Coulomb explosion of diiodomethane CH2I2 molecules irradiated by ultrashort and intense X-ray pulses from SACLA, the Japanese X-ray free electron laser facility, was investigated by multi-ion coincidence measurements and self-consistent charge density-functional-based tight-binding (SCC-DFTB) simulations. The diiodomethane molecule, containing two heavy-atom X-ray absorbing sites, exhibits a rather different charge generation and nuclear motion dynamics compared to iodomethane CH3I with only a single heavy atom, as studied earlier. We focus on charge creation and distribution in CH2I2 in comparison to CH3I. The release of kinetic energy into atomic ion fragments is also studied by comparing SCC-DFTB simulations with the experiment. Compared to earlier simulations, several key enhancements are made, such as the introduction of a bond axis recoil model, where vibrational energy generated during charge creation processes induces only bond stretching or shrinking. We also propose an analytical Coulomb energy partition model to extract the essential mechanism of Coulomb explosion of molecules from the computed and the experimentally measured kinetic energies of fragment atomic ions by partitioning each pair Coulomb interaction energy into two ions of the pair under the constraint of momentum conservation. Effective internuclear distances assigned to individual fragment ions at the critical moment of the Coulomb explosion are then estimated from the average kinetic energies of the ions. We demonstrate, with good agreement between the experiment and the SCC-DFTB simulation, how the more heavily charged iodine fragments and their interplay define the characteristic features of the Coulomb explosion of CH2I2. The present study also confirms earlier findings concerning the magnitude of bond elongation in the ultrashort X-ray pulse duration, showing that structural damage to all but C–H bonds does not develop to a noticeable degree in the pulse length of ∼10 fs.

•Recent progress of double core-hole spectroscopy was reviewed.•Results of the HF, CASSCF, and DFT methods were found to be consistent.•Wagner plots and charge density difference plots were applied to analyze data.With the advent of X-ray free electron lasers and the development of elaborate multi-coincidence methods combined with conventional synchrotron radiation, the interest in double core-hole spectroscopy revived. To describe the measured data and provide the guideline for yet coming experiments, theoretical studies are required. In this paper we review previous theoretical works on double core-hole states and discuss new theoretical results for the XHm–YHn (X, Y = C, N, O, or F; m,n = 0–3) molecules and their fluorine substituted compounds. We compute the single and double core-hole binding energies of these systems using different methods and show that the DFT results agree well with the results of other ab initio methods. In agreement with previous theoretical works, our study demonstrates that the changes in the ionization potential resulting from fluorine substitutions mainly arise due to changes in the ground state and not due to electron relaxation. Special emphasis is given to interatomic relaxation originating from creation of two core holes on different atomic sites. We demonstrate that there exists clear correlation between this quantity, the number of hydrogen atoms and the order of the bond between the heavy atoms in the systems studied.

In recent years, free-electron lasers operating in the true X-ray regime have opened up access to the femtosecond-scale dynamics induced by deep inner-shell ionization. We have investigated charge creation and transfer dynamics in the context of molecular Coulomb explosion of a single molecule, exposed to sequential deep inner-shell ionization within an ultrashort (10 fs) X-ray pulse. The target molecule was CH3I, methane sensitized to X-rays by halogenization with a heavy element, iodine. Time-of-flight ion spectroscopy and coincident ion analysis was employed to investigate, via the properties of the atomic fragments, single-molecule charge states of up to +22. Experimental findings have been compared with a parametric model of simultaneous Coulomb explosion and charge transfer in the molecule. The study demonstrates that including realistic charge dynamics is imperative when molecular Coulomb explosion experiments using short-pulse facilities are performed.

