Rovibrational Spectroscopy Research Articles

Theoretical Rovibrational Spectroscopy of Magnesium Tricarbide-Multireference Character Thwarts a Full Analysis of All Isomers.

Magnesium tricarbide isomers are studied herein with coupled cluster theory and multireference configuration interaction to support their possible detection in astrochemical environments such as the circumstellar envelope surrounding the star IRC +10216 or in terrestrial laboratories. Magnesium-bearing species may abound in the interstellar medium (ISM), but only eight (MgNC, MgCN, HMgNC, MgC2H, MgC3N, MgC4H, MgC5N, and MgC6H) have been directly identified thus far. Several possible isomers for the related MgC3 system are explored in their singlet and triplet spin multiplicities. Overall, this work offers quantum chemical insight of rovibrational spectroscopic data for MgC3 using quartic force fields (QFFs) based on the CCSD(T) and CCSD(T)-F12 levels of theory at the complete basis set (CBS) limit. Additional corrections with small basis set CCSDT(Q) and scalar relativistic effects are also included in the analysis. Salient multireference character is found in the singlet diamond electronic state, which makes a definitive assignment of the ground state challenging. Nevertheless, coupled cluster-based composite energies and multireference configuration interaction both predict that the 1A1 diamond isomer is 1.6-2.2 kcal mol-1 lower in energy than the 3A1 diamond isomer. Furthermore, highly accurate binding energies of various isomers MgC3 are provided for comparison to photodetachment experiments. Dipole moments along with harmonic infrared intensities will guide efforts for astronomical and spectroscopic characterization.

Comprehensive Quantum Chemical Characterization of the Astrochemically Relevant HCnH Chain Family: An Attempt to Aid Astronomical Observations

AbstractIn this paper, the results of a comprehensive theoretical investigation of neutral HCnH0 and charged HCnH± chains are reported using state‐of‐the‐art compound model chemistries (W1BD, G4, CBS‐QB3, CBS‐APNO, CBS‐4M) and single point methods (including “gold standard” CCSD(T)). The properties envisaged include electronic and chemical bonding structure data, enthalpies of formation, adiabatic and vertical ionization (IP), and electron attachment (EA) energies, gas phase acidities, C−H bond strengths, and chain “deprotecting” (HCnH → H + Cn + H) energies. The best theoretical estimates based on composite models reported here for the various properties investigated agree with the available experimental data and are better than all previous theoretical estimates. To exemplify, for the enthalpies of formation, the G4 composite model achieves a RSMD (root square mean deviation) of 0.28 kcal mol−1 and a MUD (maximum unsigned deviation) of 0.45 kcal mol−1. Most importantly from the astrochemical perspective, it is predicted that the stable HCnH− anions with are nonlinear. Having sufficiently large dipole moments, the anion HCnH− cis conformers are potential candidates for detection in space via rovibrational spectroscopy.

Synchronized Two-Color Time-Resolved Dual-Comb Spectroscopy for Quantitative Detection of HOx Radicals Formed from Criegee Intermediates.

Criegee intermediates, derived from ozonolysis of alkenes and recognized as key species in the production of nonphotolytic free radicals, play a crucial role in atmospheric chemistry. Here, we present a spectrometer based on synchronized two-color time-resolved dual-comb spectroscopy, enabling simultaneous spectral acquisitions in two molecular fingerprint regions near 2.9 and 7.8 μm. Upon flash photolysis of CH2I2/O2/N2 gas mixtures, multiple reaction species, involving the simplest Criegee intermediates (CH2OO), formaldehyde (CH2O), hydroxyl (OH) and hydroperoxy (HO2) radicals are simultaneously detected with microsecond time resolution. The concentration of each molecule can be determined based on high-resolution rovibrational absorption spectroscopy. With quantitative detection and simulation of temporal concentration profiles of the targeted molecules at various conditions, the underlying reaction mechanisms and pathways related to the formation of the HOx radicals, which can be generated from decomposition of initially energized and vibrationally excited Criegee intermediates, are explored. This approach capable of achieving multispectral measurements with simultaneously high spectral and temporal resolutions opens up the opportunities for quantification of transient intermediates and products, thus, enabling elucidation of complex reaction mechanisms.

