Chirped Pulse Spectrometer Operating at 200 GHz

The combination of electronic sources operating at high frequencies and modern microwave instrumentation has enabled the recent development of chirped-pulse spectrometers for the millimetre and THz bands. This type of instrument can operate at high resolution which is particularly suited to gas phase rotational spectroscopy. The construction of a chirped pulse spectrometer operating at 200 GHz is described in detail while attention is paid to the phase stability and the data accumulation over many cycles. Validation using carbonyl sulphide has allowed the detection limit of the instrument to be established as function of the accumulation. A large number of OCS transitions were identified using a 10 GHz chirped pulse and include the 6 most abundant isotopologues, the weakest line corresponding to the fundamental R(17) transition of 16 O 13 C 33 S with a line strength of 4.3 x 10-26 cm-1 /(molec.cm-2). The linearity of the system response for different degrees of data accumulation and transition line strength was confirmed over 4 orders of magnitudes. A simple analysis of the time domain data was demonstrated to provide the line broadening coefficient without the need for conversion by a Fourier transform. Finally, the pulse duration is discussed and optimal values are given for both Doppler limited and collisional regimes

Analysis of self-broadened pure rotational and rovibrational lines of methyl chloride at room temperature

International audienceRovibrational absorption spectra of methyl chloride in the spectral region between 2800 and 3200 cm 1 were recorded with a high-resolution Fourier spectrometer. A multispectrum fitting procedure was used to analyze 527 transitions of the ν1 band and to retrieve the self-broadening coefficients for various J- and K-values with an estimated accuracy around 8%. Pure rotational transitions of CH3Cl in the submillimeter/terahertz region (0.2-1.4 THz) were also investigated using two complementary techniques of frequency-multiplication and continuous-wave photomixing. 43 pure rotational self-broadening coefficients were extracted with the accuracy between 3 and 5%. The whole set of measured values was used to model the J- and K-rotational dependences of the self-broadening coefficients by second-order polynomials. In addition, semi-classical calculations were performed, based on the real symmetric-top geometry of the active molecule, an intermolecular potential model including not only the dominant electrostatic but also the short-range forces, as well as on an exact classical treatment of the relative translational motion of the colliding partners. Comparison of all experimental and theoretical results shows similar rotational dependences and no significant vibrational dependence, so that extrapolations to other spectral regions should be straightforward

Recent Developments of an Opto-Electronic THz Spectrometer for High-Resolution Spectroscopy

A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements

A THZ PHOTOMIXING SYNTHESIZER BASED ON A FIBER FREQUENCY COMB FOR HIGH RESOLUTION ROTATIONAL SPECTROSCOPY

Author Institution: Laboratoire de Physico-Chimie de l'Atmosphere; Universite du Littoral Cote d'Opale, 189A Ave. Maurice; Schumann, 59140 Dunkerque, FranceTo date the principal application for photomixing sources has been for high resolution spectroscopy of gases due to the large tuning range and spectral purity. New Developments of the Opto-Electronic THz Spectrometer have been performed in order to obtain a powerfull tool for High-Resolution Spectroscopy. The combination of two extended cavity laser diodes and fast charge carrier lifetime semiconductor materials has allowed a continuous-wave THz spectrometer to be constructed based on optical heterodyning. Unlike many THz sources, this instrument gives access to all frequencies in the range 0.3 to 3.5 THz with a resolution of 1 MHz. The main spectroscopic applications of this spectrometer were dedicated to line profile analysis of rotational transitions referenced in the spectroscopic databases., 2006, \textbf{239:} 182.}$^,$, 2008, \textbf{109:} 2857.} One limitation of the THz spectrometer was accuracy with which the generated frequency is known. Recently, this obstacle has been circled with the construction of a photomixing spectrometer where the two pump lasers are phase locked to two modes of a repetition rate stabilized frequency doubled fiber laser frequency comb., 2009, \textbf{17:} 22031.} In order to achieve a tuning range in excess to 100 MHz a third cw laser was required in the new configuration of the THz spectrometer. To assess the performances of this instrument, the frequencies of the pure rotational transitions of OCS molecules have been measured beetween 0,8 to 1,2 THz. A rms inferior to 100 kHz, deduced from the frequencies measured, demonstrates that the THz photomixing synthesizer is now able to be competitive with microwave and submillimeter techniques

CH 3 D photomixing spectroscopy up to 2.5 THz: New set of rotational and dipole parameters, first THz self-broadening measurements

