The critical stage in a chemical reaction — the progression through the transition state from reagents to products — occurs in less than a picosecond (10^(−12)s). Using laser pulses of femtosecond (10^(−15)s) duration it is possible to probe the nuclear motions throughout formation and break-up of the transition state. The coherence and very short duration of these femtosecond pulses provides a means to influence the course of the reaction during this stage if the time resolution is made sufficiently short. Here we describe a demonstration of such control of a chemical reaction on the femtosecond timescale. Using two sequential coherent laser pulses, we can control the reaction of iodine molecules with xenon atoms to form the product XeI by exciting the reactants through the transition state, in a two-step process. The yield of product XeI is modulated as the delay between the pulses is varied, reflecting its dependence on the nuclear motions of the reactants.

Femtosecond laser control of a chemical reaction

Time-Domain Fluorescence Spectroscopy Using Time-Correlated Single-Photon Counting

Mise au point. Discussion du role du principe de Franck-Condon dans l'interpretation et l'analyse des transitions lie-libre de molecules diatomiques, dans la cadre classique, semi-classique ou quantique. Transitions radiatives. Predissociation

The Franck—Condon Principle in Bound‐Free Transitions

Although the diatomic halogens are endowed with a wide range of intravalence-shell electronic states, the number and variety of these states is far from obvious in one-photon absorption from the ground state where only a few regions of structured absorption are present at visible and UV wavelengths. Selection rules on angular momentum and parity are key players in enforcing this sparse structure, but Franck- Condon effects also exercise considerable influence by directing transitions into regions where the transition dipole is small o r into a continuum where much of the detail is lost: further, a simultaneous excitation of two o r more electrons is formally forbidden in single-photon transitions. Some of the obstacles to one-photon absorption are avoided in the sequential absorption of two o r three photons using the conveniently placed, low-lying excited sQtes in the visible region for the initial step, in effect broadening the perspective from the ground state to include that from the lower ex...

Multiphoton Spectra and States of Halogens

Semiempirical model is developed for studying the electronic structure of the rare gas atom–halogen molecule systems. It is formulated in the frame of diatomics‐in‐molecule (DIM) approach and takes explicitly into account strong spin–orbit coupling pertinent to heavy halogen molecules. The consistent DIM scheme is realized for intermolecular interactions, whereas the description of valence electronic states of halogen molecule is more approximate being based on the asymptotic wave functions. The corresponding perturbation theory is also put forward. The model is applied to analysis of several features of the Ar...I2 van der Waals complex. First, the calculations on the spectroscopic constants of the B←X transition in the complex reveal the quantitative performance of the model. Second, mechanisms of nonadiabatic dynamics are examined. The results are qualitatively consistent with the current view on the Ar...I2 electronic predissociation and one‐atom cage effect. Third, the prediction is made on the valence electronic spectrum of Ar...I2 complex. These examples demonstrate the reliability of the model.

Ar–I2 interactions: The models based on the diatomics‐in‐molecule approach

Analysis of the 350–400 nm oscillatory continuum from I2 (D 1Σ+u)

Detailed studies of the reactions of electronically excited IBr[E(0+)] with Xe, translationally excited Xe(3P2) with I2, Br2 and ICl, and Kr(3P2) with Br2 are reported.Vibrationally excited levels in the E(0+) ion-pair state of IBr were pumped using tunable synchrotron radiation and the ‘vibrational’ excitation function for XeBr(B) formation determined for energies ⩽66 kJ mol–1 above threshold. Branching ratios for XeBr(B)vs. XeI(B) formation, together with data for the alternative excitation-transfer channel have also been determined over the same energy range.Fully dispersed chemiluminescence spectra and their polarizations have been measured under superthermal atomic beam [Xe(3P2), Kr(3P2)]–Maxwellian gas (I2, Br2, ICl) conditions at average collision energies Ēcm < 120 kJ mol–1. These have revealed distinctive patterns of behaviour for the collisional energy dependence of branching into alternative atom- and excitation-transfer channels and of vibrational energy and rotational angular momentum disposals.A simple global model is presented to reconcile the varying patterns of behaviour found in the rare-gas–halogen systems under both ‘collisional’ and ‘complexed’ conditions, and in particular the dependence on the initial reagent state preparation.

M. MacDonald

Papers

Analysis of the 350–400 nm oscillatory continuum from I2 (D 1Σ+u)

The dynamics of electronically excited states in the rare-gas–halogen systems

Oscillatory continuum emission from Br2 following vacuum ultraviolet laser excitation

Oscillatory continuum emission from IBr

Formation of electronically excited XeBr by reaction of excited IBr with ground-state Xe