Photolysis of ketene at 193 nm and the rate constant for H+HCCOat 297 K

The 193 nm photolysis of ketene was studied by measuring the amount of atom ic hydrogen produced when very dilute ketene/Ar and ketene/H-2 mixtures wer e irradiated by a single pulse from an ArF excimer laser. Absolute concentr ations of atomic hydrogen were monitored over a time interval of 0-2.5 ms b y using Lyman-alpha atomic resonance absorption spectroscopy (ARAS). Four d ifferent photodissociation channels of ketene were identified: H2CCO + hz, gives (a) CH2(B-3(1)) + CO; (b) CH2((1)A(1)) + CO; (c) HCCO + H; and (d) C2 O(b(1)Sigma(+)) + H-2. The quantum yields for each channel were measured as phi(a) = 0.628, phi(b) = 0.193, phi(c) = 0.107, and phi(d) = 0.072, respec tively. To explore the secondary chemistry that occurred when using higher pressure H2CCO/Ar mixtures, a mechanism was constructed that used well-docu mented reactions and, for most processes, rate constants that had already b een accurately determined. Modeling studies using this mechanism showed the [H] profile to be determined largely by the rate of the reaction I;I + HCC O --> CH2 + CO. An excellent fit to all of the experimental data was obtain ed when k(2) = (1.7 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1).