Preparation and control of electronic wave packets in neutral molecules via an attosecond X-Ray pulse

Abstract

The goal of attochemistry is to use the properties of electronic wave packets excited by an attosecond pulse to manipulate the electronic distribution within a molecule and control its chemical reactivity. This work focuses on studying the preparation of an electronic wave packet in a neutral molecule by means of an attosecond X-Ray pulse. The electronic wave packet is expanded in terms of the core- and valence-excited states of the molecule, which are coherently populated respectively via a one-photon X-Ray absorption and a two-photons impulsive stimulated X-Ray Raman process. The goal of this work is to analyse the characteristics of the electronic excitation and their dependence on the pulse polarisation and the X-Ray excitation edge. Particular attention is posed on the atom-specific properties of the X-Ray excitation, a goal being the unequivocal demonstration of the possibility of initially localising the valence wave packet (excited by the X-Ray Raman process) on the specifically pumped atom. The excitation is described within time-dependent perturbation theory, modelling the X-Ray absorption as a first-order process and the X-Ray Raman as a second-order process. The electronic structure of the considered molecules, i.e. Carbonyl sulfide and Oxazole, is described at the equation-of-motion coupled-cluster level of theory, including an accurate description of the X-Ray Raman process which avoids the customary truncation of the sum-over-states characterising second-order perturbative contributions. The evolution of the intramolecular electronic distribution due to the preparation and propagation of the electronic wave packet are analysed in terms of the time-dependent electronic density. In particular, the 'excited' part of the electronic density is decomposed in terms of the main perturbative components of the excitation, separating the contributions related to the core-excited and the valence-excited states. In the initial stages of the time-evolution the contributions relative to the core-excited states, which are characterised by an atom-specific localised character, play the dominant role. The atom-specific localisation of the core-excitation can be transferred to the manifold of valence-excited states via the X-Ray Raman process, provided a sufficiently large number of valence-excited states are included in the wave packet. This is the case of Oxazole, wherein the possibility of localising the initial 'source' of the valence electronic migration around a specific atom of the molecule is shown. This conditions the subsequent electronic migration, whose spatial distribution depends also on the pulse polarisation, which controls the symmetry of the states included in the wave packet. Owing to the ultrafast decay of the core-excited states, on a long timescale, when the nuclear motion starts to have a sizeable influence, the electronic dynamics are dominated by the component associated with the valence excitation. This makes the impulsive stimulated X-Ray Raman technique a valuable tool to control the chemical reactivity in a neutral molecule.