Observation of a continuum-continuum interference hole in ultrafast dissociating core-excited molecules

Dynamics of core-excited ammonia: disentangling fragmentation pathways by complementary spectroscopic methods

Fragmentation dynamics of core-excited isolated ammonia molecules is studied by two different and complementary experimental methods, high-resolution resonant Auger spectroscopy and electron energy-selected Auger electron-photoion coincidence spectroscopy (AEPICO). The combined use of these two techniques allows obtaining information on different dissociation patterns, in particular fragmentation before relaxation, often called ultrafast dissociation (UFD), and fragmentation after relaxation. The resonant Auger spectra contain the spectral signature of both molecular and fragment final states, and therefore can provide information on all events occurring during the core-hole lifetime, in particular fragmentation before relaxation. Coincidence measurements allow correlating Auger electrons with ionic fragments from the same molecule, and relating the ionic fragments to specific Auger final electronic states, and yield additional information on which final states are dissociative, and which ionic fragments can be produced in timescales either corresponding to the core-hole lifetime or longer. Furthermore, we show that by the combined use of two complementary experimental techniques we are able to identify more electronic states of the NH₂⁺ fragment with respect to the single one already reported in the literature.

Electron spectroscopy and dynamics of HBr around the Br 1s-1 threshold

A comprehensive electron spectroscopic study combined with partial electron yield measurements around the Br 1s ionization threshold of HBr at ≅13.482 keV is reported. In detail, the Br 1s-1 X-ray absorption spectrum, the 1s-1 photoelectron spectrum as well as the normal and resonant KLL Auger spectra are presented. Moreover, the L-shell Auger spectra measured with photon energies below and above the Br 1s-1 ionization energy as well as on top of the Br 1s-1σ* resonance are shown. The latter two Auger spectra represent the second step of the decay cascade subsequent to producing a Br 1s-1 core hole. The measurements provide information on the electron and nuclear dynamics of deep core-excited states of HBr on the femtosecond timescale. From the different spectra the lifetime broadening of the Br 1s-1 single core-hole state as well as of the Br(2s-2,2s-12p-1,2p-2)  double core-hole states are extracted and discussed. The slope of the strongly dissociative HBr 2p-2σ* potential energy curve is found to be about -13.60 eV Å-1. The interpretation of the experimental data, and in particular the assignment of the spectral features in the KLL and L-shell Auger spectra, is supported by relativistic calculations for HBr molecule and atomic Br.

Hard x-ray spectroscopy and dynamics of isolated atoms and molecules: a review

We present here a review of the most significant recent achievements in the field of HAXPES (hard x-ray photoelectron spectroscopy) on isolated atoms and molecules, and related spectroscopies. The possibility of conducting hard x-ray photoexcitation and photoionization experiments under state-of-the art conditions in terms of photon and electron kinetic energy resolution has become available only in the last few years. HAXPES has then produced structural and dynamical information at the level of detail already reached in the VUV and soft-x-ray ranges. The much improved experimental conditions have allowed extending to the hard x-ray range some methods well established in soft x-ray spectroscopies. Investigations of electron and nuclear dynamics in the femtosecond (fs, 10^-15 s) and even attosecond (as, 10^-18 s) regime have become feasible. Complex relaxation phenomena following deep-core ionization can now be enlightened in great detail. Other phenomena like e.g. recoil-induced effects are much more important in fast photoelectron emission, which can be induced by hard x-rays. Furthermore, a new kind of ionic states with double core holes can be observed by x-ray single-photon absorption. Future perspectives are also discussed.

Triple ionization of HCl via states with a 2p core hole

The triple ionization of HCl by double Auger decay and related processes has been studied using a multiparticle coincidence technique combined with synchrotron radiation. Four contributing processes are identified; direct double Auger, two indirect double Auger decay pathways, and single Auger decay from core-valence doubly ionized intermediate states. One indirect Auger process involves autoionization from superexcited states of Cl⁺. Double Auger decay from HCl⁺ (2p^-1, ²P_J), which makes up 11% ± 2% of total Auger decay, is estimated to be 40% direct, 15% indirect via atomic Cl^+* and 45% indirect via molecular intermediate doubly ionized states. The vertical triple ionization energy of HCl is determined as 73.8 ± 0.5 eV. Molecular field effects are found to affect the direct double Auger process as well as normal single Auger decay. A comparison between spectra of the HCl and DCl isotopomers indicates that electronic decay is faster in all the processes than molecular dissociation.

