Rapid and accurate measurement of the frequency-frequency correlation function.

Using an implementation of heterodyne-detected vibrational echo spectroscopy, we show that equilibrium spectral diffusion caused by solvation dynamics can be measured in a fraction of the time required using traditional two-dimensional infrared spectroscopy. Spectrally resolved, heterodyne-detected rephasing and nonrephasing signals, recorded at a single delay between the first two pulses in a photon echo sequence, can be used to measure the full waiting time dependent spectral dynamics that are typically extracted from a series of 2D-IR spectra. Hence, data acquisition is accelerated by more than 1 order of magnitude, while permitting extremely fine sampling of the spectral dynamics during the waiting time between the second and third pulses. Using cymantrene (cyclopentadienyl manganese tricarbonyl, CpMn(CO)3) in alcohol solutions, we compare this novel approach--denoted rapidly acquired spectral diffusion (RASD)--with a traditional method using full 2D-IR spectra, finding excellent agreement. Though this approach is largely limited to isolated vibrational bands, we also show how to remove interference from cross-peaks that can produce characteristic modulations of the spectral dynamics through vibrational quantum beats.