arXiv subjects

Andrey Yachmenev

Publications and source records attributed to Andrey Yachmenev.

At least 37 records · Page 2Linked to original sources

arXiv2020

Laser-induced dynamics of molecules with strong nuclear quadrupole coupling

We present a general variational approach for computing the laser-induced rovibrational dynamics of molecules taking into account the hyperfine effects of the nuclear quadrupole coupling. The method combines the general variational approach TROVE, which provides accurate rovibrational hyperfine energies and wave functions for arbitrary molecules, with the variational method RichMol, designed for generalized simulations of the rovibrational dynamics in the presence of external electric fields. We investigate the effect of the nuclear quadrupole coupling on the short-pulse laser alignment of a prototypical molecule CFClBrI, which contains nuclei with large quadrupole constants. The influence of the nuclear quadrupole interactions on the post-pulse molecular dynamics is negligible at early times, first several revivals, however at longer timescales the effect is entirely detrimental and strongly depends on the laser intensity. This effect can be explained by dephasing in the laser-excited rotational wavepacket due to irregular spacings between the hyperfine-split nuclear spin states across different rotational hyperfine bands.

physics.atm-clus↗

arXiv2020

First observation of electric-quadrupole infrared transitions in water vapour

Molecular absorption of infrared radiation is generally due to ro-vibrational electric-dipole transitions. Electric-quadrupole transitions may still occur, but they are typically a million times weaker than electric-dipole transitions, rendering their observation extremely challenging. In polyatomic or polar diatomic molecules, ro-vibrational quadrupole transitions have never been observed. Here, we report the first direct detection of quadrupole transitions in water vapor. The detected quadrupole lines have intensity largely above the standard dipole intensity cut-off of spectroscopic databases and thus are important for accurate atmospheric and astronomical remote sensing.

physics.chem-ph↗

arXiv2019

Field-induced diastereomers for chiral separation

A novel approach for the state-specific enantiomeric enrichment and the spatial separation of enantiomers is presented. Our scheme utilizes techniques from strong-field laser physics, specifically an optical centrifuge in conjunction with a static electric field, to create a chiral field with defined handedness. Molecular enantiomers experience unique rotational excitation dynamics and this can be exploited to spatially separate the enantiomers using electrostatic deflection. Notably, the rotational-state-specific enantiomeric enhancement and its handedness is fully controllable. To explain these effects, we introduce the conceptual framework of $field\text{-}induced~diastereomers$ of a chiral molecule and perform robust quantum mechanical simulations on the prototypical chiral molecule propylene oxide (C$_3$H$_6$O), for which ensembles with an enantiomeric excess of up to $30~\%$ were obtained.

physics.chem-ph↗

arXiv2019

A Variationally Computed IR Line List for the Methyl Radical CH$_3$

We present the first variational calculation of a hot temperature ab initio line list for the CH$_3$ radical. It is based on a high level ab initio potential energy surface and dipole moment surface of CH$_3$ in the ground electronic state. The ro-vibrational energy levels and Einstein $A$ coefficients were calculated using the general-molecule variational approach implemented in the computer program TROVE. Vibrational energies and vibrational intensities are found to be in very good agreement with the available experimental data. The line list comprises 9,127,123 ro-vibrational states ($J\le 40$) and 2,058,655,166 transitions covering the wavenumber range up to 10000 cm$^{-1}$ and should be suitable for temperatures up to $T= 1500$ K.

physics.chem-ph↗

arXiv2018

Climbing the rotational ladder to chirality

Molecular chirality is conventionally understood as space-inversion-symmetry breaking in the equilibrium structure of molecules. Less well known is that achiral molecules can be made chiral through extreme rotational excitation. Here, we theoretically demonstrate a clear strategy for generating rotationally-induced chirality (RIC): An optical centrifuge rotationally excites the phosphine molecule (PH$_3$) into chiral cluster states that correspond to clockwise ($R$-enantiomer) or anticlockwise ($L$-enantiomer) rotation about axes almost coinciding with single P-H bonds. Application of a strong dc electric field during the centrifuge pulse favors the production of one rotating enantiomeric form over the other, creating dynamically chiral molecules with $permanently$ oriented rotational angular momentum. This essential step toward characterizing RIC promises a fresh perspective on chirality as a fundamental aspect of nature.

