Some of the most important electronic processes in chemistry occur on truly ultrafast few-femtosecond timescales. Important examples of such processes are (i) electron transfer following absorption of a photon, and (ii) the tunneling of an electron from one electrode to another via a single molecule. Because such processes are extremely rapid, it is generally a challenge to follow such events experimentally.

In the context of electronic motion through π-conjugated organic molecules, it is the valence electrons that are most closely associated with being mobile. However, one can learn quite a bit about ultrafast electronic processes by exciting core electrons into unoccupied molecular orbitals. Such electrons are bound in energetically deep orbitals such as the 1s orbitals of C or N. The energy of the photon required to produce such an electronic excited state are in the X-ray region, which requires use of a synchrotron light source.

When a core electron is excited to a LUMO+n state, this highly energetic excited state  with a core vacancy – a core-hole – lives for a time of order few femtoseconds. At some point the photoexcited electron recombines with the core-hole. In organic molecules, the energy that is released is used to photo-eject one or two valence electron from HOMO-n states. This is known as Auger electron decay, illustrated in the cartoon below with green arrows. The spectral position and intensity of this Auger process can be used to measure rates of ultrafast electron transfer in the frequency or energy domain via a remarkable technique called the Core-Hole Clock.

Electronic structure of a conjugated molecule adsorbed onto an inorganic material. Red arrow shows excitation of a N 1s electron to the LUMO+1, followed by transfer of the electron to the inorganic substrate. Green arrows show the Auger process, which fills the vacant core-hole.

Recently we have begun applying the Core-Hole Clock method to the subject of electron flow into an inorganic substrate through a conjugated small molecule. Understanding this process is important for the field of molecular electronics, whose promise is producing electronic devices composed of a single molecule.

An interesting question is encapsulated by the diagram below. The cartoon shows a benzene derivative chemisorbed onto a gold surface via a thiol bond. The benzene ring is further functionalized with a reporter group R. This group will serve as the source of electrons, which will tunnel through the molecule to the inorganic substrate. How can the rate of this unltrafast process be manipulated via judicious placement of substituents onto the conjugated molecule in a manner that alters the tunneling paths for the electron from R? We are taking advantage of the elemental specificity of X-ray spectroscopy and the time window afforded by resonant Auger electron spectroscopy to begin to answer such questions.