Set Of Spectroscopic Constants Research Articles

Ground state spectroscopy and photochemistry of HAlOH.

Ab initio calculations were carried out in order to study the electronic structure and spectroscopy of cis-HAlOH, trans-HAlOH, H2AlO, and AlOH2. The cis structure is more stable than the trans, and both are thermodynamically stable relative to the AlOH + H dissociation limit. A set of spectroscopic constants were generated for the lowest stable isomers to help with their detection in the laboratory and in the interstellar medium. The first excited state absorbs strongly in the visible region (λ = 460nm), with a predicted transition dipole moment of 2.07D. The electronic structures of the first excited state were calculated, including the lifetime, adiabatic excitation energy, rotational constants, and frequencies. We have shown that both isomers may be suitable for laser-induced fluorescence detection. Finally, photodissociation of the cis- and trans-HAlOH isomers is a plausible mechanism for the production of AlOH and H.

The Laboratory Millimeter and Submillimeter Rotational Spectrum of Lactaldehyde and an Astronomical Search in Sgr B2(N), Orion-KL, and NGC 6334I

Abstract We present a laboratory rotational study of, and astronomical search for, lactaldehyde (CH3CH(OH)CH(O)), one of the simplest chiral molecules that could reasonably be seen in the interstellar medium (ISM), in the millimeter and submillimeter wave regions from 80 to 460 GHz. More than 5000 transitions were assigned to the most stable conformer, and a set of spectroscopic constants was accurately determined. Lactaldehyde is involved in numerous metabolic pathways used by life on Earth, and is a logical step up in complexity from glycolaldehyde (CH(O)CH2OH) which is being detected with increasing regularity in the ISM. We present an accompanying radio astronomical search for lactaldehyde in three high-mass star-forming regions (NGC 6334I, Sgr B2(N), and Orion-KL) as well as in the publicly available data from the ASAI Large Project. Neither molecule is detected in these sources, and we report corresponding upper limits to the column densities. We discuss the potential utility of lactaldehyde in combination with other members of the [C3,H6,O2] isomeric family in probing pathways of chemical evolution in the ISM.

Rotational spectroscopy of imidazole: improved rest frequencies for astrophysical searches

Context. Organic ring compounds play a key role in terrestrial biochemistry, and they were also most likely pivotal ingredients in Earth’s prebiotic chemistry. The five-membered ring imidazole, c-C3N2H4, is a substructure of fundamental biological molecules such as the purine nucleobases and the amino acid histidine. An unsuccessful search for imidazole in a sample of cold-core clouds and massive star-forming regions was performed almost 40 years ago. At that time, the spectroscopic knowledge of this species was scarce: the existing laboratory study was limited to the centimetre-wave region, and the precision of the rest frequencies in the millimetre regime was not adequate. Aims. The goal of the present work is to perform a comprehensive investigation of the rotational spectrum of imidazole in its ground vibrational state from the microwave region to the 1 mm wavelength regime. Methods. The rotational spectrum of imidazole was recorded in selected frequency regions from 2 to 295 GHz. These intervals were covered using various broadband spectrometers developed at DESY (Hamburg) and at the University of Virginia. High-level ab initio calculations were performed to obtain reliable estimates of the quartic and sextic centrifugal distortion constants. We used the EMoCA imaging spectral line survey to search for imidazole towards the hot molecular core Sgr B2(N2). Results. About 700 rotational transitions spanning a J interval from 0 to 59 and Kc interval from 0 to 30 were analysed using the Watson S-reduced Hamiltonian. These new data allowed the determination of a much extended set of spectroscopic parameters for imidazole in its vibrational ground state. The improved spectral data allow us to set an upper limit to the column density of imidazole in Sgr B2(N2). Its non-detection implies that it is at least 3400 times less abundant than ethyl cyanide in this source. Conclusions. With the new set of spectroscopic constants, it has been possible to compute reliable rest frequencies at millimetre wavelengths. We suggest a search for imidazole towards TMC-1, where the aromatic molecule benzonitrile was recently detected.

Mechanistic study of the photoexcitation, photoconversion, and photodissociation of CS2.

