Distinguishing homogeneous and heterogeneous origins of nonexponential dynamics using multiple population period transient spectroscopy.

摘要

Despite several experimental and theoretical efforts, origins of nonexponential relaxation dynamics are not well understood. We have developed a new spectroscopic technique called MUPPETS (multiple population period transient spectroscopy) which can distinguish between the origins of nonexponential dynamics. MUPPETS uses two time periods where the first time period acts as a filter time for fast decaying molecules. Time domain MUPPETS can be transferred to rate domain by using an inverse Laplace transform. Rate domain MUPPETS show remarkable similarity to existing frequency domain hole burning techniques such that instead of burning frequency lines, relaxation rates are burned in the spectra. MUPPETS was tested on two samples namely auramine (∼20ps) in methanol and a mixture of auramine and coumarin (∼4.7ns) in methanol. MUPPETS successfully distinguished the origins of nonexponential dynamics in both cases where auramine in methanol showed primarily homogeneous origins and the mixture showed heterogeneous origins for nonexponential dynamics. MUPPETS uses six ultrafast phase stabilized pulses. In order to develop MUPPETS technique mainly three challenges were addressed: first diffractive optics were used to generate passively phase stabilized pulses, second spherical aberration problem was solved by using meniscus lens and third a new balanced detection method was developed to isolate higher order resonant response. After the proof of concept and validation of MUPPETS, it was applied to solute dynamics (auramine) in 3-butyl-1-methylimidazoliumhexaflorophosphate (BMIPF6). BMIPF6 is an ionic liquid which is molten at room temperature. There has been strong but indirect evidence that structural heterogeneity in ionic liquids affect the dynamics of solute molecules. We provide more direct evidence that solute dynamics in BMIPF6 show dispersed kinetic rates, however the origins are homogeneous in nature.