1C09
放電フローへのUV-PES応用
D.E. Shallcross1, J.M. Dyke2 and M.Ghosh2
1.

University of Bristol, 2 University of Southampton, UK.

The measurement of thermal gas-phase kinetic parameters in the temperature range of 200300 K and in the pressure range from 1-760 Torr has particular importance for the Earth’satmosphere. Many reactions of atmospheric interest involve free radicals and therefore therate coefficient of the reaction may display a complex temperature dependence and may also
involve association channels that give rise to a pressure dependence also. The presence ofradicals may also lead to multiple product channels. In order to model ozone depletion or air
quality for example detailed information on these product channels is essential. Conventional
approaches have focused on two direct techniques to measure rate coefficients, dischargeflow and flash photolysis. The discharge-flow technique involves generating reactants
separately and bringing them together in a flowing gas mixture, the velocity of the flow
dictates the reaction time between reactants mixing and reaching the detector. There is norestriction on the type of detector that can be used and their time response is not a drawback.
The discharge-flow technique has until recently been restricted to low pressures (less than 10Torr) and temperatures above 250 K due to flow profile disruption and loss of species to thewalls. Flash photolysis involves the simultaneous generation of reactants from a light pulse
acting on a suitable gas mixture, here fast response detectors are required and althoughtemperature and pressure restrictions are far less the range of species that can be detected aremore limited. Photolelectron spectroscopy (PES) is a scattering technique that permits thedetection of all species in the reaction mixture. A high energy (21.22 eV, He Iα) radiation
source (UV-PES) is used to eject photolelectrons from the gases present in the sample andthese are energy analysed to produce a spectrum. The UV-PES detector is ideally suited forcoupling with a discharge-flow tube, but has not until recently been accomplished. In thispaper we describe the first results from coupling a UV-Photoelectron Spectrometer with adischarge-flow system to investigate the kinetics and products of two important reactions inthe atmosphere.1,2,3,4Cl2+ CH3SCH3 → products(1)

Reaction (1) between molecular chlorine and dimethyl
sulphide (DMS) is of potential importance in the marine
Fig. 1boundary layer where production of SO2 from DMS
oxidation can lead to cloud formation and directly
impact on the Earth’s climate through the CLAW
hypothesis. Using the new instrument the overall
kinetics of the reaction was determined at room
temperature and low pressure was k1 = (3.4 ± 0.7) x 10-14
cm3 molecule-1 s-1. The analysis of the PES spectra
revealed that the products of the reaction were HCl andCH3SCH2Cl and that a bound intermediate (CH3)2SCl2 was involved in the reaction. Reaction(1) can occur at night and allow the build up of CH3SCH2Cl (as high as 50 ppt.) Further
studies of the photochemistry of the product CH3SCH2Cl suggest that it can be photolysed
during the day and cleave a C-S bond to yield CH3S that can be readily oxidised to SO2,
thereby speeding up the oxidation of DMS and the production of SO2.

O3O3+ CH2CH2→ products+ C(CH3 )2C(CH3)2 → products

(2)
(3)

The ozonolysis of alkenes is a very important source of radicals in winter and at night in thetroposphere and has been the subject of many studies. Reaction (2) and (3) were studied andapart from well known products such as HCHO, CO and CO2 from reaction (2), O2 (1∆g) andO2 (3Σg) were also observed at long reaction times shown in figure 2. The possible source of
this excited oxygen arises from the decomposition of a secondary ozonide shown in figure3.
Interestingly no O2 (1∆g) is observed from reaction (3). Some suggestion why this might beare suggested.
Figure 3: Schematic mechanism ofozone +ethylene

Figure 2: Photoelectron spectrum
for O3+ethylene
1. J.M.Dyke etal. Phys.Chem.Chem.Phys., 7, 866 (2005).
2.. J.M.Dyke etal.. J. Chem. Phys.324, 855 (2006).
3.. C. J. Percival etal. J. Photochem. Photobiol , 176, 250 (2005).
4. H. D.E. Shallcross et al. Atmos. Environ.