Rotational Transition Frequencies Research Articles

Attainable accuracies of QH+ rotational transition frequencies (Q: 40Ca, 24Mg, 202Hg)

Molecular rotational transition frequencies are useful as frequency standard in the terahertz region. The transition frequencies of QH+ molecular ions (Q: an even isotope of Group II element) in a linear trap are useful as measurement uncertainties are low. An uncertainty given by the Stark and Zeeman shifts can be below 10−15 using 202HgH+ molecular ion.

RKR-потенциалы молекул -=SUP=-14-=/SUP=-N-=SUP=-16-=/SUP=-O, -=SUP=-16-=/SUP=-OH в основном электронном состоянии

In this work, molecular parameters of the Dunham type, obtained from the global processing of the experimental vibrational-rotational and rotational transition frequencies of ^{14}N^{16}O and ^{16}OH molecules, were used to construct the RKR potential energy curves. Pointwise defined potentials were approximated by the expansions in variable z_{S}=(r-r_{e})/r. Ab initio calculations of the potential energy functions of these molecules were carried out using the MOLPRO program. Comparisons of the obtained results with those known from the literature are given.

Theoretical investigation of potential energy surface and bound states for the N2-OCS van der Waals complex.

In this work we report an ab initio intermolecular potential energy surface and theoretically spectroscopic studies for N2-OCS complex. A four-dimensional intermolecular potential energy surface (4D PES) is constructed at the level of single and double excitation coupled-cluster method with a non-iterative perturbation treatment of triple excitations [CCSD(T)] with aug-cc-pVTZ basis set supplemented with bond functions. A global minimum corresponding to a planar and nearly T-shaped structure, which has been observed experimentally, is located at R = 3.96 Å, θ1 = 8 or 172°, θ2 = 75°, φ = 180 or 0° with a well depth of 271.078 cm-1 on the potential energy surface. The local minimum corresponding to a linear geometry is also found at R = 5.06 Å, θ1 = 2 or 178°, θ2 = 179°, φ = 0 or 180° with a well depth of 224.743 cm-1. The bound state calculations have been performed for the complex by approximating the N2 and OCS molecules as the rigid rotors. The calculated structural parameters and rotational transition frequencies are in good agreement with the experimental observed values. Based on the ab initio PES, the tunneling splitting is calculated to be 0.052 cm-1 for the ground vibrational state, which can just reproduce 64% of experimental observation (0.082 cm-1). A refined method is used to calculate the tunneling splitting, and a much better result is obtained with the value of 0.094 cm-1.

Investigating the influence of intramolecular bond lengths on the intermolecular interaction of H2-AgCl complex: Binding energy, intermolecular vibrations, and isotope effects.

In this paper, we performed a theoretical study on the influence of intramolecular bond lengths on the intermolecular interactions between H2 and AgCl molecules. Using four sets of bond lengths for the monomers of H2 and AgCl, four-dimensional intermolecular potential energy surfaces (PESs) were constructed from ab initio data points at the level of single and double excitation coupled cluster method with noniterative perturbation treatment of triple excitations. A T-shaped global minimum was found on the PES. Interestingly, both the binding energies and Ag-H2 distances present a linear relationship with the intramolecular bond lengths of H2-AgCl. The accuracy of these PESs was validated by the available spectroscopic data via the bound state calculations, and the predicted rotational transition frequencies can reproduce the experimental observations with a root-mean-squared error of 0.0003 cm-1 based on the PES constructed with r(H-H) and r(Ag-Cl) fixed at 0.795 and 2.261 Å, respectively. The intermolecular vibrational modes were assigned unambiguously with a simple pattern by analyzing the wave functions. Isotope effects were also investigated by the theoretical calculations, and the results are in excellent agreement with the available spectroscopic data. The transition frequencies for the isotopolog D2-AgCl are predicted with the accuracy of 0.3 MHz.

Prebiotic molecules in interstellar space: Rotational spectroscopy and quantum chemistry

AbstractThe starting point for the development of any astrochemical model is the knowledge of whether a molecule is present in the astrophysical environment considered, with the astronomical observations of spectroscopic signatures providing the unequivocal proof of its presence. Among the goals of astrochemistry, the detection of potential prebiotic molecules in the interstellar medium and planetary atmospheres is fundamental in view of possibly understanding the origin of life. The detection of new molecules in space requires the spectroscopic signatures (mostly, rotational transition frequencies) to be accurately determined over a large frequency range. This task is more and more often the result of a synergic interplay of experiment and theory.

