Ultrafast X-ray Auger probing of photoexcited molecular dynamics

Abstract

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.

Ultrafast isomerization initiated by X-ray core ionization

“…These light sources have been employed for molecular and material science, as well as structural biology [5][6][7][8][9][10][11][12][13][14][15] . Current and future X-ray FELs plan to deliver ultrashort X-ray pulses with sufficiently high intensity to collect diffraction images of selected targets in a single shot.…”

Photoelectron spectra of 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil

“…Also, up to our knowledge, no time-resolved photoelectron spectroscopy (TRPES) 27 has been reported for these molecules, even though this methodology has been extensively applied to study the excited-state dynamics of the canonical nucleobases [28][29][30] (see, e.g., Ref. 31 for a list of these experiments) and a small number of nucleobase analogous.…”

Detecting electronic coherence by multidimensional broadband stimulated x-ray Raman signals

“…X-ray signals allow one to probe valence excitations rather than vibrations. Furthermore, both the initial preparation and/or the preparation process can then be the product of an X-ray scattering, photoionization, or Auger process (as recently discussed in [24,40]) in addition to an X-ray pulse E 1 centered at t 1 interacts with the system and after another delay T 2 = t 2 − t 1 the second pulse E 2 centered at t 2 yields the two-dimensional spectra as a function of the two time delays. The corresponding diagrams are given in Fig.…”

“…Optical Raman techniques have been applied to study electron transfer and nonadiabatic dynamics at conical intersections. Using recently developed FEL and HHG sources [17][18][19], Raman techniques can be further extended to the X-ray regime [20][21][22][23][24] whereby the system is initially prepared in the superposition of valence electronic states and an X-ray Raman probe then reveals information about electronic, rather than vibrational, coherence.…”