Rovibrational levels and inelastic scattering of the H2O-Ar cluster in full and reduced dimensionality

The Water-Argon cluster is an important system of fundamental and practical interest. It is for example known to be one of the simplest systems capable of manifesting “hydrophobic interactions” and as such is an ideal candidate for the study of those interactions. On the fundamental level, it is a model system for the description of the intermolecular potential, rovibrational states and inelastic scattering of an atom and an asymmetric top van der Waal complex and thus may serve as a test to perform similar work on other systems. Additionally, the description of the H$_2$O-Ar intermolecular interaction is an important initial step to a deeper understanding of the static and dynamical properties of condensed phases such H$_2$O doped in large (Ar)$_N$ clusters. We investigate in this work the H$_2$O-Ar cluster on a global potential energy surface recently generated. We thus compute the rovibrational energy levels of the cluster in the rigid rotor approximation and in full dimensionality using the MCTDH improved relaxation method and compare our results with available experimental measurements and previous calculations. Then, we present inelastic scattering cross-sections of H$_2$O+Ar collisions obtained in the rigid rotor approximation using time-independent method and time-dependent method, and compare where available results with previous calculations. Finally, we will discuss the extension of the scattering calculations to the full dimensional case and the prospect of studying rovibrational relaxation within accurate time-dependent quantum calculations on similar systems or clusters

Vibrational Levels And Resonances On A New Potential Energy Surface For The Ground Electronic State Of Ozone

The isotopic ratios for ozone observed in laboratory and atmospheric measurements, known as the ozone isotopic anomaly,[1,2] have been an open question in physical and atmospheric chemistry for the past 30 years. The biggest limitation in achieving agreement between theory and experiment has been the availability of a satisfactory[3-5] ground state potential energy surface (PES). The presence of a spurious reef feature in the asymptotic region of most PESs has been associated with large discrepancies between calculated and observed rates of formation especially at low temperature. We recently proposed a new global potential energy surface for ozone[6,7] possessing 4 features that make it suitable for kinetics and dynamics studies: excellent equilibrium parameters, good agreement with experimental vibrational levels, accurate dissociation energy and a transition region with accurate topography (without the reef artifact). This PES has been used recently to simulate the temperature dependent exchange reaction (16O+16O2) with a quantum statistical model[6,7], and, for the first time, a negative temperature dependence which agrees with experiments was obtained, indicating the good quality of this global surface. A quantum description of the ozone exchange and recombination reaction requires knowledge of the resonances but also the rovibrational levels just below the dissociation. We present results of global 3-well vibrational-state calculations up to the dissociation threshold and (J = 0) resonances up to 1000 \wn beyond. The calculations were done using a large DVR basis (~24 million functions) with a symmetry-adapted Lanczos algorithm as well as MCTDH. Results indicate the presence of localized bound states at energies close to the dissociation threshold beyond which some long-lived resonances follow, contrasted with a few delocalized bound states with density at large values of the stretching coordinates. References: 1- K. Mauersberger et al., Adv. At. Mol. Opt. Phys. 50, 1 (2005) 2- R. Schinke et al., Ann. Rev. Phys. Chem. 57, 625 (2006) 3- R. Siebert et al., J. Chem. Phys. 116, 9749 (2002) 4- M. Ayouz and D. Babikov, J. Chem. Phys. 138, 164311 (2013) 5- V.G. Tyuterev et al., J. Chem. Phys. 139, 134307 (2013) 6- R. Dawes et al., J. Chem. Phys. 135, 081102 (2011) 7- R. Dawes et al., J. Chem. Phys. 139, 201103 (2013

Inelastic scattering of H+CO: influence of renner-teller coupling

Carbon monoxide is after molecular hydrogen the second most abundant molecule in the interstellar medium (ISM) and also an important molecule for processes occurring in the atmosphere, and hydrocarbon combustion. The rate coefficients of CO in collision with dominant species like H, H$_2$, He, etc are common keys to understand the CO emission spectrum or to model combustion chemistry processes. The inelastic scattering of H+CO has indeed been intensively studied in the past decades, using mainly the so-called WKS PES developed by Werner et al or recently a modified version by Song et al. Though the spectroscopic agreement of the WKS surface with experiment is known to be quite good, there is no experimental evidence that the dynamics of the system is correctly reproduced by the surface. We will present in this talk new results on a set of HCO surfaces of the ground and the excited Renner-Teller coupled electronic states with the principal objective of studying the influence of the Renner-Teller coupling on the inelastic scattering of H+CO. Our calculations done using the MCTDH algorithm cover the 0-2 eV energy range and allow one to interpret the effect of the Renner-Teller coupling on the rovibrational inelastic scattering. Additionally, vibrational bound and resonance state calculations on this new PES and comparisons with available experimental data will be presented

