Figure 1: Potential energy surfaces and ultrafast Auger probe scheme.

(a) Sketch illustrating general features of photoexcited molecular states. After a light-induced electronic transition, a nuclear wavepacket is driven by gradients in the potential energy surface (blue) towards regions containing conical intersections (CIs), allowing for ultrafast electronic transitions to lower electronic states (red and black) under violation of the Born–Oppenheimer approximation. The electronic relaxation of the photoexcited population can be intercepted by reaction barriers between the initial photoexcited reaction coordinate and CI region. For the particular case of thymine, the UV excitation (pump) pulse excites a wavepacket on the ππ* state and CIs connect to n π* and ground state (GS). The topology of the ππ* state controls the speed of electronic relaxation and the height of the sketched barrier is currently a matter of debate. The soft X-ray probing is shown by a transition to core-ionized molecular potential energy surfaces, from which the Auger decay proceeds to a manifold of dicationic states. The potential energy difference between core ionized and dicationic state determines the kinetic energy of the Auger electron. (b) Ultrafast Auger probing applied to thymine photoprotection. The images on the left side show the π , n and π* molecular valence orbitals. The oxygen 1s orbitals, in which the core hole can be created, are highly localized at one of the marked two oxygen sites. The π and n orbitals are both doubly occupied in the ground state configuration. The UV pump pulse promotes an electron from a π orbital to a π* orbital, giving rise to the chemically reactive ππ* state. We probe the electronic valence state by a delayed soft X-ray (SXR) probe pulse creating a core hole and inducing Auger decay. The emitted Auger electrons carry information about the valence electronic state at the core-hole position (oxygen in our case).