The study of the Aromatic Infrared Bands (AIBs) in astronomical environments has opened interesting spectroscopic questions on the effect of anharmonicity on the infrared (IR) spectrum of hot polycyclic aromatic hydrocarbons (PAHs) and related species in isolated conditions. The forthcoming James Webb Space Telescope will unveil unprecedented spatial and spectral details in the AIB spectrum; significant advancement is thus necessary to model the infrared emission of PAHs, their presumed carriers, with enough detail to exploit the information content of the AIBs. This requires including anharmonicity in such models, and to do so systematically for all species included, requiring a difficult compromise between accuracy and efficiency. We performed a benchmark study to compare the performances of two methods in calculating anharmonic spectra, comparing them to available experimental data. One is a full quantum method, AnharmoniCaOs, relying on an potential, and the other relies on Molecular Dynamics simulations using a Density Functional based Tight Binding potential. The first one is found to be very accurate and detailed, but it becomes computationally very expensive for increasing temperature; the second is faster and can be used for arbitrarily high temperatures, but is less accurate. Still, its results can be used to model the evolution with temperature of isolated bands. We propose a new recipe to model anharmonic AIB emission using minimal assumptions on the general behaviour of band positions and widths with temperature, which can be defined by a small number of empirical parameters. Modelling accuracy will depend critically on these empirical parameters, allowing for an incremental improvement in model results, as better estimates become gradually available.

A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH-He and CH-He 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. The spectrum of CH-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm, and thus confirms it to have a T-shaped structure. For CH-He, the C-H stretch along the symmetry axis of this oblate top results in parallel transitions.

The microwave spectrum of the doubly deuterated cyanomethyl radical (D2CCN) in its ground electronic state ( ) has been observed for the lowest four rotational transitions ( = 1-0, 2-1, 2-1 and 2-1) using a Fourier transform microwave spectrometer in combination with a pulsed discharge nozzle. A total of 394 hyperfine components were measured and subjected to a least squares analysis which allowed determining twelve hyperfine constants for nitrogen and deuterium nuclei. With this new set of molecular constants we obtained accurate predictions for low rotational transitions with hyperfine structure, and searched for this species in TMC-1.

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon rich Asymptotic Giant Branch (AGB) stars. The increased sensitivity and angular resolution of high altitude ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra - both in the ground and vibrationally excited states of the normal and rare isotopic species - of the principal molecules in the inner region which appear to contain only two or three heavy atoms Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeters-wave rotational lines that are observed in the inner envelope of carbon rich AGB stars. As an illustration we refer to the example of an initial laboratory approach for establishing whether vibrationally excited SiC and HCN are the carriers of some of the unassigned features observed in the prototypical carbon rich AGB star IRC+10216 with ALMA. Also highlighted are ongoing laboratory studies of the silicon carbides SiC and SiCSi in their ground and excited vibrational states, and SiC in the ground vibrational state. Following the initial detection of SiC and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with ≤ 48, ≤ 3 and upper level energies ≤ 484 K in the range from 178 - 391 GHz, and 35 new transitions of SiC with ≤ 38, ≤ 20 and ≤ 875 K between 315 and 440 GHz were measured in the laboratory. In addition five to six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi, and the two lowest rotational transitions in the previously unexplored C-C stretching mode ( ) of SiCC were measured in the normal and doubly substituted C isotopic species.

The 1:1 complex of tert-butyl alcohol with difluoromethane has been characterized by means of a joint experimental-computational investigation. Its rotational spectrum has been recorded by using a pulsed-jet Fourier-Transform microwave spectrometer. The experimental work has been guided and supported by accurate quantum-chemical calculations. In particular, the computed potential energy landscape pointed out the formation of three stable isomers. However, the very low interconversion barriers explain why only one isomer, showing one O-H···F and two C-H···O weak hydrogen bonds, has been experimentally characterized. The effect of the H → -butyl- group substitution has been analyzed from the comparison to the difluoromethane-water adduct.

Infrared multiple photon dissociation (IRMPD) spectroscopy and computational chemistry are applied to the ortho-, meta-, and para- positional isomers of aminobenzoic acid to investigate whether the amine or the carboxylic acid are the favored sites of proton attachment in the gas phase. The NH and OH stretching modes yield distinct patterns that establish the carboxylic acid as the site of protonation in para-aminobenzoic acid, as opposed to the amine group in ortho- and meta-aminobenzoic acid, in agreement with computed thermochemistries. The trends for para- and meta-substitutions can be rationalized simplistically by inductive effects and resonant stabilization, and will be discussed in light of computed charge distributions based from electrostatic potentials. In ortho-aminobenzoic acid, the close proximity of the amine and acid groups allow a simultaneous interaction of the proton with both groups, thus stabilizing and delocalizing the charge more effectively, and compensating for some of the resonance stabilization effects.

