Electron-ion Coincidence Measurements Research Articles

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.

Electronic state influence on selective bond breaking of core-excited nitrosyl chloride (ClNO).

The potential for selective bond breaking of a small molecule was investigated with electron spectroscopy and electron-ion coincidence experiments on ClNO. The electron spectra were measured upon direct valence photoionization and resonant core excitation at the N 1s- and O 1s-edges, followed by the emission of resonant-Auger (RA) electrons. The RA spectra were analyzed with particular emphasis on the assignment of the participator and spectator states. The states are of special relevance for investigating how distinct electronic configurations influence selective bond breaking. The electron-ion coincidence measurements provided branching fractions of the produced ion fragments as a function of electron binding energy. They explicitly demonstrate how the final electronic states created after photoionization and RA decay influence fragmentation. In particular, we observed a significantly different branching fraction for spectator states compared with participator states. In addition, it was also observed that the bonds broken for the spectator states correlate with the antibonding nature of the spectator-electron orbital.

Unimolecular Double Photoionization-Induced Processes in Iron Pentacarbonyl.

The dissociations of nascent Fe(CO)5++ ions created by 40.81 eV photoionization of iron pentacarbonyl have been examined using threefold and fourfold electron–ion coincidence measurements. The energies and forms of the ions have been explored by high-level calculations, revealing several new structures. The most stable form of Fe(CO)5++ has a quite different geometry from that of the neutral molecule. The dissociation pattern can be modeled as a sequence of CO evaporations followed by two-body charge separations. Each Fe(CO)n++ (n = 1–4) dication is stable in a restricted energy range; as its internal energy increases, it first ejects a neutral CO, then loses CO+ by charge separation at higher energy. In the initial stages, charge-retaining CO evaporations dominate over charge separation, but the latter become more competitive as the number of residual CO ligands decreases. At energies where ionization is mainly from the CO ligands, new Fe–C and C–C bonds are created by a mechanism which might be relevant to catalysis by Fe.

Electron-ion coincidence measurements of molecular dynamics with intense X-ray pulses

Molecules can sequentially absorb multiple photons when irradiated by an intense X-ray pulse from a free-electron laser. If the time delay between two photoabsorption events can be determined, this enables pump-probe experiments with a single X-ray pulse, where the absorption of the first photon induces electronic and nuclear dynamics that are probed by the absorption of the second photon. Here we show a realization of such a single-pulse X-ray pump-probe scheme on N_2 molecules, using the X-ray induced dissociation process as an internal clock that is read out via coincident detection of photoelectrons and fragment ions. By coincidence analysis of the kinetic energies of the ionic fragments and photoelectrons, the transition from a bound molecular dication to two isolated atomic ions is observed through the energy shift of the inner-shell electrons. Via ab-initio simulations, we are able to map characteristic features in the kinetic energy release and photoelectron spectrum to specific delay times between photoabsorptions. In contrast to previous studies where nuclear motions were typically revealed by measuring ion kinetics, our work shows that inner-shell photoelectron energies can also be sensitive probes of nuclear dynamics, which adds one more dimension to the study of light-matter interactions with X-ray pulses.

Role of ultrafast dissociation in the fragmentation of chlorinated methanes.

Photon-induced fragmentation of a full set of chlorinated methanes (CH3Cl, CH2Cl2, CHCl3, CCl4) has been investigated both experimentally and computationally. Using synchrotron radiation and electron-ion coincidence measurements, the dissociation processes were studied after chlorine 2p electron excitation. Experimental evidence for CH3Cl and CH2Cl2 contains unique features suggesting that fast dissociation processes take place. By contrast, CHCl3 and CCl4 molecules do not contain the same features, hinting that they experience alternative mechanisms for dissociation and charge migration. Computational work indicates differing rates of charge movement after the core-excitation, which can be used to explain the differences observed experimentally.

低エネルギー電子－分子衝突過程における電子・解離イオン同時計数実験

低エネルギー電子－分子衝突過程における電子・解離イオン同時計数実験

Doubly excited states of molecular nitrogen by scattered electron-ion coincidence measurements

Doubly excited states of molecular nitrogen by scattered electron-ion coincidence measurements

Doubly excited states of molecular hydrogen by scattered electron-ion coincidence measurements

Electron energy-loss spectroscopy and electron-ion coincidence techniques were used to determine the total generalized oscillator strength distribution (GOSD) of molecular hydrogen (H2) under 200 eV incident electron energy at a scattering angle of 6 degree. Ionic GOSDs were obtained for each formed ion at each energy-loss value and were determined by scattered electron-ion coincidence measurements. The total and partial ionic GOSDs of H2 were compared with the OOSDs from the experimental results of both photon and fast-electron impacts. We observed auto-ionization induced by excitation to the optically forbidden Q 1 1Σ+ g (1) state near 28 eV. Additionally, the non-ionic GOSD, defined as the difference between the total GOSD and the total ionic GOSD, was determined. From the non-ionic GOSD, neutral dissociation from excitation to the forbidden Q 1 1Σ+ g (1) state near 28 eV and the Q 2 series near 35 eV were identified.

