The Cl+H 2  HCl+H reaction induced by IR+UV irradiation of Cl 2 in solid parahydrogen Sharon C. Kettwich and David T. Anderson Department of Chemistry, University of Wyoming 4:06 pm Wednesday June 24, th International Symposium on Molecular Spectroscopy

Overview Cl+H 2 reaction Making pH 2 crystals Different photolysis schemes Results: – Cl atom vs. HCl production Comparison with theory Summary Acknowledgements

Introduction to Cl+H 2 →HCl +H reaction Prototypical hydrogen abstraction reaction that has been studied theoretically and experimentally for >150 years. 1,2 Cl + H 2 (v=0,J=0) H + HCl(v=0,J=0) [Cl-H-H] ‡ 1900 cm cm -1 Cl+H 2 → [Cl-H-H] ‡ → HCl+H 1 Wang et al., Science 322, 573 (2008). 2 M.H. Alexander, G. Capecchi and H.-J. Werner, Science 296, 715 (2002). Study Cl+H 2 reaction in solid parahydrogen (pH 2 ).

Cl+H 2 →HCl +H reaction in pH 2 Dope pH 2 with Cl 2 3 D. T. Anderson, R. J. Hinde, S. Tam, and M. E. Fajardo JCP 116, 594 (2002). Detect HCl via R 1 (0) transition 3 UV photodissociate Cl 2  2Cl ? pH 2 v=0J=0 pH 2 v=0J=0 Cl =355 nm

Photolysis scheme 1 Co-deposit Cl 2 and parahydrogen 4 Photodissociate Cl 2 at 355 nm: Cl 2  2Cl Record spectra using FTIR BaF 2 substrate Cl 2 /pH 2 crystal Cl 2 pH 2 FTIR beam 355 nm UV beam 4 Tam, S. and M.E. Fajardo. Rev. Sci. Instrum. 70, 4, 1926 (1999).

Photolysis scheme 2 Expose Cl 2 /pH 2 crystal to IR and UV radiation simultaneously. 5 Source of IR light is FTIR spectrometer IR pH 2 Cl nm UV 5 S.C. Kettwich, P.L. Raston, D.T. Anderson, J.Phys. Chem. A, 2009 submitted.

Comparison of photolysis schemes Photolysis schemes produce different results. Simultaneous IR+UV t=42 min t=0 UV photolysis t=0 t=41 min HCl R 1 (0)

UV only: Cl 2 photodissociation at 355 nm Cl nm → 2Cl* cm -1 Cl* E trans =4085 cm cm -1 98% Cl ( 2 P 3/2 ) 2% Cl ( 2 P 1/2 ) 355 nm 6 D. B. Kokh, A. B. Alekseyev, and R. J. Buenker, JCP 120, (2004). 7 S. A. Kandel, T. P. Rakitzis, T. Lev-On, and R. N. Zare JCP 105, 7550 (1996). 355 nm

Cl H1H1 H2H2 R H,H R Cl,HH R COM Total kinetic energy (3-atoms) in laboratory frame Solid pH 2 – Assume hydrogen at rest Fraction of energy available for reaction in COM frame 8 Korolkov, M. V.; Manz, J.; Schild, A. J. Phys. Chem. 2009, submitted. Kinematics of a 3-body collision Cl* E trans =4085 cm -1 Cl* E com =(5.4%)(4085 cm -1 )=220 cm -1 Cl* + H 2 (v=0) → HCl + H

Cl+ H 2 (v=0,j=0) H + HCl(v=0,j=0) [Cl-H-H] ‡ 1900 cm cm -1 Cl+H 2 (v=0) → [Cl-H-H] ‡ → HCl+H E com =220 cm -1 Reactive surface in COM frame 2 M.H. Alexander, G. Capecchi and H.-J. Werner, Science 296, 715 (2002). Cl* E trans = 4085 cm -1

355 nm UV photolysis results HCl R 1 (0) Before photolysis After 41 min UV photolysis 9 Raston, P. L. and Anderson, D. T. Phys. Chem. Chem. Phys. 8, 3124 (2006).

