M. N. Piancastelli

Nuclear motion driven by the Renner–Teller effect as observed in the resonant Auger decay to the X̃2Π electronic ground state of N2O+

High-resolution Auger spectroscopy applied under resonant Auger Raman conditions is shown to be a powerful tool for characterizing complex potential energy surfaces in core-excited systems. Using the example of Nt 1s–1*2 resonant Auger transition in nitrous oxide we emphasize the interplay between the nuclear motion and the electronic decay. We show how the choice of excitation energy allows selection of core-excited species of different geometries. The nuclear dynamics of these species are mapped by measuring the resonant Auger decay spectra. In addition to the changes in vibrational structure observed for the resonant Auger decay spectra, a strong influence of nuclear motion on the electronic decay is revealed, inducing the so-called dynamical Auger emission. The experimental results are supported by ab initio quantum chemical calculations restricted to a linear geometry of the core-excited state.

Mapping potential energy surfaces by core electron excitation: the resonant Auger decay spectrum of BF3

High-resolution electron spectroscopy applied under resonant Auger-Raman conditions is shown to be a powerful tool for characterizing complex potential energy surfaces of core-excited states in polyatomic systems. BF3 is used as a prototype molecule: excitation of the Bls --> 2a(2)() transition leads to an exceptional out-of-plane vibrational excitation resolved over a wide energy range (3.6 eV). The photon energy dependence of the spectra through the resonance profile is rationalized in the frame of the relative slopes of the potential energy curves along the boron out-of-plane displacement coordinate.

Evidence of ultra-fast dissociation in ammonia observed by resonant Auger electron spectroscopy

We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the Nls --> 4a(1) core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z + l approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH2* and H fragments

Molecular-frame photoelectron angular distribution imaging studies of OCS S1s photoionization

Molecular-frame photoelectron angular distribution imaging studies of OCS S1s photoionization

Acetylacetone photodynamics at a seeded free-electron laser

The first steps in photochemical processes, such as photosynthesis or animal vision, involve changes in electronic and geometric structure on extremely short time scales. Time-resolved photoelectron spectroscopy is a natural way to measure such changes, but has been hindered hitherto by limitations of available pulsed light sources in the vacuum-ultraviolet and soft X-ray spectral region, which have insufficient resolution in time and energy simultaneously. The unique combination of intensity, energy resolution, and femtosecond pulse duration of the FERMI-seeded free-electron laser can now provide exceptionally detailed information on photoexcitation–deexcitation and fragmentation in pump-probe experiments on the 50-femtosecond time scale. For the prototypical system acetylacetone we report here electron spectra measured as a function of time delay with enough spectral and time resolution to follow several photoexcited species through well-characterized individual steps, interpreted using state-of-the-art static and dynamics calculations. These results open the way for investigations of photochemical processes in unprecedented detail.The first steps in photochemical processes involve changes in electronic and geometric structure on extremely short timescales. Here, the authors report femtosecond dynamics in prototypical acetylacetone, by pump-probe photoexcitation-photoemission experiments and static and dynamics calculations.

Subfemtosecond Control of Molecular Fragmentation by Hard X-Ray Photons

Tuning hard x-ray excitation energy along Cl 1s→σ^{*} resonance in gaseous HCl allows manipulating molecular fragmentation in the course of the induced multistep ultrafast dissociation. The observations are supported by theoretical modeling, which shows a strong interplay between the topology of the potential energy curves, involved in the Auger cascades, and the so-called core-hole clock, which determines the time spent by the system in the very first step. The asymmetric profile of the fragmentation ratios reflects different dynamics of nuclear wave packets dependent on the photon energy.

Experimental setup for the study of resonant inelastic X-ray scattering of organometallic complexes in gas phase

A new setup has been designed and built to study organometallic complexes in gas phase at the third-generation Synchrotron radiation sources. This setup consists of a new homemade computer-controlled gas cell that allows us to sublimate solid samples by accurately controlling the temperature. This cell has been developed to be a part of the high-resolution X-ray emission spectrometer permanently installed at the GALAXIES beamline of the French National Synchrotron Facility SOLEIL. To illustrate the capabilities of the setup, the cell has been successfully used to record high-resolution Kα emission spectra of gas-phase ferrocene Fe(C5H5)2 and to characterize their dependence with the excitation energy. This will allow to extend resonant X-ray emission to different organometallic molecules.

Double-core-hole states in CH3CN: Pre-edge structures and chemical-shift contributions

Spectra reflecting the formation of single-site double-core-hole pre-edge states involving the N 1s and C 1s core levels of acetonitrile have been recorded by means of high-resolution single-channel photoelectron spectroscopy using hard X-ray excitation. The data are interpreted with the aid of ab initio quantum chemical calculations, which take into account the direct or conjugate nature of this type of electronic states. Furthermore, the photoelectron spectra of N 1s and C 1s singly core-ionized states have been measured. From these spectra, the chemical shift between the two C 1s-1 states is estimated. Finally, by utilizing C 1s single and double core-ionization potentials, initial and final state effects for the two inequivalent carbon atoms have been investigated.

Potential Energy Surface Reconstruction and Lifetime Determination of Molecular Double-Core-Hole States in the Hard X-Ray Regime

A combination of resonant inelastic x-ray scattering and resonant Auger spectroscopy provides complementary information on the dynamic response of resonantly excited molecules. This is exemplified for CH_{3}I, for which we reconstruct the potential energy surface of the dissociative I 3d^{-2} double-core-hole state and determine its lifetime. The proposed method holds a strong potential for monitoring the hard x-ray induced electron and nuclear dynamic response of core-excited molecules containing heavy elements, where abxa0initio calculations of potential energy surfaces and lifetimes remain challenging.