Methacrolein, CH₂=C(CH₃)CHO or MAC, is an important atmospheric molecule because it is a major product of the isoprene-OH reaction. Meanwhile the spectroscopic information on MAC is very scarse. ^{1 2} On the theoretical side, we have performed quantum calculations at different levels of theory (DFT and ab initio) to model the structure of the two conformers, the large amplitude motion associated with the methyl top, and the anharmonic vibrational structure. On the experimental side, we have at first characterized the millimeter-wave spectrum of MAC in the 150-465 GHz range using the Lille frequency multiplication chain spectrometer. In particular the ground state has been analyzed up to J, K_a = 37, 17 and the first excited states are currently investigated. Secondly, FTIR spectra have been recorded on the AILES beamline of SOLEIL using a long path cell, between 30 and 3500 cm⁻¹at medium resolution (0.5 cm⁻¹). A few bands of atmospheric interest have also been recorded at higher resolution (0.001 cm⁻¹). We will report the details of the vibrational analysis, as well as the molecular parameters derived from the analysis of the high resolution spectrum of the c-type band located around 930 cm⁻¹. Support from the French Laboratoire d'Excellence CaPPA (Chemical and Physical Properties of the Atmosphere) through contract ANR-10-LABX-0005 of the Programme d'Investissements d'Avenir is acknowledged. The experiment on the AILES beam-line of the synchrotron SOLEIL was performed under project number 20130192.
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¹M. Suzuki and K. Kozima, J. Molec. Spectrosc. 38 (1971) 314 ²J. R. Durig, J. Qiu, B. Dehoff and T. S. Little, Spectrochimica Acta 42A (1986) 89

Several monoterpenes are biogenic volatile organic compounds (BVOCS) present in the atmosphere. They can react with OH, O₃, NO_x, etc. to give rise to several oxydation and degradation products. We have studied the gas phase spectroscopy of monocyclic (limonene, γ-terpinene) and bicyclic (α-pinene, β-pinene) atmospheric monoterpenes (C₁₀H₁₆), as well as two C₁₀H₁₄O oxydation products (perillaldehyde, carvone). In the first step of this work, theoretical calculations and microwave spectroscopy were used in order to evidence the most stable conformers and their relative energies. ^{1 2 3} In the present communication we will present the results of the IR study. Infrared spectra have been recorded on the FTIR spectrometer of the AILES beamline at synchrotron SOLEIL, using the Jet-AILES molecular beam and a long path cell. Special attention was given to the 700-1300 cm⁻¹ atmospheric window, to the CH vibration region, and to the "finger print" FIR region. Quantum calculations have been performed at different levels of theory (DFT, ab initio). In particular anharmonic force fields were obtained in order to model the vibrational structures. Support from the French Laboratoire d'Excellence CaPPA (Chemical and Physical Properties of the Atmosphere) through contract ANR-10-LABX-0005 of the Programme d'Investissements d'Avenir is acknowledged. The experiment on the AILES beam-line at synchrotron SOLEIL was performed under project number 20130192.
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¹J. R. Aviles Moreno, F. Partal Urena, J. J. Lopez Gonzalez and T. R. Huet, C. Phys. Lett. 473 (2009) 17 ²J. R. Aviles Moreno, T. R. Huet, F. Partal Urena, J. J. Lopez Gonzalez, Struc. Chem. 24 (2013) 1163 ³J. R. Aviles Moreno, E. Neeman, T. R. Huet, manuscript in preparation

Rotationally resolved direct absorption spectra in the OH stretching mode were recorded using difference frequency generation infrared spectrometer and slit-jet supersonic discharge expansion source. The spectra are really complicated because of the following reasons: a) CH₂OH was generated by discharging the expansion mixed gas CH₃OH and Cl₂ in carrier gas 70% Ne and 30% He. Large signals for CH₃OH are overlapped with that for CH₂OH in the OH stretching mode. b) The large amplitude of COH torsion for CH₂OH splits each vibration state into two states labeled as "+" and "-". The "+" state has energy levels with a 3:1 nuclear spin statistic ratio for K_a=0⁺/K_a=1⁺. The "-" state has K_a=0⁻/K_a=1⁻ levels with 1:3 nuclear spin statistics. c) The OH stretch fundamental is a hybrid a/b-type band. Both a-type and b-type bands were observed. As a result of the low rotational temperature and sub-Doppler linewiths, the spin-rotation structure was also resolved for b-type transitions. The spectra were assigned and fit to a Watson A-reduced symmetric top Hamiltonian to improve the accuracy of rotational parameters for the ground vibration state and determine the rotational parameters for the OH stretch state. The spin rotational constants and the tunneling splittings for both ground and OH stretch state were also determined.

