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.

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.

Product state distributions of the CO produced in the 308-nm photolysis of acetaldehyde show clear evidence of two dissociation mechanisms. One is attributed to the conventional transition state mechanism predicted by theory, with high rotational and translational energy of the CO and a pronounced vЌJ vector correlation. However, as much as 15% of the reaction flux proceeds via another pathway that produces low CO rotational and translational energy, very high CH 4 internal energy, and no correlation between the CO velocity and angular momentum vectors. The attributes of this channel are dynamically similar to the recently reported ''roaming atom'' mechanism in formaldehyde. We therefore speculate that the second pathway in acetaldehyde also occurs via a roaming mechanism in the CH 3 ؉ HCO exit channel that decays into the CH4 ؉ CO channel.photochemistry ͉ photodissociation dynamics ͉ product state distributions ͉ roaming atom ͉ transition state T he concept of the transition state (TS) is central to chemistry. It is the transient structure at the highest point of the minimum energy pathway (reaction coordinate) between reactants and products. Reaction mechanisms are often described with reference to this structure. For example, an S N 2 reaction is defined by the transient 5-center carbon atom at the TS, or a 3-center elimination reaction refers to the transient 3-membered ring at the TS. Interpretation of the kinetics and thermodynamics of reactions is also based on the energy and entropy of the TS-from the simplest Arrhenius model to more sophisticated TS theories, including their variational forms.What would happen if reactions were found to bypass the TS? The result would be a new class of reaction mechanisms, with reaction products and kinetics that cannot be predicted by current TS theories. Recently, a mechanism of this type was reported, in which the photodissociation of H 2 CO to H 2 ϩ CO was observed to occur via a second, non-TS mechanism (1). The conventional 3-center elimination of H 2 from H 2 CO has long been known to produce very high relative translational energy of the departing fragments, highly rotationally excited CO, and modest vibrational energy of the H 2 fragment (2). The new mechanism revealed a very different signature: rotationally cold CO, coupled with very high vibrational excitation of H 2 and low relative translational energy. The mechanism was interpreted by reference to quasiclassical trajectory calculations on a high-level ab initio potential energy surface (1). These calculations revealed that all of the reactions that produced the unusual signature circumvented the TS. The reaction starts out looking like a conventional C-H bond cleavage, which would produce the radical products of H 2 CO photolysis: H ϩ HCO. In a significant number of trajectories, just enough energy is tied up in internal motion of the HCO moiety so that the H-atom cannot quite escape; it turns back at very long distance (several angstroms) from the HCO but is still loosely bound. At this range, vibrationa...

Highly vibrationally excited O(2)(X(3)sigmag(-), v >/= 26) has been observed from the photodissociation of ozone (O(3)), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the "ozone deficit" problem, or why the previously predicted stratospheric O(3) concentration is less than that observed. Recent kinetic studies have suggested that O(2)(X(3)sigmag(-), v >/= 26) can react rapidly with O(2) to form O(3) + O and have led to speculation that, if produced in the photodissociation of O(3), this species might be involved in resolving the discrepancy. The sequence O(3) + hv --> O(2)(X(3)sigmag(-), v >/= 26) + O; O(2)(X(3)sigmag(-), v >/= 26) + O(2) --> O(3) + O (where hv is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O(3) deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O(3) in the stratosphere.

We have investigated the electrochemical, spectroscopic, and electroluminescent properties of a family of diimine complexes of Ru featuring various aliphatic side chains as well as a more extended pi-conjugated system. The performance of solid-state electroluminescent devices fabricated from these complexes using indium tin oxide (ITO) and gold contacts appears to be dominated by ionic space charge effects. Their electroluminescence efficiency was limited by the photoluminescence efficiency of the Ru films and not by charge injection from the contacts. The incorporation of di-tert-butyl side chains on the dipyridyl ligand was found to be the most beneficial substitution in terms of reducing self-quenching of luminescence.