Ionization and emission spectra of the photofragments of allene excited at 193 nm

The ionic and luminous photofragments of allene excited with a single focused laser beam at 193 nm are detected using time-of-flight mass spectrometry and dispersed emission spectroscopy, respectively. The major ionic products detected are C-3(+), C-2(+), and C+. The yields of C3Hn+ (n = 1-3) are minor compared to the carbon ions. For the emission spectra, the C-2 Swan system (d(3)Pi(g) --> a(3)Pi(u)), the C-2 Mulliken system (D(1)Sigma(u)(+) --> X(1)Sigma(g)(+)), CH(A(2)Delta --> X(2)Pi), and an atomic line corresponding to the C(2p3s P-1(o)) --> C(2p(2) S-1) transition are the major features observed. The vibrational population in the C-2(d(3)Pi(g)) state is found inverted to a thermal equilibrium distribution. The laser-power dependence of each photofragment is measured, from which multiphoton dissociation and ionization mechanisms of allene are proposed. A kinetic model is developed to further illuminate the photodissociation mechanisms. Although C-n(+) (n = 1-3) ions are the major species observed in the present photodissociation experiment, C3Hn+ (n = 1-4) are most abundant in the electron impact of allene. The hydrogen elimination and carbon-carbon bond rupture in the photodissociation of allene at 193 nm, with C, and C3Hn+ (n = 1-3) photofragments, are concluded to precede the competing multiphoton ionization process, in producing C3H4+ within the laser flux (<2.7 x 10(26) photons cm(-2) s(-1)) used in this experiment.