular hydrogen has been computed through restricted coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)], with the assumption of fixed bond lengths. The computed points were fit to an...

The chemistry of fluorine in the interstellar medium is primarily dependent on a few key reactions. Fluorine is unique among the elements because the F atom is the only light atom that can react exothermically with molecular hydrogen to form its hydride, HF. The only other fluorine-containing molecule of significant abundance is the CF ⁺ ion, which...

The chemistry of fluorine in the interstellar medium is particularly simple, with only a few key species and important reactions. Of the latter, the rate of the reaction of C⁺ ions with HF is not well established but is one of the key reactions that sets the relative abundance of HF and the CF⁺ ion, the two fluorine-bearing species that have been o...

Accurate estimation of the abundance of the ethynyl (C2H) radical requires accurate radiative and collisional rate coefficients. Hyperfine-resolved rate coefficients for (de-)excitation of C2H in collisions with ortho- and para-H2 are presented in this work. These rate coefficients were computed in time-independent close-coupling quantum scattering...

The rotational excitation of the ³⁶ArH⁺ ion in collisions with hydrogen atoms is investigated in this work. The potential energy surface (PES) describing the ³⁶ArH⁺-H interaction, with the ion bond length r fixed at the average of r over the radial v = 0 vibrational state distribution, was obtained with a coupled cluster method that included single...

This chapter outlines the application of quantum statistical theory to nominally nonreactive collisions that access potential energy surfaces (PESs) having one or more deep wells. It uses the quantum statistical method to calculate transport cross sections, which are weighted averages of differential cross sections. The availability of these PESs a...

We investigate here the excitation of methylidene (CH) induced by collisions with atomic and molecular hydrogen (H and H2). The hyperfine-resolved rate coefficients were obtained from close coupling nuclear-spin-free scattering calculations. The calculations are based upon recent, high-accuracy calculations of the CH(X²∏)-H(²S) and CH(X²∏)-H2 poten...

The kinetics and dynamics of the collisional electronic quenching of O(¹D) atoms by Kr have been investigated in a joint experimental and theoretical study. The kinetics of quenching were measured over the temperature range 50–296 K using the Laval nozzle method. O(¹D) atoms were prepared by 266 nm photolysis of ozone, and the decay of the O(¹D) co...

The potential energy surface (PES) describing the interaction of the ethynyl (C2H) radical in its ground X̃2Σ+ electronic state with molecular hydrogen has been computed through restricted coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)], with the assumption of fixed molecular geometries. The c...

Accurate determination of the abundance of CH2 in interstellar media relies on both radiative and collisional rate coefficients. We investigate here the rotational/fine-structure excitation of CH2 induced by collisions with He. We employ a recoupling technique to generate finestructure- resolved cross-sections and rate coefficients from close coupl...

The excitation of fine-structure levels of O(3P) by collisions is an important cooling process in the interstellar medium. We investigate here spin-orbit (de-)excitation of O(3Pj, j = 0,1,2) induced by collisions with He, H and H2 based on quantum scat- tering calculations of the relevant rate coefficients in the 10 – 1000 K temperature range. The...

Observations of OH in molecular clouds provide crucial constraints on both the physical conditions and the oxygen and water chemistry in these clouds. Accurate modelling of the OH emission spectra requires the calculation of rate coefficients for excitation of OH by collisions with the most abundant collisional partner in the molecular clouds, name...

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X2Π) with atomic hydrogen. These calculations employed the four adiabatic potential energy surfaces correlating CH(X2Π) + H(2S), computed in this work through the multi-reference configuration interaction method [MRCISD + Q(Davidson)]. Because...

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’s was computed with the explicitly correlated multi-reference configuration interaction method [MRCISD-F12 + Q(Davidson)], and...

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X2Π) with molecular hydrogen. These calculations employed the diabatic matrix elements of the interaction potential determined by Dagdigian [J. Chem. Phys. 145, 114301 (2016)], which employed the multi-reference configuration-interaction metho...

Collisional parameters describing both the pressure-induced broadening and shifting of isolated lines in the spectrum of the hydroxyl radical in collisions with argon have been determined through quantum scattering calculations using accurate potential energy surfaces describing the OH( , )–Ar interactions. These calculations have been carried for...

