Polarization spectroscopy is a pump-probe technique that utilizes a change in the polarization of the probe beam, due to anisotropic Zeeman state populations created by the pump beam, in order to investigate the line shapes and line strengths of a target molecule. We have developed a two-color laser-induced polarization spectroscopy (TCPS) technique, with independent spectral tuning of the pump and probe beams, for investigating collision induced resonances (CIRs) in nitric oxide (NO). We distinguish CIRs from natural resonances (NRs), which occur when the transitions excited by pump and probe beams share a common upper and/or lower level. CIRs occur because of state-to-state rotational transfer during which the Zeeman state anisotropy is preserved to some extent. In these NO TCPS studies, a circularly-polarized pump beam is tuned to create a Zeeman state anisotropy in the A²Σ⁺-X²Π (0-0) band of NO molecules. A weak probe beam is then scanned over a range of transitions close to the pump frequency in order to probe NRs and CIRs. Gas mixtures of 1% NO in the buffer gases N₂ (Fig. below), He, and Ar were selected for the experiments to investigate the dependence of CIR on collision species. The strongest CIRs were observed in the He buffer gas and the weakest CIRs were observed in the Ar buffer gas. The TCPS technique may also be used for species selective and transition selective NO measurements. l0pt Figure

Characterizing and modelling the atmospheric CO₂ with a sub-percent accuracy necessitates high-quality spectroscopic lineshape parameters. For this goal, we collected the best experimentally and theoretically measured lineshape coefficients of CO₂ lines broadened by air and CO₂ to create sets of semi-empirical models for updating all the transitions of the HITRAN database [1]. Based on the available data, we estimated the air- and CO₂- broadening coefficients, their associated temperature exponents, and the speed dependence of the broadening together with its temperature dependence for every transition in HITRAN. Furthermore, the semi-empirical approach proposed by Hartmann 2009 [2] (trained by most reliable experimental data) is used to calculate the line shifts of air- and self- broadened CO₂ absorption lines. The updated data will be provided on www.hitran.org and will be used in the next edition of HITRAN. It is notable that thanks to the relational structure of the HITRAN database we provide separate consistent and complete sets of parameters in Voigt and speed-dependent Voigt parametrizations. Finally, the updated lineshape parameters are used for calculating the first-order line mixing using the program developed by Lamouroux et al. [3] which will also be provided in HITRAN 2020. For verifying the semi-empirically calculated lineshape parameters, the laboratory spectra measured by Dr. Keeyoon Sung at the Jet Propulsion Laboratory (JPL) is used to compare with the calculation of the absorption coefficient by HITRAN Application Programming Interface (HAPI) [4] which is now equipped with necessary functionality. [1] JQSRT, 203 (2017) 3-69. [2]JQSRT, 110 (18) (2009) 2019-2026. [3] JQSRT, 151 (2015) 88-96. [4] JQSRT, 177 (2016) 15-30.

l0pt Figure In 2016, Iwakuni et al. measured the ν₁ + ν₃ vibrational combination band of acetylene using a dual-frequency comb based spectrometer. They reported an alternation in the self-pressure broadening coefficients of even and odd rotational levels, which correspond to the para- and ortho- nuclear spin states. This can occur if relaxation involving resonant energy transfer between molecules with common nuclear spin symmetry is important, because ortho-ortho collisions are statistically more probable than para-para ones. Subsequently several authors have disputed these findings, however there have been no experimental results to investigate the issues. At last year's ISMS meeting, we presented some preliminary findings. The work has been considerably extended and refined using a frequency-comb stabilized laser, and no experimental evidence was found for this alternation in the R(8)−R(13) lines of the band, which reportedly showed the largest effects. Careful accounting of weak background absorptions due to hot-band and lower abundance isotopomer lines was included. The apparent alternation observed by Iwakuni et al. was due to the use of a Voigt profile function in fits to transmission representation data with very large peak absorption. Further low temperature measurements are planned to try to limit influence of the hot-band lines. Acknowledgement: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences, under Award Number DE-SC0018950.