INFLUENCE OF THE RENNER-TELLER COUPLING IN CO+H COLLISION DYNAMICS

begin{wrapfigure}{r}{0.2textwidth}_x000d_ begin{center}_x000d_ includegraphics[width=0.20textwidth]{Fig12a.eps}_x000d_ end{center}_x000d_ end{wrapfigure}_x000d_ _x000d_ Carbon monoxide is after molecular hydrogen the second most abundant molecule in the universe and an important molecule for processes occurring in the atmosphere, hydrocarbon combustion and the interstellar medium. The rate coefficients of CO in collision with dominant species like H, H$_2$, He, etc are necessary to understand the CO emission spectrum or to model combustion chemistry processes. The inelastic scattering of CO with H has been intensively studied theoretically in the past decades,$^1$ mostly using the so-called WKS PES$^6$ developed by Werner et al. or recently a modified version by Song et al.$^2$ Though the spectroscopic agreement of the WKS surface with experiment is quite good, so far the studies of scattering dynamics have neglected coupling to an electronic excited state. We present new results on a set of HCO surfaces of the ground and the excited Renner-Teller coupled electronic states$^3$ with the principal objective of studying the influence of the Renner-Teller coupling on the inelastic scattering of CO+H. Our calculations done using the MCTDH$^4$ algorithm in the 0-2 eV energy range allow evaluation of the contribution of the Renner-Teller coupling on the rovibrationally inelastic scattering and discuss the relevance and reliability of the calculations._x000d_ _x000d_ _x000d_ underline{References:}\_x000d_ 1. N. Balakrishnan, M. Yan and A. Dalgarno, Astrophys. J. 568, 443 (2002); B.C. Shepler et al, Astron. & Astroph. 475, L15 (2007); L. Song et al, J. Chem. Phys. 142, 204303 (2015); K.M. Walker et al, Astroph. J. 811, 27 (2015).\_x000d_ 2. L. Song et al, Astrophys. J. 813, 96 (2015).\_x000d_ 3. H.-M. Keller et al, J. Chem. Phys. 105, 4983 (1996).\_x000d_ 4. S. Ndengue, R. Dawes and H. Guo, J. Chem. Phys. 144, 244301 (2016). \_x000d_ 5. M.H. Beck et al., Phys. Rep. 324, 1 (2000)._x000d

MCTDH ROVIBRATIONAL STATES AND STATE-TO-STATE INELASTIC SCATTERING CALCULATIONS ON THE H2O-H2 SYSTEM

Water, an essential ingredient of life, is prevalent in space and various media. H$_2$O in the gas phase is the major polyatomic species in the interstellar medium (ISM) and a primary target of current studies of collisional dynamics. In recent years a number of theoretical and experimental studies have been devoted to H$_2$O-X (with X=He, H$_2$, D$_2$, Ar, …) elastic and inelastic collisions in an effort to understand rotational distributions of H$_2$O in molecular clouds. In this work we are following those studies and will present benchmark calculations of rovibrational states and resonances of the H$_2$O-H$_2$ cluster in the rigid rotor approximation using the MultiConfiguration Time Dependent Hartree (MCTDH) approach. We will also present the first state-to-state inelastic scattering results of the H$_2$O+H$_2$ process in the rigid rotor approximation using the MCTDH approach. These calculations will serve as a foundation for similar triatomic – linear molecule interactions which are usually computationally expensive using standard calculations methods

