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).

Hydrocarbons are of considerable importance in studies of planetary atmospheres such as Titan and Giant Planets. In Titan, ethane (C₂H₆) is the second most abundant hydrocarbon and is also widespread in Giant Planets. Neopentane (C(CH₃)₄) may be detected in these planetary atmospheres. This talk will present the absorption cross sections of ethane and neopentane. Infrared absorption spectra of ethane and neopentane were recorded in the 2600-3300 cm⁻¹ region by high resolution Fourier transform spectroscopy at the Canadian Light Source. Absorption cross sections for neopentane were obtained for pure samples and samples broadened by nitrogen. The data were taken at temperatures of 203 K, 232 K, 266 K and 293 K with broadening gas pressures of 10 Torr, 30 Torr and 100 Torr. Cross sections for ethane were obtained for pure samples and samples broadened by hydrogen and helium at the same temperatures and pressures. Calibration of cross sections for ethane was carried out using data from the Pacific Northwest National Laboratory (PNNL).

We measured temperature-dependent cross-sections of n-butane (CH3-CH2-CH2-CH3) in the mid-infrared (7-15 μm) region. For this, we obtained 28 pure and N2-mixture spectra at 180–298 K using a high-resolution Fourier transform spectrometer (Bruker IFS 125 HR) at the Jet Propulsion Laboratory. The observed spectra were fit simultaneously to generate empirical pseudoline parameters, which include line intensities and empirical lower state energies at the individual pseudoline positions. We observed that the pseudolines could reproduce the observed spectra within 4 % through line-by-line radiative transfer calculations, showing the pseudoline parameters are an excellent practical alternative until the true spectroscopic line parameters become available. The integrated intensities at 296 K were measured to be 5.06(28), 7.18(27), 0.91(4), and 49.01(20)E-19 cm⁻¹/(molecule.cm⁻²) in the 660-860, 860-1060, 1060-1200, and 1200-1538 cm⁻¹regions, respectively, by summing up the pseudoline intensity parameters. The pseudolines are electronically compiled in the HITRAN database format, which can be readily integrated with existing radiative transfer calculations. The measured cross-sections represented by the pseudolines could provide critical laboratory input in search of elusive species that might be captured in the Cassini/CIRS spectral observations.

r0.40 Figure A hydrogen bound bimolecular complex consists of a hydrogen bond donor and acceptor unit. The OH-stretching fundamental transition of the hydrogen bond donor is typically redshifted and its infrared intensity enhanced upon complexation.^[1] This facilitates detection of weak complexes even though the equilibrium is strongly shifted towards the monomers at room temperature. The ratio of a measured and calculated intensity of a vibrational band is proportional to the complex abundance, which with known monomer pressures gives the equilibrium constant.^[2] We calculate absolute transition intensities with a reduced dimensionality variational local mode model that also includes low-frequency vibrations. Calculated and experimental intensities of multiple bands are combined to give the equilibrium constant of complex formation for the water·dimethylamine and water·trimethylamine complex.^[3] The equilibrium constant obtained from different bands should be equivalent, and the detection of multiple bands therefore allows us to validate the accuracy of our combined experimental and theoretical approach.

The 20013 ← 00001 rotation-vibration band of ¹²C¹⁶O₂ near 2.06 μm is widely used for remote sensing of carbon dioxide concentration in the terrestrial atmosphere. The high accuracy target of such applications (0.3 % for the OCO-2 satellite mission) motivates reducing the uncertainties of relevant line intensities in currently available line lists. To this end, we present transition intensities measured by frequency-stabilized cavity ring-down spectroscopy for 39 transitions of the 2.06 μm CO₂ band. The relative combined standard uncertainty of the line intensities is estimated to be u_r = 0.09 %. To determine the integrated band intensity from these measurements, two approaches were used to model the J-dependence of the intensities. The first approach is based on a one-dimensional quantum mechanical model with Herman-Wallis rotation-vibration corrections, whereas in the second approach, we fit a single factor to rescale the intensities derived from the ab initio dipole moment surface (DMS) of Zak et al. [JQSRT 177, 31-42 (2016)]. Although the two approaches yield equally adequate representations of the observed J-dependence, we consider the rescaled ab initio DMS intensities to be the more physically general representation of our data. The results will be compared to existing databases and other experiments.

