Steady‐state hydrogen‐atom concentrations produced by the mercury‐photosensitized decomposition of molecular hydrogen are detected by Lyman‐α photometry. On the basis of the natural lifetime of Hg(3P1) and the mechanism for the reaction, quenching plots are constructed, and the over‐all bimolecular rate constant for Hg(3P1) + H2 is calculated. The absolute cross section for Hg(3P1) + H2 is determined to be 10.8 ± 0.4 Å2 in the limit of low mercury opacity. This result agrees with other recent experimental determinations. In addition, experimental values of imprisonment lifetimes for Hg(3P1) as a function of mercury concentration are estimated and are compared with other low‐opacity and high‐opacity results. The results are also compared to various theoretical predictions of imprisonment lifetimes.

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