International audienc

HIGH RESOLUTION SPECTROSCOPY USING A TUNABLE THZ SYNTHESIZER BASED ON PHOTOMIXING

Author Institution: Laboratoire de Physico-Chimie de l'Atmosphere; Universite du Littoral Cote d'Opale, 189A Ave. Maurice; Schumann, 59140 Dunkerque, FranceOptical heterodyning, also know as photomixing is an attractive solution as a single device able to cover the entire frequency range from 300 GHz to 3 THz. , 2008, \textbf{9:} 262-275.} %, 2009, \textbf{9:} 9039.} %However, As the THz frequency is extracted from the difference frequency of two lasers, the accuracy with which the generated frequency is known is directly determined by the frequency accuracy of the lasers. %Standard wavelength meters operating in the near infrared typically offer an accuracy of 50 MHz, or $\left(\frac{\Delta\nu}{\nu}\right)_{IR}\simeq 10^{-7}$ in the near infrared corresponding to $\left(\frac{\Delta\nu}{\nu}\right)_{THz}\simeq 5\times 10^{-5}$ for a beatnote frequency generated at 1 THz. In order to fully characterize the spectral fingerprint of a given molecule an accuracy approximately one order of magnitude finer than the Doppler linewidth is required, around 100 kHz for smaller polar compounds. To generate accurate cw-THz the frequency spacing of the modes of a Frequency Comb (FC) has been employed to constrain the emission frequency of a photomixing source., 2009, \textbf{17:} 22031.} Two phase locked loops are implemented coherently locking the two cw-lasers (CW1 and CW2) to different modes of the FC. Although this solution allows accurate generation of narrowband THz the continuous tuning of the frequency presents some obstacles. To overcome these difficulties a system architecture with a third cw-laser (CW3) phase locked to CW2 has been implemented. The beatnote between CW2 and CW3 is free from the FC modes therefore the PLL frequency can be freely scanned over its entire operating range, in our case around 200 MHz. The most of polar compounds may be studied at high resolution in the THz domain with this synthesizer. Three different examples of THz analysis with atmospherical and astrophysical interests will be presented: \begin{itemize} \item The ground and vibrationnally excited states of H$_2$CO revisited in the 0.5-2 THz frequency region \item The rotational dependences of the broadening coefficients of CH$_3$Cl studied at high J and K values \item The molecular discrimination of a complex mixture containing methanol and ethanol. \end{itemize

A COMPACT CW-THz PHOTOMIXING SPECTROMETER : HCN LINES STRENGTHS AND BROADENING COEFFICIENTS AT FREQUENCIES UP TO 3.26 THz

Author Institution: Laboratoire de Physico-Chimie de l'Atmosphere, UMR CNRS 8101, Universite du Littoral Cote d'Opale, 189A Av. Maurice Schumann,59140 Dunkerque, France; Laboratoire de Physique des Lasers, Atomes et Molecules,UMR CNRS 8523, Centre d'Etudes et Recherches Lasers et Applications, Universite de Lille 1,59655 Villeneuve d'Ascq Cedex, FranceA continuous-wave THz spectrometer has been constructed using a photomixing source to provide a spectral resolution of 3 MHz (0.0001 cm$^{-1}$) at all frequencies from 300 GHz to 3.3 THz (10 cm$^{-1}$ to 110 cm$^{-1}$). The source is composed of two extended cavity diode lasers, a tapered semiconductor optical amplifier and a photomixer element. The fine spectral resolution of this instrument is particularly suited to the analysis of molecules in the gas phase. In addition, the high species discrimination of the THz waveband promises much potential for the simultaneous detection of multiple industrial pollutants such as HCN, H$_2$S, H$_2$CO and C$_2$H$_3$Cl. The quantification of species requires reliable knowledge of transition line strengths and broadening coefficients. In this context the absorption profile of HCN has been measured in the presence of oxygen and nitrogen as gaseous perturbers for pressures up to 35 mbar. Rotational J$\leftarrow$J+1 transitions in the ground (0,0,0) state from 531 GHz to 3.26 THz (J=5 to J=36) were observed along with the upper degenerated (0,$1_2^1$,0) bending state from 712 to 1246 GHz (J=7 to J=13). The broadening coefficients show little difference between the fundamental and excited transitions hence no vibrational dependence is observed. The availability of oxygen and nitrogen broadening coefficients allows a comparison to be made with the air broadening coefficient contained in the HITRAN database and shows a significant difference for transitions J$\geq$29. This demonstrates the attractiveness of this technique to provide spectral information at frequencies from 2 to 3 THz where commercial interferometers and electronic sources are not a viable alternative

CONTINUOUS-WAVE TERAHERTZ SPECTROSCOPY APPLIED TO MAINSTREAM CIGARETTE SMOKE ANALYSIS

Author Institution: Laboratoire de Physico-Chimie de l'Atmosphere, CNRS UMR-8101,; Universite du Littoral Cote d'Opale, 189A Ave. Maurice; Schumann, 59140 Dunkerque, FranceThe analysis of the mainstream cigarette smoke is an appropriate situation of detection and quantification of chemical compounds in a hostile environment. The first application of a Continuous-Wave THz spectrometer using photomixing was used to provide detection and quantitative information in this kind of medium. The spectral signatures of hydrogen cyanide, carbon monoxide, formaldehyde and water have been detected and analysed. HCN and CO present a large series of pure rotational J lines in the 600 GHz - 2300 GHz frequency range. The average concentrations of HCN and CO have been evaluated to be 210 ppm and 1.7 $\%$ respectively by intensity measurements. These concentration have been validated by the measurement of calibrated gases. The trace gas detection potential of the CW-THz spectrometer has been demonstrated with the estimation of a detection limit of 7 ppm.m for HCN. Finally, pure rotational lines of H$_2$CO were observed with a very low pressure of cigarette smoke in the cell