Nerine J. Cherepy

Effective Rejection of Fluorescence Interference in Raman Spectroscopy Using a Shifted Excitation Difference Technique

Resonance Raman spectroscopy is a powerful technique for probing the vibrations of particular chromophores in multicomponent systems. By tuning the wavelength of the excitation laser into resonance with an electronic absorption band of only one molecular species, the vibrational Raman scattering from this species can be selectively enhanced. Thus, resonance Raman spectroscopy can provide structural information for chromophores in solution or biological chromophores within their functionally active protein environment. However, since the very nature of the experiment requires that the excitation light be absorbed by the sample, the measurement of resonance Raman spectra is often made difficult by a large fluorescence background in the same spectral region as the Raman scattering. The problem of fluorescence interference from intrinsic sample emission is often further exacerbated in biological samples, where low-concentration impurities with large fluorescence yields can be difficult to remove. Even weak fluorescence, with an effective fluorescence quantum yield of ≈10−4, completely overwhelms resonance Raman signals, which have typical quantum yields of ≈10−7.

Nature of the power-dependent ultrafast relaxation process of photoexcited charge carriers in II-VI semiconductor quantum dots: Effects of particle size, surface, and electronic structure

The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the p...