Photofragment ion imaging (or product imaging) is an experimental technique for making measurements of the velocity of product molecules or particles following a chemical reaction or the photodissociation of a parent molecule. The method uses a two-dimensional detector, usually a microchannel plate, to record the arrival positions of state-selected ions created by resonance-enhanced multiphoton 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).
Background Many problems in molecular reaction dynamics demand the simultaneous measurement of a particle's speed and angular direction; the most demanding require the measurement of this velocity in coincidence with internal energy. Studies of molecular reactions, energy transfer processes and photodissociation can only be understood completely if the internal energies and velocities of all products can be specified. Product imaging approaches this goal by determining the three-dimensional velocity distribution of one state-selected product of the reaction. For a reaction producing two products, because the speed of the unobserved sibling product is related to that of the measured product through conservation of momentum and energy, the internal state of the sibling can often be inferred.
Example A simple example illustrates the principle. Ozone (O3) dissociates following ultraviolet excitation to yield an oxygen atom and an oxygen molecule. Although there are (at least) two possible channels, the principle products are O(1D) and O2(1Δ); that is, both the atom and the molecule are in their first excited electronic state (see atomic term symbol and molecular term symbol for further explanation). At a wavelength of 266 nm, the photon has enough energy to dissociate ozone to these two products, to excite the O2(1Δ) vibrationally to a maximum level of v = 3, and to provide some energy to the recoil velocity between the two fragments. Of course, the more energy that is used to excite the O2 vibrations, the less will be available for the recoil. The O(1D) atom's REMPI, combined with the product imaging technique, yields an image that can be used to calculate the O(1D) three-dimensional velocity distribution. A slice through this cylindrically symmetric distribution is shown in the figure, where an O(1D) atom that has zero velocity in the center-of-mass frame would arrive at the center of the figure. Note that there are four rings, corresponding to four main groups of O(1D) speeds. These correspond to O2(1) production at vibrational levels v = 0, 1, 2, and 3. The ring corresponding to v = 0 is the outer one, since production of the O2(1Δ) in this level leaves the most energy for recoil between the O(1D) and O2(1Δ). Thus, the product imaging technique immediately shows the vibrational distribution of the O2(1Δ). 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. In this case, the north-south axis is parallel to the polarization direction of the light that dissociated the ozone. Ozone molecules that absorb the polarized light are those in a particular alignment distribution, with a line connecting the end oxygen atoms in O3 roughly parallel to the polarization. Because the ozone dissociates more rapidly than it rotates, the O and O2 products recoil predominantly along this polarization axis. But there is more detail as well. 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. More detail can be found elsewhere. There are other dissociation channels available to ozone following excitation at this wavelength. One produces O(3P) and O2(3Σ), indicating that both the atom and molecule are in their ground electronic state. The image above has no information on this channel, since only the O(1D) is probed. 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).
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