Oxygen, nitrogen and air broadening of HCN spectral lines at terahertz frequenciesIntroductionExperimental procedureResults and data analysisTheoretical study of oxygen-broadening coefficientsAnalysis of isotropic potential parameterization and comparison with experimental dataAir-broadened half-widthsConclusionAcknowledgmentsReferencesOxygen, nitrogen and air broadening of HCN spectral lines atterahertz frequenciesChun Yanga, Jeanna Buldyrevab, Iouli E. Gordonc, Franc- ois Rohartd, Arnaud Cuisseta,Gae¨l Moureta, Robin Bocqueta, Francis Hindlea,aLaboratoire de Physico-Chimie de l’Atmosphe`re, UMR CNRS 8101, Universite´du Littoral Coˆte d’Opale, 189A Av. Maurice Schumann, 59140 Dunkerque, FrancebInstitut UTINAM, UMR CNRS 6213, Universite´de Franche-Comte, 16, Route de Gray, 25030 Besanc- on Cedex, FrancecHarvard-Smithsonian Center for Astrophysics, Atomic and Molecular Physics Division, 60 Garden Street, Cambridge, MA 02138-1516, USAdLaboratoire de Physique des Lasers, Atomes et Mole´cules, UMR CNRS 8523, Baˆtiment P5-135, Universite´de Lille 1, 59655 Villeneuve d’Ascq Cedex, Francearticle infoArticle history:Received 25 July 2008Received in revised form25 August 2008Accepted 26 August 2008Keywords:HCNOxygenNitrogenBroadeningTerahertzPhotomixingabstractThe room-temperature nitrogen- and oxygen-broadening coefficients of hydrogencyanide spectral lines have been measured in the 0.5–3 THz (17–100 cm1) frequencyrange (purely rotational transitions with 5pJp36) by a continuous-wave terahertzspectrometer based on a photomixing source. An improved version of the Robert andBonamy semiclassical formalism has been used to calculate the oxygen-broadeningcoefficients and resulted in a good agreement with these measurements. The nitrogenand oxygen data are combined to provide the air-broadening coefficients as used by theHITRAN database. A significant difference is observed between the measured andtabulated values for transitions with high values of the rotational quantum number.A new polynomial representation is suggested for inclusion in HITRAN. A similarpolynomial expression has been derived for the nitrogen broadening to aid the studiesof Titan’s atmosphere.& 2008 Elsevier Ltd. All rights reserved.1. IntroductionHydrogen cyanide (HCN) is an important gas in astrophysical and atmospheric research. It is present in comets,interstellar clouds and the atmospheres of some planets and moons such as Titan for example [1,2]. Therefore, an extensiveknowledge of its spectroscopic parameters is very important and they are tabulated in the major spectroscopic databasessuch as HITRAN [3]. The knowledge of HCN spectral parameters is also important for the star formation models as well asfor industrial pollution monitoring and burning of bio-mass. The HCN line broadening by various foreign gases is ofparticular interest. For example, the values of the foreign gas broadening parameters can be used to estimate temperaturesand concentrations of the gas in different astrophysical objects [4]. There have been extensive experimental studies of thesebroadening parameters in the infrared region for air and N2perturbation [5–8] and references therein. The terahertz (THz)region has been studied far less extensively and only the N2-broadening parameters have been measured using a far-infrared Fourier spectrometer [9]. The O2-broadening data in any spectral region are very limited. The purpose of theexperiments described in this paper is to extend the available oxygen-broadening information. The N2-broadenedcoefficients have also been measured allowing the air-broadening coefficients to be determined.Contents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/jqsrtJournal of Quantitative Spectroscopy &Radiative TransferARTICLE IN PRESS0022-4073/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.jqsrt.2008.08.005Corresponding author.E-mail address: [email protected] (F. Hindle).Journal of Quantitative Spectroscopy & Radiative Transfer 109 (2008) 2857–2868Computations of collisional broadening coefficients for HCN–N2system are currently available for (0,1, 0) band at room[10] and low [11] temperatures in the framework of the traditional Robert and Bonamy (RB) semiclassical approach withparabolic trajectories for the relative molecular motion [12]. Recently, the room-temperature coefficients had been alsoevaluated [13] by the improved version of the RB formalism [14] involving the exact solutions of the classical equations ofmotion [15] (RBE). In contrast, the oxygen-broadening coefficients for HCN (vib)rotational lines to our knowledge havenever been computed. We present therefore their theoretical values by the semiclassical RBE approach. Since for the case oftwo linear molecules this approach had been already discussed in detail elsewhere [14], in Section 4 we briefly present itskey features and main formulae relative to the linewidth calculation. The numerical results are given and compared withthe experimental data in Section 5. In Section 6, the air-broadening coefficients deduced from the measured data arecompared to existing infrared data and are used to determine a new polynomial function which is suggested for inclusionin HITRAN as it extends to higher rotational levels.2. Experimental procedureThe use of ultra-fast electronic components and optical heterodyning or ‘‘photomixing’’ can provide access tofrequencies from 0.1 to 3 THz (3.3–100 cm1). Photomixing [16] is a frequency down-conversion technique where twolasers close in frequency are used to create a beatnote at the target frequency. A semiconductor material with a suitablyshort carrier lifetime is employed to convert the beatnote into an electrical current which is subsequently radiated by anantenna. Photomixing sources have already demonstrated their utility for the spectroscopy of gases [17–20] along with thequantitative analysis of complex samples such as cigarette smoke containing many chemical species and a high degree ofaerosols [21].A continuous-wave terahertz (CW-THz) spectrometer was constructed using a photomixing radiation source, Fig. 1. Theinstrument can be divided into the following functional units: a dual-frequency optical source, the photomixer element, aTHz beam propagation path including the sample cell and a detector (bolometer). The optical source contains two (TopticaDL-100) extended cavity diode lasers