Spectrotemporal imaging of ultrafast electron dynamics
We demonstrate a spectrotemporal imaging technique that maps ultrafast electron dynamics onto energy- and angle-resolved photoelectron distributions in a single measurement. The underlying principle is to encode the probing time into the kinetic energy of the emitted photoelectrons, while the evolving quantum dynamics is retrieved from their angular distributions. We experimentally demonstrate this concept by imaging spin-orbit wave packet dynamics in resonance-enhanced multiphoton ionization of potassium atoms using two complementary approaches. In the first, three spectrally distinct probe pulses sample the dynamics at selected delays and map the corresponding wave packet configurations onto separate photoelectron energy channels. In the second, a single broadband chirped probe pulse establishes a continuous time-to-energy mapping through its time-dependent instantaneous frequency, thereby encoding the evolution of the wave packet across a broad photoelectron energy band. The resulting photoelectron momentum distribution exhibits a characteristic fork-shaped pattern. Combining supercontinuum pulse shaping with velocity map imaging, we experimentally resolve the spin-orbit dynamics over a time window exceeding 300 fs, in good agreement with quantum dynamical simulations. Our results establish spectrotemporal photoelectron imaging as a versatile approach to time-resolved spectroscopy, replacing sequential delay scans by parallel acquisition of ultrafast quantum dynamics.