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Dynamic Spectroscopies

Prof. J. Hauer, TUM

The Dynamic Spectroscopy group is a team of experimental physicists and physical chemists. We measure, analyze and simulate time-resolved optical signals of molecules and molecular complexes. Our experimental toolbox contains techniques such as pump-probe, heterodyned and homodyned transient grating, and as the most comprehensive method two-dimensional electronic spectroscopy, both in single- and double-quantum variants.


Pump-probe or transient absorption spectroscopy is the by far most wide spread of ultrafast techniques. The transmission of a probe pulse through the investigated sample is recorded with and without the presence of a preceding pump-pulse. By scanning the delay of the probe with respect to the pump, transient molecular species such electronic excited states or vibrational modes can be monitored in their time behaviour. We use pump-probe as a first view on molecular dynamics and to determine lifetimes of electronically excited states. It is also a reference technique for our central method, two-dimensional electronic spectroscopy.

Figure 1Figure 1: Scheme for a transient absorption or pump-probe experiment. The output of a laser source is split into two time-delayed pulses. After the sample, the transmission of one of the pulses is recorded as a function of time delay. This reveals molecular relaxation dynamics.

Two-dimensional electronic spectroscopy

Two-dimensional electronic spectroscopy (2D-ES) and pump-probe are directly related techniques with many similarities. From a theoretical point of view, 2D-ES and pump-probe share the same non-linear signal, but differ in its representation: in pump-probe one integrates over all excitation frequencies and plots the resulting signal in its time dependence. In 2D-ES on the other hand, the signal gets resolved in excitation frequency. What used to be a simple transient in pump-probe, becomes a time-dependent energy landscape in 2D-ES. The coordinates of these landscapes are excitation and emission frequencies. This increase in dimensionality leads to a much richer data basis for model building. Specifically, 2D-ES highlights energy transfer and coherence pathways which could remain elusive in excitation frequency-integrated techniques such as pump-probe.

Figure 2Figure 2: Scheme for a 2D electronic spectroscopy experiment. The output of a laser source is split into four pulses with time delays t1, t2 and tLO. At the sample, this pulse sequence leads to non-linear signal along a background-free direction. Spectrally dispersed detection and Fourier transform t1 to ω1 leads to a spectrally multidimensional and time-resolved signal.