physics.chem-ph↗

arXiv2018

The ExoMol database: molecular line lists for exoplanet and other hot atmospheres

The ExoMol database (www.exomol.com) provides extensive line lists of molecular transitions which are valid over extended temperatures ranges. The status of the current release of the database is reviewed and a new data structure is specified. This structure augments the provision of energy levels (and hence transition frequencies) and Einstein $A$ coefficients with other key properties, including lifetimes of individual states, temperature-dependent cooling functions, Landé $g$-factors, partition functions, cross sections, $k$-coefficients and transition dipoles with phase relations. Particular attention is paid to the treatment of pressure broadening parameters. The new data structure includes a definition file which provides the necessary information for utilities accessing ExoMol through its application programming interface (API). Prospects for the inclusion of new species into the database are discussed.

astro-ph.GA↗

arXiv2018

Hyperfine-resolved rotation-vibration line list of ammonia (NH$_3$)

A comprehensive, hyperfine-resolved rotation-vibration line list for the ammonia molecule ($^{14}$NH$_3$) is presented. The line list, which considers hyperfine nuclear quadrupole coupling effects, has been computed using robust, first principles methodologies based on a highly accurate empirically refined potential energy surface. Transitions between levels with energies below $8000$~cm$^{-1}$ and total angular momentum $F\leq14$ are considered. The line list shows excellent agreement with a range of experimental data and will significantly assist future high-resolution measurements of NH$_3$, both astronomically and in the laboratory.

physics.chem-ph↗

arXiv2018

A global \textit{ab initio} dipole moment surface for methyl chloride

A new dipole moment surface (DMS) for methyl chloride has been generated at the CCSD(T)/aug-cc-pVQZ(+d for Cl) level of theory. To represent the DMS, a symmetry-adapted analytic representation in terms of nine vibrational coordinates has been developed and implemented. Variational calculations of the infrared spectrum of CH$_3$Cl show good agreement with a range of experimental results. This includes vibrational transition moments, absolute line intensities of the $ν_1$, $ν_4$, $ν_5$ and $3ν_6$ bands, and a rotation-vibration line list for both CH$_{3}{}^{35}$Cl and CH$_{3}{}^{37}$Cl including states up to $J=85$ and vibrational band origins up to $4400{\,}$cm$^{-1}$. Across the spectrum band shape and structure are well reproduced and computed absolute line intensities are comparable with highly accurate experimental measurements for certain fundamental bands. We thus recommend the DMS for future use.

physics.chem-ph↗

arXiv2018

Accurate \textit{ab initio} vibrational energies of methyl chloride

Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level \textit{ab initio} theory for the two main isotopologues of methyl chloride, CH$_{3}{}^{35}$Cl and CH$_{3}{}^{37}$Cl. The respective PESs, CBS-35$^{\,\mathrm{HL}}$ and CBS-37$^{\,\mathrm{HL}}$, are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY$_3$Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35$^{\,\mathrm{HL}}$ and CBS-37$^{\,\mathrm{HL}}$ PESs reproduce the fundamental term values with root-mean-square errors of $0.75$ and $1.00{\,}$cm$^{-1}$ respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH$_3$Cl without empirical refinement of the respective PESs.

physics.chem-ph↗

arXiv2018

A global potential energy surface and dipole moment surface for silane

A new nine-dimensional potential energy surface (PES) and dipole moment surface (DMS) for silane have been generated using high-level \textit{ab initio} theory. The PES, CBS-F12$^{\,\mathrm{HL}}$, reproduces all four fundamental term values for $^{28}$SiH$_4$ with sub-wavenumber accuracy, resulting in an overall root-mean-square (rms) error of $0.63{\,}$cm$^{-1}$. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit, and incorporates a range of higher-level additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, and scalar relativistic effects. Systematic errors in computed intra-band rotational energy levels are reduced by empirically refining the equilibrium geometry. The resultant Si-H bond length is in excellent agreement with previous experimental and theoretical values. Vibrational transition moments, absolute line intensities of the $ν_3$ band, and the infrared spectrum for $^{28}$SiH$_4$ including states up to $J=20$ and vibrational band origins up to $5000{\,}$cm$^{-1}$ are calculated and compared with available experimental results. The DMS tends to marginally overestimate the strength of line intensities. Despite this, band shape and structure across the spectrum are well reproduced and show good agreement with experiment. We thus recommend the PES and DMS for future use.

