Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation-X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the pp* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the np* state.

Rapid proton migration is a key process in hydrocarbon photochemistry. Charge migration and subsequent proton motion can mitigate radiation damage when heavier atoms absorb X-rays. If rapid enough, this can improve the fidelity of diffract-before-destroy measurements of biomolecular structure at X-ray-free electron lasers. Here we study X-ray-initiated isomerization of acetylene, a model for proton dynamics in hydrocarbons. Our time-resolved measurements capture the transient motion of protons following X-ray ionization of carbon K-shell electrons. We Coulomb-explode the molecule with a second precisely delayed X-ray pulse and then record all the fragment momenta. These snapshots at different delays are combined into a 'molecular movie' of the evolving molecule, which shows substantial proton redistribution within the first 12 fs. We conclude that significant proton motion occurs on a timescale comparable to the Auger relaxation that refills the K-shell vacancy.

Attosecond electron wave packet in a molecule is captured by the pump-probe method with a-few-pulse attosecond pulse train.

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Recent experiments on double photoionization of H2 with photon energies between 160 and 240 eV have revealed body-frame angular distributions that suggest classical two-slit interference effects may be present when one electron carries most of the available energy and the second electron is not observed. We report precise quantum mechanical calculations that reproduce the experimental findings. They reveal that the interpretation in terms of classical diffraction is only appropriate at substantially higher photon energies. At the energies considered in the experiment we offer an alternative explanation based on the mixing of two non-diffractive contributions by circularly polarized light. [5,6]. In particular, Fernández et al [3] have shown by explicit inclusion of electron correlation and nuclear motion, that the observed interferences in H 2 can indeed be interpreted as resulting from diffraction of a single electron by the two nuclei, the second electron being a mere spectator. Surprisingly, in one-photon double ionization of H 2 , a process that is only possible through electron correlation [7], very recent experimental results by Akoury et al. [8] and Kreidi et al. [9] have suggested that a similar interpretation is still appropriate when one electron is much faster than the other. On the theoretical front, over the past four years new computational developments have made it possible to solve the Schrödinger equation numerically for double ionization of two-electron molecules to produce effectively exact wave functions and cross sections [7,[10][11][12][13][14]. In this letter we report such calculations at the photon energies used in the experiments [8,9], and show that at these energies there is almost no trace of double slit diffraction patterns and that the apparent interference patterns arise from the use of circularly polarized light. However, we are able to predict that the effects sought in these experiments can indeed be observed at higher photon energies.In the experiments of Akoury et al.[8], the central observation was a four-lobed angular distribution seen for the faster of the two ejected electrons when it carries most of the available kinetic energy and when the other electron is not detected. These experiments use the cold target recoil ion momentum spectroscopy (COLTRIMS) method of coincident detection of the electrons and the protons released by the Coulomb explosion that follows complete ionization of the H 2 molecule. For that reason the experiment is able to give kinematically complete information about double photoionization of molecules whose orientation is known. Of course, it is the knowledge of that orientation that makes the discussion of angular diffraction effects possible, and this is one of the unique qualities of this powerful momentum imaging technique. The photon energies used were 160 eV and 240 eV, corresponding to maximum available energies (from a vertical transition to the doubly ionized state) to be shared by the two outgoing electrons of 109 eV and 189 eV respectiv...

Coincident measurement of the Auger electron and fragment ion momenta emitted after carbon core-level photoionization of acetylene has yielded new understanding of how the dication fragments. Ab initio calculations and experimental data, including body-frame Auger angular distributions, are used to identify the parent electronic states and together yield a comprehensive map of the dissociation pathways which include surface crossings and barriers to direct dissociation. The Auger angular distributions for certain breakup channels show evidence of core-hole localization.

We investigate the photodouble ionization of H 2 molecules with 400 eV photons. We find that the emitted electrons do not show any sign of two-center interference fringes in their angular emission distributions if considered separately. In contrast, the quasiparticle consisting of both electrons (i.e., the "dielectron") does. The work highlights the fact that nonlocal effects are embedded everywhere in nature where many-particle processes are involved.