1. Champaign-Urbana June 22-26, 2015 International Symposium of Molecular Spectroscopy 70 th Meeting Abdelaziz OMAR, Arnaud CUISSET, SOPHIE ELIET, Francis HINDLE, Robin BOCQUET and Gaël MOURET Laboratoire de Physico-Chimie de l'Atmosphère, EA-CNRS 4493, Université du Littoral Côte d'Opale, 189A Av. Maurice Schumann, 59140 Dunkerque, France

2. H 2 CO Photolysis Motivation CH 3 CHO Photolysis Conclusion & Prospects 70 th ISMS FB01 Arnaud Cuisset

3. THz gas phase spectroscopy: an attractive approach for trace gas detection, identification and quantification of polar molecules (high selectivity, straightforward quantification, versatility of electronic sources…) « Submillimeter spectroscopy for chemical analysis » Medvedev & al., Opt. Lett., 35, 10, (2010) « CP-THz spectroscopy for broadband trace gas sensing » Gerecht & al., Opt. Expr., 19, 8973, (2011) « Versatile sub-THz spectrometer for trace gas analysis », Mouret & al., IEEE sensors, 13, (2013) Potentialities of THz spectroscopy in chemical reactivity: from multi- components time-resolved quantification to heterogen chemistry studies « Millimeter & Submillimeter –wave kinetic spectroscopy of reaction intermediates » Endo & al., Laser Chem., 7, 61, (1987) « Chirped-pulse millimeter-wave spectroscopy for dynamics and kinetics studies of pyrolysis reactions» Prozument & al., Phys. Chem. Chem. Phys., 16, 15739, (2014) « Rotational spectral studies of O( 1 D) insertion reactions with methane and ethylene: Methanol and vinyl alcohol in a supersonic expansion », Hays & al., Chem. Phys. Lett., 630, 18, (2015) H2CO Photolysis CH3CHO Photolysis Conclusion & prospects 70 th ISMS meeting Atmos. Env. 36, 5947- 5052(2002) THz spectroscopy should be able to answer ?

4. Sub-THz study of the formaldehyde photolysis H 2 CO is one of the most abundant pollutant in the inner atmosphere and a key compound for the OH production in the troposphere 250-360 nm Radical way (<330nm) Molecular way (<370nm) Monitoring in real time the photolysis products with a good sensitivity Determine the kinetic rates of both reactions H2CO Photolysis CH3CHO Photolysis Conclusion & prospects

5. Simple cell: Quartz cell 135 cm in length 50 mm diameter 600-900 GHz continuously tunable < 100 kHz resolution 10 μW power level AM & FM detection LS-1000 Solar Simulator: Xenon arc lamp (1000W) AM 0 (sun @ zenith) & AM 1.5 (sun @ earth surface) filters Paraformaldehyde heating at 90°C Typical working pressure: 30 μbar Schottky diode detector (VDI, WR 1.2 600-900 GHz)Motivation CH3CHO Photolysis Conclusion & prospects

6. The measured data contain strong baseline variations that are easily and reliably removed by polynomial fitting of the measured baseline at frequencies where no molecular absorption is expected. CO formation induced by H 2 CO photolysis a single measurement cycle providing one H 2 CO and CO absorption profile every 37 seconds. Two isolated rotational transitions of H 2 CO and CO were probed in the spectral range of 600- 900 GHz to determine the molecular concentrations as a function of the photolysis duration. An automatic data acquisition program was written to continuously measure each absorption line.Motivation CH3CHO Photolysis Conclusion & prospects

7. integrated intensity Gaussian fit of Doppler limited rotational transitions Time profiles of [CO] & [H 2 CO] concentrations for the 2 filters: AM 0 (blue: sun @ zenith) & AM 1,5 (red: sun @ earth surface) (cm -1 )Motivation CH3CHO Photolysis Conclusion & prospects

8. Although the HCO concentration is not directly probed, the value of K 1 is determined. Kinetic ratePathwayAM 0 (×10 -5 s -1 )AM 1,5 (×10 -5 s -1 ) Reference data: Röth et al. Atmos Chem Phys Discuss, 15 (5), p. 7239, (2015) K1K1 radical6.2 ± 0.40.8 ± 0.40.8 < K 1 < 11 K2K2 molecular2.2 ± 0.11.1 ± 0.21.6 < K 2 < 14 K 1 + K 2 8.4 ± 0.31.9 ± 0.2Motivation CH3CHO Photolysis Conclusion & prospects