physics.chem-ph↗

arXiv2018

The rotation-vibration spectrum of methyl fluoride from first principles

Accurate \textit{ab initio} calculations on the rotation-vibration spectrum of methyl fluoride (\ce{CH3F}) are reported. A new nine-dimensional potential energy surface (PES) and dipole moment surface (DMS) have been generated using high-level electronic structure methods. Notably, the PES was constructed from explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit and considered additional energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms beyond perturbative triples, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. The PES and DMS are evaluated through robust variational nuclear motion computations of pure rotational and vibrational energy levels, the equilibrium geometry of \ce{CH3F}, vibrational transition moments, absolute line intensities of the $ν_6$ band, and the rotation-vibration spectrum up to $J=40$. The computed results show excellent agreement with a range of experimental sources, in particular the six fundamentals are reproduced with a root-mean-square error of 0.69~cm$^{-1}$. This work represents the most accurate theoretical treatment of the rovibrational spectrum of \ce{CH3F} to date.

physics.chem-ph↗

arXiv2018

Analyzing laser-induced alignment of weakly-bound molecular aggregates

The rotational and torsional dynamics of the prototypical floppy $\text{indole}(\text{H}_2\text{O})$ molecular cluster was theoretically and computationally analyzed. The time-dependent Schrödinger equation was solved for a reduced-dimensionality description of the cluster, taking into account overall rotations and the internal rotation of the water moiety. Based on our results, it became clear that coupling between the internal and the overall rotations are small, and that for typical field strengths in alignment and mixed-field orientation experiments the rigid rotor approximation can be employed to describe the investigated dynamics. Furthermore, the parameter space over which this is valid and its boundaries where the coupling of the internal and overall rotation can no longer be neglected were explored.

physics.chem-ph↗

arXiv2018

Coherent Control of the Rotation Axis of Molecular Superrotors

The control of ultrafast molecular rotational motion has benefited from the development of innovative techniques in strong-field laser physics. Here, we theoretically demonstrate a novel type of coherent control by inducing rotation of an asymmetric-top molecule about two different molecular axes. An optical centrifuge is applied to the hydrogen sulfide (H$_2$S) molecule to create a molecular superrotor, an object performing ultrafast rotation about a well-defined axis. Using two distinct pulse envelopes for the optical centrifuge, we show that H$_2$S can be excited along separate pathways of rotational states. This leads to stable rotation about two entirely different molecular axes while ensuring rotation is about the propagation direction of the centrifuge, i.e., the laboratory-fixed $Z$-axis. The presented scheme to control the angular momentum alignment of a molecule will, for instance, be useful in studies of molecule-molecule or molecule-surface scattering, especially due to the large amounts of energy associated with superrotors, which can even be controlled by changing the duration of the optical centrifuge pulse.

physics.chem-ph↗

arXiv2018

ExoMol molecular line lists - XXVII: spectra of C2H4

A new line list for ethylene, $^{12}$C$_2$$^1$H$_4$ is presented. The line list is based on high level ab initio potential energy and dipole moment surfaces. The potential energy surface is refined by fitting to experimental energies. The line list covers the range up to 7000 cm$^{-1}$ (1.43 $μ$m) with all ro-vibrational transitions (50 billion) with the lower state below 5000 cm$^{-1}$ included and thus should be applicable for temperatures up to 700 K. A technique for computing molecular opacities from vibrational band intensities is proposed and used to provide temperature dependent cross sections of ethylene for shorter wavelength and higher temperatures. When combined with realistic band profiles (such as the proposed three-band model), the vibrational intensity technique offers a cheap but reasonably accurate alternative to the full ro-vibrational calculations at high temperatures and should be reliable for representing molecular opacities. The C$_2$H$_4$ line list, which is called MaYTY, is made available in electronic form from the CDS.