High-resolution double resonance action spectroscopy in ion traps: vibrational and rotational fingerprints of CH2NH2.

By applying various action spectroscopic techniques in a 4 K cryogenic ion trap instrument, protonated methanimine, CH2NH2+, has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 39 rovibrational transitions within the fundamental band of the ν2 symmetric C-H stretch were measured around 3026 cm-1, which were used to predict pure rotational transition frequencies of CH2NH2+ in the ground vibrational state. Based on these predictions, nine rotational transitions were observed between 109 and 283 GHz using a novel double resonance method, which significantly improved the sensitivity of the rotational measurements. This double resonance method consists of rotational excitation followed by vibrational excitation, which is finally detected as a dip in the number of CH2NH2+-He complexes formed in the 4 K He bath of the trap. The new measurements and the derived predictions of pure rotational transitions will enable the first radio-astronomical search for CH2NH2+.

Sub-Doppler rovibrational spectroscopy of the [formula omitted] fundamental band of D2H+

The molecular ion D2H+ is an important species in cold interstellar environments and a key benchmark for ab initio calculations. We have measured 37 transitions in the ν1 fundamental band using the sub-Doppler technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy. Using an optical frequency comb for accurate frequency calibration, the uncertainties for most transitions were improved by more than an order of magnitude. These new measurements lead to improved predictions of pure rotational transition frequencies which have not been observed by THz observatories or laboratory spectrometers.

Accurate Rotational Rest Frequencies for Ammonium Ion Isotopologues.

We report rest frequencies for rotational transitions of the deuterated ammonium isotopologues NH3D+, and , measured in a cryogenic ion trap machine. For the symmetric tops NH3D+ and one and three transitions are detected, respectively, and five transitions are detected for the asymmetric top . While the lowest frequency transition of NH3D+ was already known in the laboratory and space, this work enables the future radio astronomical detection of the two other isotopologues.

Mass-correlated rotational Raman spectra with high resolution, broad bandwidth, and absolute frequency accuracy

We present mass-correlated rotational alignment spectroscopy, based on the optical excitation of a coherent rotational quantum wave and the observation of temporal wave interferences in a mass spectrometer. Combined electronic and opto-mechanical delays increased the observation time and energy resolution by an order of magnitude compared with preceding time-domain measurements. Rotational transition frequencies were referenced to an external clock for accurate absolute frequency measurements. Rotational Raman spectra for six naturally occurring carbon disulfide isotopologues were resolved with 3 MHz resolution over a spectral range of 500 GHz. Rotational constants were determined with single-kilohertz accuracy, competitive with state-of-the-art frequency domain measurements.

First Laboratory Detection of Vibration-Rotation Transitions of 12CH+ and 13CH+ and Improved Measurement of their Rotational Transition Frequencies.

The long-searched C-H stretches of the fundamental ions CH+ and 13CH+ have been observed for the first time in the laboratory. For this, the state-dependent attachment of He atoms to these ions at cryogenic temperatures has been exploited to obtain high-resolution rovibrational data. In addition, the lowest rotational transitions of CH+, 13CH+ and CD+ have been revisited and their rest frequency values improved substantially.

Line Lists for LiF and LiCl in the X1Σ+ Ground State

Abstract Vibration–rotation line lists for 6LiF, 7LiF, 6Li35Cl, 6Li37Cl, 7Li35Cl, and 7Li37Cl in the X 1Σ+ ground states have been prepared. The rovibrational energy levels have been calculated using potential energy surfaces determined by direct potential-fitting employing the rotational and rovibrational transition frequencies of all isotopologues, and required the inclusion of Born–Oppenheimer breakdown terms. Dipole moment functions calculated ab initio at the MRCI/aug-cc-pwCV5Z level have been used for line strength calculations. Partition functions for temperatures up to 5000 K have been calculated. LiF and LiCl are predicted to be present in the atmospheres of hot rocky exoplanets, brown dwarfs, and cool stars.