9. Molecular photolysis products have been monitored Molecular and radicalar kinetic rates have been determined Comparison with recent work at atmospheric pressure shows no pressure dependence and a strong sensitivity to the UV emmission of the radicalar channel, this is not the case for the molecular channel Kinetic ratePathwayAM 0 (×10 -5 s -1 )AM 1,5 (×10 -5 s -1 ) Reference data: Röth et al. Atmos Chem Phys Discuss, 15 (5), p. 7239, (2015) K1K1 radical6.2 ± 0.40.8 ± 0.40.8 < K 1 < 11 K2K2 molecular2.2 ± 0.11.1 ± 0.21.6 < K 2 < 14Motivation CH3CHO Photolysis Conclusion & prospects

10. Experience is not sufficiently sensitive to detect the HCO radical The precursor is impure. Only 30 % of formaldehyde Other absorption lines disturb the measurement Kinetic ratePathwayAM 0 (×10 -5 s -1 )AM 1,5 (×10 -5 s -1 ) Reference data: Röth et al. Atmos Chem Phys Discuss, 15 (5), p. 7239, (2015) K1K1 radical6.2 ± 0.40.8 ± 0.40.8 < K 1 < 11 K2K2 molecular2.2 ± 0.11.1 ± 0.21.6 < K 2 < 14 Changing precursor H 2 CO (paraformaldehyde) by CH 3 CHO(acetaldehyde) Changing UV lampes (UVB->UVC) in order to boost the radical formation by Hg photosensitization increase detection sensitivity : Bolometer, Frequency & Zeeman modulationsMotivation CH3CHO Photolysis Conclusion & prospects

11. photosensitization, the process of initiating a reaction through the use of a substance capable of absorbing light and transferring the energy to the desired reactants. Photosensitization by mercury vapor (H. M. Pickett and T. L. Boyd, Chem. Phys. Lett., vol. 58, no. 3, pp. 446–449, Oct. 1978.) UVC Light Centred at 251.4 nm Hg CH 3 CHO CH 3 HCO CH 4 CO Collisional energyMotivation CH3CHO Photolysis Conclusion & prospects

12. Simple cell: Quartz cell 135 cm in length 50 mm diameter Cu coil every 1 cm (I=1.5A B=0.182mT) Osram UVC lamps: 36 W 6 Hg lamps Centred at 251.4 nm Liquide phase Acetaldehyde 99,99 % purity Typical working pressure: 30 μbar Bolometer 4K 600-900 GHz continuously tunable < 100 kHz resolution 10 μW power level AM & FM detectionMotivation CH3CHO Photolysis Conclusion & prospects

13. Both CO & HCO produced by the acetaldehyde photolysis were detected in a FM detection scheme thanks to the Hg photosensitization Quantification of HCO was performed relatively to the CO quantification P=30μbar P=30μbarMotivation CH3CHO Photolysis Conclusion & prospects

14. Detection limits: The Zeeman detection gives us the ability to measure only the HCO lines and cleans the baseline of non-paramagnetic molecules. With Zeeman modulation, the detection threshold of HCO has been improved by one order of magnitude HCO hyperfine components resolved (J'=19/2 ← J"=18/2 : F’=9 ← F’’=8 and F'=8 ← F"=7).Motivation CH3CHO Photolysis Conclusion & prospects

15. Motivation H2CO Photolysis CH3CHO Photolysis We demonstrate the potential of our terahertz spectrometer to monitor in real time the evolution of the gas phase concentration of photolysis products. Thanks to the versatility of electronic subTHz sources, the kinetic of chemical processes involving stable and unstable compounds may be studied at low pressure from rotational transitions Spectroscopic study of HCO > 300 GHz (Frequency multiplication / photomixing) in order to update the spectroscopic databases. As the relatively long wavelength of the THz radiation is less sensitive to scattering by particles, it presents a unique opportunity to measure the catalytic effect of aerosols on gas phase reaction kinetics. Prospects: Conclusion:

16. 70 th ISMS FB01 Arnaud Cuisset