We previously presented the least-squares fit of the benzonitrile (C₆H₅CN, C_2v, μ_a = 4.5 D) ground vibrational state, which was fit to a partial octic Hamiltonian, as well as the first analysis of its two Coriolis-coupled, lowest-energy fundamental states, ν₂₂ and ν₃₃.¹ Herein, we present the continuation of this work: the analysis and fitting of the Coriolis- and Fermi-coupled triad of the overtone and combination states (ν₂₂ = 2, ν₃₃ = 2, and ν₂₂ + ν₃₃) near 315 cm⁻¹. Approximately 3000 transitions are assigned for each state, including multiple resonances, and their least-squares fitting results in precise energy separations that are in agreement with those expected based on the energy separation determined for ν₂₂ and ν₃₃. 1. Zdanovskaia, M. A.; Esselman, B. J.; Lau, H. S.; Bates, D. M.; Woods, R. C.; McMahon, R. J.; Kisiel, Z. The 103 - 360 GHz Rotational Spectrum of Benzonitrile, the First Interstellar Benzene Derivative Detected by Radioastronomy. J. Mol. Spectrosc. 2018, 351, 39-48.

The rotational spectra of 2-, 3-, and 4-cyanopyridine (C₆H₄N₂) were recorded from 130 – 360 GHz, and an analysis of the ground vibrational state and two lowest-energy excited vibrational states for each isomer was completed. Each ground state was least-squares fit to a partial octic, distorted-rotor Hamiltonian with several thousand distinct transitions assigned (σ_fit < 50 kHz). The B3LYP/6-311+G(2d,p)-predicted quartic and sextic distortion constants are in excellent agreement with their experimentally determined values for all three isomers. Given the large dipole moments of 2-, 3-, and 4-cyanopyridine (μ = 5.8 D, 3.7 D, 2.0 D, respectively) and the astrochemical community’s interest in detecting nitriles, the cyanopyridines are an attractive set of aromatic species for detection by radioastronomy. This work provides the fundamental constants needed to support an astronomical search of the cyanopyridine isomers in the interstellar medium. We also examined the two lowest-energy fundamentals of each isomer, which correspond to the in-plane and out-of-plane CN bending modes for all three species. These states are  ∼  150 cm⁻¹higher in energy than the ground state and are separated by less than 30 cm⁻¹, an energy separation small enough that each corresponding pair exhibits Coriolis coupling. The dyads display a-type Coriolis coupling for 4-cyanopyridine (C_2v) and both a− and b-type Coriolis coupling in 2- and 3-cyanopyridine (C_s), resulting in both global and local perturbations to the rotational transitions. Each isomer required the inclusion of several coupling terms in the two-state least-squares fit in order to fit all observed rotational transitions, including numerous resonances. Fitting each dyad’s spectroscopic parameters allowed for the precise experimental determination of each dyad’s energy separation, the Coriolis coupling coefficients, and the deperturbed vibration-rotation interaction constants. Combined with the recently published work on the analogous dyad of benzonitrile, this coupling in the cyanopyridines provides an opportunity to compare the Coriolis interactions of cyano-substituted aromatic molecules in greater detail.

A discrepancy between theoretical and experimental values of the rotational barrier in benzaldehyde has been observed, which was attributed to inaccurate experimental results in part.I.A. Godunov, V.A. Bataev, A. V. Abramenkov, V.I. Pupyshev, J. Phys. Chem.A, 2014, 118, 10159−10165, and references therein. Here, we report results on the –CHO torsion of benzaldehyde (C_6H_5CHO) based on a high resolution spectroscopic investigation in the far−infrared range in an effort to remove the experimental ambiguity. The rotationally−resolved vibrational spectra were measured with an unapodized resolution of 0.00064 cm⁻¹using synchrotron−based Fourier transform infrared (FTIR) spectroscopy at the Canadian Light Source. The torsional fundamental ν_t = 109.415429(20) cm⁻¹was unambiguously assigned via rovibrational analysis, followed by the tentative assignment of the first (2ν_t – ν_t) and second (3ν_t – 2ν_t) hot bands at 107.58 cm⁻¹and 105.61 cm⁻¹, respectively, by comparison of the observed Q branch structures at high resolution with simulation based on a previous microwave study.R. K. Kakar, E. A. Rinehart, C. R. Quade and T. Kojima, J. Chem. Phys, 1970, 52, 3803-3813 This assignment is different from any previous low resolution infrared studies in which the intensity patterns were misleading. The key result of the assignment of the first three transitions allowed for the determination of the barrier to internal rotation of (hc)1533.6 cm⁻¹(4.38 kcal mol⁻¹). When compared with calculated results from vibrational second-order perturbation theory (VPT2) and quasiadiabatic channel reaction path Hamiltonian (RPH), the experimental value is still too low and this suggests that the discrepancy between theory and experiment remains despite the best experimental efforts.