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-refe...

Transport properties for collisions of hydrogen atoms with molecular nitrogen and methane were calculated through close-coupling quantum scattering calculations. For these calculations, potential energy surfaces for the interaction of H atoms with these molecules, with their geometries fixed at the respective equilibrium structures, were obtained w...

We have determined four-dimensional ab initio quasi-diabatic potential energy surfaces describing the interaction of CH(X ²Π) with H2, under the assumption of fixed CH and H2 internuclear separations. These calculations employed the multi-reference configuration interaction method [MRCISD+Q(Davidson)]. The computed points were fit to an analytical...

Cavity-enhanced spectroscopy is a sensitive optical absorption technique but one where the practical applications have been limited to studying small wavelength ranges. This Letter shows that wideband operation can be achieved by combining techniques usually reserved for the communications community with that of cavity-enhanced spectroscopy, produc...

The dynamics of the O(1 D) + 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(3 P) and Xe. The shape and relative phases of the oscillatory structure depend strongly on c...

We present the quantum close-coupling treatment of spin-orbit induced transitions between the 1 D and 3 P 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 eleme...

Transport properties for collisions of hydrogen atoms with CO and CO2 have been computed by means of quantum scattering calculations. The carbon oxides are important species in hydrocarbon combustion. The following potential energy surfaces (PES's) for the interaction of the molecule fixed in its equilibrium geometry were employed: for H-CO, the PE...

We present theoretical studies on the scattering resonances in rotationally inelastic collisions of NH3 and ND3 molecules with H2 molecules. We use the quantum close-coupling method to compute state-to-state integral and differential cross sections for the NH3/ND3–H2 system for collision energies between 5 and 70 cm−1, using a previously reported p...

Differential cross sections (DCSs) are reported for rotationally inelastic scattering of ND3 with H2, measured using a crossed molecular beam apparatus with velocity map imaging (VMI). ND3 molecules were quantum-state selected in the ground electronic and vibrational levels and, optionally, in the j±k = 14- rotation-inversion level prior to collisi...

The effect of employing accurate transport properties in simulations of freely propagating H2/O2/He and CH4/O2 flames is investigated. Transport properties for the collision pairs H2–H2, H–H2, and H–H are computed from published tables of collision integrals. Previous calculations in our group of transport properties for OH–He, H–H2O and H–O2 have...

We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X 2Π, v = 0, j = 3/2 F 1 f 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 75–155 cm−1 using a Stark dece...

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 recove...

We present quantum dynamical calculations that describe the rotational excitation of H$_2$O due to collisions with H atoms. We used a recent, high accuracy potential energy surface, and solved the collisional dynamics with the close-coupling formalism, for total energies up to 12 000 cm$^{-1}$. From these calculations, we obtained collisional rate...

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 machin...

We have earlier determined the dependence on the bending angle of the interaction of the methylene radical (CH2) in its X̃B13 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 c...

For the interaction of OH(X(2)Π) 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 c...

Collisional energy transfer between the ground ([Formula: see text]) and first excited ([Formula: see text]) 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...

Transport properties for collisions of the oxygen molecule with hydrogen atoms are computed by means of quantum scattering calculations. Since two potential energy surfaces (PES's) arise from the interaction of H((2)S) with O2(X(3)Σ(-)g), namely (2)A'' and (4)A'', collision integrals were computed using both PES's and then averaged with weighting b...

Comparisons are presented of experimental and theoretical studies of the rotationally inelastic scattering of CD3 radicals with H2 and D2 collision partners at respective collision energies of 680 ± 75 and 640 ± 60 cm(-1). Close-coupling quantum-mechanical calculations performed using a newly constructed ab initio potential energy surface (PES) pro...

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 o...

Quantum beat spectroscopy has been used to measure rate coefficients at 300 K for collisional depolarization for NO(A (2)Σ(+)) and OH(A (2)Σ(+)) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr....