The study and modelling of the N₂-perturbed lines in O₂ is of great importance for Earth's atmosphere studies. However, collisional systems with the ground-state molecular oxygen, O₂ (X³Σ⁻_g), either as perturbing or active molecule, are difficult to handle because of its non-zero spin that needs to be taken into account in the description of quantum scattering. Here, we present the methodology as well as the results of our quantum scattering calculations and the line-shape parameters for the O₂ immersed in molecular nitrogen bath. It is the first theoretical ab initio investigation of this collisional system in the context of the shapes of molecular lines. The PES for this study was constructed automatically using the AUTOSURF code Quintas-Sánchez, Ernesto, and Richard Dawes. ÄUTOSURF: A freely available program to construct potential energy surfaces." Journal of chemical information and modeling 59, no. 1 (2018): 262-271 The data provided through this investigation is important for the terrestrial atmospheric measurements and can be used for populating the spectroscopic databases such as HITRAN or GEISA.

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Footnotes:

Quintas-Sánchez, Ernesto, and Richard Dawes. ÄUTOSURF: A freely available program to construct potential energy surfaces." Journal of chemical information and modeling 59, no. 1 (2018): 262-271.

Line-by-line spectroscopic databases of CO₂ such as those in HITRAN include so many transitions and bands that it is intractable to provide reference data for every transition and relevant collisional partner. To address this dilemma, theoretical line parameters enabling widespread coverage need to be validated in terms of accurate spectroscopic measurements having well-established uncertainties. In this work, we present frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS) measurements of CO₂ (30012) ← (00001) band transitions up to J" = 50. These data correspond to foreign broadening by four gas mixtures: air, N₂, and two Ar-enriched synthetic air samples, with each sample containing CO₂ at a mole fraction near atmospheric levels. The measured O₂, N₂, and Ar foreign broadening parameters were determined using a NIST-developed multi-spectrum fitting algorithm which used appropriate constraints for pressure, temperature, and sample composition. These experimental results are compared to theoretical semiclassical half-widths¹ as well as to literature values. 1. R. R. Gamache, J. Lamouroux, A. L. Laraia, J.-M. Hartmann and C. Boulet, Journal of Quantitative Spectroscopy and Radiative Transfer 113 (11), 976-990 (2012).

We present fully ab initio calculations of rovibrational line shapes of carbon monoxide perturbed by argon. The quantum mechanical scattering problem between the CO molecule and the Ar atom is solved numerically for an ab initio interaction potential. We use the generalized Hess method to determine the spectroscopic cross sections which describe the effect of collisions on a spectral lineL. Monchick and L. W. Hunter, JCP 85, 713 (1986); G. Kowzan, P. Wcisło, M. Słowiński, P. Masłowski, A. Viel, F. Thibault, JQSRT 243, 106803 (2020). Using these cross sections, we determine line-shape parameters of the Hartmann-Tran profile and the speed-dependent billiard ball profile and generate the line profiles at the experimental conditions of the reference measurements. We compare the generated line shapes with high-quality experimental line profilesG. Kowzan, K. Stec, M. Zaborowski, S. Wójtewicz, A. Cygan, D. Lisak, P. Masowski, and R. S. Trawi\'nski, JQSRT 191, 46 (2017).btained at five pressures between 0.01 and 1 atm and obtain sub-percent agreement in the whole pressure range. Calculations on the P(9) line are used to test the accuracy of a second Ar-CO interaction potential. The discrepancies between the results for both potentials and the experimental data are explained within the presented theoretical framework. Based on these results and quantum mechanical calculations of mass diffusion cross sections, we clarify the relations between the frequency of velocity-changing collisions and the complex Dicke parameter. L. Monchick and L. W. Hunter, JCP 85, 713 (1986); G. Kowzan, P. Wcisło, M. Słowiński, P. Masłowski, A. Viel, F. Thibault, JQSRT 243, 106803 (2020).. G. Kowzan, K. Stec, M. Zaborowski, S. Wójtewicz, A. Cygan, D. Lisak, P. Masowski, and R. S. Trawi\'nski, JQSRT 191, 46 (2017).o