We performed rotational analyses of previously reported¹ vibronic bands belonging to ~B²A′← ~X²A′ and ~B²A′← ~A²A" electronic transitions of isopropoxy radical. It is noted that certain vibronic bands belonging to both ~B²A′← ~X²A′ and ~B²A′← ~A²A" electronic transitions exhibit unusual rotational contours inconsistent with the electronic symmetry designation of the connecting levels, and the orientation of the electronic transition dipole moment with respect to the principal axis system of the molecule is inconsistent with expectations from the molecule's electronic structure. A coupled, three-electronic-state, vibronic Hamiltonian has been used to account for vibronic interactions between the ~B electronic state and the ~X/ ~A states, whereas an effective rotational Hamiltonian developed earlier² has been used to describe the rovibronic eigenstates within the ~X²A′ and ~A² A" twofold. We show that inclusion of the vibronic coupling of the ground twofold to the upper electronic ~B state is necessary to account for the observed rotational structure anomalies and present molecular parameters resulting from the rotational analysis of the vibronic spectra.
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¹R. Chhantyal-Pun and T. A. Miller, TD03, 68^th Molecular Spectroscopy Symposium, Columbus, 2013 ²J. Liu, D. Melnik and T. A. Miller, J. Chem. Phys., 139, 094308, (2013)

An estimated 10,000 to 100,000 different compounds have been measured in the atmosphere, each one undergoes many oxidation reactions that may or may not degrade air quality. To date, the fate of even some of the most abundant hydrocarbons in the atmosphere is poorly understood. One difficulty is the detection of atmospheric oxidation products that are very labile and decompose during analysis. To study labile species under atmospheric conditions, a highly sensitive, non-destructive technique is needed. Here we describe a near-IR incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) setup that we are developing to meet this end. We have chosen to utilize the near-IR, where vibrational overtone absorptions are observed, due to the clean spectral windows and better spectral separation of absorption features. In one spectral window we can simultaneously and continuously monitor the composition of alcohols, hydroperoxides, and carboxylic acids in an air mass. In addition, we have used our CEAS setup to detect organoamines. The long effective path length of CEAS allows for low detection limits, even of the overtone absorption features, at ppb and ppt levels.

Fluorine- and chlorine-containing molecules in the atmosphere are very strong greenhouse gases, meaning that even small amounts of these gases contribute significantly to the radiative forcing of climate. Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are regulated by the 1987 Montreal Protocol because they deplete the ozone layer. Hydrofluorocarbons (HFCs), which do not deplete the ozone layer and are not regulated by the Montreal Protocol, have been introduced as replacements for CFCs and HCFCs. HFCs have global-warming potentials many times greater than carbon dioxide, and are increasing in the atmosphere at a very fast rate. Various satellite instruments monitor many of these molecules by detecting infrared radiation that has passed through the Earth's atmosphere. However, the quantification of their atmospheric abundances crucially requires accurate quantitative infrared spectroscopy. This talk will focus on new and improved laboratory spectroscopic measurements for a number of important halogenated species.

Accurate modeling of atmospheric trace gases requires detailed knowledge of spectroscopic line parameters at temperatures and pressures relevant to the atmospheric layers where the spectroscopic signatures form. Pressure-broadened line shapes, frequency shifts, and their temperature dependences, are critical spectroscopic parameters that limit the accuracy of state-of-the-art atmospheric remote sensing. In order to provide temperature dependent parameters from controlled laboratory experiments, a 20.946 ± 0.001 m long path Herriott cell and associated transfer optics were designed and fabricated at Connecticut College to operate in the near infrared using a Bruker 125 HR Fourier transform spectrometer. The cell body and gold coated mirrors are fabricated with Oxygen-Free High Conductivity (OFHC) copper. Transfer optics are through-put matched for entrance apertures smaller than 2 mm. A closed-cycle Helium refrigerator cools the cell and cryopumps the surrounding vacuum box. This new system and its transfer optics are fully evacuated to ~ 10 mTorr (similar to the pressure inside the interferometer). Over a period of several months, this system has maintained extremely good stability in recording spectra at gas sample temperatures between 75 and 250 K. The absorption path length and cell temperatures are validated using CO spectra. The characterization of the Herriott cell is described along with its performance and future applications.^{1 2}
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¹We thank Drs. V. Malathy Devi and D. Chris Benner at The College of William and Mary for helpful discussion. ²Research described in this paper was performed at Connecticut College, the Jet Propulsion Laboratory, California Institute of Technology, and NASA Langley Research Center, under contracts and cooperative agreements with the National Aeronautics and Space Administration.

