  Triple sum-frequency (TSF) spectroscopy measures multidimensional spectra by
resonantly exciting multiple quantum coherences of vibrational and electronic
states. In this work we demonstrate pump-TSF-probe spectroscopy in which a pump
excites a sample and some time later three additional electric fields generate
a probe field which is measured. We demonstrate pump-TSF-probe spectroscopy on
polycrystalline, smooth, thin films and spiral nanostructures of both MoS2 and
WS2. The pump-TSF-probe spectra are qualitatively similar to the more
conventional transient-reflectance spectra. While transient-reflectance
sensitivity suffers under low surface coverage, pump-TSF-probe sensitivity is
independent of the sample coverage and nanostructure morphologies. Our results
demonstrate that pump-TSF-probe is a valuable methodology for studying
microscopic material systems.
