1. © 2009 SRI International TEMPERATURE DEPENDENCE OF THE VIBRATIONAL RELAXATION OF OH(  = 1, 2) by O, O 2, AND CO 2 Constantin Romanescu, Henry Timmers, Gregory P. Smith, Konstantinos S. Kalogerakis, and Richard A. Copeland SRI International, Molecular Physics Laboratory, Menlo Park, CA 94025

2. © 2009 SRI International OH(  ) in the terrestrial atmospheres Source:  Emission from OH(  ) dominates the visible and infrared emissions of the atmospheric nightglow (Meinel bands).  Collisional energy transfer between OH(  ) and other atmospheric species significantly influences the mesospheric heat budget.

3. © 2009 SRI International Why are we studying OH(  = 1 and 2)?  Recent identification of OH(  = 1, 2) in the atmosphere of Venus requires a better understanding of the dynamics of the vibrational relaxation of OH by CO 2 ;  Measurements for relaxation by CO 2 at Venus mesospheric temperatures (T=160–200 K) are needed. Venus temperature and OH(  = 1, 2) emission rate vertical profiles (adapted from Piccioni et al., 2008 )

4. © 2009 SRI International Goals  Measure the vibrational relaxation rates of OH(  = 1, 2) by O-atoms, CO 2, and O 2 at temperatures between 160 – 300 K.  Estimate the  = -1 /  = -2 branching ratio for the vibrational relaxation of OH(  = 2) by CO 2.  Resolve the disagreement between the current literature values for the vibrational relaxation of OH(  = 2) by O 2 at room temperature.

5. © 2009 SRI International Experimental approach

6. © 2009 SRI International Vibrationally excited OH detection  Excite the Q 1 (1) line of the diagonal bands of A – X transition;  Monitor the excited state population via the  = -1 transition LIF.

7. © 2009 SRI International Experimental set-up

8. © 2009 SRI International Vibrational relaxation of OH(  = 2) by O-atoms T = 300K T = 240K First measurement of this rate constant at a lower temperature. T = 210K

9. © 2009 SRI International Vibrational relaxation of OH(  = 2) by CO 2 and O 2 ColliderThis workPrevious results CO 2 O2O2 Room temperature measurements ( k x 10 -13 cm 3 s -1 ) T = 240K (a) Rensberger et al., 1989. (b) Raiche, G. et al. 1990. (c) Dodd, J.A. et al. 1991. (d) D’Ottone, L. et al. 2004. T = 210K

10. © 2009 SRI International Relaxation of OH(  = 2) – Temperature dependence

11. © 2009 SRI International OH(  = 1) room temperature relaxation data ColliderThis workPrevious results CO 2 (*) O (**) (*) rate constant x 10 -13 cm 3 s -1 ; (**) rate constant x 10 -11 cm 3 s -1 Experimental data for the relaxation of OH(  = 1 and 2) by CO 2 and O-atoms (a) Dodd, J.A. et al. 1991. (b) Raiche, G. et al. 1990. (c) Khachatrian et al., 2005. CO 2 Branching ratio

12. © 2009 SRI International Conclusions We measured the removal rate constants of OH(  = 2) by O-atoms, CO 2, and O 2 at T = 210, 240, and 300K; We resolved the discrepancy between the removal rate constants of OH(  = 2) by O 2 at room temperature; The extracted branching value,  CO 2, points to a predominantly  = -1 vibrational relaxation of OH(  = 2).

13. © 2009 SRI International Acknowledgements Dr. Dušan Pejaković, SRI International Dr. Robert Robertson, SRI International Funding This work is supported by the NASA Geospace Science and Planetary Atmospheres Programs Participation of Henry Timmers was made possible through the NSF Research Experience for Undergraduate (REU) program.

14. © 2009 SRI International

15. Kinetic equations  The strong coupling between the nascent vibrational population ratio (f) and the branching ratios (  i ) does not allow for the fitting of both parameters;  Estimation of the nascent vibrational distribution for the actual experimental conditions is needed.

16. © 2009 SRI International Vibrational quenching of OH(  = 1, 2) 1. OH(  = 1, 2) nascent distributions 2. OH(  = 2) cascading 3. OH(  = 1) quenching ( * ), Robertson and Smith, J. Phys. Chem. A, 2006

17. © 2009 SRI International Why are we studying OH(  = 1 and 2)?  There is a disagreement in the OH(  = 2) room temperature rate constants;  A better understanding of the vibrational relaxation pathways of OH(  ≥ 2), i.e. single or multiple quanta energy loss, is needed for the analysis of the nightglow of the terrestrial atmospheres. Current literature values for the relaxation of OH(  = 2) by CO 2 and O 2

18. © 2009 SRI International Additional plots