Nearly 1000 line intensities of ¹⁶O¹²C¹⁷O between 4604 and 5126 cm⁻¹were measured using an isotopically-enriched mixture sample having 40 % (determined by mass spectrometry). Spectra were recorded at 0.0056 cm⁻¹resolution with a Fourier transform spectrometer (Bruker IFS-125HR at JPL) configured to a Herriott cell with a 20.946 m absorption path. Since collisional narrowing effects were observed, the Rautian profile was systematically applied (instead of the Voigt profile) using a multispectrum retrieval procedure. Transition dipole moments and Herman-Wallis factors were derived for 15 bands, and a global comparison with theoretical calculations and predictions was obtained. Accuracies for the line intensities ranged between 2 – 3 % for strong bands and 6 – 30 % for weak bands. Retrieved line positions were calibrated using CO, HCl and some well-known ¹⁶O¹²C¹⁶O transitions. For both measured and calculated line positions, the accuracies fell between 0.1 – 1×10⁻³ cm⁻¹. Self-broadening was also obtained for a few bands. Complete line lists were generated to support atmospheric remote sensing of the Earth (e.g., OCO-2 mission), Mars and Venus.¹
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¹Research described in this paper was performed at Connecticut College, the Jet Propulsion Laboratory, and California Institute of Technology, and NASA Langley Research Center, under contracts and cooperative agreements with the National Aeronautics and Space Administration.

The optical frequency comb provided in the output of a femtosecond, mode-locked laser has been employed for many applications, including broadband spectroscopic measurements of trace gases using a variety of detection techniques. One environmentally significant trace gas is CO₂, which has characteristic absorption bands near 1.6 μm and 2.0 μm. Continuous wave (cw) lasers have typically been used to measure CO₂ at atmospheric-level concentrations. However, a broadband frequency comb source can provide rapid, simultaneous and accurate measurements of multiple transitions without the need for mechanical scanning or frequency tuning. Previously, precision broadband spectroscopy was performed on CO₂ bands near 1.6 μm. However, the CO₂ absorption bands near 2 μm have nearly a ten-fold increase in line strength compared to the bands near 1.6 μm, making the 2 μm bands attractive candidates for precision measurements of CO₂ with improved signal-to-noise and reduced uncertainty. Here, broadband quantitative spectroscopy of CO₂ bands near 2 μm is pursued. The source that was developed consists of an Er:fiber oscillator, Er:doped fiber amplifier, and highly nonlinear optical fiber, which generates a broadband spectrum spanning from 1 to 2.2 μm with an average power of 270 mW. Over 70 mW of the optical power is contained in the 1.8-2.2 μm region relevant to the CO₂ measurement. After generation, the laser light is passed through laboratory gas cells or open air where the absorption features from the sample gas are imprinted onto the laser light. Initial detection efforts involve a virtually imaged phased array- (VIPA-)based spectrometer whose output is subsequently imaged on a InSb array detector. The bandwidth of the measured spectrum is 50 nm, limited by the size of the detector array. The characteristics of the spectrometer, including the detection limits and temporal resolution, will be presented. In addition, the progress towards the use of the present spectrometer and related frequency comb technology for quantitative measurement of CO₂ on a 2 km open-air path on the NIST-Boulder campus will be discussed.

