  We present a new method for ultrafast spectroscopy of molecular photoexcited
dynamics. The technique uses a pair of femtosecond pulses: a photoexcitation
pulse initiating excited state dynamics followed by a soft x-ray (SXR) probe
pulse that core ionizes certain atoms inside the molecule. We observe the Auger
decay of the core hole as a function of delay between the photoexcitation and
SXR pulses. The core hole decay is particularly sensitive to the local valence
electrons near the core and shows new types of propensity rules, compared to
dipole selection rules in SXR absorption or emission spectroscopy. We apply the
delayed ultrafast x-ray Auger probing (DUXAP) method to the specific problem of
nucleobase photoprotection to demonstrate its potential. The ultraviolet
photoexcited \pi\pi* states of nucleobases are prone to chemical reactions with
neighboring bases. To avoid this, the single molecules funnel the \pi\pi*
population to lower lying electronic states on an ultrafast timescale under
violation of the Born-Oppenheimer approximation. The new type of propensity
rule, which is confirmed by Auger decay simulations, allows us to have
increased sensitivity on the direct relaxation from the \pi\pi* state to the
vibrationally hot electronic ground state. For the nucleobase thymine, we
measure a decay constant of 300 fs in agreement with previous quantum chemical
simulations.
