Computer simulation of proton channelling in silicon

The channelling of 3 MeV protons in the 〈110〉 direction of silicon has been simulated using Vineyard model taking into account thermally vibrating nuclei and energy loss due to ion-electron interactions. A beam made up of constant energy particles but with spatial divergence has been simulated for the purpose. The values of the minimum scattering yield and half width of the channelling dip are shown to be depth sensitive and agree well with the measured values. The dependence of yield on the angle of incidence has been found to give information of all three types of channelling. The critical angles for the three types of channelling and wavelength of planar oscillations are consistent with the previous calculations.     

Neutral pion production at 350 MeV/u in the system20Ne +27Al

π⁰ -production probability of (9± 2)· 10^{? 3} per²⁰Ne+Al reaction has been measured at 350 MeV/u using the Two Arm Photon Spectrometer TAPS. This yield is consistent with an interpolation of published π-production rates in heavy ion collisions.     

A femtosecond time-resolved investigation of dual fluorescence from N6,N6-dimethyladenine

The excited electronic state dynamics of N(6),N(6)-dimethyladenine (DMAde), a molecule known to emit dual fluorescence, has been studied in aqueous solution using femtosecond fluorescence up-conversion spectroscopy. Time profiles of the fluorescence of DMAde excited at lambda= 258 nm were measured at a series of wavelengths in the range 320 nm or= 500 nm), which appeared slightly delayed compared to the UV fluorescence, the long-lived fluorescence component (tau(3)) dominated, the second component (tau(2)) disappeared. The results are consistent with the assumption that DMAde is primarily excited to a short-lived local excited (LE) electronic state that fluoresces mostly in the UV and decays rapidly, on a approximately 0.5 ps timescale, to an intramolecular charge transfer (ICT) state that emits only at longer wavelengths in the visible spectrum. The fluorescence-time profiles and transient fluorescence spectra reconstructed from the time profiles provided further information on secondary relaxation processes within and between the excited states and their non-radiative relaxation to the electronic ground state.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

Electron lifetime in a Shockley-type metal-organic interface state

The lifetimes of electrons at the interface between 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and Ag(111) have been studied by means of time- and angle-resolved two-photon photoemission. We observe a dispersing unoccupied state 0.6 eV above the Fermi level with an effective electron mass of 0.39m{e} at the Gamma[over ] point. The short lifetime of 54 fs is indicative of a large penetration of the wave function into the metal. Supported by model calculations this interface state is interpreted as predominantly arising from an upshift of the occupied Shockley surface state of the clean metal due to the interaction with the PTCDA overlayer.