For some temperatures of atmospheric interest from 200 to 298 K, the self-broadening coefficients of OCS-OCS and HCN-HCN collisional systems, at different strengths of electrostatic interactions, were calculated respectively for ν and ν bands for a wide range of rotational quantum numbers J. In particular, we have considered some lines that were not studied previously. We have employed the approximation of bi-resonance functions (Starikov, 2012) in the frame of the semiclassical model of Robert and Bonamy with exact trajectory (RBE). The calculated results are found to be fully consistent with the available experimental values of self-broadening coefficients of OCS and HCN. A comparative study shows that the RBE calculations reproduce the dependence of broadening coefficients on quantum number J much better than the simpler Robert and Bonamy model with parabolic trajectory (RB) for all considered temperatures.

The microwave spectrum of the molecular complex of sulfur dioxide (SO) with carbon monoxide (CO) has been studied with a pulsed-beam Fourier Transform Microwave Spectrometer (FTMW) from a pair of gas samples of 1 % by volume of SO and CO in Ar, and introduced via separate capillary inputs to the flow nozzle. The frequency coverage was about 7 GHz to 16 GHz for various isotopomers. The molecular structure was determined with the aid of spectral studies of isotopically substituted monomers containing C, O and S. The rotational analyses provide the rotational and centrifugal distortion constants for all of the isotopomers analyzed. The structure determination is compared to detailed structural calculations. The electric dipole moment components along the - and -axis were determined from Stark effect measurements.

We present a thorough theoretical and experimental study of the electronic structure of RbCa. The mixed alkali-alkaline earth molecule RbCa was formed on superfluid helium nanodroplets. Excited states of the molecule in the range of 13 000-23 000 cm were recorded by resonance enhanced multi-photon ionization time-of-flight spectroscopy. The experiment is accompanied by high level calculations of ground and excited state properties, utilizing a multireference configuration interaction method based on multiconfigurational self consistent field calculations. With this approach the potential energy curves and permanent electric dipole moments of 24 electronic states were calculated. In addition we computed the transition dipole moments for transitions from the ground into excited states. The combination of experiment and theory allowed the assignment of features in the recorded spectrum to the excited [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] states, where the experiment allowed to benchmark the calculation. This is the first experimental work giving insight into the previously unknown RbCa molecule, which offers great prospects in ultracold molecular physics due to its magnetic and electronic dipole moment in the [Formula: see text] ground state.

Previously unobserved nitrous oxide transitions around 2.5 μm are measured by intracavity laser absorption spectroscopy (ICLAS) analyzed by time-resolved Fourier transform (TRFT) spectrometer. With an accuracy of the order of 10(-3) cm(-1), measured positions of 1637 assigned weak transitions are provided. They belong to 42 vibrational transitions, among which 33 are observed for the first time. These data are believed to be useful in particular to monitoring atmosphere purposes.

The electric dipole moments of symmetric top tertiary butyl derivatives were determined from Stark effect measurements made with cavity Fourier transform microwave spectroscopy under conditions of supersonic expansion. The resulting values are 1.9562(15), 2.1817(16), 2.2574(17) and 2.2122(17) D for tertiary butyl fluoride, chloride, bromide, and iodide, and 4.0129(30), and 4.0640(31) for tertiary butyl cyanide and isocyanide, respectively. The experimental values are compared with ab initio calculations and with experimental and calculated dipole moments for the corresponding methyl derivatives. Copyright 2001 Academic Press.

In a classic paper by G. Herzberg and J. W. C. Johns entitled "The Spectrum and Structure of Singlet CH(2)" (Proc. Roy. Soc. A 295, 107-128 (1966)) the analysis of the &btilde;(1)B(1)<--ã(1)A(1) red absorption band system of CH(2) is discussed in detail for the first time. In addition to that band system the observation of a fragment of a weak near ultraviolet absorption band system is reported. The three observed bands of the system could not be vibrationally assigned or rotationally analyzed but it was pointed out that they probably involve absorption into the second excited singlet state, &ctilde;(1)A(1). We show this supposition to be true here by simulation. In order to simulate the spectrum we have calculated ab initio the &ctilde;-ã and &ctilde;-&btilde; transition moment surfaces and used the MORBID and RENNER program systems with previously determined potential energy surfaces for the ã, &btilde;, and &ctilde; states in a calculation of the energy levels and wavefunctions. We find that the three bands seen by Herzberg and Johns are part of the &ctilde;<--(ã/&btilde;) system but that all of the bands of the system above about 31 000 cm(-1) are missing as a result of &ctilde; state predissociation. We vibrationally assign the bands but the weakness of the spectrum, and the presence of perturbations, make it impossible for us to analyze the rotational structure fully. Further experimental and theoretical studies are suggested. Copyright 2001 Academic Press.

