K–L resonant X-ray Raman scattering as a tool for potential energy surface mapping S. Carniato a, * , R. Taı ¨eb a , R. Guillemin a , L. Journel a , M. Simon a , F. Gel’mukhanov b a Laboratoire de Chimie Physique-Matie `re et Rayonnement, Universite ´ Pierre et Marie Curie (Paris VI), UMR 7614 du CNRS, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b Theoretical Chemistry, Roslagstullsbacken 15, Royal Institute of Technology, S-106 91 Stockholm, Sweden Received 22 February 2007; in final form 22 March 2007 Available online 30 March 2007 Abstract With help of theoretical calculations, we demonstrate that original pump-probe experiments, using IR laser as a pump and K–L X-ray Raman scattering as a probe, offer a powerful tool to map core-excited potential energy curves with very high energy resolution, beyond vibrational and lifetime limitations. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Computing potential energy surfaces (PES) is a major and essential task in theoretical physical chemistry. While the technique is now well understood for ground and low lying excited states, accurate ab initio calculations for core-excited states is complex. However, knowledge of these PES is crucial for molecular dynamics studies after X-ray absorption. Experimental determination of the potentials would be extremely valuable but is yet to be achieved. A key point for experimental exploration of core-excited PES from X-ray measurements is high spectral resolution, which is limited both by physical and instrumental factors. Continuous technical advances have improved the spectral resolution of X-ray spectrometers [1,2], leaving only two physical limitations as major factors, namely, lifetime and vibrational broadening. However, the lifetime broadening is very large ([1 eV) for inner shells with binding energies in the hard X-ray region, i.e., 10 times larger than for inner shells in the soft X-ray region, and is comparable to, or lar- ger than, the vibrational spacing of small molecules. This, in such case, reduces strongly the resolution obtainable for mapping PES using regular X-ray absorption tech- niques. X-ray Raman scattering in hard X-ray region, on the other hand, has an great advantage over X-ray absorp- tion: it provides lifetime-broadening removed resolution [3,4]. Furthermore, vibrational broadening in Resonant X-ray Raman scattering (RXRS) is quenched when PES involved in the process are parallel: (1) away from reso- nance if both ground and final states potential surfaces are parallel; (2) on resonance if the core-excited and final states are parallel [5]. As recently reported [6,7], RXRS measurements can greatly take advantage of this effect to investigate ultrafast molecular dynamics. High resolution decay spectra obtained with long-pulse synchrotron light pulses coupled with calculations based on the concept of an effective duration time of the scattering process provide a unique look at very short time nuclear dynamics a poste- riori [9,8]. This opens great opportunities for super-high resolution X-ray spectroscopy beyond vibrational and life- time limitations. The main purpose of this Letter is to propose an original method which combines X-ray Raman techniques with infrared laser providing a tool to map the PES of core- excited states, using the concept of effective duration time of the X-ray scattering process. We propose an experimen- tal scheme where selected vibrational states are populated 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.03.100 * Corresponding author. E-mail address: carniato@moka.ccr.jussieu.fr (S. Carniato). www.elsevier.com/locate/cplett Chemical Physics Letters 439 (2007) 402–406