1. © 2009 SRI International Laboratory measurement of the CO Cameron bands and visible emissions following EUV photodissociation of CO 2 Konstantinos S. Kalogerakis, Constantin Romanescu, Tom G. Slanger SRI International, Molecular Physics Laboratory, Menlo Park, CA 94025; Long C. Lee San Diego State Univ., Dept. Elect. & Comp. Engn., San Diego, CA 92182; Musa Ahmed and Kevin R. Wilson Lawrence Berkeley Lab., Div. Chem. Sci., Berkeley, CA 94720

2. © 2009 SRI International Motivation CO 2 is an important component of the atmospheres of terrestrial planets and comets: Venus96% CO 2, 4% N 2 90 atm. Earth80%N 2, 20% O 2, 350 ppm CO 2 1 atm. Mars96% CO 2, 4% N 2 0.01 atm. The most prominent features in the dayglow of Mars are associated with EUV absorption by CO 2 ; There are no spectral measurements in the visible; The study of the photodissociation of CO 2 at energies below the first ionization potential can provide a marker for ground- based detection of CO 2.

3. © 2009 SRI International CO (a – X) Cameron bands UV spectrum of the dayglow of Mars measured by SPICAM on Mars Express (Leblanc, F. et al., J. Geophys. Res. 111: E09S11, 2006) CO (a 3  ) sources: CO 2 EUV dissociation; CO 2 and CO electron impact excitation; CO 2 + - electron recombination.

4. © 2009 SRI International CO 2 photodissociation Solar extreme ultraviolet irradiance (Woods, T.N. et al., Solar Phys. 177: 133-146, 1998) CO 2 absorption cross section (Chan, W.F. et al., Chem. Phys. 178: 401-413, 1993)

5. © 2009 SRI International CO 2 photodissociation products below the IP

6. © 2009 SRI International Experimental set-up ~10 Torr ~40 mTorr ~5 mTorr low 10 -5 Torr low 10 -7 Torr 1 mm2 mm 3 mm ALS Gate Valve Dry Scroll Pump with Roots Blower (200 l/s) Turbomolecular Pump (550 l/s) Turbomolecular Pump (550 l/s) Dry Scroll Pump (10 l/s) Gas Photodiode PMT Monochromator Collection Optics Light Baffles

7. © 2009 SRI International CO excitation spectrum

8. © 2009 SRI International CO combined UV-VIS-NIR spectrum Burke, M.L. et al., J. Phys. Chem. 1996: 100(1), 138-148.

9. © 2009 SRI International CO bands ratio – CO 2 pressure dependence

10. © 2009 SRI International Conclusions A major CO 2 photodissociative channel is represented by CO* (a 3 , a’ 3    d 3  or  e 3  - ) + O( 3 P); The upper triplet states cascade into the lower CO(a 3  ) state, generating the CO(a’-a, d-a, e-a) transitions in the visible and IR regions; The CO(a 3  ) produces the well known CO(a-X) Cameron band radiation; There have been no observations of the predicted visible/ IR emission in the dayglows of Venus or Mars because the proper instrumentation has not been flown.

11. © 2009 SRI International Acknowledgements Bill Olson, Gabe Hernandez, SRI International Sarah J. Ferrell, ALS Berkeley ALS Technical Team Funding This work was supported by the NASA Outer Planets Research Program under grant NNX06AB82G. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231

12. © 2009 SRI International

14. CO 2 photodissociation - fluorescence spectrum