Photofragment-ion imaging
[1] The method uses a two-dimensional detector, usually a microchannel plate, to record the arrival positions of state-selected ions created by resonantly enhanced multi-photon ionization (REMPI).
The first experiment using photofragment ion imaging was performed by David W Chandler and Paul L Houston in 1987 on the phototodissociation dynamics of methyl iodide (iodomethane, CH3I).
Note that the angular distribution of the O(1D) is not uniform – more of the atoms fly toward the north or south pole than to the equator.
A close examination shows that the peak in the angular distribution is not actually exactly at the north or south pole, but rather at an angle of about 45 degrees.
This has to do with the polarization of the laser that ionizes the O(1D), and can be analyzed to show that the angular momentum of this atom (which has 2 units) is aligned relative to the velocity of recoil.
However, by tuning the ionization laser to the REMPI wavelength of O(3P) one finds a completely different image that provides information about the internal energy distribution of O2(3Σ).
Because ejection of an electron by the ionization laser does not change the recoil velocity of the CH3 fragment, its position at any time following the photolysis is nearly the same as it would have been as a neutral.
The advantage of converting it to an ion is that, by repelling it with a set of grids (represented by the vertical solid lines in the figure), one can project it onto a two-dimensional detector.
The detector is a double microchannel plate consisting of two glass discs with closely packed open channels (several micrometres in diameter).
Since multiple electrons are ejected for each one that hits the wall, the channels act as individual particle multipliers.
The electrons are then accelerated to a phosphor screen, and the spots of light are recorded with a gated charge-coupled device (CCD) camera.
One can imagine the ions produced by the dissociation and ionization lasers as expanding outward from the center-of-mass with a particular distribution of velocities.
Fortunately, for systems with an axis of cylindrical symmetry parallel to the surface of the detector, the three-dimensional distribution may be recovered from the two-dimensional projection by the use of the inverse Abel transform.
The ability to selectively detect a single mass by gating the detector electronically is thus an important advantage in reducing noise.
[6] A difficulty that limits the resolution in the position-sensing version is that the spot on the detector is no smaller than the cross-sectional area of the ions excited.
This dimension is much larger than the limit of a channel width (10 μm) and is substantial compared to the radius of a typical detector (25 mm).
This technique allows one to measure the three-dimensional product momentum vector distribution without having to rely on mathematical reconstruction methods which require the investigated systems to be cylindrically symmetric.
[9] Chang et al.,[10] realized that further increase in resolution could be gained if one carefully analyzed the results of each spot detected by the CCD camera.
(D. Townsend, S. K. Lee and A. G. Suits, “Orbital polarization from DC slice imaging: S(1D) alignment in the photodissociation of ethylene sulfide,” Chem.
