Laser-induced fluorescence studies of C3 formation and isomerization in the 193 nm photolysis of allene and propyne

The quantum state distributions of the C, radical from the UV multiphoton dissociation of allene and propyne in a pulsed molecular beam have been measured. The observed distributions for both molecules are identical and are much colder than a phase space model would predict, implying that direct dissociation is occurring in one step and isomerization in another. It is concluded that both allene and propyne undergo internal conversion from the S1 state to the vibrationally excited Sa ground state of allene followed by molecular elimination of Hr. The observed Cs is then produced by molecular elimination of H2 from the secondary photolysis of CsH2.