Ionization and Emission Spectra of the Photofragments of Allene Excited at 193 nm

Abstract

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 C3+, C2+, and C+. The yields of C3Hn+ (n = 1−3) are minor compared to the carbon ions. For the emission spectra, the C2 Swan system (d3Πg → a3Πu), the C2 Mulliken system (D1Σu+ → X1Σg+), CH(A2Δ → X2Π), and an atomic line corresponding to the C(2p3s 1Po) → C(2p2 1S) transition are the major features observed. The vibrational population in the C2(d3Πg) 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 Cn+ (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 Cn and C3Hn (n = 1−3) photofragments, are concluded to precede the competing multiphoton ionization process, in producing C3H4+, within the laser flux (<2.7 × 1026 photons cm-2 s-1) used in this experiment.