Temporal mapping of photochemical reactions and molecular excited states with carbon specificity

Photochemical reactions are essential to a large number of important industrial and biological processes. A method for monitoring photochemical reaction kinetics and the dynamics of molecular excitations with spatial resolution within the active molecule would allow a rigorous exploration of the pathway and mechanism of photophysical and photochemical processes. Here we demonstrate that laser-excited muon pump-probe spin spectroscopy (photo-μSR) can temporally and spatially map these processes with a spatial resolution at the single-carbon level in a molecule with a pentacene backbone. The observed time-dependent light-induced changes of an avoided level crossing resonance demonstrate that the photochemical reactivity of a specific carbon atom is modified as a result of the presence of the excited state wavefunction. This demonstrates the sensitivity and potential of this technique in probing molecular excitations and photochemistry

Hyperfine fields in a Ag/Fe magnetic multilayer probed with low energy spin polarized Li-8

A beam of low energy Li-8 has been used to probe the hyperfine field distribution in a magnetic multilayer composed of Au(4nm)/Ag(80nm)/Fe(2nm) grown on a GaAs substrate. The beta-NMR frequency spectrum is shown to be a strong function of the implantation energy reflecting the depth dependence of the local hyperfine fields. Correlating the spectra with TRIM.SP implantation profiles allows us to identify signals from the GaAs, Ag and Au layers. The frequency spectrum in Ag is a strong function of energy. A very narrow line, and corresponding distribution of hyperfine fields, is observed when the beam in centered well away front the Fe whereas a much broader distribution is observed when the Li stops close to the Fe. These results demonstrate that low energy beta-NMR can act as a sensitive probe of the induced magnetism in the otherwise non-magnetic Ag spacer layers. (c) 2005 Elsevier B.V. All rights reserved

Early Li-8+beta-NMR investigations in GaAs and Ge

In this paper, we describe initial studies of the structure and dynamics associated with Li-8(+) in bulk crystalline GaAs and Ge. At low temperatures in GaAs, the amplitude of the Li-8(+) resonance signal at approximate to 3 T indicates that a large fraction (at least 70%) of the Li end up in locations with cubic symmetry (i.e. the tetrahedral interstitial and substitutional sites). The linewidth of the beta-NMR Li-8(+) resonance increases dramatically above 150 K, reaches a maximum at about 290 K, and decreases again. This suggests that the Li starts to change its location, probably from an interstitial to a substitutional site, at approximate to 150 K. Experiments in Ge are also described. In this sample, a narrow resonance is seen at low temperatures that is likely due to Li located at an interstitial site. Near room temperature, it appears that Li is converting to another site. (c) 2005 Elsevier B.V. All rights reserved