MCTDH CALCULATION OF INELASTIC COLLISION OF COMPLEX MOLECULAR SYSTEMS

The details of energy transfer between colliding atoms and molecules is essential in order to understand chemical processes such as those in Earth’s or other atmospheres, the Interstellar Medium (ISM) or for combustion chemistry processes in industrial and aeronautic applications. The most accurate and reliable data for these applications are mostly based on fully quantum mechanical calculations (classical treatments of the nuclei usually lead to inaccuracies, in particular when low temperature processes (common in the ISM) are of interest). To counter the curse-of-dimensionality (COD) problem that arises when cross-section calculations are performed on molecular systems with large masses (and thus dense state densities), two particularly promising methods have been developed recently: the Statistical Adiabatic Channel Model (SACM) and the Mixed Quantum-Classical Trajectory (MQCT) approach. We have also recently highlighted the effectiveness of the MultiConfiguration Time Dependent Hartree (MCTDH) approach to overcome the COD issue arising in the calculation of cross-sections of inelastic collisions. Indeed, since its inception, the MCTDH approach has allowed characterization, using a fully quantum dynamical approach and with an excellent accuracy, the spectroscopy and the dynamics for several challenging molecular systems and is thus known to push the boundaries of traditional quantum dynamical calculations. We will present our recent progress on this topic and discuss recent (and some preliminary) results on the collisions of H$_2$O with Ar, H$_2$ and H$_2$O using the MCTDH approach. We will also present the results of other collisions processes (HCOOCH$_3$+He, CH$_3$CH$_2$COH+He, N$_2$H$^+$+H$_2$, …) which remain challenging with standard computation approaches. We will also discuss the limitations of the approach and possible routes for improvements

TOWARDS A QUANTUM DYNAMICAL STUDY OF THE H2O+H2O INELASTIC COLLISION: REPRESENTATION OF THE POTENTIAL AND PRELIMINARY RESULTS

Water, an essential ingredient of life, is prevalent in space and various media. H$_2$O in the gas phase is the major polyatomic species in the interstellar medium (ISM) and a primary target of current studies of collisional dynamics. In recent years a number of theoretical and experimental studies have been devoted to H$_2$O-X (with X=He, H$_2$, D$_2$, Ar, …) elastic and inelastic collisions in an effort to understand rotational distributions of H$_2$O in molecular clouds. Although those studies treated several abundant species, no quantum mechanical calculation has been reported to date for a nonlinear polyatomic collider. We present in this talk the preliminary steps toward this goal, using the H$_2$O molecule itself as our collider, the very accurate MB-Pol surface to describe the intermolecular interaction and the MultiConfiguration Time Dependent (MCTDH) algorithm to study the dynamics. One main challenge in this effort is the need to express the Potential Energy Surface (PES) in a sum-of-products form optimal for MCTDH calculations. We will describe how this was done and present preliminary results of state-to-state probabilities

The Rotational Spectrum and Potential Energy Surface of the Ar-SiO Complex

The rotational spectra of five isotopic species of the Ar-SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si-O stretch, in which lines up to v = 12 (~14 500 cm-1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar-Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar-CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar-SiO is predicted to be T-shaped with a well depth of 152 cm-1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm-1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar-SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar-SiO, and the putative Ka = ±1 levels for Ar-28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential

MCTDH ROVIBRATIONAL STATES AND STATE-TO-STATE INELASTIC SCATTERING CALCULATIONS ON THE H2O-H2 SYSTEM

Water, an essential ingredient of life, is prevalent in space and various media. H$_2$O in the gas phase is the major polyatomic species in the interstellar medium (ISM) and a primary target of current studies of collisional dynamics. In recent years a number of theoretical and experimental studies have been devoted to H$_2$O-X (with X=He, H$_2$, D$_2$, Ar, …) elastic and inelastic collisions in an effort to understand rotational distributions of H$_2$O in molecular clouds. In this work we are following those studies and will present benchmark calculations of rovibrational states and resonances of the H$_2$O-H$_2$ cluster in the rigid rotor approximation using the MultiConfiguration Time Dependent Hartree (MCTDH) approach. We will also present the first state-to-state inelastic scattering results of the H$_2$O+H$_2$ process in the rigid rotor approximation using the MCTDH approach. These calculations will serve as a foundation for similar triatomic – linear molecule interactions which are usually computationally expensive using standard calculations methods

Influence of Renner-Teller Coupling between Electronic States on H + CO Inelastic Scattering

We examine the excitation of carbon monoxide from its rovibrational ground state via collisions with a hydrogen atom. Calculations employ the Multi-Configuration Time-Dependent Hartree method and treat the nonadiabatic dynamics with the inclusion of both the ground and the Renner-Teller coupled first excited electronic states. For this purpose, a new set of recently presented global HCO Potential Energy Surfaces (PESs) that cover the 0-3 eV range of energy is used. The results obtained here considering only the ground state (without the Renner-Teller coupling) are in qualitative agreement with those available in the literature. The Renner-Teller effect is known to have an important effect on the spectroscopy of the system, and its inclusion and effects on the dynamics for the processes described in this paper are fairly significant also. The results of this study indicate that for certain very particular initial conditions rather dramatic effects can be observed