In this paper we show that the rotational structure of the nu(5)/2nu(9) infrared band near 11 &mgr;m can be synthesized to high accuracy from pure rotational measurements in the millimeter- and submillimeter-wave region. The analysis uses an internal axis system Hamiltonian that accounts for the rotational dependence of the torsional splitting in 2nu(9), the induced torsional splitting in nu(5), and all of the infrared line positions, including those in the regions of strongest mixing and of highest excitation. This model also predicts the strength of the 2nu(9) infrared band due to the strong Fermi mixing with nu(5). The analysis is based on the 2317 millimeter/submillimeter lines and uses the well-documented SPFIT routines of JPL. Copyright 2001 Academic Press.

Quantum chemistry packages can be used to predict with reasonable accuracy spin-rotation hyperfine interaction constants for methanol, which contains one methyl-top internal rotor. In this work we use one of these packages to calculate components of the spin-rotation interaction tensor for acetaldehyde. We then use torsion-rotation wavefunctions obtained from a fit to the acetaldehyde torsion-rotation spectrum to calculate the expected magnitude of hyperfine splittings analogous to those observed at relatively high values in the E symmetry states of methanol. We find that theory does indeed predict doublet splittings at moderate values in the acetaldehyde torsion-rotation spectrum, which closely resemble those seen in methanol, but that the factor of three decrease in hyperfine spin-rotation constants compared to methanol puts the largest of the acetaldehyde splittings a factor of two below presently available Lamb-dip resolution.

Rotational-vibrational transitions of the fundamental vibrational modes of the CN and CN cations have been observed for the first time using a cryogenic ion trap apparatus with an action spectroscopy scheme. The lines (3) to (3) of CN and (1) to (3) of CN have been measured, limited by the trap temperature of approximately 4 K and the restricted tuning range of the infrared laser. Spectroscopic parameters are presented for both isotopologues, with band origins at 2000.7587(1) and 1970.321(1) cm, respectively, as well as an isotope independent fit combining the new and the literature data.

We present here the first experimental study of the microwave spectrum of deuterated 5-methyltropolone, a molecule which exhibits two large-amplitude motions: an intramolecular hydrogen transfer (deuterium transfer in the current case of deuterated 5-methyltropolone) and a methyl torsion. The main goal of this study was to get information on the isotopic dependence of the main tunneling parameters of 5-methyltropolone in the framework of the two dimensional tunneling formalism, which previously has shown some counterintuitive results for isotopic dependence of tunneling parameters in 2-methylmalonaldehyde. Measurements were carried out by Fourier-transform microwave spectroscopy in the 9 GHz to 26 GHz frequency range. Theoretical analysis was carried out using a tunneling-rotational Hamiltonian based on a G extended-group-theory formalism. Our global fit of 384 transitions to 17 molecular parameters gave a weighted root-mean-square deviation of 0.8. The current study on the isotopic dependence of the main tunneling parameters in 5-methyltropolone supports the assumption of possible "leakage" between tunneling parameters in the two-dimensional tunneling formalism in use.

We present a complex-valued electric field model for experimentally observed cavity transmission in coherent cavity-enhanced (CE) multiplexed spectroscopy (i.e., dual-comb spectroscopy, DCS). The transmission model for CE-DCS differs from that previously derived for Fourier-transform CE direct frequency comb spectroscopy [Foltynowicz et al., 163-175 (2013)] by the treatment of the local oscillator which, in the case of CE-DCS, does not interact with the enhancement cavity. Validation is performed by measurements of complex-valued near-infrared spectra of CO and CO by an electro-optic frequency comb coherently coupled to an enhancement cavity of finesse = 19600. Following validation, we measure the 30012 ← 00001 CO vibrational band origin with a combined standard uncertainty of 770 kHz (fractional uncertainty of 4 × 10).

In this paper we examine the two-dimensional tunneling formalism used previously to fit the hydrogen-transfer and internal-rotation splittings in the microwave spectrum of 2-methylmalonaldehyde in an effort to determine the origin of various counterintuitive results concerning the isotopic dependence of the internal-rotation splittings in that molecule. We find that the cause of the problem lies in a "parameter contamination" phenomenon, where some of the numerical magnitude of splitting parameters from modes with large tunneling splittings "leaks into" the parameters of modes with smaller tunneling splittings. We show that such parameter contamination, which greatly complicates the determination of barrier heights from the least-squares-fitted splitting parameters, will be a general problem in spectral fits using the multi-dimensional tunneling formalism, since it arises from subtle mathematical features of the non-orthogonal framework functions used to set up the tunneling Hamiltonian. Transforming to a physically less intuitive orthonormal set of basis functions allows us to give an approximate numerical estimate of the contamination of tunneling parameters for 2-methylmalonaldehyde by combining a dominant tunneling path hypothesis with results recently given for the hydrogen-transfer-internal-rotation potential function for this molecule.