Selective gating to vibrational modes through resonant X-ray scattering

Abstract

The dynamics of fragmentation and vibration of molecular systems with a large number of coupled degrees of freedom are key aspects for understanding chemical reactivity and properties. Here we present a resonant inelastic X-ray scattering (RIXS) study to show how it is possible to break down such a complex multidimensional problem into elementary components. Local multimode nuclear wave packets created by X-ray excitation to different core-excited potential energy surfaces (PESs) will act as spatial gates to selectively probe the particular ground-state vibrational modes and, hence, the PES along these modes. We demonstrate this principle by combining ultra-high resolution RIXS measurements for gas-phase water with state-of-the-art simulations.

Infrared-pump–x-ray-probe spectroscopy of vibrationally excited molecules

“…1(b)). Computational details can be found in our previous RIXS study of the vibrationally cold water molecule [15] which confirms the applicability of the model. The normal mode picture describes well with respect to its minimum.…”

“…In the ground electronic state, the two stretching modes are nearly degenerate and strongly coupled via Darling-Dennison coupling [23][24][25], which have to be considered together for a proper system description. In contrast, the bending mode is only weakly coupled to the stretching modes, and can thus be considered separately in an orthonormal approximation [15]. Previous studies [15,26] have shown that the bending mode is only weakly excited in XAS and RIXS near the two lowest core-excited states.…”

“…In contrast, the bending mode is only weakly coupled to the stretching modes, and can thus be considered separately in an orthonormal approximation [15]. Previous studies [15,26] have shown that the bending mode is only weakly excited in XAS and RIXS near the two lowest core-excited states. Due to this circumstance, we neglect here the bending motion and focus fully on the two coupled stretching modes using a two-dimensional (2D) approach, as described below.…”

“…The proposed method is applied to the water molecule, in the xray spectral region near the two lowest core-excited states of dissociative 1a −1 1 4a 1 and bound 1a −1 1 2b 2 character [12,13]. We show advantages of the proposed schemes with the help of numerical simulations using two-dimensional wave packet techniques [14,15] combined with high level ab initio calculations of the PESs. We conclude that XAS from a vibrationally excited molecule allows to probe an extended region of the core-excited PES, accordingly to the reflection principle [16].…”

“…For the water molecule µ 1 = µ 2 = µ H , while for isotopic substitution of water HDO, studied here as well, µ 1 = µ H and µ 2 = µ D . In order to compute the XAS and RIXS profiles we employ the time-dependent wave packet formalism [29] in the 2D representation [6,15] with the Hamiltonian (2), as it is described in the following subsections.…”