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

Abstract

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.

Vibronic Structure of the Formyloxyl Radical (HCO2) via Slow Photoelectron Velocity-Map Imaging Spectroscopy and Model Hamiltonian Calculations

“…The photoelectron cloud formed was then coaxially extracted down a 50 cm flight tube and mapped onto a detector comprising a chevron-mounted pair of timegated, imaging quality microchannel plates coupled to a phosphor screen, as is typically used in photofragment imaging experiments. 33 Events on the screen were collected by a 1024 × 1024 charge-coupled device (CCD) camera and sent to a computer. Electron velocity-mapped images resulting from 50 000 to 100 000 laser pulses were summed, quadrant symmetrized, and inverse-Abel transformed.…”

Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Studies carried out on timescales spanning the nanosecond to femtosecond range have yielded insights into energy partitioning, interactions between excited states, product angular momentum polarisation, and a range of other phenomena. Starting with the first demonstration of ion 35 imaging by Chandler and Houston in 1987, 8 methyl iodide has also proven to be a popular molecule for demonstrating new experimental imaging methods in chemical dynamics, and has been widely studied by velocity-map imaging (VMI) and related techniques. [10][11][12][13] Unsurprisingly, interest has extended to the 40 photodynamics of other methyl halides (CH 3 X), and there have been several studies on CH 3 Cl, CH 3 Br, and CH 3 I in both their neutral (see 16 and references therein) and cationic forms [17][18][19][20][21][22][23][24] ; these earlier studies provide a valuable point of reference for the present investigations into the photofragmentation of two ethyl 45 halide cations, CH 3 CH 2 Br + and CH 3 CH 2 I + .…”

Effects of resonant excitation, pulse duration and intensity on photoelectron imaging of a dianion

“…The angular distribution of photoelectron (PE) emission relative to the polarisation vector of the light field, ε, has been extensively 20 studied in neutrals and anions, and has become routine with the advent of charged particle imaging 1 and especially velocity map imaging. 2 In simple systems the PE angular distribution (PAD) can be well described by the Cooper-Zare formalism 3 in which the partial waves and their interference determine the observed 25 PE anisotropy.…”