Pump Probe Spectroscopy
Pump probe is one of the most common ultrafast spectroscopic measurement. A laser pump pulse first excites a sample and a time-delayed probe pulse monitors the subsequent photo-induced dynamics. We measure the differential transmission of the probe spectrum after the sample, calculated with and without the pump excitation for each delay between the two pulses. Measurements of this type obtain information on the rates of decay of photoexcited populations, when we use narrowband pump pulses. When broadband pulse are involved they can excite a coherent superposition of multiple chromophore sites.
Information on the coherences can then be observed in oscillations of the signal amplitude. The broadband-width pump pulse presents challenges in the interpretation of the data though, as many transitions are excited simultaneously and many energy transfer pathways contribute to the evolution of the signal amplitude as a function of pump-probe time delay.
Two-dimensional Electronic Spectroscopy (2DES)
2DES has emerged as an optical technique capable of creating and probing coherences while still preserving information on both the excitation and emission energies. In addition to elucidating typical optical properties of molecular systems, 2DES can distinguish homogeneous and in homogeneous broadening and detect correlations between excitonic states.
2DES is similar to transient absorption spectroscopy, however two excitation pulses are used cooperatively to excite the sample followed by the third ‘probe-pulse’ which interacts with the sample after a time delay and initiates the radiation of a signal depending on the optical response of the sample. By controlling the delay between the two excitation pulses, information on the excitation energy is obtained in the data analysis and the resulting spectrum plots the optical response as a function of the excitation frequency and detection frequency for a given time delay.