Two-dimensional imaging of state-selected photodissociation products detected by multiphoton ionization

A new technique is presented that makes it possible, with a single laser pulse, to determine the three‐dimensional spatial distribution of state‐selected photoproducts. Initially, absorption of a photon from a laser beam causes fragmentation of a molecule. Multiphoton ionization is used to select the internal state of a desired fragment without perturbing its velocity. Following a short delay, the three‐dimensional spatial distribution caused by the fragment velocities is projected onto two dimensions by accelerating the state‐selected fragment ions into the surface of a channel plate particle multiplier. Electrons emerging from the multiplier are imaged onto a phosphorescent screen for analysis by a digital‐image processing device such as a two‐dimensional optical multichannel analyzer. The three‐dimensional spatial distribution is reconstructed by taking the Hankel transform of the Fourier transform of the projection. The technique is illustrated by recording the spatial distribution of methyl fragments produced in their vibrational ground state by the 266 nm photodissociation of CH3I. From this study it is determined that the fraction of CH3(v=0) formed in coincidence with I(2P1/2) is greater than 0.95, the rest being formed in coincidence with I(2P3/2) ground state.