The proliferation and increased abilities of remote sensing missions for the monitoring of planetary atmospheric gas species has spurred the need for complete and accurate spectroscopic reference standards. As a part of our ongoing effort toward creating a global carbon dioxide (CO₂) frequency reference standard, we report new FTS measurements of the ¹⁷O enriched isotopologues of CO₂. The first measurements were taken in the ν₃ region (2200 - 2450 cm⁻¹,  65 - 75 THz), have absolute calibration accuracies of 100 kHz (3E-6 cm⁻¹), comparable to the uncertainties for typical sub-millimeter/THz spectroscopy. Such high absolute calibration accuracy has become regular procedure for the cases of linear molecules such as CO₂ and CO for FTS measurements at JPL, and enables us to produce measured transition frequencies for entire bands with accuracies that rival those of early heterodyne measurements for individual beat notes. Additionally, by acquiring spectra of multiple carbon dioxide isotopologues simultaneously, we have begun to construct a self-consistent frequency grid based on CO₂ that extends from 20 - 200 THz. These new spectroscopic reference standards are a significant step towards minimizing CO₂ retrieval errors from remote sensing applications, especially those involving targets with predominantly CO₂ atmospheres such as Mars, Venus and candidate terrestrial exoplanets where minor isotopologues will make significant contributions to the radiance signals.

An empirical line list has been constructed for “natural” water vapor at 296 K in the 5850 – 7920 cm⁻¹region. It was obtained by gathering separate line lists recently published on the basis of spectra recorded by high sensitivity Continuous Wave Cavity Ring Down Spectroscopy (CW-CRDS) of natural water, complemented with literature data for the strongest lines. The list includes 38318 transitions of four major water isotopologues (H₂¹⁶O, H₂¹⁸O, H₂¹⁷O and HD¹⁶O) with an intensity cut-off of 1x10⁻²⁹ cm/molecule at 296 K. The list is made mostly complete over the whole spectral region by including a large number of weak lines with positions calculated using experimentally determined energy levels and intensities obtained from variational calculations. In addition, we provide HD¹⁸O and HD¹⁷O lists in the same region for transitions with intensities larger than 1x10⁻²⁹ cm/molecule. The HD¹⁸O and HD¹⁷O lists (1972 lines in total) were obtained using empirical energy levels available in the literature and variational intensities. The global list (40290 transitions) for water including the contribution of the six major isotopologues will be adopted for the next edition of the GEISA database in the region. The advantages and drawbacks of our list are discussed in comparison with the list provided for the same region in the 2012 edition of the HITRAN database. The direct comparison of the CRDS spectra to simulations based on the HITRAN list has revealed some insufficiencies which could easily be corrected: missing HDO lines, duplicated lines, inaccurate line positions or line intensities from variational calculations.

Since its discovery one century ago, a deep and unresolved controversy remains on the nature of the water vapor continuum. Several interpretations are proposed: accumulated effect of the distant wings of many individual spectral lines, metastable or true bound water dimers, collision-induced absorption. The atmospheric science community has largely sidestepped this controversy, and has adopted a pragmatic approach: most radiative transfer codes used in climate modelling, numerical weather prediction and remote sensing use the MT_CKD model which is a semi-empirical formulation of the continuum¹. The MT_CKD cross-sections were tuned to available observations in the mid-infrared but in the absence of experimental constraints, the extrapolated near infrared (NIR) values are much more hazardous. Due to the weakness of the broadband absorption signal to be measured, very few measurements of the water vapor continuum are available in the NIR windows especially for temperature conditions relevant for our atmosphere. This is in particular the case for the 1.6 μm window where the very few available measurements show a large disagreement. Here we present the first measurements of the water vapor self-continuum cross-sections in the 1.6 μm window² by cavity ring down spectroscopy (CRDS). The pressure dependence of the absorption continuum was investigated during pressure cycles up to 12 Torr for selected wavenumber values. The continuum level is observed to deviate from the expected quadratic dependence with pressure. This deviation is interpreted as due to a significant contribution of water adsorbed on the super mirrors to the cavity loss rate. The pressure dependence is well reproduced by a second order polynomial. We interpret the linear and quadratic terms as the adsorbed water and vapour water contribution, respectively. The derived self-continuum cross sections, measured between 5875 and 6450 cm⁻¹, shows a minimum value around 6300 cm⁻¹. These cross sections will be compared to the existing experimental data and models, especially to recent FTS measurements and to the last version of the MT_CKD 2.5 model.
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¹Mlawer, E.J., V.H. Payne, J.L. Moncet, et al. (2012), Phil. Trans. R. Soc. A, 370, 2520–2556. ²Mondelain, D., A. Aradj, S. Kassi, et al. (2013), JQSRT, 130, 381–391.