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Footnotes:

I.A. Godunov, V.A. Bataev, A. V. Abramenkov, V.I. Pupyshev, J. Phys. Chem.A, 2014, 118, 10159-10165, and references therein.\end R. K. Kakar, E. A. Rinehart, C. R. Quade and T. Kojima, J. Chem. Phys, 1970, 52, 3803−3813

We conducted multiple high-level ab initio calculations to obtain improved semi-experimental equilibrium structures (r_e^SE) of hydrazoic acid (HN₃) using previously reported experimental data and to obtain improved, purely theoretical predictions of the structure. For the r_e^SE structures, we used the isotopologue-dependent vibrational corrections to the ground rotational constants obtained from anharmonic VPT2 calculations at CCSD(T) using cc-pVXZ and cc-pCVXZ basis sets (where X = D, T, Q, 5), as well as the isotopologue-dependent electron-mass corrections at the same level of theory, to carry out nonlinear least-squares fits of the experimentally determined rotational constants for all but two of the HN₃ isotopologues. The extent of corrections that were calculated allowed for extrapolation of the r_e^SE structure to the complete basis set limit. To achieve an improved theoretical prediction, we obtained a CCSD(T)/cc-pCV6Z optimization and included corrections for extrapolation to the complete basis set limit, for effects of electron correlation, for relativistic effects, and for the Born-Oppenheimer approximation. The parameters of the resulting theoretical structure agrees to within the 2σ uncertainties of the r_e^SE.

Spectral complexity can prove to be a challenge when investigating the rotational spectra of complicated systems and developments in automated fitting tools can reduce the time spent fitting these spectra. In this talk, we discuss our progress on improving various aspects of Autofit (algorithmic, including incorporating double resonance information, and usability, e.g. improved graphical interface). Our main test cases are spectra collected from our lab of small molecules near room temperature where contributions from multiple vibrational states are present, but we will also discuss results from molecular beam spectra.

A chirped pulse experiment is a powerful means to rapidly obtain an high-resolution spectrum of molecules on a large frequency band. The theoretical paper from McGurk et al.J. C. McGurk, T. G. Schmalz, and W. H. Flygare, J. Chem. Phys. 60, 4181 (1974).s the main reference paper to describe the polarization induced by fast chirped pulses generated with microwave sources. We built a chirped pulse spectrometer operating at 200 GHz for astrophysical applications.F. Hindle, C. Bray, K. Hickson, D. Fontanari, M. Mouelhi, A. Cuisset, G. Mouret and R. Bocquet, J. Infrared Millim. Te. 39, 105 (2018).t works in the millimeter domain with slower chirped pulses. In such a situation, the paper of McGurk et al. does not capture all the physics involved in the polarization step. In particular, the intensity of a molecular transition is dependent on its temporal position inside the chirped pulse, as discovered by Abeysekera et al.C. Abeysekera, L. N. Zack, G. B. Park, B. Joalland, J. M. Oldham, K. Prozument, N. M. Ariyasingha, I. R. Sims, R. W. Field, and A. G. Suits, J. Chem. Phys. 141, 214203 (2014). theoretical study of the polarization of molecules subjected to a slow chirped pulse is presented for three typical cases: the cell, the uniform flow and the molecular beam. Analytical expressions are proposed alongside the numerical solution and are used in the expression of the free induction decay signal. We test the analytical expression on the rotational emission spectra of OCS molecules. In the thermalized case, a relation between the pulse duration, the line position in the chirped pulse, and the signal amplitude is proposed to correct the line intensities.D. Fontanari, C. Bray, G. Dhont, G. Mouret, A. Cuisset, F. Hindle, R. Bocquet, and K. M. Hickson, Phys. Rev. A 100, 043407 (2019).html:<hr /><h3>Footnotes: J. C. McGurk, T. G. Schmalz, and W. H. Flygare, J. Chem. Phys. 60, 4181 (1974).i F. Hindle, C. Bray, K. Hickson, D. Fontanari, M. Mouelhi, A. Cuisset, G. Mouret and R. Bocquet, J. Infrared Millim. Te. 39, 105 (2018).I C. Abeysekera, L. N. Zack, G. B. Park, B. Joalland, J. M. Oldham, K. Prozument, N. M. Ariyasingha, I. R. Sims, R. W. Field, and A. G. Suits, J. Chem. Phys. 141, 214203 (2014).A D. Fontanari, C. Bray, G. Dhont, G. Mouret, A. Cuisset, F. Hindle, R. Bocquet, and K. M. Hickson, Phys. Rev. A 100, 043407 (2019).