Carbon disulfide is a prototype molecular system for studies of photophysical processes in molecules at different time scales and is also relevant to astrophysics. Here, reliable molecular properties are computed for linear SCS, bent cyc-CS2, and linear CSS forms using highly correlated post Hartree-Fock methods in conjunction with large basis sets. Structures are identified using explicitly correlated and standard coupled cluster techniques. Evolution of the lowest-lying singlet and triplet electronic states of the three isomers along the SS and CS stretching coordinates and along the bending angle are mapped at the multireference configuration interaction (MRCI)/aug-cc-pV(5+d)Z level of theory. The computations suggest that the 1B2(1Σ+) electronic state of the SCS isomer plays an important role in the photoconversion of CS2 to cyc-CS2 and CSS. Photoconversion competes with photodissociation. Plausible mechanisms for the production of S2 and CS diatomics after the photoexcitation of SCS are proposed. To aid in the identification of CSS in the laboratory and in astrophysical media, a set of spectroscopic constants and rovibrational levels for CSS are reported.

Accurate Laboratory Measurement of the Complete Fine Structure of the N = 1 − 0 Transition of 15NH

Abstract The imidogen radical is an important molecule of the chemistry of nitrogen in the interstellar medium and is thought to be a key intermediate in the gas-phase synthesis of ammonia. The full fine structure of the rotational transition of has been observed for the first time by pure rotational spectroscopy around 1 THz. The radical has been produced by means of low-pressure glow discharge of H2 and -enriched nitrogen. A number of hyperfine components have been observed and accurately measured. The analysis of the data provided very precise spectroscopic constants, which include rotational, centrifugal distortion, electron spin–spin interaction, and electron spin–rotation terms in addition to the hyperfine parameters relative to the isotropic and anisotropic electron spin–nuclear spin interactions for 15N and H and to the nuclear spin–rotation for 15N. The efficiency of the discharge system allowed us to observe several components of the same rotational transition in the excited vibrational state v = 1, for which a set of spectroscopic constants has also been determined.

FTIR spectrum of vinyl fluoride near 3.6 μm: rovibrational analysis of the ν4+ν7 band and modelling Coriolis resonances in a seven-level polyad

ABSTRACTThe Fourier transform infrared (FTIR) spectrum of vinyl fluoride, H2C=CHF, has been widely investigated in the region of the ν4+ν7 combination band around 2800 cm−1 at a resolution of 0.005 cm−1. This vibration of A' symmetry gives rise to an a/b-hybrid band with a predominant a-type component. The rovibrational structure is strongly perturbed and the analysis has been rather complicated since this combination band is involved at least in a seven-level interacting polyad, including the ν8+2ν10, 2ν8+ν10, 2ν7+ν9, ν7+ν8+ν12, ν5+ν9+ν10 and ν7+ν10+ν12 vibrational states. The study has been further complicated by the absence of transitions coming from the perturbers that were considered as dark states. The spectral analysis resulted in the identification of 936 transitions with J" ≤ 46 and Ka" ≤ 11, all belonging to the a-type component. Most of the assigned data have been fitted using the Watson's A-reduction Hamiltonian in the Ir representation and proper Coriolis perturbation operators. The model employed includes seven different resonances within a complex polyad resonant system and a set of spectroscopic constants for the ν4+ν7 combination band, for the dark states, and Coriolis coupling coefficients have been determined.

MILLIMETER WAVE SPECTRUM AND ASTRONOMICAL SEARCH FOR VINYL FORMATE

ABSTRACT Previous detections of methyl and ethyl formate make other small substituted formates potential candidates for observation in the interstellar medium. Among them, vinyl formate is one of the simplest unsaturated carboxylic ester. The aim of this work is to provide direct experimental frequencies of the ground vibrational state of vinyl formate in a large spectral range for astrophysical use. The room-temperature rotational spectrum of vinyl formate has been measured from 80 to 360 GHz and analyzed in terms of Watson’s semirigid rotor Hamiltonian. Two thousand six hundred transitions within J = 3–88 and K a = 0–28 were assigned to the most stable conformer of vinyl formate and a new set of spectroscopic constants was accurately determined. Spectral features of vinyl formate were then searched for in Orion KL, Sgr B2(N), B1-b, and TMC-1 molecular clouds. Upper limits to the column density of vinyl formate are provided.

