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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.

The spin-crossover phenomenon is a cooperative low-spin (LS) to high-spin (HS) transition which can be initiated using temperature or light irradiation. We have used muon-spin relaxation (μSR) to study this transition in two salts which show this effect. μSR provides local magnetic information and hence a means of examining this transition from a local perspective. For both salts, the LS phase gives rise to root-exponential relaxation which we associate with a dilute distribution of fluctuating moments resulting from incomplete spin crossover. The low temperature HS fraction which remains is small but can be altered by rapid cooling. We relate the observed muon relaxation to the underlying fluctuating moment distribution and compare our results to Monte-Carlo simulations of these distributions.

We present zero-field muon spin relaxation (ZF-$\mu^{+}$SR) measurements on two examples of a new series of hybrid organometallic-inorganic layered magnets, namely ferromagnetic [ FeCp$^{*}_{2}$] [ MnCr(ox)3] and ferrimagnetic [ CoCp$^{*}_{2}$] [ FeFe(ox)3] (where ox = oxalate and Cp*= pentame- thylcyclopentadienyl). Both materials show multi-component muon spin precession signals characteristic of quasistatic magnetic fields at several distinct muon sites. The temperature dependence of the precession frequencies allow critical exponents to be extracted. Possible muon sites are discussed on the basis of dipole field calculations. Key words.Muon-spin relaxation, layered magnets, oxalate complexes.

The response of the superconducting charge carriers to an applied magnetic field has been probed in a number of organic superconductors using μSR. Systematic studies have been made of the in-plane superconducting penetration depth $\lambda $in superconductors based on the ET and BETS molecules. These studies have revealed a strong correlation between $\lambda $and Tc. This type of correlation was first noted in high Tccuprates by Uemura et alwho found a linear relation between Tcand $\lambda ^{-2}$. In the case of organic superconductors, however, we find that the overall correlation is better described as T$_{c}\propto \lambda ^{-3}$. Understanding this characteristic trend should provide some important clues about the nature of the superconductivity in the organic superconductors; several possible models are considered here for describing this behaviour. It is concluded that models in which the superconducting carrier concentration is tied to the nominal band filling are inconsistent with the observed scaling behaviour. Key words.London penetration depth – Superconducting transition temperature.

Muon radical states were studied in the electron donor molecules TTF and BEDT-TTF and in the electron acceptor molecule TCNQ. Muonium addition to double bond carbon sites results in hyperfine coupling contants ranging from 350 MHz in TTF down to 80 MHz for TCNQ. In TTF and BEDT-TTF additional radical states with extremely low hyperfine coupling constants in the 5 MHz region are observed; these are assigned to muon addition at sulphur sites accompanied by C — S bond cleavage. Copyright © 2000 John Wiley & Sons, Ltd.

Polarized infrared reflectance measurements are reported for crystals of La_2−xSrxCuO_4−δ and YBa_2Cu_3O_7−δ. Free-carrier reflectance is observed in the copper oxide planes, and strong phonons are observed in the perpendicular direction. The interpretation of the infrared optical properties is discussed.

Muon repolarization in a longitudinal field is calculated for anisotropic muonium with arbitrary orientation of magnetic field to the muonium principal axis. In the absence of superhyperfine interactions, the low-field polarization is shown to be highly dependent on the hyperfine anisotropy. The repolarization is calculated in the quasistatic limit for some different types of orientational averaging, which might result from static or dynamic disorder. Approximate analytical forms for the repolarization curve are obtained and some experimental applications are discussed.

A procedure has been developed to extract qualitative and quantitative information on the muonium fractions, in particular the Mu* fraction, in polycrystalline and amorphous materials from their longitudinal field repolarization curves. Preliminary results for amorphous silicon suggests that both the Mu* and Mu* fractions here are generally lower than in crystalline silicon at temperatures below 200K, but the Mu* fraction may survive to room temperature in this disordered host.

Polarized infrared reflectance measurements are reported for crystals of La_2−x,Sr_xCuO_4−δ and YBa_2Cu_3O_7−δ. Free-carrier reflectance is observed in the copper oxide planes, and strong phonons are observed in the perpendicular direction. The interpretation of the infrared optical properties is discussed.

We used a genomic fingerprinting protocol to characterize 59 Staphylococcus aureus strains and a single S. intermedius isolate, all of which were previously typed by 13 different methods (F. C. Tenover et al., J. Clin. Microbiol. 32:407-415, 1994). These 60 strains were divided into three groups of 20 strains each, with each group including internal controls. Two of the three groups (groups SB and SC) included 29 strains from four relatively well-defined outbreaks. The epidemiological relationships of the strains in the third group (group SA) were unclear. Fingerprints were established by Southern blotting with HaeIII-digested genomic DNA and a probe mixture consisting of DNA fragments corresponding to the S. aureus collagen adhesin (cna), fibronectin-binding protein (fnbA and fnbB), and beta-toxin (hlb) genes. An unambiguous fingerprint was obtained for all S. aureus isolates. No hybridization signal was observed with S. intermedius. Twenty-seven of the 29 related strains in the SB and SC groups were correctly identified as belonging to one of the four epidemiologically related groups. Our protocol was less successful with respect to the exclusion of unrelated strains. Specifically, only 6 of 11 unrelated strains in the SB and SC groups had a fingerprint that was distinct by comparison to the fingerprints of the outbreak strains. Nevertheless, our protocol was relatively accurate by comparison to the accuracies of the other methods and was one of only six methods that accurately identified all of the repetitive strains included as internal controls.

We used genomic fingerprinting to investigate an outbreak of methicillin-resistant Staphylococcus aureus in the neonatal intensive care units (NICUs) of two hospitals. The hospitals are located in the same city and are part of the same medical care system. Fingerprinting was done by Southern blot hybridization with DNA probes for the genes encoding the S. aureus collagen adhesin (cna), fibronectin-binding proteins (fnbA and fnbB), and beta-toxin (hlb). Genomic DNA was digested with HaeIII (cna and fnbA-fnbB probes) or HindIII (hlb probe). Hybridization patterns could be distinguished on the basis of (i) the presence or absence of cna, (ii) the size of the restriction fragment containing the cna gene, (iii) restriction fragment length polymorphisms within fnbA and fnbB, (iv) the presence of a lysogenic phage within hlb, and (v) the sizes of the restriction fragments containing the phage-bacterial DNA junction fragments. Over a period of 4 months we examined a total of 46 isolates obtained from various wards within each hospital. Among these 46 isolates, we observed a total of 4 cna patterns, 11 fnbA-fnbB patterns, and 11 hlb patterns. Southern blots with HaeIII-digested genomic DNA and a combination of all three gene probes revealed a total of 16 clearly distinguishable patterns. A total of 22 of the 46 isolates were identical with respect to every genomic marker examined. A total of 21 of these 22 isolates were obtained from patients within an NICU. Nineteen of 21 isolates also exhibited identical antibiotic resistance profiles (antibiogram). Although 5 of the remaining 24 strains exhibited an antibiogram identical to those of the NICU isolates, all 24 strains could be distinguished from the NICU isolates by at least one genomic marker. These results suggest that the NICU isolates had a common origin and that genomic fingerprinting with the cna, fnbA, fnbB, and hlb gene probes can provide an important epidemiological tool for the identification of clinical isolates of S. aureus.

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