Effect of correlated hyperfine theory errors in the determination of rotational and vibrational transition frequencies in HD+

We revisit the problem of separating the purely rovibrational and hyperfine contributions to observed rovibrational transition frequencies. The separation of hyperfine frequency shifts played an essential role in recent works in which the value of the proton-electron mass ratio was determined from Doppler-free rovibrational spectroscopy and high-precision theory of the molecular ion. Here, we present an alternative approach to removing theoretically computed hyperfine shifts from a limited set of observed rovibrational transition frequencies, based on a least-squares adjustment. The adjustment facilitates a quantitative study of the effect of correlations between the uncertainties of lower- and upper-state hyperfine energies on the derived value of the rovibrational transition frequency. We find that correlations may lead to significant shifts of the extracted rovibrational transition frequencies. In addition, we propose and demonstrate a method to quantify the additional shift and uncertainty, which is of importance to other applications of the spectroscopic data, such as the determination of fundamental physical constants.

Rotational analysis and hyperfine structures of A2Π1/2 → X2Σ1/2 transition in 25,26MgF isotope molecules

High resolution ro-vibrational spectroscopy of the A2Π1/2(v′ = 0) → X2Σ1/2(v = 0) band, for the buffer-gas-cooled magnesium monofluoride (25,26MgF) isotope molecules, has been measured by the in-cell direct absorption. Due to the decent populations in the low-lying rotational states (X2Σ1/2(v = 0, N = 0, 1)) for the cooled 25,26MgF molecules, the rotational levels and constants of the A2Π1/2(v′ = 0) state of 25,26MgF are accurately determined. Furthermore, hyperfine-structure levels of X2Σ1/2(v = 0, N = 1) of 25,26MgF are successfully measured by the laser induced fluorescence. Such studies complement previous studies on the more abundant 24MgF molecule and are useful for precision measurement and relevant laser cooling experiments.

Toward a global model of the interactions in low-lying states of methyl cyanide: Rotational and rovibrational spectroscopy of the [formula omitted] state and tentative interstellar detection of the [formula omitted] state in Sgr B2(N)

Rotational spectra of methyl cyanide were recorded newly and were analyzed together with existing spectra to extend the global model of low-lying vibrational states and their interactions to υ4=1 at 920 cm−1. The rotational spectra cover large portions of the 36–1439 GHz region and reach quantum numbers J and K of 79 and 16, respectively. Information on the K level structure of CH3CN is obtained from IR spectra. A spectrum of 2ν8 around 717 cm−1, analyzed in our previous study, covered also the ν4 band. The assignments in this band cover 880–952 cm−1, attaining quantum numbers J and K of 61 and 13, respectively.The most important interaction of υ4=1 appears to be with υ8=3, ΔK=0,Δl=+3, a previously characterized anharmonic resonance. We report new analyses of interactions with ΔK=-2 and Δl=+1, with ΔK=-4 and Δl=-1, and with ΔK=-6 and Δl=-3; these four types of interactions connect all l substates of υ8=3 in energy to υ4=1. A known ΔK=-2,Δl=+1 interaction with υ7=1 was also analyzed, and investigations of the ΔK=+1, Δl=-2 and ΔK=+3,Δl=0 resonances with υ8=2 were improved, as were interactions between successive states with υ8⩽3, mainly through new υ8⩽2 rotational data.A preliminary single state analysis of the υ4=υ8=1 state was carried out based on rotational transition frequencies and on ν4+ν8-ν8 hot band data. A considerable fraction of the K levels was reproduced within uncertainties in its entirety or in part, despite obvious widespread perturbations in υ4=υ8=1.In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to υ4=1, we report the tentative detection of υ4=υ8=1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.

Rovibrational spectroscopy of the CH+-He and CH+-He4 complexes

A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH+-Heand CH+-He4 complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C–H stretching vibrations are recorded in the range of 2720–2800 cm−1. The spectrum of CH+-Heexhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm−1, and thus confirms it to have a T-shaped structure. For CH+-He4, the C-H stretch along the symmetry axis of this oblate top results in parallel transitions.

Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths

Even if on-board mm-wave/THz heterodyne receivers have been developed to measure greenhouse gases (GHGs) atmospheric profiles, rotational spectroscopy rests under-exploited for their monitoring unlike IR rovibrational spectroscopy. The present study deals with the ability of THz spectroscopy using long interaction path-lengths for GHG laboratory investigations. High-resolution THz signatures of non-polar greenhouse molecules may be observed by probing very weak centrifugal distortion induced rotational transitions. To illustrate, new measurements on CH4 and CF4 have been carried out. For CH4, pure rotational transitions, recorded by cw-THz photomixing up to 2.6 THz in a White type cell adjusted to 20 m, have allowed to update the methane line list of atmospheric databases. Concerning CF4, Fabry-Perot THz absorption spectroscopy with a km effective pathlength was required to detect line intensities lower than 10−27 cm−1/(moleccm−2). Contrary to previous synchrotron-based FT-FIR measurements, the tetrahedral splitting of CF4 THz lines is fully resolved. Finally, quantitative measurements of N2O and O3 gas traces have been performed in an atmospheric simulation chamber using a submm-wave amplified multiplier chain coupled to a Chernin type multi-pass cell on a 200 m path-length. The THz monitoring of these two polar GHGs at tropospheric and stratospheric concentrations may be now considered.

Intermolecular dynamics of NH3-rare gas complexes in the ν2 umbrella region of NH3 investigated by rovibrational laser jet-cooled spectroscopy and ab initio calculations.

High resolution jet-cooled spectroscopy experiments have been realized to investigate the intermolecular dynamics of van der Waals (vdW) heterodimers between NH3 and rare gas (Rg) atoms in the ν2 umbrella mode region of NH3. With respect to a previous study dedicated to NH3-Ar [Asselin et al. Mol. Phys. 116, 3642 (2018)], the sensitivity and spectral resolution of our laser spectrometer coupled to a pulsed supersonic jet have been significantly improved to derive more accurate excited state spectroscopic parameters from rovibrational analyses. In addition, we calculated the ground and ν2 excited vibration-rotation-tunneling (VRT) states of these complexes on the four-dimensional ab initio potential energy surfaces from Loreau et al. [J. Chem. Phys. 141, 224303 (2014), ibid. 143, 184303 (2015).] Transition frequencies and intensities of the allowed ν2 = 1 ← 0 transitions obtained from the calculated energy levels and wave functions agree well with the experimental data and are helpful in their analysis. By means of a pseudodiatomic model with the assumption of weak Coriolis coupling, the rovibrational analysis of both the Πe/f(j = 1,k = 0) ←Σf(j = 0,k = 0) and Σf(j = 1,k = 0) ←Σf(j = 0,k = 0) transitions in ortho NH3-Rg (Rg = Ne, Ar, Kr, Xe) complexes enabled us to determine reliable excited state parameters and derive accurate values of the effective vdW bond length Reff, force constant ks, and vdW stretching frequency νs. Comparison between the experimental structural parameters and those from the ab initio calculated VRT levels shows good agreement for NH3-Ne, NH3-Ar and NH3-Kr, and a similar variation of Reff, ks, and νs with the polarizability of Rg in the ground and ν2 excited states. Anomalously small values of νs and ks derived for NH3-Xe in the Πe/f(j = 1,k = 0) state suggest that the applied model is not valid in this case, due to the presence of another state coupling to the perturbed Πf state. Such a state could not be found, however.

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene.

We review the recent developments in precision ro-vibrational spectroscopy of buffer-gas-cooled neutral molecules, obtained using infrared frequency combs either as direct probe sources or as ultra-accurate optical rulers. In particular, we show how coherent broadband spectroscopy of complex molecules especially benefits from drastic simplification of the spectra brought about by cooling of internal temperatures. Moreover, cooling the translational motion allows longer light-molecule interaction times and hence reduced transit-time broadening effects, crucial for high-precision spectroscopy on simple molecules. In this respect, we report on the progress of absolute frequency metrology experiments with buffer-gas-cooled molecules, focusing on the advanced technologies that led to record measurements with acetylene. Finally, we briefly discuss the prospects for further improving the ultimate accuracy of the spectroscopic frequency measurement.

