The distribution of local enhancement factors in surface enhanced Raman-active substrates and the vibrational dynamics in the liquid phase

Surface-enhanced Raman scattering (SERS) is observed on the nano-structured noble metallic surface, where Raman scattering is enhanced by a factor G which is frequently about one million, but underlying the factor G is a broad distribution of local enhancement factors eta. To reveal this distribution and obtain more information about structure-enhancement relationships, a method employing photochemical hole-burning is developed. A series of laser pulses with increasing electric fields burned away molecules at sites with progressively decreasing electromagnetic enhancement factors. We have measured this distribution for benzenethiolate molecules on 330 nm silver-coated nanospheres using incident light of wavelength 532 nanometers. The enhancement distribution was found to be a reversed power-law, with minimum and maximum cutoffs. The hottest sites account for just 63 in one million of the total, but contribute 24% to the overall SERS intensity. The substrate with non-close-packed (NCP) nanospheres of similar size has been tested for the comparison purpose. The distributions revealed additional information about their differences. Later, six different substrates have been compared with the excitation wavelength of 790nm laser. On the second project, vibrational dynamics in the liquid phase has been investigated thoroughly with ultrafast time-resolved anti-Stokes Raman spectroscopy. With the only method up-to-date that provides real-time state-resolved measurements of vibrational energy flow in molecules, vibrational energy transfer (VET) and vibrational relaxation (VR) processes are disclosed. Previously our group focused on nontoxic pure liquids, but now we are able to study aqueous solutions and precious or toxic liquids with the use of an upgraded laser system. In particular, I worked with my colleagues to successfully compare the vibrational dynamics between benzene and benzene-d6, as well as to study the vibrational dynamics of small biologically relevant molecules (i.e., glycine, N-methylacetamide and sodium benzoate) in D2O by probing both the solute and solvent. In both cases, the vibrational energy relaxation to the bath was monitored by either ultrafast Raman calorimetry for benzene or molecule thermometer for aqueous solution