High-resolution FTIR spectroscopy of HCFC-31 in the 950−1160 cm−1 region: rovibrational analysis and resonances in the ν4, ν9 and ν5+ν6 bands of CH235ClF

The FTIR spectrum of CH2ClF (natural isotopic mixture) was investigated in the ν4, ν9 and ν5+ν6 band region between 950 and 1160 cm−1 at the resolution of 0.004 cm−1. The ν4 and ν5+ν6 vibrations of A′ symmetry give rise to a/b hybrid bands with a predominant a-type component. The ν9 vibration of A″ symmetry, expected with a c-type band contour, shows an intense Coriolis-induced parallel component (ΔKa = 0, ΔKc = 0) derived from mixing with the v4 = 1 vibrational state. The high-resolution spectra of ν9 and ν5+ν6 have been analyzed for the first time, while the assignments of the ν4 band, previously investigated, have been extended to higher J and Ka values in the b-type component. The spectral analysis resulted in the identification of 1508, 809 and 349 transitions for the ν4, ν9 and ν5+ν6 bands of CH235ClF, respectively. Besides the strong first-order a- and b-type Coriolis resonances between ν4 and ν9, the ν5+ν6 vibration was found to interact through a c-type Coriolis with the ν4 and 3ν6. High-order anharmonic resonance (ΔKa = ±2) between ν4 and ν5+ν6 was also established. All the assigned data were simultaneously fitted using the Watson's A-reduction Hamiltonian in the Ir representation and the relevant perturbation operators. The model employed includes five types of resonances within the tetrad ν4/ν9/ν5+ν6/3ν6. Α set of spectroscopic constants for ν4, ν9 and ν5+ν6 bands as well as parameters for the dark state 3ν6 and seven coupling terms have been determined. The simulations performed in different spectral regions satisfactorily reproduce the experimental data.

Ab initio study of the low-lying electronic states of [Be–Kr

The potential energy curves of the seven Ω states generated from the X2Σ+, 22Σ+, 32Σ+, A2Π and 22Π Λ–S states correlating with the first three dissociation channels of [Be–Kr]+ are calculated using the internally contracted multireference configuration interaction method with Davidson correction and relativistic correction. The spin–orbit coupling is computed using the state interaction approach with Breit–Pauli Hamiltonian. Based on the calculated potential energy curves, a set of spectroscopic constants are determined. Finally, the transition dipole moments and radiative lifetimes of transitions from all the Ω excited states to ground state are predicted for future laboratory research.

FOURIER TRANSFORM EMISSION SPECTROSCOPY OF THE B 2 Σ + - X 2 Σ + (VIOLET) SYSTEM OF 13 C 14 N

Emission spectra of the B {sup 2}{Sigma}{sup +}-X {sup 2}{Sigma}{sup +} transition of {sup 13}C{sup 14}N have been observed at high resolution using the Fourier transform spectrometer associated with the McMath-Pierce Solar Telescope of the National Solar Observatory. The spectra have been measured in the 21000-30000 cm{sup -1} region and a total of 52 vibrational bands involving vibrational levels up to v = 15 of the ground and excited states have been rotationally analyzed to provide a much improved set of spectroscopic constants. An experimental line list and calculated term values are provided. The results of the present analysis should prove useful in the identification of additional {sup 13}C{sup 14}N lines in comets and cool stars, and will help in the determination of the {sup 12}C/{sup 13}C abundance ratio.

Electronic structure and spectra of a new molecular species: SI. A theoretical contribution

A high level theoretical approach is used to characterize for the first time a manifold of doublet and quartet Λ+S and Ω states correlating with the first two dissociation channels of an as yet experimentally unknown molecular species, SI, sulfur monoidide. A set of spectroscopic constants is determined, including vibrationally averaged spin–orbit coupling constants, vibrationally averaged dipole moments, and dissociation energies. The transition dipole moment function for the spin-forbidden transition a4Σ−−X2Π, and the associated radiative lifetimes were also evaluated. Two possibilities to detect transitions experimentally and to derive spectroscopic constants are suggested.