New measurements of the a¹∆_g ← X³Σ_g⁻ band of oxygen (¹⁶O₂) at 1.27 μm will be presented, improving on a previous study [Mendonca et al., Atmos. Meas. Tech. 12, 35-50 (2019)]. Spectra were acquired by frequency-stabilized cavity ring-down spectroscopy over a 160 cm⁻¹ wave number range (7792 cm⁻¹ to 7952 cm⁻¹). The frequency axis was anchored to a Cs-clock-referenced optical frequency comb through a heterodyne beat note between the comb and the probe laser at about 20 points across the wave number range. The probe laser was phase locked to the frequency comb prior to measuring the beat note frequency in order to improve the accuracy, yielding a 10-Hz uncertainty in the ring-down cavity free spectral range and a 30-kHz absolute frequency uncertainty for all mode orders. Six air-broadened spectra were recorded, at pressures ranging from 3.3 kPa to 100 kPa. They were analyzed with custom multi-spectrum fitting software based on the HAPI python library, using the speed-dependent Nelkin-Ghatak profile. The resulting line shape parameters reveal important discrepancies with the HITRAN2016 values. These results will also be compared to those reported in recent studies by the Grenoble group [Konefał et al., JQSRT 241, 106653 (2020) and Tran et al., JQSRT 240, 106673 (2020)].

The simple structure of molecular hydrogen isotopologues provides a great opportunity for testing the ab initio quantum scattering theory on the ultra-accurate experimental spectra and analysis of their spectral lines plays a crucial role in studies of atmospheres of gas giants. We present the results of ab initio calculations of collisional cross sections and line-shape parameters for the simple benchmark systems of D₂ perturbed by He and H₂. In our analysis, we use the state-of-the-art statistical model of the collision-perturbed molecular lineshapes and we obtain all the parameters for this model from quantum scattering calculations with use of the highly accurate ab initio potential energy surface.

Light propagation in a passively decaying optical resonator occurs exclusively at the cavity resonance frequencies because only these fields satisfy the source-free wave equation. This mechanism occurs regardless of the frequency spectrum of the light source. Likewise, after switching on a light source to build up power within a resonator, transient fields at the cavity resonance frequencies are induced which prevent the instantaneous buildup of intra-cavity circulating power. For the case of mode-matched excitation by a single-frequency laser, the transient response of the system involves fields at the incident laser frequency and the cavity resonance frequency. To illustrate, we show that a frequency-agile probe laser which maintains phase lock to an optical resonator during both decay and buildup events yields exponentially damped heterodyne beat signals between the incident laser field and the local cavity resonance. These signals have a damping rate equal to one half the conventional ring-down value and occur at the frequency difference of the source laser and resonance fields. This technique enables the simultaneous measurement of absorption (decay rate) and dispersion (mode shifting) associated with light-matter interaction in a ring-down cavity. We illustrate how these complementary data can be used to measure complex-valued line shapes and validate associated line profiles.

Optical amplifiers that rely on free-free molecular transition for both pumping and amplification will be introduced. A gaseous medium comprised of Cs vapor and Ar is photoexcited through a free-free molecular transition, B ²Σ_1/2⁺ ← X ²Σ_1/2⁺, with a pulse duration of 8 ns. The gain medium is characterized by scanning the probe wavelength and changing the time delay between the pump and probe pulses. It will be shown that the large bandwidth ( > 150 GHz) and reduced lifetime ( ∼ 5 ns) make the amplifiers more suitable for high power IR sources. The optical-to-optical efficiency exceeds 28% in a single pass amplification. The circular polarization of the optical pump increases the efficiency by 20%. Furthermore, the weak atomic interaction was identified in the gain spectrum of a Cs-Xe gas mixture by implementing the pump-probe method and compared to the simulation based on the Franck-Condon principle. The spectral feature, also known as the red satellite, had been hard to be resolved experimentally because of the wing structure of A²Π_3/2⁺ ← X ²Σ_1/2⁺ transition. In the simulation, the Franck-Condon overlap integral develops in the region as the phases of the wavefunctions are stationary to each other, which shows that even a shallow feature in the potential curve can contribute the resulting spectrum significantly. This technique can be utilized in precise atomic spectroscopy.