Abstract
We report an ab-initio study of a pump-probe experiment on the amino-acid glycine. We consider an UV pump followed by an X-ray probe tuned to carbon K-edge and study the vibronic structure of the core transition. The simulated experiment is feasible using existing free electron laser or high harmonic generation sources and thanks to the localization of the core orbitals posseses chemical selectivity. The present theory applies to other experimental schemes, including the use of a THz probe, available with present soft X-ray free electron lasers and/or high harmonic generation sources.
Highlights
Spectroscopic techniques aim at revealing different and detailed matter properties
We only considered the lowest energy core-hole state, this approximation is well motivated by the fact that existing Free Electron Laser (FEL) sources[30] can have sub-1 eV transform limited pulses that can be used to discriminate among the different states
As the one provided by HHG16, may be considered as well at the cost of longer computational times and a richer dynamic that has to be interpreted
Summary
Spectroscopic techniques aim at revealing different and detailed matter properties. The simplest technique, linear absorption, can determine the energy levels of a system[1] and by tuning the photon energy, different degrees of freedom can be investigated, e.g. vibrational and rotational transitions (in the infra-red spectral region)[2,3] and electronic transitions[4,5] (visible/X-ray spectral regions). Broad wavelength tunability and multicolor options are available at existing FELs25–27, stimulating the theoretical description of multi-dimensional X-rays experiments. These theoretical approaches often assume pulses at the cutting edge, and even beyond, current technologies (sub-fs duration, heterodyne detection, multiple pulses with sub-fs delays and independent central wavelength, etc.) driving the development of new sources[27,28,29]. The non-linear response formalism[9] to simulate the optical pump/X-ray probe signal of the simplest amino acid glycine (C2H5NO2), Fig. 1, in order to monitor impulsively excited vibrations: this experimental scheme is achievable within the current FEL technology. We have tuned the probe wavelength to the carbon K-edge and calculated the signal due to four vibrational modes
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