Surface-enhanced resonance Raman scattering using pulsed and continuous-wave laser excitation

Pulsed and continuous-wave (CW) lasers were compared as excitation sources for surface-enhanced resonance Raman scattering (SERRS). CW excitation provided SERRS spectra with a greater signal-to-noise ratio and more sensitive detection by a factor of 50 compared with the high peak power, low repetition rate pulsed configuration used. The SERRS intensity using a pulsed laser produced a non-linear response with respect to changes in power of the laser. At powers of less than similar to 0.012 mW, the absolute intensity under the peaks of the CW and pulsed SERRS spectra converged, suggesting that lower peak power, high repetition rate systems may be more effective excitation sources for SERRS. Transmission electron microscopy of pulsed laser-irradiated silver particles showed significant sample damage and morphological changes. This problem was overcome with the use of a recirculating large-volume flow cell system, providing a fresh sample for each measurement. A picosecond-resolved time delay experiment found that SERRS intensity decreased by similar to 60% when exposed to a 400 nm pump pulse and probed with a 529 nm pulse. As the time delay between pump and probe increased the system recovered gradually to similar to 60% of the original SERRS intensity after 50 ps, where it remained constant. This suggests that the surface bonding between the silver and the dye is significantly perturbed, with some nanoscale diffusion occurring of the dye away from the metal surface. Hence chemical enhancement is temporarily prevented and electromagnetic enhancement is reduced as a function of 1/r(3) as the dye moves away from the surface. Additionally, transient heating of the colloidal particles caused by the pulsed laser may also lead to plasmon shifts and changes in absorption intensity. Other factors such as surface annealing or decomposition of the silver particle or dye due the extreme temperature conditions may account for the permanent loss in SERRS intensity. Copyright (c) 2005 John Wiley & Sons, Ltd.