r0pt Figure Doppler-free saturated-absorption Lamb dips were observed for weak vibration-rotation transitions of acetylene between 7167 and 7217 cm⁻¹, using frequency comb assisted cavity ring-down spectroscopy. We measured the absolute center frequency of 16 lines of the 2ν₃+ν₅¹ band, targeting ortho and para states of the molecule. Line pairs of the P and Q branches were selected so as to form a V-scheme, sharing the lower energy level. Such a choice made it possible to determine the rotational energy separations of the excited vibrational state for J-values from 11 to 20. The spectrometer is an upgraded version of that described by Fasci et al.E. Fasci et al., Phys. Rev. A 98, 022516, (2018). the main novelty being the use of a BOA - Booster Optical Amplifier, to increase the optical power injected into the high-finesse cavity. Our data provide a very stringent test of the MARVEL analysis recently performed on acetyleneK.L. Chubb et al. J. Quant. Spectrosc. Radiat. Transfer 204, 42–55, (2018). Figure on the right reports an example of observed Lamb dips recorded in correspondence of the line doblet already exploited for low-uncertainty Doppler-broadening gas thermometryA. Castrillo et al. Phys. Rev. Appl. 11, 064060, (2019)

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Footnotes:

E. Fasci et al., Phys. Rev. A 98, 022516, (2018)., K.L. Chubb et al. J. Quant. Spectrosc. Radiat. Transfer 204, 42–55, (2018).. A. Castrillo et al. Phys. Rev. Appl. 11, 064060, (2019).

I recently published a paper¹ that analyzed near-resonant two-photon absorption (TPA) spectra of polyatomic molecules excited in the vibrational fundamentals. For each molecule, a very small number of TPA lines are greatly enhanced due to near resonance. It is well known that TPA in a standing wave is dominated by a Doppler Free absorption. In this talk, the theory will be summarised and extensions to the published paper will be presented, including: (1) Theory of high finesse optical cavities with TPA loss, including the peak transmission and spectral shape of cavity modes (which are not Lorenztian with TPA).² (2) Extension of the theory of saturation that includes the consequences of slow vibrational relaxation, which invalidates the traditional optical Bloch equations. (3) Theory of the TPA lineshape in the limit of low pressure such that the transition width is limited by the finite time of flight of the absorbers through the TEM₀₀ mode of the cavity.³ 1. K.K. Lehmann, J. Chem. Phys. 151, 144201 (2019) 2. K.K. Lehmann, J. Opt. Soc. Am. A 37, 3055 (2020). 3. K.K. Lehmann, to be published in J. Chem. Phys.

As recently discussed by K.K. Lehmann in a theoretical analysis of two-photon absorption (TPA) [1], the combination of high circulating power and counterpropagating fields associated with the cavity ring-down spectroscopy (CRDS) technique can yield relatively prominent, two-photon, Doppler-free absorption features for near-resonant, three-level rovibrational systems. Here, we present CRDS measurements of TPA by N₂O near λ =4.5 μm. The experiment comprises a quantum cascade laser which is frequency-coupled to a high-finesse optical resonator through controlled optical feedback. Consistent with Lehmann's model, we observe CRDS decay signals that depend upon two characteristic decay rates: a constant single-photon value and an intracavity-power-dependent two-photon absorption rate. The resulting spectra exhibit strong Doppler-free features, saturation effects at low pressure, and TPA cross-section and collisional broadening coefficient consistent with theoretical predictions. 1. K.K. Lehmann, "Resonance enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: A proposal," J Chem Phys 151, 144201 (2019).

