Champaign-UrbanaJune 22-26, 2015

International Symposium of Molecular Spectroscopy70th Meeting

Continuous monitoring of photolysis products by THz

spectroscopyAbdelaziz 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

Outline :

H2CO Photolysis

Motivation

CH3CHO Photolysis

Conclusion & Prospects

70th ISMS FB01Arnaud Cuisset

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(1D) insertion reactions with methane and ethylene: Methanol and vinyl alcohol in a supersonic expansion », Hays & al., Chem. Phys. Lett., 630, 18, (2015)

70th ISMS meetingAtmos. Env. 36, 5947-

5052(2002)

THz spectroscopy

should be able to answer ?

Sub-THz study of the formaldehyde photolysis

H2CO 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

Simple cell:Quartz cell135 cm in length50 mm diameter

600-900 GHz continuously tunable< 100 kHz resolution10 μW power levelAM & FM detection

LS-1000 Solar Simulator:Xenon arc lamp (1000W)AM0 (sun @ zenith) &

AM1.5 (sun @ earth surface) filters

Paraformaldehyde heating at 90°CTypical working pressure: 30 μbar

Schottky diode detector (VDI, WR 1.2 600-900 GHz)

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 H2CO photolysis

a single measurement cycle providing one

H2CO and CO absorption profile every 37 seconds.

Two isolated rotational transitions of H2CO 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.

integrated intensity

Gaussian fit of Doppler limited rotational transitions

Time profiles of [CO] & [H2CO] concentrations for

the 2 filters: AM0 (blue: sun @ zenith) &

AM1,5 (red: sun @ earth surface)

(cm-1)

 

Although the HCO concentration is not directly

probed, the value of K1 is determined.

Kinetic rate Pathway AM0 (×10-5s-1) AM1,5 (×10-5s-1) Reference data:Röth et al. Atmos Chem Phys Discuss, 15 (5), p.

7239, (2015)

K1 radical 6.2 ± 0.4 0.8 ± 0.4 0.8 < K1 < 11

K2 molecular 2.2 ± 0.1 1.1 ± 0.2 1.6 < K2 < 14

K1 + K2 8.4 ± 0.3 1.9 ± 0.2

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 rate Pathway AM0 (×10-5s-1) AM1,5 (×10-5s-1) Reference data:Röth et al. Atmos Chem Phys Discuss, 15 (5), p.

7239, (2015)

K1 radical 6.2 ± 0.4 0.8 ± 0.4 0.8 < K1 < 11

K2 molecular 2.2 ± 0.1 1.1 ± 0.2 1.6 < K2 < 14

Experience is not sufficiently sensitive to detect the HCO radicalThe precursor is impure. Only 30 % of formaldehydeOther absorption lines disturb the measurement

Kinetic rate Pathway AM0 (×10-5s-1) AM1,5 (×10-5s-1) Reference data:Röth et al. Atmos Chem Phys Discuss, 15 (5), p.

7239, (2015)

K1 radical 6.2 ± 0.4 0.8 ± 0.4 0.8 < K1 < 11

K2 molecular 2.2 ± 0.1 1.1 ± 0.2 1.6 < K2 < 14

Changing precursor H2CO (paraformaldehyde) by CH3CHO(acetaldehyde)Changing UV lampes (UVB->UVC) in order to boost the radical formation by Hg photosensitizationincrease detection sensitivity : Bolometer, Frequency & Zeeman modulations

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 LightCentred at 251.4 nm

Hg CH3CHO

CH3HCO

CH4CO

Collisional energy

Simple cell:Quartz cell135 cm in length50 mm diameterCu coil every 1 cm(I=1.5A B=0.182mT)

Osram UVC lamps:36 W6 Hg lampsCentred at 251.4 nm

Liquide phase Acetaldehyde 99,99 % purityTypical working pressure: 30 μbar

Bolometer 4K

600-900 GHz continuously tunable< 100 kHz resolution10 μW power levelAM & FM detection

Both CO & HCO produced by the acetaldehyde photolysis were detected in a FM detection scheme thanks to the Hg photosensitizationQuantification of HCO was performed relatively to the CO quantification

P=30μbar

P=30μbar

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).

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:

Thank you for your attention

70th ISMS FB01Arnaud Cuisset