Terahertz Spectroscopy and Global Analysis of the Rotational Spectrum of Doubly Deuterated Amidogen Radical ND2

Abstract The deuteration mechanism of molecules in the interstellar medium is still being debated. Observations of deuterium-bearing species in several astronomical sources represent a powerful tool to improve our understanding of the interstellar chemistry. The doubly deuterated form of the astrophysically interesting amidogen radical could be a target of detection in space. In this work, the rotational spectrum of the ND2 radical in its ground vibrational and electronic state has been investigated between 588 and 1131 GHz using a frequency modulation millimeter/submillimeter-wave spectrometer. The ND2 molecule has been produced in a free-space glass absorption cell by discharging a mixture of ND3 and Ar. Sixty-four new transition frequencies involving J values from 2 to 5 and K a values from 0 to 4 have been measured. A global analysis including all the previous field-free pure rotational data has been performed, allowing for a more precise determination of a very large number of spectroscopic parameters. Accurate predictions of rotational transition frequencies of ND2 are now available from a few gigahertz up to several terahertz.

High-resolution Fourier transform emission spectroscopy of the A∼2Πi-X∼2Πi band of the OCS+ ion.

High resolution Fourier transform emission spectroscopy of the A∼2Πi-X∼2Πi band of the OCS+ ion was performed in the UV region to observe the ν1 (CO stretch) progression bands (υ1 = 0 → 2-5) for both the Ω=3/2 and 1/2 spin components. Accurate molecular constants including the rotational constants, B0 = 0.194 765(13) and 0.187 106(13) cm-1, and the spin-orbit interaction constants, A0 = -381.0(56) and -126.5(56) cm-1, were determined for the X∼2Π and A∼2Π states, respectively, by the simultaneous analysis of the observed progression bands. The CO bond length (rCO = 1.2810 Å) for the A∼2Π state, derived from the rotational constant B0 and Franck-Condon factors, is longer by 0.1756 Å than that (1.1054 Å) for the X∼2Π state, while the CS bond length for the A∼2Π state is shorter by 0.0905 Å than that for the X∼2Π state. Pure rotational transition frequencies in the ground X∼2Π state are predicted, as well as transition frequencies of the ν1 fundamental band, with the present molecular constants.

Methanol absorption in PKS B1830−211 at milliarcsecond scales

Observations of the frequencies of different rotational transitions of the methanol molecule have provided the most sensitive probe to date for changes in the proton-to-electron mass ratio, over space and time. Using methanol absorption detected in the gravitational lens system PKS B1830-211, changes in the proton-to-electron ratio over the last 7.5 billion years have been constrained to a fractional change less than 1.1e-07. Molecular absorption systems at cosmological distances present the best opportunity for constraining or measuring changes in the fundamental constants of physics over time, however, we are now at the stage where potential differences in the morphology of the absorbing systems and the background source, combined with their temporal evolution, provide the major source of uncertainty in some systems. Here we present the first milliarcsecond resolution observations of the molecular absorption system towards PKS B1830-211. We have imaged the absorption from the 12.2-GHz transition of methanol (which is redshifted to 6.45 GHz) toward the southwestern component and show that it is possibly offset from the peak of the continuum emission and partially resolved on milliarcsecond scales. Future observations of other methanol transitions with similar angular resolution offer the best prospects for reducing systematic errors in investigations of possible changes in the proton-to-electron mass ratio on cosmological scales.

THz spectroscopy of D2H+

We extended the measurements of the rotational transitions of D2H+ up to 3THz by using the JPL frequency multiplier chains and a TuFIR system at Toyama. D2H+ was generated in an extended negative glow discharge cell cooled to liquid nitrogen temperature. We observed five new THz lines. All the available rotational transition frequencies together with the combination differences derived from the three fundamental bands were subject to least square analysis to determine the molecular constants. New THz measurements are definitely useful for better characterization of spectroscopic properties. The improved molecular constants provide better predictions of other unobserved rotational transitions.

Millimeter-Wave Spectroscopy, X-ray Crystal Structure, and Quantum Chemical Studies of Diketene: Resolving Ambiguities Concerning the Structure of the Ketene Dimer.

