This approach grants access to states which are unoccupied in equilibrium and allows studying the elementary scattering processes directly in the electronic band structure with fs temporal resolution. Studying the time and energy scales of the decay of an excited state population allows disentangling relevant interactions, which are inherently coupled in equilibrium. We also employ optical excitation as a method to manipulate and control electronic states. For example, light can be used to initiate electronic and structural phase transitions and to stabilize phases not accessible in equilibrium.

For optimal experimental flexibility, we combine in our setup a conventional hemispherical analyzer and a novel time-of-flight-based momentum microscope. The momentum microscope allows simultaneous acquisition of the entire 3D photoelectron signale across the full momentum and energy range, which our pump-probe scheme extends to the 4-dimensional multidimensional photoelectronspectroscopy (MPES) signal I(E_kin, k_x, k_y, t). This approach facilitates unprecedented mapping of band structure dynamics and exhaustive comparison to band structure theory. As a complementary tool, the hemispherical analyzer provides superb statistics in a narrow 2D energy-momentum window I(E_kin, k_II), and enables sensitive detection of very small transient signals.

The benefits of our novel higher-harmonic-generation-based trARPES setup are multifold: The high repetition rate XUV source in combination with the momentum microscope allows mapping the electronic structure dynamics across the full Brillouin zone. The simultaneously improved time resolution of < 40 fs facilitates detection of ever-faster phenomena such as high-frequency coherent modes. As an example, Fig. 4 shows the occupied and unoccupied electronic band structure of the quasi-2D semiconductor WSe₂ after excitation with intense 800 nm pulses at pump-probe delay t = 0 fs.