QCT calculations of O2 + O collisions: Comparison to molecular beam experiments

We present quasiclassical trajectory simulations of O2 + O collisions under conditions representative of a crossed molecular beam experiment [Lahankar et al., J. Phys. Chem. A 120, 5348-5359 (2016)]. These calculations are compared to experimental data in order to further validate Potential Energy Surfaces (PESs) recently developed at the University of Minnesota [Z. Varga, Y. Paukku, and D. G. Truhlar, J. Chem. Phys. 147, 154312 (2017)]. Spin and spatial symmetries allow nine adiabatic PESs to represent the ground state interactions of O2 + O. We simulate trajectories adiabatically on all nine surfaces and perform analysis on data from each surface separately and the combined set of nine. It is shown that aggregated adiabatic calculations from nine surfaces agree better with the experiment for both inelastic and exchange collisions than a previous study that used a single surface and lie within the reported experimental uncertainty at almost all points. Distributions for exchange reactions using all nine PESs are interpreted using each surface's opacity function and activation energy for exchange. Rovibrationally resolved product distributions are then studied for a better understanding of energy relaxation in exchange collisions and may prove useful for further quasiclassical trajectory study and new experiments that use rovibrational spectroscopy to resolve the internal energy of the scattered products.

Profiling astrophysically relevant MgC4H chains. An attempt to aid astronomical observations

ABSTRACT In this paper, we report results of an extensive theoretical study on MgC4H chains conducted at DFT and CCSD(T) levels motivated by the recent discovery of this species in IRC+10216. A detailed characterization of both neutral and charged species is presented, which include structural, chemical bonding and vibrational properties, rotational, centrifugal distortion and Watson l-type doubling constants, dipole moments, Fermi contact, and spin-rotation constants. In addition, we present ab initio estimates needed for subsequent astrochemical evolution modelling (e.g. dissociation energies, acidity, electron attachment, and ionization energies and related chemical reactivity indices). Possible formation pathways are also discussed. They comprise exchange, (radiative) association, dissociative recombination, and ion neutralization reactions. As an important result aiming at stimulating further observational searching, we suggest that MgC4H− anions should also be observable via rovibrational spectroscopy. The reason is twofold: (i) Neutral MgC4H0 chains possess a sufficiently large dipole moment consistent with dipole-bound anion states and large electron attachment cross-sections. (ii) MgC4H− anions possess a dipole substantially larger than MgC4H0 neutrals (and also larger than that estimated earlier for the longest astronomically detected C8H− anion). This makes MgC4H− anion intensities in rovibrational spectrum experimentally accessible even in the unlikely case of a relative abundance MgC4H−/MgC4H0 comparable to that of CH4, whose anion has the lowest relative abundance observed so far in space because weakly polar C4H0 chains do not support dipole-bound anion states. A suggestion on why, counterintuitively, the MgC2H abundance found in IRC+10216 was lower than that of the longer MgC4H is also presented.

Ro-vibrational spectroscopy of 2,5-norbornadiene between 400 and 850 cm−1

Rotationally-resolved infrared absorption spectra of the two lowest-energy fundamental bands for each of the three infrared active symmetry species of 2,5-norbornadiene (C2v point group symmetry; asymmetry parameter κ = −0.2203), ν11 (a1), ν12(a1), ν28(b1), ν29(b1), ν38(b2), and ν39(b2), have been measured, assigned and fit to a semi-rigid rotor Hamiltonian to produce a set of spectroscopic constants for each band. The fit band centers are ν11 (a1), c-type transitions, 728.852357(3) cm−1; ν12 (a1), c-type transitions, 417.31604(1) cm−1; ν28 (b1), a-type transitions, 656.719741(3) cm−1; ν29 (b1), a-type transitions, 500.31425(1) cm−1; ν38 (b2), b-type transitions, 800.82243(2) cm−1; and ν39 (b2), b-type transitions, 539.23110(6) cm−1. Using ground state combination differences from the ν11 and ν28 bands and previously published microwave data, an improved set of ground state spectroscopic constants was fit for the parent isotopologue of the molecule.