Different Time Scales in the Dissociation Dynamics of Core-Excited CF_{4} by Two Internal Clocks

Fragmentation processes following C 1s→lowest unoccupied molecular orbital core excitations in CF_{4} have been analyzed on the ground of the angular distribution of the CF_{3}^{+} emitted fragments by means of Auger electron-photoion coincidences. Different time scales have been enlightened, which correspond to either ultrafast fragmentation, on the few-femtosecond scale, where the molecule has no time to rotate and the fragments are emitted according to the maintained orientation of the core-excited species, or dissociation after resonant Auger decay, where the molecule still keeps some memory of the excitation process before reassuming random orientation. Potential energy surfaces of the ground, core-excited, and final states have been calculated at the ab initio level, which show the dissociative nature of the neutral excited state, leading to ultrafast dissociation, as well as the also dissociative nature of some of the final ionic states reached after resonant Auger decay, yielding the same fragments on a much longer time scale.

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 ab initio calculations of potential energy surfaces and lifetimes remain challenging.

Imaging the Temporal Evolution of Molecular Orbitals during Ultrafast Dissociation

We investigate the temporal evolution of molecular frame angular distributions of Auger electrons emitted during ultrafast dissociation of HCl following a resonant single-photon excitation. The electron emission pattern changes its shape from that of a molecular σ orbital to that of an atomic p state as the system evolves from a molecule into two separated atoms.

Electronic state-lifetime interference in resonant Auger spectra: a tool to disentangle overlapping core-excited states

Thanks to a new fit approach, electronic state-lifetime interference terms are extracted and used to disentangle overlapping states.

High-resolution photoelectron spectroscopy with angular selectivity - a tool to probe valence-Rydberg states and couplings in HCl(+)

Due to strong electron correlation effects and electron coupling with nuclear motion, the molecular inner-valence photoionization is still a challenge in electron spectroscopy, resulting in several interesting phenomena such as drastic changes of angular dependencies, spin-orbit induced predissociation, and complex interplay between adiabatic and nonadiabatic transitions. We investigated the excited electronic states of HCl(+) in the binding energy range 27.5-30.5 eV using synchrotron radiation based high-resolution inner-valence photoelectron spectroscopy with angular resolution and interpreted the observations with the help of ab initio calculations. Overlapping electronic states in this region were disentangled through the analysis of photoelectron emission anisotropies. For instance, a puzzling transition, which does not seem to obey either an adiabatic or a nonadiabatic picture, has been identified at ∼28.6 eV binding energy. By this study, we show that ultrahigh-resolution photoelectron spectroscopy with angular selectivity represents a powerful tool to probe the highly excited ionic molecular electronic states and their intricate couplings.

Ultrafast dynamics in C 1s core-excited CF4 revealed by two-dimensional resonant Auger spectroscopy

Following core excitation in an isolated molecule, ultrafast dissociation of one particular chemical bond can occur, where "ultrafast" is defined as taking place during the lifetime of the core hole, of the order of few femtoseconds. The signature of such phenomenon can be observed in resonant Auger spectra following core excitation. We present here an investigation of ultrafast dissociation following C 1s-to-σ* core excitation in CF4, with high-resolution resonant Auger spectroscopy. We are able to characterize final states of both the molecular ion and the CF3 (+) fragment. We use two-dimensional (2D) maps to record resonant Auger spectra across the resonance as a function of photon energy and to characterize ultrafast dynamics. This method provides immediate visual evidence of one of the important characteristics of the study of spectral features related to molecular versus fragment ionic final states, and namely their dispersion law. In the 2D maps we are also able to identify the dissociation limit for one of the molecular final states.