This chapter discusses the electronic, vibrational, and rotational spectroscopy of atoms and molecules, and the relationship between the concentration of a species and its spectral intensities. The different types of radiative transitions and spectral broadening mechanisms are discussed. A brief review of energy levels and selection rules for radia...

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 doub...

Rotationally inelastic scattering of methyl radicals (CD3 and CH3) in collisions with helium is examined by a combination of velocity map imaging experiments and quantum scattering calculations. In the experiments a beam of methyl radicals seeded in Ar intersects a beam of He atoms at 90° at a collision energy of 440 ± 35 cm−1 (CD3 + He) or 425 ± 3...

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...

Electronic quenching of OH A (2)Σ(+) by Kr is investigated through experimental studies of the collision cross sections and the OH X (2)Π product distribution. The quenching cross sections decrease with increasing rotational excitation in the excited OH A (2)Σ(+) electronic state. The OH X (2)Π products of quenching exhibit a significant degree of...

Quantum scattering calculations on collisional rotational and vibrational energy transfer in small hydrocarbon reactive intermediates are highlighted. This review focuses on recent theoretical studies of energy transfer in methylene (CH2), in both its ground triplet 3B 1 and low-lying singlet 1A 1 electronic states, and in the methyl (CH3) radical....

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 energie...

Transport properties for collisions of methylene, in both its ground 3B1 and low-lying 1A1 electronic states, with helium have been computed using recently computed high-quality ab initio potential energy surfaces (PESs). Because of the difference in the orbital occupancy of the two electronic states, the anisotropies of the PESs are quite differen...

We report a theoretical investigation of the relaxation of the umbrella vibrational mode (the ν2 mode) of the CH3 molecule in its ground X̃(2)A2 ('') 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...

Transport properties for OH-He are computed through quantum scattering calculations using the ab initio potential energy surfaces determined by Lee et al. [J. Chem. Phys. 113, 5736 (2000)]. To gauge the importance of the open-shell character of OH and the anisotropy of the potential on the transport properties, including the collision integrals Ω((...

Following our earlier work on collisions of He with the methylene radical in its excited ã(1)A(1) state [L. Ma, M. H. Alexander, and P. J. Dagdigian, J. Chem. Phys. 134, 154307 (2011)], we investigate here the analogous relaxation of CH(2) in its ground X(3)B(1) electronic state. The molecule is treated as semi-rigid, with fixed bond lengths but a...

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 t...

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...

We present detailed calculations on resonances in rotationally and spin-orbit inelastic scattering of OH ($X\,^2\Pi, j=3/2, F_1, f$) radicals with He and Ne atoms. We calculate new \emph{ab initio} potential energy surfaces for OH-He, and the cross sections derived from these surfaces compare favorably with the recent crossed beam scattering experi...

A combined theoretical and experimental study of the depolarization of selected NO(X(2)Π, v = 0, j, F, ɛ) levels in collisions with a thermal bath of Ar has been carried out. Rate constants for elastic depolarization of rank K = 1 (orientation) and K = 2 (alignment) were extracted from collision-energy-dependent quantum scattering calculations, alo...

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 e...

Rotationally inelastic collisions of the CH(3) molecule in its ground X(2)A(2)'' electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(3), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as w...

Rotationally inelastic collisions of the CH(2) molecule in its ã(1)A(1) electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(2)(ã), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as...

Spectrally resolved bound–free fluorescence emission spectra for the excitation of several vibrational levels in the excited B2Σ+ electronic state of the van der Waals molecule 11BAr are presented. This excited state emits to the ground X2Π and low-lying A2Σ+ states, both of which correlate with the ground state atomic asymptote B(2p2P) + Ar. Becau...

A kinetic model previously developed to predict the relative intensities of atomic emission lines in laser-induced breakdown spectroscopy has been extended to include processes related to CN and C2 molecular emissions. Simulations with this model were performed to predict the relative excited-state populations. The results from the simulations are...

The Stark deceleration method exploits the concepts of charged particle accelerator physics to produce molecular beams with a tunable velocity. These tamed molecular beams offer interesting perspectives for precise crossed beam scattering studies as a function of the collision energy. The method has advanced sufficiently to compete with state-of-th...