Calculation of electric quadrupole linestrengths for diatomic molecules: Application to the H2, CO, HF and O2 molecules

We present a unified variational treatment of the electric quadrupole (E2) matrix elements, Einstein coefficients, and line strengths for general open-shell diatomic molecules in the general purpose diatomic code \Duo. Transformation relations between the Cartesian representation (typically used in electronic structure calculations) to the tensorial representation (required for spectroscopic applications) of the electric quadrupole moment components are derived. The implementation has been validated against accurate theoretical calculations and experimental measurements of quadrupole intensities of 1H2 available in the literature. We also present accurate electronic structure calculations of the electric quadrupole moment functions for the X 1Σ+ electronic states of CO and HF at the CCSD(T) and MRCI levels of theory, respectively, as well for the a 1Δg - b 1Σg+ quadrupole transition moment of of O2 with MRCI level of theory. Accurate infrared E2 line lists for 12C16O and 1H19F are provided. A demonstration of spectroscopic applications is presented by simulating E2 spectra for 12C16O, 1H19F and 16O2 (Noxon a 1Δg - b 1Σg+ band)

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

A global 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 CH3Cl show good agreement with a range of experimental results. This includes vibrational transition moments, absolute line intensities of the ν1, ν4, ν5 and 3ν6 bands, and a rotation-vibration line list for both CH3 35Cl and CH3 37Cl 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. Keywords: Line-lists, Radiative transfer, Databases, HITRA

Ro-vibrational averaging of the isotropic hyperfine coupling constant for the methyl radical

© 2015 AIP Publishing LLC. We present the first variational calculation of the isotropic hyperfine coupling constant of the carbon-13 atom in the CH3 radical for temperatures T = 0, 96, and 300 K. It is based on a newly calculated high level ab initio potential energy surface and hyperfine coupling constant surface of CH3 in the ground electronic state. The ro-vibrational energy levels, expectation values for the coupling constant, and its temperature dependence were calculated variationally by using the methods implemented in the computer program TROVE. Vibrational energies and vibrational and temperature effects for coupling constant are found to be in very good agreement with the available experimental data. We found, in agreement with previous studies, that the vibrational effects constitute about 44% of the constant's equilibrium value, originating mainly from the large amplitude out-of-plane bending motion and that the temperature effects play a minor role

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

In this paper, 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

ExoMol line lists - XXII. The rotation-vibration spectrum of silane up to 1200K

A variationally computed 28SiH4 rotation-vibration line list applicable for temperatures up to T = 1200 K is presented. The line list, called OY2T, considers transitions with rotational excitation up to J = 42 in the wavenumber range 0–5000 cm−1 (wavelengths λ > 2 μm). Just under 62.7 billion transitions have been calculated between 6.1 million energy levels. Rovibrational calculations have utilized a new ‘spectroscopic’ potential energy surface determined by empirical refinement to 1452 experimentally derived energy levels up to J = 6, and a previously reported ab initio dipole moment surface. The temperature-dependent partition function of silane, the OY2T line list format, and the temperature dependence of the OY2T line list are discussed. Comparisons with the PNNL spectral library and other experimental sources indicate that the OY2T line list is robust and able to accurately reproduce weaker intensity features. The full line list is available from the ExoMol data base and the CDS data base

ExoMol line lists – XXIX. The rotation-vibration spectrum of methyl chloride up to 1200 K

Comprehensive rotation-vibration line lists are presented for the two main isotopologues of methyl chloride, $^{12}$CH$_3{}^{35}$Cl and $^{12}$CH$_3{}^{37}$Cl. The line lists, OYT-35 and OYT-37, are suitable for temperatures up to $T=1200\,$K and consider transitions with rotational excitation up to $J=85$ in the wavenumber range $0$--$6400\,$cm$^{-1}$ (wavelengths $\lambda> 1.56\,\mu$m). Over 166 billion transitions between 10.2 million energy levels have been calculated variationally for each line list using a new empirically refined potential energy surface, determined by refining to 739 experimentally derived energy levels up to $J=5$, and an established {\it ab initio} dipole moment surface. The OYT line lists show excellent agreement with newly measured high-temperature infrared absorption cross-sections, reproducing both strong and weak intensity features across the spectrum. The line lists are available from the ExoMol database and the CDS database

A 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 ≤ 14 are considered. The line list shows excellent agreement with a range of experimental data and will significantly assist future high-resolution measurements of NH3, both astronomically and in the laboratory

Electric-quadrupole and magnetic-dipole contributions to the ν₂+ν₃ band of carbon dioxide near 3.3 µm

The recent detections of electric-quadrupole (E2) transitions in water vapor and magnetic-dipole (M1) transitions in carbon dioxide have opened a new field in molecular spectroscopy. While in their present status, the spectroscopic databases provide only electric-dipole (E1) transitions for polyatomic molecules (H_{2}O, CO_{2}, N_{2}O, CH_{4}, O_{3}…), the possible impact of weak E2 and M1 bands to the modeling of the Earth and planetary atmospheres has to be addressed. This is especially important in the case of carbon dioxide for which E2 and M1 bands may be located in spectral windows of weak E1 absorption. In the present work, a high sensitivity absorption spectrum of CO_{2} is recorded by Optical-Feedback-Cavity Enhanced Absorption Spectroscopy (OFCEAS) in the 3.3 µm transparency window of carbon dioxide. The studied spectral interval corresponds to the region where M1 transitions of the ν_{2}+ν_{3} band of carbon dioxide were recently identified in the spectrum of the Martian atmosphere. Here, both M1 and E2 transitions of the ν_{2}+ν_{3} band are detected by OFCEAS. Using recent ab initio calculations of the E2 spectrum of {12}^C^{16}O_{2}, intensity measurements of five M1 lines and three E2 lines allow us to disentangle the M1 and E2 contributions. Indeed, E2 intensity values (on the order of a few 10^{–29} cm/molecule) are found in reasonable agreement with ab initio calculations while the intensity of the M1 lines (including an E2 contribution) agree very well with recent very long path measurements by Fourier Transform spectroscopy. We thus conclude that both E2 and M1 transitions should be systematically incorporated in the CO_{2} line list provided by spectroscopic databases

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: An optical centrifuge rotationally excites the phosphine molecule (PH3) into chiral cluster states that correspond to clockwise (R enantiomer) or anticlockwise (L enantiomer) rotation about axes almost coinciding with single P─H bonds. The 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 rotationally induced chirality promises a fresh perspective on chirality as a fundamental aspect of nature