The focus of this paper is the calculation of radiative transition rates between doublet electronic levels in diatomic molecules. A different formulation for listing Hönl-London factors is introduced that enables the straightforward inclusion of higher-order correction factors for effects such as centrifugal distortion and Lambda-doubling in the calculation of the radiative transition rates. The formulae for the Hönl-London factors are developed using Hund’s case (a) basis states and the correction factors for spin-orbit splitting, centrifugal distortion, spin-rotation interactions, and Lambda-doubling and included in the Hund’s case (a) coefficients for the state wavefunctions. Inclusion of Herman-Wallis effects in the calculation of the radiative transition rates is also illustrated for the hydroxyl radical and nitric oxide. The Herman-Wallis correction factors are incorporated in a straightforward fashions in the tables of line strength factors.

r0pt Figure To address the notable lack of knowledge on high-temperature absorption cross sections of important molecular species in combustion and exoplanets, a new strategy is proposed and deployed to collect broad-band absorption spectra in shock-heated gases. The methodology utilizes a broad-scan, rapid-tuning external-cavity quantum cascade laser in conjunction with a shock tube and is capable of providing quantitative spectroscopic information across full vibrational bands spanning over 200 cm⁻¹within 6 ms ( > 30,000 cm⁻¹/s), with a spectral resolution between 0.3 – 0.6 cm⁻¹. This experimental approach is demonstrated with absorption spectra measurements on the ν₇ vibrational band of ethylene (C₂H₄) from 8.4 μm to 11.7 μm at temperature/pressure conditions between 800 – 1600 K, 1 – 5 atm. The measured spectra are compared against spectral simulations using existing spectroscopic databases, showing better agreement with the line list of Rey et al.M. Rey, T. Delahaye, A. V. Nikitin, and V. G. Tyuterev, “First theoretical global line lists of ethylene (¹²C₂H₄) spectra for the temperature range 50-700 K in the far-infrared for quantification of absorption and emission in planetary atmospheres,” Astron. Astrophys., vol. 594, pp. 1–16, 2016.han of HITRAN 2016I. E. Gordon et al., “The HITRAN2016 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf., vol. 203, pp. 3–69, 2017. With the current set of instruments available, this methodology could be applied to numerous gas-phase molecules that have attractive absorption features in the spectral range of 3.6 – 11.7 μm and opens an efficient pathway towards improving knowledge on radiation absorption in the mid-infrared at high temperatures.

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Footnotes:

M. Rey, T. Delahaye, A. V. Nikitin, and V. G. Tyuterev, “First theoretical global line lists of ethylene (¹²C₂H₄) spectra for the temperature range 50-700 K in the far-infrared for quantification of absorption and emission in planetary atmospheres,” Astron. Astrophys., vol. 594, pp. 1–16, 2016.t I. E. Gordon et al., “The HITRAN2016 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf., vol. 203, pp. 3–69, 2017..

l0pt Figure An infrared absorption cross-section database for gas-phase molecules at high-temperatures and high-pressures is under construction to address a growing cross-disciplinary need for experimental data at these conditions. Recently developed broad-scan, rapid-tuning external-cavity quantum cascade lasers (QCL) have enabled the application of shock tube facilities, commonly used to study high-temperature chemical kinetics, to the efficient acquisition of absorption spectra under short-duration shock-heated test gas conditions. Available shock tube facilities can produce temperatures from 500 to 10,000 K and pressures from 0.1 to 1000 atm with test time durations ranging from 500 μs to 50 ms. Uncertainties in the known thermodynamic conditions as low as ±1% can be achieved. Presently available laser systems enable the rapid acquisition ( < 10 ms) of approximately 300 cm⁻¹ wide spectral regions at any location within the QCL-accessible wavelength region of 3.6 -11.7 μm (850 - 2800 cm⁻¹). The resulting spectra are composed of discrete data points at a spectral interval ranging from 0.3 - 0.6 cm⁻¹ and an instrument broadening function defined by the laser linewidth ( ≤ 0.0033 cm⁻¹). Present efforts are focused on studying large polyatomic molecules (4+ atoms) dilute in a bath gas of argon under conditions for which dissociation is negligible and test time durations are favorable (T < 1600K and P < 5atm). The database currently includes ethylene, methanol, and ethanol with over a dozen more species measured and being prepared for inclusion soon. Database permanent URL: https://purl.stanford.edu/cy149sv5686