The low-lying electronic states of BeP: a reliable and accurate quantum mechanical prediction

A very high level of theoretical treatment (complete active space self-consistent field CASSCF/MRCI/aug-cc-pV5Z) was used to characterize the spectroscopic properties of a manifold of quartet and doublet states of the species BeP, as yet experimentally unknown. Potential energy curves for 11 electronic states were obtained, as well as the associated vibrational energy levels, and a whole set of spectroscopic constants. Dipole moment functions and vibrationally averaged dipole moments were also evaluated. Similarities and differences between BeN and BeP were analysed along with the isovalent SiB species. The molecule BeP has a X 4Σ− ground state, with an equilibrium bond distance of 2.073 Å, and a harmonic frequency of 516.2 cm−1; it is followed closely by the states 2Π (Re = 2.081 Å, ωe = 639.6 cm−1) and 2Σ− (Re = 2.074 Å, ωe = 536.5 cm−1), at 502 and 1976 cm−1, respectively. The other quartets investigated, A 4Π (Re = 1.991 Å, ωe = 555.3 cm−1) and B 4Σ− (Re = 2.758 Å, ωe = 292.2 cm−1) lie at 13 291 and 24 394 cm−1, respectively. The remaining doublets (2Δ, 2Σ+(2) and 2Π(3)) all fall below 28 000 cm−1. Avoided crossings between the 2Σ+ states and between the 2Π states add an extra complexity to this manifold of states.

Improved Rest Frequencies of HCO + at 1 THz

The ground-state rotational spectrum of HCO+, produced in a negative-glow discharge cell, has been recorded up to 892 GHz. The sources of systematic error have been carefully accounted for, thus obtaining accurate rotational and centrifugal distortion constants. The present measurements have allowed us to determine a value of the sextic distortion constant, HJ, which, unlike the recent determination of Lattanzi et al., is consistent with the values determined for DCO+ and for the isoelectronic molecule HCN. The new set of spectroscopic constants allows us to predict the rotational spectrum of the formyl ion with an accuracy of 1 part in 108 near 1 THz. Such an accuracy is important for kinematic studies of dense cores in molecular clouds and for future far-infrared observations.

Radical SBr: A challenge to spectroscopists?

A theoretical description of the electronic states of the SBr radical at a high level of correlation effects treatment is presented for the first time. Most of its potential energy curves have a repulsive nature, but a set of bound (2)Pi states should provide a means for the experimental characterization of this species. The ground state can support over 40 vibrational states, but transitions from the 2 (2)Pi state, with a very shallow well (four levels), should have intensities spread over a maximum around v(")=10; lower vibrational levels can only be accessed for transitions from two high-lying Rydberg states. A whole set of spectroscopic constants, including vibrationally averaged spin-orbit coupling constants, and radiative transition probabilities and lifetimes quantify the theoretical description, thus supplying reliable results to guide the experimental investigation of this species.

Laser infrared spectroscopy of vinyl fluoride in the 1280–1400 cm−1 region

The infrared spectra of CH2=CHF have been investigated in the ν5 and ν6 band regions between 1280 and 1400 cm−1, at a resolution of about 0.002 cm−1, using a tunable diode laser spectrometer. These vibrations of symmetry species A′ give rise to a/b-hybrid bands with different contributions from both the components. Spectral analysis resulted in the identification of 1565 (J≤46, K a ≤11) and 1651 (J≤48, K a ≤15) transitions of the ν5 and ν6 fundamentals, respectively. Both bands are perturbed by the nearby states ν8 + ν9 and ν9 + ν11 through different Coriolis resonances and an anharmonic interaction. Using Watson's A-reduction Hamiltonian in the Ir representation and perturbation operators almost all the transitions have been fitted simultaneously to a model including six resonances within the tetrad ν5/ν6/ν8 + ν9/ν9 + ν11. A set of spectroscopic constants for the ν5 and ν6 bands, as well as parameters for the dark states ν8 + ν9 and ν9 + ν11 and coupling constants, have been determined. From spectral simulations the dipole moment ratio |Δμ a /Δμ b | was estimated to be 0.6 ± 0.1 and 2.0±0.3 for the ν5 and ν6 bands, respectively.