The pure rotational spectrum of diketene has been studied in the millimeter-wave region from ∼240 to 360 GHz. For the ground vibrational state and five vibrationally excited satellites (ν24, 2ν24, 3ν24, 4ν24, and ν16), the observed spectrum allowed for the measurement, assignment, and least-squares fitting a total of more than 10 000 distinct rotational transitions. In each case, the transitions were fit to single-state, complete or near-complete sextic centrifugally distorted rotor models to near experimental error limits using Kisiel's ASFIT. Additionally, we obtained less satisfactory least-squares fits to single-state centrifugally distorted rotor models for three additional vibrational states: ν24 + ν16, ν23, and 5ν24. The structure of diketene was optimized at the CCSD(T)/ANO1 level, and the vibration-rotation interaction (αi) values for each normal mode were determined with a CCSD(T)/ANO1 VPT2 anharmonic frequency calculation. These αi values were helpful in identifying the previously unreported ν16 and ν23 fundamental states. We obtained a single-crystal X-ray structure of diketene at -173 °C. The bond distances are increased in precision by more than an order of magnitude compared to those in the 1958 X-ray crystal structure. The improved accuracy of the crystal structure geometry resolves the discrepancy between previous computational and experimental structures. The rotational transition frequencies provided herein should be useful for a millimeter-wave or terahertz search for diketene in the interstellar medium.

Vibrational energies and full analytic potential energy functions of PbI and InI from pure microwave data

Pure rotational spectra of PbI and InI are interpreted to yield a full analytic potential energy function for each molecule. Rotational spectra for PbI have been retrieved from literature sources to perform the analysis. Rotational transition frequencies for excited vibrational states of InI (0<v<11) are measured during this work. Ignoring hyperfine splittings, Bv and Dv values are used to generate a set of “synthetic” pure R(0) transitions for each vibrational level. These are then fitted to an “Expanded Morse Oscillator” (EMO) potential using the direct-potential-fit program, dPOTFIT. The well-depth parameter, De, is fixed at a literature value, while values of the equilibrium distance re and EMO exponent-coefficient expansion (potential-shape) parameters are determined from the fits. Comparison with potential functions determined after including older mid-IR and visible electronic transition data shows that our analysis of the pure microwave data alone yields potential energy functions that accurately predict (to better than 1%) the overtone vibrational energies far beyond the range spanned by the levels for which the microwave data is available.

FIRST LABORATORY MEASUREMENT OF THE J = 1 − 0 TRANSITIONS OF 36ArH+ AND 38ArH+: NEW, IMPROVED REST FREQUENCIES FOR ASTRONOMICAL SEARCHES

ABSTRACT The recent detections of 36 ArH &plus; ?> and 38 ArH &plus; ?> in the interstellar medium have triggered interest in argonium. This molecular cation has proved to be widespread in our galaxy in the diffuse medium and may serve as a very specific tracer of atomic gas and as a proxy for the cosmic-ray ionization rate. Pure rotational transitions of the two isotopologues detected in the ISM have never been recorded in the laboratory until this work. Here, we report on the first detection of the J = 1 &larr; 0 ?> transition of both 36 ArH &plus; ?> and 38 ArH &plus; ?> produced in a low-pressure glow discharge. All of the rotational and ro-vibrational transition frequencies available have been analyzed in global multi-isotopologues fit to give a comprehensive set of isotopic independent spectroscopic parameters. These data allow us to compute an improved set of rotational and ro-vibrational rest frequencies to assist astronomical searches of ArH &plus; ?> .

Terahertz-visible two-photon rotational spectroscopy of coldOD−

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, J=1<-0 and J =2<-1 of OD- anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598596.08(19) MHz for J=1<-0 and 1196791.57(27) MHz for J=2<-1 of OD-, in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.

HIGH PRECISION ROVIBRATIONAL SPECTROSCOPY OF OH+

ABSTRACT The molecular ion OH+ has long been known to be an important component of the interstellar medium. Its relative abundance can be used to indirectly measure cosmic ray ionization rates of hydrogen, and it is the first intermediate in the interstellar formation of water. To date, only a limited number of pure rotational transitions have been observed in the laboratory making it necessary to indirectly calculate rotational levels from high-precision rovibrational spectroscopy. We have remeasured 30 transitions in the fundamental band with MHz-level precision, in order to enable the prediction of a THz spectrum of OH+. The ions were produced in a water cooled discharge of O2, H2, and He, and the rovibrational transitions were measured with the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy. These values have been included in a global fit of field free data to a 3Σ− linear molecule effective Hamiltonian to determine improved spectroscopic parameters which were used to predict the pure rotational transition frequencies.