We have investigated interatomic Coulombic decay (ICD) after resonant Auger decay in Ar2, ArKr, and ArXe following 2p3/2 → 4s and 2p3/2 → 3d excitations in Ar, using momentum-resolved electron–ion–ion coincidence. The results illustrate that ICD induced by the resonant Auger decay is a well-controlled way of producing energy-selected slow electrons at a specific site.Keywords: interatomic Coulombic decay; rare-gas dimers; resonant Auger decay;

Using density functional theory (DFT) methods, we have calculated ionization potential (IP) for K-shell single core hole (SCH) creation and double ionization potential (DIP) for K-shell double core hole (DCH) creation for XHmYHn (X, Y = C, N, O, F, m,n = 0–3), NX2CXO (X = H or F) and C60. For these molecules, we estimated the relaxation energies (a measure of the electron density flow to the core-hole site) and the interatomic relaxation energies (a measure of the electron density flow to the two core-hole sites) from the calculated IPs and DIPs. For XHmYHn, we find that the interatomic relaxation energy for the DCH states having two holes at X and Y atoms decreases with the increase in the bond order between X and Y. For NX2CXO (X = H or F), we find that the substitution of the hydrogen atoms by the fluorine atoms affects the initial-state-bonding shifts but less influences the relaxation energy. For DCH states having two holes at two carbon atoms in C60, we find that the interatomic relaxation energy decreases with the increase in the hole–hole distance.Highlights► Interatomic relaxation energy can be extracted from double core-hole spectroscopy. ► Interatomic relaxation is sensitive to the chemical environment. ► Interatomic relaxation decreases with the decrease in bond order. ► Interatomic relaxation decrease with the distance between the two core holes. ► F substation of H atoms induces the initial bonding shift and less affects the relaxation.

A photoelectron spectrum of H2O has been recorded at a resolution of 2 meV under Doppler-free conditions. Complex rotational structures appear in the individual vibrational states of the electronic X̃+ 2B1 and Ã+ 2A2 states in H2O+. The rotational structures are analyzed and well reproduced using a spectator orbital model developed for rotationally resolved photoelectron spectroscopy.

Interatomic Coulombic decay (ICD) in Ar2, ArKr and Kr2 following Ar 2p or Kr 3d Auger decay has been investigated by means of momentum-resolved electron–ion–ion coincidence spectroscopy. This sequential decay leads to Coulombic dissociation into dication and monocation. Simultaneously determining the kinetic energy of the ICD electron and the kinetic energy release between the two atomic ions, we have been able to unambiguously identify the ICD channels. We find that, in general, spin-conserved ICD, in which the singlet (triplet) dicationic state produced via the atomic Auger decay preferentially decays to the singlet (triplet) state, transferring the energy to the other atom, is faster than spin-flip ICD, in which the Auger final singlet (triplet) dicationic state decays to the triplet (singlet) state. However, spin-flip ICD may take place when spin-conserved ICD becomes energetically forbidden. Dipole-forbidden ICDs from Kr2+(4s−2 S1)–B (B = Ar or Kr) to Kr2+(4p−2 D1, P3)–B+ are also observed.

Vibrationally resolved Nc and Nt K-shell photoelectron spectra of the dinitrogen oxide have been studied experimentally and theoretically. Vibrational frequencies for the Nc and Nt 1s ionized states obtained from the 2D potential surfaces computed by the CCSD(T) method within the equivalent core approximation reasonably agree with the experimental values. Experimental relative intensities of the vibrational structure are reasonably reproduced by the multi-channel Schwinger configuration interaction method (MCSCI) with the computed 2D potential surfaces. Improved relaxed geometries of these core–hole states are obtained from fitting the experimental spectra using the MCSCI calculations and regarding the bond lengths as fitting parameters.Vibrationally resolved Nc and Nt K-shell photoelectron spectra of the dinitrogen oxide have been studied experimentally and theoretically.

Angle-resolved energetic-ion yield spectra have been measured in the O 1s excitation region of N2O. Franck–Condon analysis based on ab initio two-dimensional potential energy surfaces of the core-excited Rydberg states reproduces well the observed vibrational excitations specific to the individual Rydberg states. The irregular Rydberg behavior in the Σ-symmetry absorption spectrum is attributed the valence-Rydberg coupling in light of the second moment analysis.Franck–Condon analysis based on the ab initio two-dimensional potential surfaces of the core excited Rydberg states reproduces well the observed vibrational excitations specific to the individual Rydberg states.

