The existence of a mediating state for the ultrafast electronic relaxation in ethylenic-like molecules has been shown by femtosecond electron imaging techniques. This state is of Rydberg character, and its high efficiency is due to its ability to link the electron distributions of the initial and final electronic states.

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 combination of a standard velocity map imaging setup with three different slicing methods is demonstrated for the dissociation of O2(X3Σg-) (v = 0) at 157 nm leading to the O(1D) + O(3P) channel, and I2(X1Σg) (v = 0−4) at 490 nm leading to the I(2P3/2) + I(2P1/2) channel. The kinetic energy and angular distributions of the products are compared with the results from conventional (nonsliced) velocity mapping. In addition, one of the methods is applied for the study of field-free dissociation of molecular iodine ion I2+ by two-photon excitation at 522 nm.

The photofragmentation of state-selected BrCl+ cations has been investigated by velocity-map ion-imaging methods. Detailed analyses of the 79Br+ fragment velocities lead to the most precise values yet reported for (i) the spin−orbit coupling constant for the X2Π state, A = −2070 ± 4 cm-1; (ii) the bond dissociation energies of both states of 79Br35Cl+, 25019 ± 4 cm-1 (X2Π3/2) and 22949 ± 2 cm-1 (X2Π1/2); and (iii) the adiabatic ionization thresholds for forming 79Br35Cl+ parent ions in their X2Π3/2 and X2Π1/2 states, 88292 ± 6 and 90362 ± 4 cm-1, respectively.

We have studied dissociative multiphoton ionization of NO2 by time-resolved velocity map imaging in a two-color pump-probe experiment using the 400 and 266 nm harmonics of a regeneratively amplified titanium-sapphire laser. We observe that most of the ion signal appears as NO+ with approximately 0.28 eV peak kinetic energy. Approximately 600 fs period oscillations indicative of wave packet motion are also observed in the NO+ decay. We attribute the signal to two competitive mechanisms. The first involving three-photon 400 nm absorption followed by dissociative ionization of the pumped state by a subsequent 266 nm photon. The second involving one-photon 400 nm absorption to the 2B2 state of NO2 followed by two-photon dissociative ionization at 266 nm. This interpretation is derived from the observation that the total NO+ ion signal exhibits biexponential decay, 0.72 exp(-t/90+/-10)+0.28 exp(-t/4000+/-400), where t is the 266 nm delay in femtoseconds. The fast decay of the majority of the NO+ signal suggests a direct dissociation via the bending mode of the pumped state. .

The photodissociation dynamics of state selected OD radicals has been examined at 243 and 226 nm using velocity map imaging to probe the angle–speed distributions of the D(2S) and O(3P2) products. Both experiment and complementary first principle calculations demonstrate that photodissociation occurs by promotion of OD from high vibrational levels of the ground X 2Π state to the repulsive 1 2Σ− state.

Slice imaging is used to measure the alignment of Br(2P3/2) and Cl(2P3/2) photofragments from the photodissociation of HBr and HCl, respectively, at 193 nm. In both cases the A1Π1 state is predominantly optically excited, which correlates to mJ=±3/2 for the halogen atom photofragments. However, both the Br and Cl photofragments are measured to populate mostly mJ=±1/2, showing that for both HBr and HCl nonadiabatic transitions transfer more population to the a3Π1 state (which correlates to mJ=±1/2) during dissociation than remains in the A1Π1 state. The interference between these two pathways is also measured.

We illustrate the use of a three-dimensional (x,y,t) charge-coupled-device (CCD) camera detection system in an ion imaging experiment. The time measurement is based on the decay characteristics of the phosphor screen, which is recorded in two successive images by a double exposure CCD camera. The strength of the method is illustrated in a velocity map imaging experiment on iodine molecules that are ionized and dissociated by intense femtosecond laser pulses. Singly and doubly charged iodine fragments are detected and their (x,y) coordinates and arrival time are recorded in an event counting routine. We estimate the time resolution of the system to be 1.3 ns. We show that the fragment velocity distribution derived from the (x,y,t) data is similar and in some conditions more accurate than the distribution obtained by a mathematical inversion of the (x,y) data only. This principle of detection can be used in all situations in which inversion methods are impossible, for example, when the particle distribution does not have an axis of symmetry.