We present an edge-specific scheme for calculating near edge x-ray absorption fine structure (NEXAFS) spectra using core-valence separated equation-of-motion coupled-cluster (CVS-EOM-CC) theory. [1,2] Standard correlation-consistent basis set for the atom where targeted core excitation takes place is systematically augmented with diffuse s-, p-, and d-type functions to accurately describe Rydberg-type core excitations with diffuse character. It is also shown that triple excitations in CVS-EOM-CC methods not only are important for obtaining accurate absolute values of core excitation energies, but also make significant contributions to relative shifts between local and Rydberg core excitations. Quadruples excitations are shown to be relevant when aiming at high-accuracy calculations of absolute values.

The density dependence of pressurized and supercritical carbon dioxide (CO₂) electronic absorption spectra was explored by vacuum ultraviolet (VUV) spectroscopy over the wavelength range 1455-2000 Å. The peak energy position of the lowest absorption band of gaseous CO₂ does not change upon pressurizing up to and beyond the thermodynamic critical point (137 bar, 308 K). Upon increase of density, the known diffuse vibrational structure inherent to the VUV absorption spectrum of gaseous CO₂ gradually diminishes in magnitude and nearly disappears upon reaching a density of 0.767 g cm⁻³. This loss of spectral detail cannot be explained solely by collisional broadening and/or dimerization. We suggest that perturbation of the monomer potential energy surfaces belonging to multiple upper electronic states leads to increased coupling between binding and dissociative states. This perturbation is caused by the near proximity of CO₂ molecules at high densities and a variety of orientations without the need for equilibrated dimers, and in turn gives rise to the observed spectral changes. Based on a high-quality CO₂ dimer potential energy surface, we estimate a critical radius of 4.1 +/- 0.2 Å between molecules necessary to cause such perturbations.

Nitromethane, the smallest organic-nitro compound, is commonly studied to model ignition and detonation of energetic materials. Using pump-probe femtosecond laser photoionization mass spectroscopy, coupled-cluster theory, and ab initio molecular dynamics, we study nitromethane cation (NM⁺) fragmentation into CH₃⁺, NO₂⁺, and NO⁺. From theoretical analysis, NO₂⁺ and CH₃⁺ are formed through direct cleavage of the C-N bond, whereas NO⁺ forms spontaneously upon nitro-nitrite rearrangement (NNR) of the NM⁺ cation. Direct ionization into the electronically excited D₁ or D₂ states by the pump pulse provides sufficient excess energy to initiate the NNR pathway. With excess energy stored in the NNR transition state, molecular dynamics simulations indicate that NNR requires 660±230 fs and is typically followed by rapid NO⁺ loss  100-200 fs later. Experimentally, the fragmentation pathways to NO⁺ and CH₃⁺ are in competition, with associated decay timescale of  ∼ 480±200 fs that is similar to the computed NNR timescale. This result suggests that CH₃⁺ is formed by further excitation of the NM⁺ initially ionized into the D₁ or D₂ states before it undergoes NNR. Finally, the dissociation to NO₂⁺ from NM⁺ can be assigned to a D₀ → D₂ transition.

Far-infrared opacity of tropospheric and lower stratospheric layers in Titan’s atmosphere is dominated by collision-induced absorption (CIA) in various molecular pairs containing N₂, CH₄, and H₂. The presently available spectra of these pairs are often insufficiently accurate for reliably analyzing observed Titan spectra. Analysis of emission spectra from Titan’s atmosphere recorded with the Cassini Composite Infrared Spectrometer (CIRS)Bézard et al. (2020). Icarus. 344, 113261.uggests that the Borysow et al. modelBorysow, et al. (1993). Icarus. 105, 175.or CH₄−N₂ CIA coefficients underestimates absorption by about 50%. We present the trajectory-based study of the rototranslational CIA band in CH₄−N₂. Assuming rigid monomers, potential energy and induced dipole are characterized quantum-chemically at the CCSD(T)/CCSD(T)-F12b levels of theory. The Monte Carlo strategy is adopted to obtain the dipole autocorrelation function. The original procedure is employed to sample initial conditions throughout the phase space of a molecular pair. The autocorrelation function is derived from an ensemble of 5 to 10 million classical trajectories obtained through the solution of Hamilton equations in the space-fixed reference frame. The Fourier transform of the autocorrelation function yields the CIA band profile. We propose a new semiempirical model for CH₄−N₂ CIA that allows us to reproduce CIRS spectra recorded at low and high emission angles in the equatorial region^a. This work is partially supported by RFBR-CNRS grant 18-55-16006 and NASA HITRAN grant.

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Footnotes:

Bézard et al. (2020). Icarus. 344, 113261.s Borysow, et al. (1993). Icarus. 105, 175.f