Due to its importance for atmospheric and astrophysical science, a spectroscopic investigation of the rotational spectrum of the singly deuterated water molecule (HDO) was performed, spanning a large frequency range: from the millimeter-wave region up to the THz frequency domain. Sub-Doppler resolution was obtained by exploiting the Lamb-dip technique, thus allowing us to resolve the hyperfine structure of the rotational lines due to deuterium and hydrogen. The experimental determination of the hyperfine parameters involved was supported by highly accurate quantum-chemical calculations. On the whole, the present measurements allowed us to carry out an accurate spectroscopic characterization of HDO.

The isotopically invariant Dunham fit of O₂ was updated with newly reported literature transitions to derive (1) the energy levels, band-by-band molecular constants and RKR potentials for the X³Σ⁻_g, a¹∆_g and b¹Σ⁺_g states of the six O₂ isotopologues, ¹⁶O¹⁶O, ¹⁶O¹⁷O, ¹⁶O¹⁸O, ¹⁷O¹⁷O, ¹⁷O¹⁸O and ¹⁸O¹⁸O; (2) the line positions and Franck-Condon factors for their a¹∆_g−X³Σ⁻_g, b¹Σ⁺_g−X³Σ⁻_g and a¹∆_g−b¹Σ⁺_g band systems. The best available experimental and theoretical data were used as input for calculating intensities of lines involving the X³Σ⁻_g, a¹∆_g and b¹Σ⁺_g states. The newly calculated positions and intensities are combined to provide HITRAN-format linelists.

The oxygen A band is used for numerous atmospheric experiments, but spectral line parameters that sufficiently describe the spectrum to the level required by OCO2 and other high precision/accuracy experiments are lacking. Fourier transform spectra from the Jet Propulsion Laboratory and cavity ring down spectra from the National Institute of Standards and Technology were fitted simultaneously using the William and Mary multispectrum nonlinear least squares fitting technique¹ into a single solution including the entire band. In addition, photoacoustic spectra already available from the California Institute of Technology will be added to the solution. The three types of spectrometers are complementary allowing the strengths of each to fill in the weaknesses of the others. With this technique line positions, intensities, widths, shifts, line mixing, Dicke narrowing, temperature dependences and collision induced absorption have been obtained in a single physically consistent fit.²
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¹D. Chris Benner, C. P. Rinsland, V. M. Devi, M. A. H. Smith, and D. Atkins, JQSRT 1995;53:705-21. ²Part of the research described in this paper was performed at The College of William and Mary, the, Jet Propulsion Laboratory, California Institute of Technology, under contracts and cooperative agreements with the National Aeronautics and Space Administration and the Jet Propulsion Laboratory. Support for the National Institute of Standards and Technology was provided by the NIST Greenhouse Gas Measurements and Climate Research Program and a NIST Innovations in Measurement Science (IMS) award.

Accurate knowledge of spectral line shape parameters is important for infrared transmission and radiance calculations in the terrestrial atmosphere. We report the self- and air-broadened Lorentz widths, shifts and line mixing coefficients along with their temperature dependences for methane absorption lines in the 2.2 μm spectral region. For this, we obtained a series of high-resolution, high S/N spectra of 99.99% ¹²C-enriched samples of pure methane and its dilute mixtures in dry air at cold temperatures down to 150 K using the Bruker IFS 125HR Fourier transform spectrometer at JPL. The coolable absorption cell had an optical path of 20.38 cm and was specially built to reside inside the sample compartment of the Bruker FTS¹. The 13 spectra used in the analysis consisted of seven pure ¹²CH₄ spectra at pressures from 4.5 to 169 Torr and six air-broadened spectra with total sample pressures of 113-300 Torr and methane volume mixing ratios between 4 and 9.7%. These 13 spectra were fit simultaneously using the multispectrum least-squares fitting technique². The results will be compared to existing values reported in the literature.³
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¹K. Sung, A. W. Mantz, L. R. Brown, et al., J. Mol. Spectrosc. 162 (2010) 124-134. ²D. C. Benner, C. P. Rinsland, V. Malathy Devi, M. A. H. Smith and D. Atkins, JQSRT 53 (1995) 705-721. ³Research described in this paper was performed at Connecticut College, the College of William and Mary, NASA Langley Research Center and the Jet Propulsion Laboratory, California Institute of Technology, under contracts and cooperative agreements with the National Aeronautics and Space Administration.