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Absorption Cross Sections of Atmospheric Pollutants

Atomic, Molecular and Laser Physics

P.A. Hatherly

Figure 1. Absolute photoabsorption cross section for SF₅CF­₃ as a function of photon energy from Chim et al [3] using sychrotron radiation and this work [2] using discrete photon energies.

Of the many atmospheric pollutants, amongst the most potent and damaging are the halocarbons, including the common refrigerants, the CFCs (Chlorofluorocarbons). Although production and use of these substances is falling under international agreements, substantial amounts remain in the atmosphere and it is important to know how long they will remain. To help atmospheric modellers determine this, it is important to know how these molecules are broken down. The halocarbons are chemically inert, hence they survive long enough to be transported to the upper atmosphere, where breakdown mechanisms include electron impact and, importantly for this study, photon induced ionisation or dissociation.

To find the importance of photon-induced loss mechanisms, the absolute photo-absorption cross section is required. At Reading, we have recently started a programme using a commercial helium discharge lamp, combined with a Samson double ion chamber [1] to measure to high precision absorption cross sections of a range of halocarbons in the vacuum ultraviolet regime of the electromagnetic spectrum. Principally, we have measured at He I (21.2 eV) and Ne I (16.7eV^*) energies, with plans to include Ar I (11.7 eV^*) and Kr I (10.4 eV^*). As an example, figure 1 above shows our work [2] in relation to that of Chim et al [3] for the absorption cross section of SF₅CF_3­­, performed using synchrotron radiation. As well as providing confirmation and calibration of the cross sections in this case, our work is also informing the debate as to the cross section at the important H Lyman a energy of 10.2 eV. As well as providing valuable information for atmospheric science, this project is also suitable as a training vehicle for MPhys students doing their final year projects. The success of this approach is illustrated by the fact that the second author (A.J. Flaxman) in ref. [2] was such a student.

^*These discharge lines are actually doublets – the mean energy has been quoted.

[1]Samson JAR, “Techniques of Vacuum Ultraviolet Spectroscopy”,1967

[2] Hatherly PA and Flaxman AJ, Chem. Phys. Lett. 380 (2003) 512–515

[3] Chim RYL, Kennedy RA, Tuckett RP, Chem. Phys. Lett. 367 (2003) 697.

Page last updated June 01, 2008