Medha Rele, Sudhir Kapoor, and Tulsi Mukherjee

Radiation Chemistry & Chemical Dynamics Division
Bhabha Atomic Research Centre, Mumbai 400085, INDIA

(email:mukherji@magnum.barc.ernet.in)

Nanoparticles have a variety of unique spectroscopic, electronic and chemical properties due to their small sizes and high surface to volume ratios. In the case of noble metal nanoparticles the surface plasmon absorption band appears in the visible region. Because of this, the optical properties of noble nanoparticles have received considerable attention. The study of dissipation processes of photoexcited carriers in metals has been the subject of intense investigations. In metals unoccupied orbitals are not restricted by a band gap, therefore the photoexcited electrons relax directly to a thermal equilibrium with the electron gas. In this process both the energy and momentum of the electrons get randomised. Due to this the lifetime of individual electronic excited states is always short, typically in the order of only a few femtoseconds to few tens of femtoseconds. The radiation resulting from decaying Surface plasmon in small metallic particles has been observed in Isopropanol:Cyclohexane mixtures. The surface plasmon of silver sol was excited by irradiation with 7Mev electron pulses of duration 50ns or 2µs. The silver sol was found to emit light in the range of 300 to 400 nm with a maximium at around 340nm. The intensity of 340 nm emission was found to decrease with decrease in concentration of Ag particles in the sol. The steady-state absorbtion spectrum of the sol showed a maximum at 410 nm and was brownish in colour. The silver particles were characterised by XRD and TEM. The size of particles was found to be in the range of 20 nm. The TEM images show that Ag particles consist of nearly spherical crystallites. However, at many places twinned particles were also observed. On absorption of Cerenkov light or energy from excited states of solvents the following fundamental excitations may take place (1) direct excitations of a single particle state and (2) excitation of surface plasmon. Energy transfer processes by Cerenkov light are similiar to that observed in photochemical experiments although the excitation field generated involving the passage of a fast electron and laser pulse is different. On illumination with light,the light field oscillates with the surface plasmon frequency. This facilitates a resonant multiphoton excitation. In contrast a fast electron generates a time- dependent electric field with a broad frequency distribution. This leads to a direct excitation of a multiply excited plasmon.