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Quantitative (υ, N, K_a) Product State Distributions near the Triplet Threshold for the Reaction H₂CO → H + HCO Measured by Rydberg Tagging and Laser-Induced Fluorescence^†

W. Scott Hopkins^‡^#, Hans-Peter Loock*^‡, Br

d Cronin^§, Michael G. D. Nix^§, Adam L. Devine^§, Richard N. Dixon^§, Michael N. R. Ashfold*^§, Hong-Ming Yin ⁺, Steven J. Rowling, Alexander B

ll and Scott H. Kable*

Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada, School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and School of Chemistry, University of Sydney, NSW 2006, Australia

J. Phys. Chem. A, 2008, 112 (39), pp 9283–9289

DOI: 10.1021/jp8021826

Publication Date (Web): August 16, 2008

†

Part of the “Stephen R. Leone Festschrift”.

, * To whom correspondence should be addressed. E-mail: hploock@chem.queensu.ca (H.-P.L.); mike.ashfold@bris.ac.uk (M.N.R.A.); s.kable@chem.usyd.edu.au (S.H.K.)., ‡

Queen’s University.

, §

University of Bristol.

University of Sydney.

, #

Current address: Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.

Current address: School of Chemistry, University of Oxford, Oxford OX1 3TA, U.K.

, +

Current address: State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Dalian 116023, Peoples Republic of China.

This article is part of the

A: Stephen Leone Festschrift

special issue.

Abstract

In this paper, we report quantitative product state distributions for the photolysis of H₂CO → H + HCO in the triplet threshold region, specifically for several rotational states in the 2²4³ and 2³4¹ H₂CO vibrational states that lie in this region. We have combined the strengths of two complementary techniques, laser-induced fluorescence for fine resolution and H atom Rydberg tagging for the overall distribution, to quantify the υ, N, and K_a distributions of the HCO photofragment formed via the singlet and triplet dissociation mechanisms. Both techniques are in quantitative agreement where they overlap and provide calibration or benchmarks that permit extension of the results beyond that possible by each technique on its own. In general agreement with previous studies, broad N and K_a distributions are attributed to reaction on the S₀ surface, while narrower distributions are associated with reaction on T₁. The broad N and K_a distributions are modeled well by phase space theory. The narrower N and K_a distributions are in good agreement with previous quasi-classical trajectory calculations on the T₁ surface. The two techniques are combined to provide quantitative vibrational populations for each initial H₂CO vibrational state. For dissociation via the 2³4¹ state, the average product vibrational energy (15% of E_avail) was found to be about half of the rotational energy (30% of E_avail), independent of the initial H₂CO rotational state, irrespective of the singlet or triplet mechanism. For dissociation via the 2²4³ state, the rotational excitation remained about 30% of E_avail, but the vibrational excitation was reduced.