Near infrared absorptions of CH 4/He plasma: the Phillips band system of C 2

Zero-background laser absorption spectroscopy using concentration modulation has been applied to study the near infrared absorption spectrum of the discharge of CH 4 and He at a ratio of about 1:170. In the 10 300–14 250 cm −1 region, a total of 13 vibronic bands have been observed with 11 of them assigned to the very weak Phillips band system of C 2. Combining these transitions with those observed in emission by Douay et al. [J. Mol. Spectrosc. 131 (1988) 250], a set of spectroscopic constants with high accuracy have been obtained.

Line Intensities and Molecular Opacities of the FeHF4Δi–X4ΔiTransition

We calculate new line lists and opacities for the $F^4\Delta_i-X^4\Delta_i$ transition of FeH. The 0-0 band of this transition is responsible for the Wing-Ford band seen in M-type stars, sunspots and brown dwarfs. The new Einstein A values for each line are based on a high level ab initio calculation of the electronic transition dipole moment. The necessary rotational line strength factors (H\"onl-London factors) are derived for both the Hund's case (a) and (b) coupling limits. A new set of spectroscopic constants were derived from the existing FeH term values for v=0, 1 and 2 levels of the $X$ and $F$ states. Using these constants extrapolated term values were generated for v=3 and 4 and for $J$ values up to 50.5. The line lists (including Einstein A values) for the 25 vibrational bands with v$\leq$4 were generated using a merged list of experimental and extrapolated term values. The FeH line lists were use to compute the molecular opacities for a range of temperatures and pressures encountered in L and M dwarf atmospheres. Good agreement was found between the computed and observed spectral energy distribution of the L5 dwarf 2MASS-1507.

An ab initio anharmonic force field of SiHCI3

An ab initio quartic force field of SiHCl3 is derived using the second-order Møller-Plesset perturbation theory and Dunning's correlation consistent triple-zeta basis set. After a minor empirical adjustment for the six diagonal quadratic force constants, most fundamentals of SiHCl3 and SiDCl3 agree with the experimental values within 1 cm−1. Additionally the observed overtones, combinations and hot band centres can also be well reproduced. Vibrational analysis based on the second-order perturbation theory is carried out with the calculated force constants. Two sets of spectroscopic constants are predicted for 28SiH35Cl3 and 28SiD35Cl3, respectively.

Accurate Rovibrational Spectroscopic Properties of Cyanogen and Its Isotopomers

The six-dimensional potential energy surface (PES) of the most stable isomer of C2N2 is investigated using the coupled cluster single and double approach with perturbative treatment of triple excitations (CCSD(T)). The full quartic force fields of this molecule and some of its isotopomers are calculated in dimensionless normal coordinates. This quartic force field has been optimized in order to reproduce the known overtones within 1 cm−1. A set of spectroscopic constants is derived for these isotopomers using this optimized quartic force field. The full rovibrational spectrum is also given up to 1500 cm−1 with an accuracy better than 1 cm−1.

Rotational spectroscopy of C-cyanophosphaethyne, NCCP, in states of multiple vibrational excitation

The rotational spectrum of the unstable NCCP molecule has been investigated in the millimetre-wave region for all the levels of multiple vibrational excitation below 1200 cm−1. More than 600 rotational transitions in the J range from 19 to 39 have been measured and assigned to 13 new vibrational states derived from excitation of the two bending modes 4 and 5 and of the low-energy stretching mode 3 . Transitions up to J = 141 have been included in the fit for 3 = 1 yielding a precise value for the sextic centrifugal distortion constant. Vibrational and rotational l-type resonance effects have been taken into account in the analyses of the observed spectra together with the anharmonic interactions which originate from the normal coordinate force constants k345, k344, k35555 and k34555. The simultaneous analyses of the rotational spectra of the interacting levels have yielded sets of spectroscopic constants with unambiguous physical meaning, including the unperturbed value of the α3 vibration–rotation coupling constant, and the xL(44), xL(55) and xL(45) anharmonic constants.