Photofragment imaging by sections for measuring state-resolved angle-velocity differential cross sections

We describe a two-dimensional (2D) imaging technique for recording state-specific photofragment angle-velocity (θ,v) distributions. In these experiments the photofragment images are recorded as 2D sections of the 3D angular distributions using state-specific ionization in a time-of-flight mass spectrometer. We compare this method to previous methods that record 2D projections of the 3D distribution. The 2D sections represent cartesian flux-velocity maps in the center of mass and are related to angle-velocity differential cross sections by a simple geometric factor. Two studies are highlighted. In the first, new results are presented for the A state photodissociation of CH3I to CH3+I. (θ,v) images are presented for I atom in the 2P3/2 and 2P1/2 spin–orbit states following photodissociation at 266 and 304 nm. The principal result is detection of the weak perpendicular transitions to the 3Q1 state (at 304 nm) and the 1Q state (at 266 nm) that underlie the strong parallel transition to the 3Q0 state. We also report the ratio of cross sections σ⊥/σ∥, the anisotropy and branching ratio for I(2P3/2) and I(2P1/2), and the 3Q0–1Q surface crossing probability. In a second study the photodissociation of O3 to O2(v)+O(3Pj=2,1,0) was measured. A bimodal anisotropic velocity distribution was measured for O(3P) corresponding to maximum in the O2(v) vibrational distribution of v=15 and 27, in general agreement with a previous measurement. The anisotropies of the high- and low-velocity components were measured to be β≊1.1 and 0.4, respectively.