The 193 nm photolysis of ketene was studied by measuring the amount of atomic hydrogen produced when very dilute ketene/Ar and ketene/H 2 mixtures were irradiated by a single pulse from an ArF excimer laser. Absolute concentrations of atomic hydrogen were monitored over a time interval of 0-2.5 ms by using Lyman-R atomic resonance absorption spectroscopy (ARAS). Four different photodissociation channels of ketene were identified: H 2 CCO + hν gives (a) CH 2 ( 3 B 1 ) + CO; (b) CH 2 ( 1 A 1 ) + CO; (c) HCCO + H; and (d) C 2 O(b 1 Σ + ) + H 2 . The quantum yields for each channel were measured as φ a ) 0.628, φ b ) 0.193, φ c ) 0.107, and φ d ) 0.072, respectively. To explore the secondary chemistry that occurred when using higher pressure H 2 CCO/Ar mixtures, a mechanism was constructed that used well-documented reactions and, for most processes, rate constants that had already been accurately determined. Modeling studies using this mechanism showed the [H] profile to be determined largely by the rate of the reaction H + HCCO f CH 2 + CO. An excellent fit to all of the experimental data was obtained when k 2 ) (1.7 ( 0.3) × 10 -10 cm 3 molecule -1 s -1 .

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