Doubly excited states of ammonia by scattered electron–ion coincidence measurements

To obtain information on the optically forbidden doubly excited states of ammonia (NH3), we performed scattered electron–ion coincidence measurements. First, we observed scattered electrons using electron energy-loss spectroscopy and determined the generalized oscillator strength distribution (GOSD) under 200 eV incident electron energy at a scattering angle of 8°. Ionic GOSDs were also determined by combination with the coincidence signal, which was observed by the time-of-flight mass spectrometer at each energy-loss value, for each ion. The total and partial ionic GOSDs were compared with the experimental results of both photon and fast electron impact. Moreover, the neutral GOSD determined by subtracting the total ionic GOSD from the total was compared with previous results. In addition to the optically forbidden doubly excited states, which were identified by Kato et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 3541) and Ishikawa et al (2008 J. Phys. B: At. Mol. Opt. Phys. 41 195204), we found a new optically forbidden doubly excited state at around 35 eV.

Auger electron spectra of hydrogenated Si(111)-1×1 surface obtained from Si-L23VV Auger electron Si-2p photoelectron coincidence measurements

The Si-L23VV Auger electron Si-2p photoelectron coincidence spectra (Si-L23VV-Si-2p APECS) of a hydrogenated Si(111)-1×1 surface are measured with an analyzer that is used for electron-electron and electron-ion coincidence measurements of surfaces. The Si-L23VV-Si-2p APECS show a sharp peak, a small shoulder, and a broad large peak at electron kinetic energies of about 85, about 80.5, and about 70 eV, respectively. The clarity of the last two peaks is lower in the Si-L23VV-Si-2p APECS of a clean Si(111)-7×7 surface. We attribute the three peaks to the Si-L23V3pV3p, Si-L23V3pV3s, and Si-L23V3sV3s Auger lines, respectively. The three-peak structure indicates that the local density of states of the 3s + 3p band is reduced in the case of the hydrogenated Si(111)-1×1 surface. This finding is consistent with that of a previous study on valence electronic states of hydrogenated Si(111) using ultraviolet photoemission spectroscopy.

Measurements of molecular-frame Auger electron angular distributions at the CO C 1s−1 2π* resonance with high energy resolution

The molecular-frame Auger electron angular distributions (MFAEADs) of resonantly excited CO 1s → π molecules in the gas phase were determined with high energy resolution using a novel experimental approach. We investigated the process of excitation, Auger decay and fragmentation in unprecedented detail. We confirmed theoretical predictions for the different MFAEADs of close lying Auger final states. By examining the dependence of the MFAEADs on the vibrational state of the excitation and on the fragmentation energy we found that the measured MFAEADs can be considered independent of the vibrational excitation and the fragmentation process. The method used to obtain molecular-frame angular distributions of Auger electrons is based on electron–ion coincidence measurements using two high-resolution electron spectrometers with limited acceptance angles mounted at fixed positions.

H 2 S ultrafast dissociation probed by energy-selected resonant Auger electron–ion coincidence measurements

We have studied the ultrafast dissociation of the H2S molecule upon S 2p3/2-->6a1 inner-shell excitation by combining high-resolution resonant Auger spectroscopy and energy-selected Auger electron-ion coincidence measurements. Auger final states have been correlated to the different fragmentation pathways (S+, HS+, and H2S+ ions). As an original result, we evidence a three-step mechanism to describe the resonant production of S+: the Auger recombination in the HS* fragment is followed for the A 3Pi and c 1Pi states by the S++H fragmentation mechanism.

EICO measurements of inner-core excited mixed rare-gas clusters

The spectra of deep inner-core excited mixed rare-gas clusters were recorded by using electron ion coincidence (EICO) and multi-hit momentum imaging (MHMI) techniques. The EICO spectra for Ar99Kr1 clusters reveal that singly charged ions are emitted from the inner-core excited clusters in addition to the multiple charged ions. The dependence of the EICO spectra on photon energy and cluster size suggests that the holes created through vacancy cascade on the krypton atoms are transferred to the surrounding atoms, and that the singly charged ions are the primary product of the krypton photoabsorption. Charge localization is suggested for the inner-core excited mixed rare-gas clusters from the analysis of the EICO peak width. The MHMI measurements give us direct evidence for the strong charge migration from X-ray absorbing atoms to surrounding atoms. The photon energy dependence of the PSD image for fragment ions suggests that the momentum of the fragment ions depends on the number of charges generated by the vacancy cascade.

