Energy Transfer into Molecular Vibrations and Rotations by Recoil in Inner-Shell Photoemission.

A mixture of CF_{4} and CO gases is used to study photoelectron recoil effects extending into the tender x-ray region. In CF_{4}, the vibrational envelope of the C 1s photoelectron spectrum becomes fully dominated by the recoil-induced excitations, revealing vibrational modes hidden from Franck-Condon excitations. In CO, using CF_{4} as an accurate energy calibrant, we determine the partitioning of the recoil-induced internal excitation energy between rotational and vibrational excitation. The observed rotational recoil energy is 2.88(28) times larger than the observed vibrational recoil energy, well in excess of the ratio of 2 predicted by the basic recoil model. The experiment is, however, in good agreement with the value of 2.68 if energy transfer via Coriolis coupling is included.

Femtosecond charge and molecular dynamics of I-containing organic molecules induced by intense X-ray free-electron laser pulses.

We studied the electronic and nuclear dynamics of I-containing organic molecules induced by intense hard X-ray pulses at the XFEL facility SACLA in Japan. The interaction with the intense XFEL pulse causes absorption of multiple X-ray photons by the iodine atom, which results in the creation of many electronic vacancies (positive charges) via the sequential electronic relaxation in the iodine, followed by intramolecular charge redistribution. In a previous study we investigated the subsequent fragmentation by Coulomb explosion of the simplest I-substituted hydrocarbon, iodomethane (CH3I). We carried out three-dimensional momentum correlation measurements of the atomic ions created via Coulomb explosion of the molecule and found that a classical Coulomb explosion model including charge evolution (CCE-CE model), which accounts for the concerted dynamics of nuclear motion and charge creation/charge redistribution, reproduces well the observed momentum correlation maps of fragment ions emitted after XFEL irradiation. Then we extended the study to 5-iodouracil (C4H3IN2O2, 5-IU), which is a more complex molecule of biological relevance, and confirmed that, in both CH3I and 5-IU, the charge build-up takes about 10 fs, while the charge is redistributed among atoms within only a few fs. We also adopted a self-consistent charge density-functional based tight-binding (SCC-DFTB) method to treat the fragmentations of highly charged 5-IU ions created by XFEL pulses. Our SCC-DFTB modeling reproduces well the experimental and CCE-CE results. We have also investigated the influence of the nuclear dynamics on the charge redistribution (charge transfer) using nonadiabatic quantum-mechanical molecular dynamics (NAQMD) simulation. The time scale of the charge transfer from the iodine atomic site to the uracil ring induced by nuclear motion turned out to be only ∼5 fs, indicating that, besides the molecular Auger decay in which molecular orbitals delocalized over the iodine site and the uracil ring are involved, the nuclear dynamics also play a role for ultrafast charge redistribution. The present study illustrates that the CCE-CE model as well as the SCC-DFTB method can be used for reconstructing the positions of atoms in motion, in combination with the momentum correlation measurement of the atomic ions created via XFEL-induced Coulomb explosion of molecules.

Fragmentation network of doubly charged methionine: Interpretation using graph theory.

The fragmentation of doubly charged gas-phase methionine (HO2CCH(NH2)CH2CH2SCH3) is systematically studied using the self-consistent charge density functional tight-binding molecular dynamics (MD) simulation method. We applied graph theory to analyze the large number of the calculated MD trajectories, which appears to be a highly effective and convenient means of extracting versatile information from the large data. The present theoretical results strongly concur with the earlier studied experimental ones. Essentially, the dication dissociates into acidic group CO2H and basic group C4NSH10. The former may carry a single or no charge and stays intact in most cases, whereas the latter may hold either a single or a double charge and tends to dissociate into smaller fragments. The decay of the basic group is observed to follow the Arrhenius law. The dissociation pathways to CO2H and C4NSH10 and subsequent fragmentations are also supported by ab initio calculations.

Vibrationally Resolved B 1s Photoionization Cross Section of BF3.

Photoelectron diffraction is a well-established technique for structural characterization of solids, based on the interference of the native photoelectron wave with those scattered from the neighboring atoms. For isolated systems in the gas phase similar studies suffer from orders of magnitude lower signals due to the very small sample density. Here we present a detailed study of the vibrationally resolved B 1s photoionization cross section of BF3 molecule. A combination of high-resolution photoelectron spectroscopy measurements and of state-of-the-art static-exchange and time-dependent DFT calculations shows the evolution of the photon energy dependence of the cross section from a complete trapping of the photoelectron wave (low energies) to oscillations due to photoelectron diffraction phenomena. The diffraction pattern allows one to access structural information both for the ground neutral state of the molecule and for the core-ionized cation. Due to a significant change in geometry between the ground and the B 1s(-1) core-ionized state in the BF3 molecule, several vibrational final states of the cation are populated, allowing investigation of eight different relative vibrationally resolved photoionization cross sections. Effects due to recoil induced by the photoelectron emission are also discussed.

