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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2008-10, Vol.112 (39), p.9283-9289
• Main Authors: Hopkins, W. Scott, Loock, Hans-Peter, Cronin, Bríd, Nix, Michael G. D, Devine, Adam L, Dixon, Richard N, Ashfold, Michael N. R, Yin, Hong-Ming, Rowling, Steven J, Büll, Alexander, Kable, Scott H
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp8021826

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.