Vibrationally resolved C 1s and O 1s photoelectron spectra of carbon dioxide have been measured with photon energies up to 500 and 700 eV, respectively. Vibrational branching ratios are nearly constant for the C 1s and O 1s photoelectron spectra recorded with the photon energies in the regions 400–500 and 600–700 eV, respectively, where neither shape resonance effect nor photoelectron recoil effect is significant. The information about the potential curves for the C 1s and O 1s ionized states are extracted from these spectra, using the Franck–Condon approximation. The experimental potential curves thus obtained are well reproduced by the present ab initio calculations based on the symmetry adapted cluster-configuration interaction (SAC-CI) method.

Electron–ion–ion coincidence momentum spectroscopy, a well-established tool to study the molecular-frame core-level photoelectron angular distribution, has been applied to investigate interatomic electronic decay processes in argon dimers Ar2 after the creation of a 2p inner-shell vacancy. Some interatomic Coulombic decay (ICD) processes from an Auger-final dicationic state are identified from the coincidence measurement between the kinetic energy of the ICD electron and the kinetic energy release between Ar+ and Ar2+. The interatomic character of the dissociation processes into Ar+–Ar+ is also discussed.

Core-level photoemission from N2 can be considered as an analogue of Young’s double-slit experiment (YDSE) in which the double-slit is replaced by a pair of N 1s orbitals. N 1s photoelectron spectra of N2 are measured in the extended photon energy region up to ∼∼ 1 keV at unprecedented resolution. The measured ratio between the 1σgσg and 1σuσu photoionization cross-sections oscillates as a function of electron momentum due to interference effects analogue to YDSE. We found a shift of the interference pattern with respect to a prediction by a simple model for coherent two-center emission, the Cohen–Fano formula, and attributed it to photoelectron scattering by the neighboring atom. We demonstrate that the shift can be used to determine the scattering phase of the photoelectron.

Auger electron spectra recorded in coincidence with two Ar+ ions produced from Ar22+ suggest that the bound one-site two-hole state, Ar2+(3p−2)–Ar, decays further only via radiative charge transfer to the dissociative two-site two-hole states Ar+(3p−1)–Ar+(3p−1). The measured kinetic energy release of Ar22+ agrees well with the theoretical estimate based on this process.The Auger spectra of Ar2 dimers recorded in coincidence with the Ar+–Ar+ ion pairs and of the Ar atoms recorded in coincidence with the Ar2+ ions. These Auger electron spectra suggest that the Ar2+(3p−2)–Ar decays further only via radiative charge transfer to Ar+(3p−1)–Ar+(3p−1).

Angle-resolved ion yield (ARIY) spectroscopy has been carried out on high-temperature N2O molecules. From spectra recorded at 300 K (room temperature) and at 700 K, symmetry-resolved absorption profiles of the N 1s−1 3π and O 1s−1 3π resonances have been extracted from molecules in the vibrational ground state and in bending-vibration excited states. The spectra show dramatic differences, and the Renner–Teller effect becomes more evident for the vibrationally excited molecules.Angle-resolved ion yield (ARIY) spectroscopy has been carried out on high-temperature N2O molecules. From spectra recorded at 300 K (room temperature) and at 700 K, symmetry-resolved absorption profiles of the N 1s−1 3π and O 1s−1 3π resonances have been extracted from molecules in the vibrational ground state and in bending-vibration excited states

We have studied site-specific fragmentation caused by F 1s photoionization of free CF3SF5 molecules. Energy-resolved electrons and mass-resolved ions were detected in coincidence. We found an enhancement of the CF3+ ion production in coincidence with photoelectrons emitted from F atoms in the SF5 group due to a reaction path leading to CF3+–SF+ pair production. We found an enhancement of the CF+ and C+ ions due to F 1s electron emission from the CF3 group. Site-selectivity was also observed for the CF+–SF+, CF+–SF2+,CF+–SF3+, C+–F+, C+–S+ and C+–SF+ ion pair production.Site-specific fragmentation caused by F 1s photoionization of CF3SF5 molecules has been studied by electron–ion coincidence spectroscopy. Site-specific production of CF3+, CF+ and C+ ions was found. Site-specific ion pair production was also observed.