Fragmentation dynamics of doubly charged methionine in the gas phase.

The dependence of the fragmentation of doubly charged gas-phase methionine (C5H11NO2S) on the electronic-state character of the parent ion is studied experimentally by energy-resolved electron ion-ion coincidence spectroscopy. The parent dication electronic states are populated by Auger transitions following site-specific sulfur 2p core ionization. Two fragmentation channels are observed to be strongly dependent on the electronic states with vacancies in weakly bound molecular orbitals. All-electron calculations are applied to assign doubly charged final states of sulfur 2p core ionized methionine. In addition, the Car-Parrinello method is applied to model fragmentation dynamics of doubly charged methionine molecules with various initial temperatures to understand the typical characteristics of the molecular dissociation and partly to support the interpretation of experimental data.

Soft x-ray ionization induced fragmentation of glycine.

X-ray absorption commonly involves dissociative core ionization producing not only momentum correlated charged fragments but also low- and high-energy electrons capable of inducing damage in living tissue. This gives a natural motivation for studying the core ionization induced fragmentation processes in biologically important molecules such as amino acids. Here the fragmentation of amino acid glycine following carbon 1s core ionization has been studied. Using photoelectron-photoion-photoion coincidence technique, a detailed analysis on fragmentation of the sample molecule into pairs of momentum correlated cations has been carried out. The main characteristics of core ionization induced fragmentation of glycine were found to be the rupture of the C-Cα bond and the presence of the CNH(2)(+) fragment.

Deep inner-shell multiphoton ionization by intense x-ray free-electron laser pulses.

We have investigated multiphoton multiple ionization dynamics of xenon atoms using a new x-ray free-electron laser facility, SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan, and identified that Xe(n+) with n up to 26 is produced at a photon energy of 5.5 keV. The observed high charge states (n≥24) are produced via five-photon absorption, evidencing the occurrence of multiphoton absorption involving deep inner shells. A newly developed theoretical model, which shows good agreement with the experiment, elucidates the complex pathways of sequential electronic decay cascades accessible in heavy atoms. The present study of heavy-atom ionization dynamics in high-intensity hard-x-ray pulses makes a step forward towards molecular structure determination with x-ray free-electron lasers.

Intramolecular photoelectron diffraction in the gas phase.

We report unambiguous experimental and theoretical evidence of intramolecular photoelectron diffraction in the collective vibrational excitation that accompanies high-energy photoionization of gas-phase CF4, BF3, and CH4 from the 1s orbital of the central atom. We show that the ratios between vibrationally resolved photoionization cross sections (v-ratios) exhibit pronounced oscillations as a function of photon energy, which is the fingerprint of electron diffraction by the surrounding atomic centers. This interpretation is supported by the excellent agreement between first-principles static-exchange and time-dependent density functional theory calculations and high resolution measurements, as well as by qualitative agreement at high energies with a model in which atomic displacements are treated to first order of perturbation theory. The latter model allows us to rationalize the results for all the v-ratios in terms of a generalized v-ratio, which contains information on the structure of the above three molecules and the corresponding molecular cations. A fit of the measured v-ratios to a simple formula based on this model suggests that the method could be used to obtain structural information of both neutral and ionic molecular species.

Isotope effects in dynamics of water isotopologues induced by core ionization at an x-ray free-electron laser.

Dynamical response of water exposed to x-rays is of utmost importance in a wealth of science areas. We exposed isolated water isotopologues to short x-ray pulses from a free-electron laser and detected momenta of all produced ions in coincidence. By combining experimental results and theoretical modeling, we identify significant structural dynamics with characteristic isotope effects in H2O2+, D2O2+, and HDO2+, such as asymmetric bond elongation and bond-angle opening, leading to two-body or three-body fragmentation on a timescale of a few femtoseconds. A method to disentangle the sequences of events taking place upon the consecutive absorption of two x-ray photons is described. The obtained deep look into structural properties and dynamics of dissociating water isotopologues provides essential insights into the underlying mechanisms.

Multiphoton ionization as a clock to reveal molecular dynamics with intense short x-ray free electron laser pulses.

We investigate molecular dynamics of multiple ionization in N2 through multiple core-level photoabsorption and subsequent Auger decay processes induced by intense, short x-ray free electron laser pulses. The timing dynamics of the photoabsorption and dissociation processes is mapped onto the kinetic energy of the fragments. Measurements of the latter allow us to map out the average internuclear separation for every molecular photoionization sequence step and obtain the average time interval between the photoabsorption events. Using multiphoton ionization as a tool of the multiple-pulse pump-probe scheme, we demonstrate the modification of the ionization dynamics as we vary the x-ray laser pulse duration.

Experimental verification of the chemical sensitivity of two-site double core-hole states formed by an x-ray free-electron laser.

We have performed x-ray two-photon photoelectron spectroscopy using the Linac Coherent Light Source x-ray free-electron laser in order to study double core-hole (DCH) states of CO2, N2O, and N2. The experiment verifies the theory behind the chemical sensitivity of two-site DCH states by comparing a set of small molecules with respect to the energy shift of the two-site DCH state and by extracting the relevant parameters from this shift.

