Towards observing site-selective chemistry in real time: controlling the formation of molecular double-core-hole states*

The development of x-ray free electron laser (XFEL) light sources, and third generation synchrotron radiation (SR) facilities allowed for the experimental observation of the elusive double-core-hole (DCH) continuum states. More specifically, for molecules, when two electrons bound at two different core-shells have been ejected to the continuum, enhanced chemical shifts can be measured [1]. Using an XFEL the creation of a DCH state relies on sequential multi-photon absorption [2], while their formation using SR can be achieved through electron correlations [3]. The newly offered ω/2ω x-ray pump-probe scheme, offered by the Linac Coherent Light Source (LCLS), can achieve a minimum delay (0 fs) between the pump and the probe pulses, and allows for more control in the creation of a DCH state by optimizing the ionization cross-sections. By varying the delay between the pump and the probe pulses, the time-evolution of the chemical shift along with the processes at play upon the creation of a core-hole can be studied, both before and after Auger decay takes place. Experiments were performed in the newly commissioned TMO endstation equipped with a Velocity Map Imaging (VMI) electron spectrometer for efficient collection of the photoemission signals and a secondary Time-of-Flight (ToF) electron spectrometer used for single shot characterization of the pump and probe pulses. The experimental results have been supported by ab initio calculations performed at the Δ-CCSD(T) level of theory. First studies on a class of fluorocarbon molecules will be presented.

1. Cederbaum et al., J. Chem. Phys. 85, 6513 (1986)

2. Berrah et al., PNAS 108, 16912 (2011)

3. Eland et al., Phys. Rev. Lett. 105, 213005 (2010)