Molecular Rovibrational Spectroscopy with Undetected Photons via Single-Photon Interferometry

Interferometric spectroscopy with undetected photons (ISUP) utilizing quantum mechanically path-entangled photon pairs has received considerable attention as an optical-measurement platform. Recently, we carried out a proof-of-concept experiment to show that ISUP can be used to measure the transmission spectrum of a Fabry-Perot resonator. Here, we demonstrate that ISUP with dual stimulated parametric down-conversion (StPDC) processes, which allows us to perform infrared rovibrational spectroscopy with visible photons. In our ISUP method, quantum coherence between two independent signal photons from each StPDC crystal is induced by the indistinguishability of conjugate idler fields and results in high visibility of the signal single-photon interferometry at the nondegenerate wavelength. If the seed-beam intensity is imbalanced due to the sample absorption, the corresponding envelope modulation of the interference fringe as a function of the seed-beam frequency reveals the absorption spectrum of the optical sample. As a proof-of-principle experiment, the full rovibrational transmission spectrum at 1550 nm of the hydrogen cyanide (${\mathrm{H}}^{13}{\mathrm{C}}^{14}\mathrm{N}$) molecules in a gas cell is measured from the single-photon interference fringe of the signal fields at 807 nm. We thus anticipate that the single-photon ISUP technique with dual StPDC crystals will find broad use for the development of high-resolution atomic and molecular spectroscopy with undetected photons.

High-resolution infrared and ab initio investigation of CHD2 79Br: Ro-vibrational analysis of the ν6 fundamental and the 2ν6−ν6 hot band

The high-resolution infrared spectrum of CHD2 79Br has been investigated by Fourier transform spectroscopy in the range 540–615 cm−1 at an unapodised resolution of 0.0035 cm−1. This spectral region is characterised by the ν6 fundamental (584.8510 cm−1), corresponding to C–Br stretching mode, and its hot band 2ν6−ν6 (578.4333 cm−1). The spectral analysis resulted in the identification of 3430 transitions (J’ ≤ 73 and K'a ≤ 18) for the ν6 fundamental and 1212 transitions (J’ ≤ 49 and K'a ≤ 11) for the hot band 2ν6−ν6. The assigned data have been fitted using the Watson’s S-reduced Hamiltonian in the Ir representation and new constants for the ground state from about 24,600 combination differences and sets of parameters for the v 6 = 1 and 2 vibrational states have been obtained. From spectral simulations the intensity ratio between 2ν6−ν6 and ν6 has been estimated to be 0.15 ± 0.02. High-quality ab initio calculations have also been performed at the CCSD(T) level of theory in order to support the experimental investigation through the calculation of molecular parameters relevant to ro-vibrational spectroscopy.

Rovibrational transition properties of the X1Σ+ and A1Π states of carbon monosulfide

Carbon monosulfide was detected in outer space by rovibrational spectroscopy of the X 1Σ+ state and A 1Π – X 1Σ+ system. This work calculated the potential energy curves and dipole moment functions of the X 1Σ+ 0+ and A 1Π1 states, and computed the transition dipole moments between the two states employing the CASSCF method, followed by the valence icMRCI approach. Core-valence correlation and scalar relativistic corrections were included. The extrapolation of potential energies to the complete basis set limit was performed. The spin-orbit coupling effect was included. The Einstein A coefficients, band origins, and oscillator strengths were calculated for the rovibrational transitions when J ≤ 150. The rovibrational transitions of the X 1Σ+ 0+ and A 1Π1 states became very weak when Δυ ≥ 6. The Einstein A coefficients of vibronic emissions of the A 1Π1 – X 1Σ+ 0+ system were large, indicating that the emissions were able to be measured easily through spectroscopy. Several rovibrational transitions of the A 1Π1 – X 1Σ+ 0+ system were analysed in detail. The distribution of radiative lifetime varying as rotational quantum number was calculated. The results obtained in this work agree well with the available experimental values.

