Standard spectroscopic methods such as absorption, emission, and fluorescence excitation spectroscopy reveal information about the energy levels of the investigated sample. Time-resolved techniques decipher energy transfer processes between those levels. Among them, transient absorption (TA) spectroscopy is the most widespread method to monitor the time evolution of ultrafast phenomena.

A conventional TA experiment with femtosecond pulses employs a spectrally well-defined pump pulse (along wavevector k₁), followed by a weaker broadband probe pulse (k₂) covering all relevant molecular transitions. The TA signal is then recorded as the difference between the transmitted probe spectra with and without the pump pulse. The system's evolution is monitored by repeating the procedure for different pump-probe delays. A schematic representation of TA is shown below. We have implemented several specializations of TA, see sub-sections.