Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

The CH(3)I A-state-assisted photofragmentation of the (CH(3)I)(2) van der Waals dimer at 248 nm and nearby wavelengths has been revisited experimentally using the time-of-flight mass spectrometry with supersonic and effusive molecular beams and the "velocity map imaging" technique. The processes underlying the appearance of two main (CH(3)I)(2) cluster-specific features in the mass spectra, namely, I(2)(+) and translationally "hot" I(+) ions, have been studied. Translationally hot I(+) ions with an average kinetic energy of 0.94+/-0.02 eV appear in the one-quantum photodissociation of vibrationally excited I(2)(+)((2)Pi(32,g)) ions (E(vib)=0.45+/-0.11 eV) via a "parallel" photodissociation process with an anisotropy parameter beta=1.55+/-0.03. Comparison of the images of I(+) arising from the photoexcitation of CH(3)I clusters versus those from neutral I(2) shows that "concerted" photodissociation of the ionized (CH(3)I)(2)(+) dimer appears to be the most likely mechanism for the formation of molecular iodine ion I(2)(+), instead of photoionization of neutral molecular iodine.

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X -O 2 ͑X=CH 3 I, C 3 H 6 , C 6 H 12 , and Xe͒. A single laser operating at wavelengths around 226 nm is used for both photodissociation of the van der Waals complex and simultaneous detection of the O͑ 3 P J , J =2,1,0͒ atom photoproduct via ͑2+1͒ resonance enhanced multiphoton ionization. The kinetic energy distribution ͑KED͒ and angular anisotropy of the product O atom recoil in this dissociation are measured using the velocity map imaging technique configured for either full ͑"crush"͒ or partial ͑"slice"͒ detection of the three-dimensional O͑ 3 P J ͒ atom product Newton sphere. The measured KED and angular anisotropy reveal a distinct difference in the mechanism of O atom generation from an X -O 2 complex compared to a free O 2 molecule. The authors identify two one-photon excitation pathways, the relative importance of which depends on IPx, the ionization potential of the X partner. One pathway, observed for all complexes independent of IPx, involves a direct transition to the perturbed covalent state X -O 2 ͑AЈ 3 ⌬ u ͒ with excitation localized on the O 2 subunit. The predominantly perpendicular character of this channel relative to the laser polarization detection, together with data on the structure of the complex, allows us to confirm that X partner induced admixing of an X + -O 2 − charge transfer ͑CT͒ state is the perturbing factor resulting in the well-known enhancement of photoabsorption within the Herzberg continuum of molecular oxygen. The second excitation pathway, observed for X -O 2 complexes with X =CH 3 I and C 3 H 6 , involves direct excitation into the 3 ͑X + -O 2 − ͒ CT state of the complex. The subsequent photodissociation of this CT state by the same laser pulse gives rise to the superoxide anion O 2 − , which then photodissociates, providing fast ͑0.69 eV͒ O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O 2 ͑b 1 ⌺ g + ͒ are also observed when the CH 3 I-O 2 complex was irradiated. Potential energy surfaces ͑PES͒ for the ground and relevant excited states of the X -O 2 complex have been constructed for CH 3 I-O 2 using the results of CASSCF calculations for the ground and CT states of the complex as well as literature data on PES of the subunits. These model potential energy surfaces allowed us to interpret all of the observed O͑ 3 P J ͒ atom production channels.

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Photodissociation of singlet oxygen, O2 a(1)Δg, by ultraviolet radiation in the region from 200 to 240 nm has been investigated using velocity map imaging of the atomic oxygen photofragments. Singlet oxygen molecules are generated in a pulsed discharge and studied by one-laser photodissociation and detection around 226 nm as well as two color photodissociation at various wavelengths in the range from 200 to 240 nm. A simple model of the discharge on and off signal indicates efficient conversion of O2 X(3)Σg(-)(v = 0) in the parent beam to O2 a(1)Δg(v = 0-2). Minute amounts of highly excited vibrational levels of ground state O2 X(3)Σg(-)(v > 0) are detected but no evidence is found for production of the O2 b(1)Σg(+) state. Over the decreasing wavelength range 240-200 nm the a(1)Δg-state signal relative to the X(3)Σg(-)(v = 0) signal decreases strongly. Around 226 nm the a(1)Δg(v = 0-2) states averaged branching ratio percentage for O((3)Pjj = 2 : 1 : 0) is found to be 56 : 36 : 8 (±5%), respectively. The anisotropy parameter for photodissociation of a(1)Δg(v = 0-2) averages to β = 1.3 ± 0.4. The a(1)Δg(v = 0) photodissociation cross section is found to 3-10 times stronger than theory predicts. Furthermore, the photodissociation image shows a strong parallel character, (i.e., transition moment parallel to the molecular axis) while theory predicts a predominantly negative character.