Nuclear spin conversion of water confined in solid parahydrogen

Rovibrational spectroscopy is used to monitor the nuclear spin conversion (NSC) kinetics of H217O and H216O trapped in solid parahydrogen (pH2) at low temperatures. Non-equilibrium water nuclear spin populations are produced via short-term in situ photolysis of 17O2-doped pH2 solids. The time constant for ortho-to-para conversion is measured to be 367(6) s for H217O and 1770(30) s for H216O. Repeated FTIR measurements indicate that the H2O NSC kinetics is insensitive to temperature (1.6 – 4.0 K), the orthohydrogen content (300 ppm – 3%), and IR radiation from the spectrometer over the stated ranges.

On neglecting Coriolis and related couplings in first-principles rovibrational spectroscopy: considerations of symmetry, accuracy, and simplicity

The rotation-vibration (Coriolis) coupling contribution to variationally computed rovibrational energy levels is investigated, employing triatomic AB{}_{2} molecules as models. In particular, calculations are performed for H{}_{2}{}^{16}O, across a range of vibrational and rotational excitations, both with and without the Coriolis contribution. A variety of different embedding choices are considered, together with a hierarchy of increasingly severe approximations culminating in a generalized version of the so-called “centrifugal sudden” method. Several surprising and remarkable conclusions are found, including that the Eckart embedding is not the best embedding choice.

Jet-cooled rovibrational spectroscopy of methoxyphenols using two complementary FTIR and QCL based spectrometers.

Methoxyphenols (MPs) are a significant component of biomass burning emissions which mainly exists in our atmosphere in the gas phase where they contribute to the formation of secondary organic aerosols (SOAs). Rovibrational spectroscopy is a promising tool to monitor atmospheric MPs and infer their role in SOA formation. In this study, we bring a new perspective on the rovibrational analysis of MP isomers by taking advantage of two complementary devices combining jet-cooled environments and absorption spectroscopy: the Jet-AILES and the SPIRALES setups. Based on Q-branch frequency positions measured in the Jet-AILES Fourier-transform infrared (FTIR) spectra and guided by quantum chemistry calculations, we propose an extended vibrational and conformational analysis of the different MP isomers in their fingerprint region. Some modes such as far-IR out-of-plane -OH bending or mid-IR in-plane -CH bending allow us to assign individually all the stable conformers. Finally, using the SPIRALES setup with three different external cavity quantum cascade laser sources centered on the 930-990 cm-1 and the 1580-1690 cm-1 ranges, it was possible to proceed to the rovibrational analysis of the ν18 ring in-plane bending mode of the MP meta isomer providing a set of reliable excited state parameters, which confirms the correct assignment of two conformers. Interestingly, the observation of broad Q-branches without visible P- and R-branches in the region of the C-C ring stretching bands was interpreted as being probably due to a vibrational perturbation. These results highlight the complementarity of broadband FTIR and narrowband laser spectroscopic techniques to reveal the vibrational conformational signatures of atmospheric compounds over a large infrared spectral range.

Ultrafast background-free ro-vibrational fs/ps-CARS thermometry using an Yb:YAG crystal-fiber amplified probe.

A novel laser system for ro-vibrational spectroscopy using coherent anti-Stokes Raman Scattering in hybrid fs/ps regime is presented. A single Yb:KGW laser source is used as a master laser to generate the three CARS laser beams, namely the pump and Stokes femtosecond pulses and a 58 ps probe pulse. Master oscillator power amplifier (MOPA) architecture is implemented to increase the probe output power using a custom two stage free space linear amplifier. The probe is 0.37 cm-1 in width and 100 µJ in energy to allow resolving the Q-branch ro-vibrational lines of N2 and recording single shot CARS spectra at kHz repetition rate in flames. An original and simple technique based on the study of the influence of probe delay and polarization has been setup to optimize nonresonant background rejection, with no loss in resonant contribution. CARS performances are reported for N2 thermometry between 300 K and 3000 K, demonstrating state of the art precision.