Reaction probability for UV photolysis a UV =max R 1 (0) integrated absorbance for UV photolysis (y o IR +a IR )=max integrated absorbance after sequential UV and IR irradation y=y 0 +a(1-e -kt ) 9 Raston, P. L. and Anderson, D. T. Phys. Chem. Chem. Phys. 8, 3124 (2006). Cl* + H 2 (v=0) → HCl + H

Compare HCl production for UV vs. IR+UV photolysis schemes Simultaneous IR + UV photolysis Sequential UV followed by IR experimental data fit to simple 1 st order exponential growth expression y=y 0 +a(1-e -kt ). 10 IR+UV experimental data is more complicated than simple 1 st order expression. UV photolysis followed by IR irradiation 10 Kettwich, S. C.; Paulson, L. O.; Raston, P. L.; Anderson, D. T. J. Phys. Chem. A 112, (2008).

Simultaneous IR+UV photolysis IR+UV shows additional mechanism that contributes to HCl production at early times  Different than step-wise photolysis! IR+UV data fits to expression appropriate for 2 sets of consecutive reactions with common initial step Laidler, K. Chemical Kinetics, 3 rd Ed.; Harper & Row, Publishers, Inc.: New York, 1987.

IR+UV analysis Possible reactionsDescription of reaction Cl 2 + hν 2Cl*UV photodissociation Cl* + H 2 (v=0) HCl + HCl* reaction with H 2 (v=0) → HCl Cl* + H 2 (v=1) HCl + H Cl* reaction with H 2 (v=1) → HCl Cl + H 2 (v=1) HCl + H Cl equilibration, reaction with H 2 (v=1) 11 Laidler, K. Chemical Kinetics, 3 rd Ed.; Harper & Row, Publishers, Inc.: New York, 1987.

Measuring IR+UV reaction probabilities Simultaneous IR+UV irradiation of Cl 2 in solid pH 2 under the conditions studied resulted in >15% of the photo-ejected Cl* atoms reacting with the pH 2 host. Cl* + H 2 (v=1) → HCl + H Cl + H 2 (v=1) → HCl + H

Collaboration with Manz group: Theory 8 Korolkov, M. V.; Manz, J.; Schild, A. J. Phys. Chem. 2009, submitted. 2-Dimensional wavepacket simulation Simple 3 atom system (Cl, H-H) Collinear collision geometry 3-step quantum model to match experiments: (1) Vibrational pre-excitation of of H 2 (v=1) (2) Photodissociation of Cl 2  2 Cl ( 2 P 3/2 ) (3) Cl reacts with H 2 (v=1) or H 2 (v=0)

t = 0 fs t = 485 fs t = 125 fs 8 Korolkov, M. V.; Manz, J.; Schild, A. J. Phys. Chem. 2009, submitted. 355 nm UV wavepacket simulation

t = 0 fs t = 95 fs t = 395 fs IR+UV wavepacket simulation 8 Korolkov, M. V.; Manz, J.; Schild, A. J. Phys. Chem. 2009, submitted. 12 Polanyi, J. C. Acc. Chem. Res. 5, 161 (1972).

Conclusions 2 different photolysis schemes allows us to study vibrational dependence of Cl+H 2 reaction. Experimental and theoretical results agree with Polanyi’s rules for late barrier reactions. Results show that gas phase reaction is reproduced in quantum solid. Cl*+H 2 (ν=1) → HCl+HCl*+H 2 (ν =0) → HCl+H X

Acknowledgements J. Manz Freie Universität Berlin, Germany Paul L. Raston University of Alberta Alberta, Canada (post-doc: W. Jaeger) David Anderson, Leif Paulson Thanks for listening! Elsbeth Klotz UniversityofWyoming