Translational and internal energy partitioning in the methyl and iodine fragments formed from photodissociation of methyl iodide in the A-band region is measured using velocity mapping. State-selective detection combined with the very good image quality afforded by the two-dimensional imaging technique allow a detailed analysis of the kinetic energy and angular distributions. Product vibrational energy is, as previously known, mainly partitioned into ν2, the umbrella mode of the methyl fragment, but a substantial fraction of molecules is also excited with one quantum of ν1, the symmetric C–H stretch, especially at higher dissociation energies. Preliminary evidence is also presented for excitation of several quanta of ν4, the asymmetric deformation mode. Rotational energy partitioning is similar for CH3 products formed in both the ground-state I(2P3/2) and the spin–orbit excited I*(2P1/2) channel for photodissociation across the full A-band spectrum. Dissociation of vibrationally excited molecules plays an increasingly important role at longer dissociation wavelengths. Two CH3I modes remain populated in the pulsed beam expansion, ν2(a1), the C–I stretch, and ν6(e), the methyl rock. Each reactant vibrational mode couples in a very specific manner into the I and I* dissociation channels. Trends in vibrational and rotational energy disposal are compared with recent theoretical predictions. Readjustment of many aspects of the ab initio multidimensional potential energy surfaces which have recently been calculated for CH3I appears to be necessary. The improved resolution offered by velocity mapping also allows a more accurate determination of the C–I bond energy. A dissociation energy of 2.41±0.02 eV is found.

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O(3Pj), j=0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.

Two-step photodissociation of O2 with a total excitation energy of 7.7 eV yields O(1D) atoms with their angular momenta aligned perpendicular to the fragment recoil axis. Correlation rules linking the bound molecule with the separated atoms predict such strong alignment only for an adiabatic process.

Velocity imaging(1), an improvement of the ion imaging method(2), is described and applied to a study of photodissociation of molecular oxygen. The electrostatic immersion lens introduced in this technique has the special property of projecting out the velocity information of a fragment formed in a photodissociation process, independent of the initial position of the fragment. This results in better image quality, thus more detailed information on the dynamics of collision and half-collision events. Photodissociation of molecular oxygen in the region of the Herzberg and Schumann-Runge continua using velocity imaging is discussed.

The methyl iodide A-band photodissociation process CH3I+hν→CH3(v,N,K)+I(2P3/2), I*(2P1/2) has been studied in a cold molecular beam. Full three-dimensional state-specific speed and angular distributions of the nascent fragments were recorded using (2+1) resonance-enhanced multi-photon ionization (REMPI) and velocity imaging, a new variant of ion imaging. By combining the I* quantum yield and anisotropy parameters for both I and I* channels, the relative absorption strength to the contributing electronic states (3Q0, Q13 and Q11) as well as the probability for curve crossing (3Q0→1Q1) are determined for excitation wavelengths across the full A band (240–334 nm). Parallel excitation to the Q03 state turns out to dominate the A band even more than previously thought.

Velocity map imaging is used to characterise the angular distributions of atoms which are formed from the sequential two photon excitation of NO via the A2Σ+ state. Molecules excited at 226 nm and dissociated at 339 nm from the same laser source yield two sets of products, ground state O(3P) atoms in conjunction with N(2D) and N(4S). The angular distributions are found to be well described by β parameters. The use of this method as a source of monoenergetic aligned atoms for studies of reaction dynamics is discussed.

A substantial improvement in the photofragment imaging technique is illustrated in a study of molecular oxygen photodynamics. In this method, labeled velocity map imaging, electrostatic ion lenses are shown to allow mapping of all particles with the same initial velocity vector onto the same point on a 2D detector, irrespective of their position of creation in the ionization volume. This leads to a dramatic increase in image resolution. Velocity map imaging of photoelectrons from molecular ionization is also demonstrated and applied along with O+ imaging to identify the processes leading to O+ formation when using (2+1) resonantly enhanced multiphoton ionization (REMPI) detection for O2. Oxygen molecules prepared in the (v=2, N=2) level of the 3dπ(3Σ1g−) Rydberg state by two-photon excitation at 11.02 eV are excited by a third photon to an energy near v=24 of ground-state O2+ (equivalent to one-photon excitation at 75 nm). All energetically accessible excited oxygen atoms and an extensive range of vibrationally excited O2+ ions result, with the primary dissociation/ionization events taking place at the third-photon level. Competition between dissociation into excited atoms and formation of O2+ is gauged by comparing images for e− and O+ products. Trends in the photoelectron and O+ fragment angular distributions are discussed for each active channel.

The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and support the appealing properties of this velocity mapping technique.