astro-ph.EP↗

arXiv2018

RichMol: A general variational approach for rovibrational molecular dynamics in external electric fields

A general variational approach for computing the rovibrational dynamics of polyatomic molecules in the presence of external electric fields is presented. Highly accurate, full-dimensional variational calculations provide a basis of field-free rovibrational states for evaluating the rovibrational matrix elements of high-rank Cartesian tensor operators, and for solving the time-dependent Schrödinger equation. The effect of the external electric field is treated as a multipole moment expansion truncated at the second hyperpolarizability interaction term. Our fully numerical and computationally efficient method has been implemented in a new program, RichMol, which can simulate the effects of multiple external fields of arbitrary strength, polarization, pulse shape and duration. Illustrative calculations of two-color orientation and rotational excitation with an optical centrifuge of NH$_3$ are discussed.

physics.chem-ph↗

arXiv2017

General variational approach to nuclear-quadrupole coupling in rovibrational spectra of polyatomic molecules

A general algorithm for computing the quadrupole-hyperfine effects in the rovibrational spectra of polyatomic molecules is presented for the case of ammonia (NH$_3$). The method extends the general variational approach TROVE by adding the extra term in the Hamiltonian that describes the nuclear quadrupole coupling, with no inherent limitation on the number of quadrupolar nuclei in a molecule. We applied the new approach to compute the nitrogen-nuclear-quadrupole hyperfine structure in the rovibrational spectrum of NH$_3$. These results agree very well with recent experimental spectroscopic data for the pure rotational transitions in the ground vibrational and $ν_2$ states, and the rovibrational transitions in the $ν_1$, $ν_3$, $2ν_4$, and $ν_1+ν_3$ bands. The computed hyperfine-resolved rovibrational spectrum of ammonia will be beneficial for the assignment of experimental rovibrational spectra, further detection of ammonia in interstellar space, and studies of the proton-to-electron mass variation.

physics.chem-ph↗

arXiv2017

Symmetry adapted ro-vibrational basis functions for variational nuclear motion calculations: TROVE approach

We present a general, numerically motivated approach to the construction of symmetry adapted basis functions for solving ro-vibrational Schrödinger equations. The approach is based on the property of the Hamiltonian operator to commute with the complete set of symmetry operators and hence to reflect the symmetry of the system. The symmetry adapted ro-vibrational basis set is constructed numerically by solving a set of reduced vibrational eigenvalue problems. In order to assign the irreducible representations associated with these eigenfunctions, their symmetry properties are probed on a grid of molecular geometries with the corresponding symmetry operations. The transformation matrices are re-constructed by solving over-determined systems of linear equations related to the transformation properties of the corresponding wavefunctions on the grid. Our method is implemented in the variational approach TROVE and has been successfully applied to a number of problems covering the most important molecular symmetry groups. Several examples are used to illustrate the procedure, which can be easily applied to different types of coordinates, basis sets, and molecular systems.

physics.atm-clus↗

arXiv2017

Simulating electric field interactions with polar molecules using spectroscopic databases

Ro-vibrational Stark-associated phenomena of small polyatomic molecules are modelled using extensive spectroscopic data generated as part of the ExoMol project. The external field Hamiltonian is built from the computed ro-vibrational line list of the molecule in question. The Hamiltonian we propose is general and suitable for any polar molecule in the presence of an electric field. By exploiting precomputed data, the often prohibitively expensive computations associated with high accuracy simulations of molecule-field interactions are avoided. Applications to strong terahertz field-induced ro-vibrational dynamics of PH$_3$ and NH$_3$, and spontaneous emission data for optoelectrical Sisyphus cooling of H$_2$CO and CH$_3$Cl are discussed.

physics.chem-ph↗