In action detected 2D four collinear pulses are focused on the sample and an incoherent "action" signal is detected. In the figure fluorescence is detected. The optical phases of the pulses are modulated at radio frequencies. In other words, the spectra of the pulses are slightly shifted in respect of each other. Therefore, while the time delays t1, t2, t3 are fixed, the detected fluorescence has beating component (AC) which is oscillating at all possible modulation combination frequencies. The signal is disentangled to the rephasing, nonrephasing and the double quantum coherence components by applying Fourier transform.
We have applied the fluorescence detected 2D spectroscopy to investigate the electronic structure and the excited state dynamics in photosynthetic antenna complex LH2.