Electron–Ion-Coincidence Measurements for K-Shell Excited Free Krypton Clusters

The decay processes of inner-core excited krypton atom and free krypton clusters up to = 1640, being the average cluster size, have been studied by means of electron ion coincidence (EICO) measurements utilizing hard X-ray. Unlike the soft X-ray absorption measurements, highly charged daughter ions are observed. With increasing cluster size, the relative abundance of the multiply charged ions decreases and that of singly charged dimer or trimer ions increases. The results may be interpreted if one takes site-dependent decay processes into account: when the core hole is generated on the cluster surface, the proliferated holes due to the vacancy cascade are strongly localized within the X-ray absorbing atom, while the charges are migrated to surrounding atoms when the X-ray absorption occurs inside the cluster.

Interchannel interference in resonant Auger scattering from fixed-in-space molecules as a technique for structure determination

A method for structure determination of polyatomic molecules with equivalent atoms is suggested. The method is based on an interference pattern in the resonant Auger scattering process. This pattern is caused by interference of resonant Auger channels corresponding to a core hole localized on different equivalent atoms. The predicted effect can be observed in angular resolved electron-ion coincidence measurements or, alternatively, using the ordinary Auger technique on surface-oriented molecules.

X-ray Doppler spectroscopy of ultrafast fragmentation

Doppler effects are now commonly observed for Auger resonances of dissociating atoms in randomly oriented molecules. The physics behind the, yet not observed, Doppler effect for fixed-in-space molecules is different in that there will appear extra Doppler resonances resulting from the diffractional scattering of the Auger electrons by the surrounding atoms. It is argued that as these resonances will show maxima in the bond directions their measurement by current energy and angular resolved electron-ion coincidence experiments will provide structural probing. It is also shown that the electronic Doppler effect caused by nuclear vibrations can be observed also for bound nuclear states making use of electron-ion coincidence measurements. Optimal conditions for such measurements prevail when the scattering duration is comparable with a vibrational period.

Development of a four-element conical electron lens dedicated to high resolution Auger electron–ion(s) coincidence experiments

A four-element conical electron lens has been developed in view of its integration to a double toroidal electron energy analyzer (DTA) dedicated to Auger electron–ion coincidence measurements. The lens design, using electron trajectory numerical simulations, was entirely guided by the perspective of analyzing energetic electrons with high resolution in the multicoincidence regime. The design, construction, and experimental characterization stages of this electron optics system are described in this article. Emphasis is put on the importance of third generation synchrotron radiation sources when performing such multicoincidence experiments.

SELECTIVE FRAGMENTATION OF VALENCE- AND CORE-ELECTRON-EXCITED CD4 and SF6 MOLECULES

Electron–ion coincidence measurements with energy-resolved electrons are a powerful tool in studies of molecular fragmentation processes, since fragmentation from a specific doorway state can be monitored while the remaining reactions are discriminated. Presently, we have applied this technique in measurements of coincidence spectra of the CD 4 and SF 6 molecules after valence and core electron excitation. The newly constructed energy-resolved electron–ion coincidence station developed for this project has been used. Our results on CD 4 are the first demonstration of a correlation between vibrations and molecular dissociation observed in this molecule. The SF 6 spectra reveal strong selectivity in dissociation following distinct electronic states of the molecule. This selectivity reflects the bonding properties of the potential surfaces involved in the studied processes. The obtained results exemplify the potential of the new multicoincidence station used in this work.

The fragmentation of CCl4 dications studied by Auger electron-ion coincidence experiments

The formation and decay of dication states of tetrachloromethane has been studied in an electron impact experiment by Auger electron-ion coincidence measurements. The fragmentation following the Cl2p ionization has been studied in three different binding energy regions of the dication between 33.5 and 48.5 eV. In the experimental data no bound stable or metastable CCl42+ states have been observed and the fragmentation pattern of the dications resulted to be strongly dependent on the state and geometry of the dication.

Separation of neutral versus cationic dissociation processes in an ultracompact double time-of-flight spectrometer: first results on CH 3I

We present a simple experimental setup, which allows simultaneous detection and analysis of photoions and photoelectrons. In short, the apparatus is a hybrid of a time-of-flight mass spectrometer and a time-of-flight magnetic bottle photoelectron spectrometer. For ions and electrons the same field-free drift region as well as the same multichannel plate detector is used. Mass resolution m/Δ m is about 1000. Electron energy resolution Δ E decreases with absolute energy and is 50 meV at 0.5 eV. For lower electron energies the resolution is limited by the bandwidth of the 200 fs laser pulse. The apparatus is tested at the well-known à state dynamics of methyl iodide by femtosecond pump–probe excitation and ionization. The observed fragment ions can have two different origins. They can be formed (1) either by parent cation photodissociation or (2) by ultrafast neutral fragmentation with subsequent fragment ionization by the intense probe laser. Because of the different ionization energies of CH 3I and iodine, neutral, and cationic dissociation pathways can be distinguished by means of electron energy analysis. This tool is very important for interpretation of mass and femtosecond laser pump–probe spectra of short-lived molecular systems. Because of the comparable detection probability for ions and electrons the apparatus would allow electron–ion coincidence measurements at kilohertz laser repetition rates.