  Molecular absorption and photo-electron spectra can be efficiently predicted
with real-time time-dependent density-functional theory (TDDFT). We show here
how these techniques can be easily extended to study time-resolved pump-probe
experiments in which a system response (absorption or electron emission) to a
probe pulse, is measured in an excited state. This simulation tool helps to
interpret the fast evolving attosecond time-resolved spectroscopic experiments,
where the electronic motion must be followed at its natural time-scale. We show
how the extra degrees of freedom (pump pulse duration, intensity, frequency,
and time-delay), which are absent in a conventional steady state experiment,
provide additional information about electronic structure and dynamics that
improve a system characterization. As an extension of this approach,
time-dependent 2D spectroscopies can also be simulated, in principle, for
large-scale structures and extended systems.