The lowest-energy satellite bands accompanying the F 1s mainline of the SF6 molecule have been investigated using angle-resolved photoelectron spectroscopy. We found that a conjugate shake-up is dominant while the internal inelastic scattering also plays a significant role.

Using density functional theory (DFT) methods, we have calculated ionization potential (IP) for K-shell single core hole (SCH) creation and double ionization potential (DIP) for K-shell double core hole (DCH) creation for XHmYHn (X, Y = C, N, O, F, m,n = 0–3), NX2CXO (X = H or F) and C60. For these molecules, we estimated the relaxation energies (a measure of the electron density flow to the core-hole site) and the interatomic relaxation energies (a measure of the electron density flow to the two core-hole sites) from the calculated IPs and DIPs. For XHmYHn, we find that the interatomic relaxation energy for the DCH states having two holes at X and Y atoms decreases with the increase in the bond order between X and Y. For NX2CXO (X = H or F), we find that the substitution of the hydrogen atoms by the fluorine atoms affects the initial-state-bonding shifts but less influences the relaxation energy. For DCH states having two holes at two carbon atoms in C60, we find that the interatomic relaxation energy decreases with the increase in the hole–hole distance.Highlights► Interatomic relaxation energy can be extracted from double core-hole spectroscopy. ► Interatomic relaxation is sensitive to the chemical environment. ► Interatomic relaxation decreases with the decrease in bond order. ► Interatomic relaxation decrease with the distance between the two core holes. ► F substation of H atoms induces the initial bonding shift and less affects the relaxation.

We have developed a dead-time free ion momentum spectroscopy technique that allows us to extract 3D momentum for each of up to 100 ions produced by a single free-electron-laser (FEL) shot, by reading signals from the three-layer delay-line detector by the multichannel digitizer and employing the software constant-fraction discrimination method.

We report observations of the interatomic Coulombic decay (ICD) and electron-transfer-mediated decay (ETMD) from the triply charged states in Ne2 and NeAr dimers. The ICD processes leading to fragmentation of Ne3+-Ne into Ne3+-Ne+ and Ne3+-Ar into Ne3+-Ar+, and ETMD processes leading to fragmentation of Ne3+-Ne into Ne2+-Ne2+ are unambiguously identified by electron–ion-ion coincidence spectroscopy in which the kinetic energy of the ICD or ETMD electron and the kinetic energy release between the two fragment ions are measured in coincidence.

Coulomb explosion of diiodomethane CH2I2 molecules irradiated by ultrashort and intense X-ray pulses from SACLA, the Japanese X-ray free electron laser facility, was investigated by multi-ion coincidence measurements and self-consistent charge density-functional-based tight-binding (SCC-DFTB) simulations. The diiodomethane molecule, containing two heavy-atom X-ray absorbing sites, exhibits a rather different charge generation and nuclear motion dynamics compared to iodomethane CH3I with only a single heavy atom, as studied earlier. We focus on charge creation and distribution in CH2I2 in comparison to CH3I. The release of kinetic energy into atomic ion fragments is also studied by comparing SCC-DFTB simulations with the experiment. Compared to earlier simulations, several key enhancements are made, such as the introduction of a bond axis recoil model, where vibrational energy generated during charge creation processes induces only bond stretching or shrinking. We also propose an analytical Coulomb energy partition model to extract the essential mechanism of Coulomb explosion of molecules from the computed and the experimentally measured kinetic energies of fragment atomic ions by partitioning each pair Coulomb interaction energy into two ions of the pair under the constraint of momentum conservation. Effective internuclear distances assigned to individual fragment ions at the critical moment of the Coulomb explosion are then estimated from the average kinetic energies of the ions. We demonstrate, with good agreement between the experiment and the SCC-DFTB simulation, how the more heavily charged iodine fragments and their interplay define the characteristic features of the Coulomb explosion of CH2I2. The present study also confirms earlier findings concerning the magnitude of bond elongation in the ultrashort X-ray pulse duration, showing that structural damage to all but C–H bonds does not develop to a noticeable degree in the pulse length of ∼10 fs.