Dissociative x-ray lasing

X-ray lasing is predicted to ensue when molecules are pumped into dissociative core-excited states by a free-electron-laser pulse. The lasing is due to the population inversion created in the neutral dissociation product, and the process features self-trapping of the x-ray pulse at the gain ridge. Simulations performed for the HCl molecule pumped at the 2p(1/2)→6σ resonance demonstrate that the scheme can be used to create ultrashort coherent x-ray pulses.

Vibrational scattering anisotropy generated by multichannel quantum interference

Based on angularly and vibrationally resolved electron spectroscopy measurements in acetylene, we report the first observation of anomalously strong vibrational anisotropy of resonant Auger scattering through the C 1s→π* excited state. We provide a theoretical model explaining the new phenomenon by three coexisting interference effects: (i) interference between resonant and direct photoionization channels, (ii) interference of the scattering channels through the core-excited bending states with orthogonal orientation of the molecular orbitals, (iii) scattering through two wells of the double-well bending mode potential. The interplay of nuclear and electronic motions offers in this case a new type of nuclear wave packet interferometry sensitive to the anisotropy of nuclear dynamics: whether which-path information is available or not depends on the final vibrational state serving for path selection.

Constant-atomic-final-state filtering of dissociative states in the O1s→σ* core excitation in O2

The below-threshold region in core-excited O2 is very complex, consisting of a multitude of exchange-split states with mixed molecular orbital-Rydberg character. We have investigated the nature of these intermediate states by resonant Auger spectroscopy. In particular, we have obtained constant-atomic-final-state yield curves for several atomic peaks in the electron decay spectra which are stemming from ultrafast dissociation. The relative intensity of Auger decay leading to atomic final states is considered a signature of the relative weight of the sigma* character. This method allows one to "filter out" intermediate states with dissociative character. Extensive calculations have been performed by multi-reference configuration interaction at different interatomic distances in order to evaluate the potential curves of the core-excited states and propose a qualitative description of the dissociative molecular dynamics. The calculations show that the core-excited states have a relevant admixture of excitations to orbitals with Rydberg character and excitations to the sigma* orbital with different spin couplings. A diabatization of the adiabatic potential curves shows that the coupling between Rydberg and sigma* diabatic states is very different at the different crossing points and ultrafast dissociation occurs more easily on the lowest sigma* diabatic potential curve. As a consequence, the observation of atomic peaks only in the lower-energy region of the absorption curve is well justified.

Direct observation of electron dynamics in the attosecond domain

Dynamical processes are commonly investigated using laser pump-probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10(-18) s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10(-15) s). Here we monitor the dynamics of ultrafast electron transfer--a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices--on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.

Core excitations of naphthalene: Vibrational structure versus chemical shifts

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.

Intramolecular electron scattering and electron transfer following autoionization in dissociating molecules

Resonant Auger decay of core-excited molecules during ultrafast dissociation leads to a Doppler shift of the emitted electrons depending on the direction of the electron emission relative to the dissociation axis. We have investigated this process by angle-resolved electron-fragment ion coincidence spectroscopy. Electron energy spectra for selected emission angles for the electron relative to the molecular axis reveal the occurrence of intermolecular electron scattering and electron transfer following the primary emission. These processes amount to approximately 25% of the resonant atomic Auger intensity emitted in the studied transition.

Strong, polarized Balmer-alpha fluorescence after resonant core excitation of HCl

Visible-UV fluorescence has been analyzed after resonant Cl 2p core excitation of HCl molecules. The dispersed fluorescence spectra are dominated by emissions from atomic fragments. In particular, an intense and polarized Balmer H(alpha) line is observed after photoexcitation of the 2p(-1)nl Rydberg states. The excited hydrogen atoms are efficiently produced in the resonant Auger process and the subsequent dissociation of high lying HCl+ states. The experimental results, complemented by a time-resolved measurement of the H(alpha) decay, point to a universal mechanism for the production of H( n = 3) atoms in the dissociation of innershell excited HCl molecules.