Size selective spectroscopy of Se microclusters.

The electronic structure and photofragmentation in outer and inner valence regions of Se(n) (n ≤ 8) clusters produced by direct vacuum evaporation have been studied with size-selective photoelectron-photoion coincidence technique by using vacuum-ultraviolet synchrotron radiation. The experimental ionization potentials of these clusters were extracted from the partial ion yield measurements. The calculations for the possible geometrical structures of the Se(n) microclusters have been executed. The ionization energies of the clusters have been calculated and compared with the experimental results. In addition, theoretical fragment ion appearance energies were estimated. The dissociation energies of Se(n) clusters were derived from the recurrent relation between the gas phase enthalpies of the formation of corresponding cationic clusters and experimental ionization energies.

Experimental observation of rotational Doppler broadening in a molecular system.

The first experimental evidence of rotational Doppler broadening in photoelectron spectra, reported here, show good agreement with recently described theoretical predictions. The dependence of the broadening on temperature and photoelectron kinetic energy is quantitatively predicted by the theory. The experiments verify that the rotational contributions to the linewidth are comparable to those from translational Doppler broadening and must be considered in the analysis of high-resolution photoelectron spectra. A classical model accounting for this newly observed effect is presented.

Metastable states in NO2+ probed with Auger spectroscopy.

High-resolution N 1s and O 1s photoelectron spectra (PES) of NO are presented together with spectra of the subsequent Auger decay. The PES are analyzed by taking spin-orbit splitting of the (2)Π ground state into account providing detailed information on equilibrium distances, vibrational energies, and lifetime widths of the core-ionized states. In the Auger electron spectra (AES) transitions to five metastable dicationic final states are observed, with two of them previously unobserved. A Franck-Condon analysis of the vibrational progressions belonging to these transitions provides detailed information on the potential-energy curves of the dicationic final states as well as on the relative Auger rates. The present calculations of the potential-energy curves of NO(2+) agree well with the experimental results and allow an assignment of the two hitherto unresolved Auger transitions to excited states of NO(2+), C(2)Σ(+)and c(4)Π.

Double core-hole production in N2: beating the Auger clock.

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.

Ultraintense x-ray induced ionization, dissociation, and frustrated absorption in molecular nitrogen.

Sequential multiple photoionization of the prototypical molecule N2 is studied with femtosecond time resolution using the Linac Coherent Light Source (LCLS). A detailed picture of intense x-ray induced ionization and dissociation dynamics is revealed, including a molecular mechanism of frustrated absorption that suppresses the formation of high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for single-pulse imaging applications.

Fragmentation patterns of core-ionized thymine and 5-bromouracil.

Photofragmentation of thymine and 5-bromouracil into cation and neutral fragments following the core ionization by soft x-rays using photoelectron-photoion-photoion coincidence technique has been studied. The fragment ion mass spectra were recorded in coincidence with the C 1s photoelectron spectra. In the case of thymine, deuterated samples were used to identify fragments. Deuteration or bromination allowed us to study not only the main fragmentation channels of these pyrimidine bases, but also to investigate if replacement of an exocyclic functional group affects molecular fragmentation. We found that the dominant fragmentation channels involve only one starting geometry, and the base ring and other bond cleavages, leading to the detected fragments, are essentially identical between thymine and 5-bromouracil. In addition, the relative intensities of the strongest fragmentation channels were determined and compared with calculated appearance energies using ab initio unrestricted Hartree-Fock theory.

Valence photoelectron spectroscopy of N2 and CO: recoil-induced rotational excitation, relative intensities, and atomic orbital composition of molecular orbitals.

Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N(2)(+) and CO(+) over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73×10(-5) to 1.40×10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory.

Fragmentation patterns of doubly charged acrylonitrile molecule following carbon core ionization.

Dissociation of acrylonitrile into pairs of cations and neutral fragments following molecular core ionization was investigated using the photoelectron-photoion-photoion coincidence (PEPIPICO) technique. The fragment ion mass spectra were recorded in coincidence with the carbon 1s photoelectrons. Deuterated and (13)C-substituted samples were used for resolving fragment mass ambiguities. Slope analysis of the PEPIPICO patterns was used in determining the fragment separation sequences in case of multiparticle processes. The results show that there are several fragmentation channels producing a wide range of charged coincident fragments. The dynamics of the dominant fragmentation processes is investigated in detail.

Dissociation pathways in the cysteine dication after site-selective core ionization.

A photoelectron-ion-ion coincidence experiment has been carried out on the amino acid molecule cysteine after core-ionization of the O 1s, N 1s, C 1s, and S 2p orbitals. A number of different dissociation channels have been identified. Some of them show strong site-selective dependence that can be attributed to a combination of nuclear motion in the core-ionized state and Auger processes that populate different final electronic states in the dication.