Collisional excitation of water by hydrogen atoms

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 coefficients for the first 45 energy levels of both ortho- and para-H$_2$O and for temperatures in the range T = 5-1500 K. These rate coefficients are subsequently compared to the values previously published for the H$_2$O / He and H$_2$O / H$_2$ collisional systems. It is shown that no simple relation exists between the three systems and that specific calculations are thus mandatory

Depolarization of rotational angular momentum in CN(A²Π, v = 4) + Ar collisions

Angular momentum depolarization and population transfer in CN(A2?, v = 4, j, F1e) + Ar collisions have been investigated both experimentally and theoretically. Ground-state CN(X2S+) 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(A2?, v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F1e) 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(A2?, v = 4, j = 1.5–10.5, F1e) 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.</p

Differential and integral cross sections for the rotationally inelastic scattering of methyl radicals with H-2 and D-2

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) provide initial-to-final CD3 rotational level (n, k → n′, k′) integral and differential cross sections (ICSs and DCSs). The DCSs are compared with crossed molecular beam and velocity map imaging measurements of angular scattering distributions, which serve as a critical test of the accuracy of the new PES. In general, there is very good agreement between the experimental measurements and the calculations. The DCSs for CD3 scattering from both H2 and D2 peak in the forward hemisphere for n′ = 2-4 and shift more to sideways and backward scattering for n′ = 5. For n′ = 6-8, the DCSs are dominated by backward scattering. DCSs for a particular CD3 n → n′ transition have a similar angular dependence with either D2 or H2 as collision partner. Any differences between DCSs or ICSs can be attributed to mass effects because the PES is unchanged for CD3-H2 and CD3-D2 collisions. Further comparisons are drawn between the CD3-D2 scattering and results for CD3-He presented in our recent paper [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)]. These systems have the same reduced mass, but are governed by different PESs.</p

Resonances in rotationally inelastic scattering of OH(X2ΠX^2\Pi) with helium and neon

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 experiment of Kirste \emph{et al.} [Phys. Rev. A \textbf{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.Comment: 14 pages, 15 Figure

Laser cooling of a diatomic molecule

It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields. Unfortunately laser cooling has not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for many applications. For example, heteronuclear molecules possess permanent electric dipole moments which lead to long-range, tunable, anisotropic dipole-dipole interactions. The combination of the dipole-dipole interaction and the precise control over molecular degrees of freedom possible at ultracold temperatures make ultracold molecules attractive candidates for use in quantum simulation of condensed matter systems and quantum computation. Also ultracold molecules may provide unique opportunities for studying chemical dynamics and for tests of fundamental symmetries. Here we experimentally demonstrate laser cooling of the molecule strontium monofluoride (SrF). Using an optical cycling scheme requiring only three lasers, we have observed both Sisyphus and Doppler cooling forces which have substantially reduced the transverse temperature of a SrF molecular beam. Currently the only technique for producing ultracold molecules is by binding together ultracold alkali atoms through Feshbach resonance or photoassociation. By contrast, different proposed applications for ultracold molecules require a variety of molecular energy-level structures. Our method provides a new route to ultracold temperatures for molecules. In particular it bridges the gap between ultracold temperatures and the ~1 K temperatures attainable with directly cooled molecules (e.g. cryogenic buffer gas cooling or decelerated supersonic beams). Ultimately our technique should enable the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bialkalis.Comment: 10 pages, 7 figure

Interaction of CH3_3CN and CH3_3NC with He : potential energy surfaces and low-energy scattering

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH$_3$CN) and methyl isocyanide (CH$_3$NC) 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 H$_2$ molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH$_3$NC-He and CH$_3$CN-He van der Waals complexes by means of the explicitly correlated coupled cluster approach with single, double and perturbative triple excitation CCSD(T)/F12a in conjunction with the aug-cc-pVTZ basis set. We find a global minimum with $D_e= 55.10$ and 58.61 cm$^{-1}$ for CH$_3$CN-He and CH$_3$NC-He, respectively, while the dissociation energy $D_0$ of the complexes are 18.64 and 18.65 cm$^{-1}$, respectively. Low energy scattering calculations of pure rotational (de-)excitation of CH$_3$CN and CH$_3$NC by collision with He atoms are carried out with the close-coupling method and the collisional cross sections of $ortho-$ and $para-$CH$_3$NC and CH$_3$CN are computed for kinetic energies up to 100 cm$^{-1}$. While the PESs for both complexes are qualitatively similar, that of CH$_3$NC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CH$_3$NC-He, we find that |$\Delta j$| = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd $\Delta j$ transitions is observed, whereas for CH$_3$CN the dominant cross sections are associated to transitions with |$\Delta j$| = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments

Absorption Cross Sections and Kinetics of Formation of AlO at 298 K

The rate coefficient of the Al + O2 reaction has been measured in a laser ablation-fast flow tube apparatus by monitoring atomic Al resonance absorption and AlO laser induced fluorescence (LIF). The rate constant has been found to be k(298 K) = (1.68 ± 0.24) × 10-10 cm3 molecule-1 s-1. Under conditions of near-stoichiometric conversion of Al into AlO, the absorption cross section of AlO at the bandhead of the B2Σ+(v'=0)←X2Σ+(v''=0) transition has been determined to be σ(298 K, 1 hPa) = (6.7 ± 1.6) × 10-15 cm2 molecule-1 (0.003 nm resolution), in very good agreement with theoretical predictions

Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes

We review the different excitation processes in the interstellar mediumComment: Accepted in Chem. Re