We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx+ fragments (with x = 0, …, 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations.

Molecular Auger spectra typically consist of many overlapping lines that are hard or impossible to resolve experimentally even with highest-resolution electron spectroscopy techniques. Since molecular Auger energies depend on non-adibatatic dynamics, they can also be difficult to calculate precisely, such that the assignment of molecular Auger transitions is often ambiguous. Here we show that by measuring Auger-electron multi-ion coincidences, we can disentangle the spectra into the contributions from specific ionic final states, which helps significantly with their interpretation and assignment.

Conformational isomerism plays a crucial role in defining the physical and chemical properties and biological activity of molecules ranging from simple organic compounds to complex biopolymers. However, it is often a significant challenge to differentiate and separate these isomers experimentally as they can easily interconvert due to their low rotational energy barrier. Here, we use the momentum correlation of fragment ions produced after inner-shell photoionization to distinguish conformational isomers of 1,2-dibromoethane (C2H4Br2). We demonstrate that the three-body breakup channel, C2H4+ + Br+ + Br+, contains signatures of both sequential and concerted breakup, which are decoupled to distinguish the geometries of two conformational isomers and to quantify their relative abundance. The sensitivity of our method to quantify these yields is established by measuring the relative abundance change with sample temperature, which agrees well with calculations. Our study paves the way for using Coulomb explosion imaging to track subtle molecular structural changes.

Intermolecular processes offer unique decay mechanisms for complex systems to internally relax. Here, we report the observation of an intermolecular Coulombic decay channel in an endohedral fullerene, a holmium nitride complex (Ho_{3}N) embedded within a C_{80} fullerene, between neighboring holmium ions, and between the holmium complex and the carbon cage. By measuring the ions and the electrons in coincidence after XUV photoabsorption, we can isolate the different decay channels, which are found to be more prevalent relative to intra-atomic Auger decay.

We report on the design and performance of a double-sided coincidence velocity map imaging spectrometer optimized for electron-ion and ion-ion coincidence experiments studying inner-shell photoionization of gas-phase molecules with soft X-ray synchrotron radiation. The apparatus employs two microchannel plate detectors equipped with delay-line anodes for coincident, time- and position-resolved detection of photoelectrons and Auger electrons with kinetic energies up to 300 eV on one side of the spectrometer and photoions up to 25 eV per unit charge on the opposite side. We demonstrate its capabilities by measuring valence photoelectrons and ion spectra of neon and nitrogen and by studying channel-resolved photoelectron and Auger spectra along with fragment-ion momentum correlations for chlorine 2p inner-shell ionization of cis- and trans-1,2-dichloroethene.

Absolute single photoionization cross section measurements for Br3+ ions are reported in the photon energy range 44.79–59.54 eV at a photon energy resolution of 21 ± 3 meV. Measurements were performed at the Advanced Light Source at Lawrence Berkeley National Laboratory using the merged-beams technique. Numerous resonance features in the experimental spectrum are assigned and their energies and quantum defect values are tabulated. The cross-section measurements are also compared with Breit–Pauli R-matrix calculations with suitable agreement over the photon energy range investigated. Analysis of the measured spectrum including Rydberg resonance series identifications produced a new emperical determination of the ionizational potential of Br3+ of 46.977 ± 0.050 eV, which is 805 meV lower than the most recently published value of 47.782 eV. This disparity between our determination and the earlier published value is similar to an 843 meV shift in the accepted ionization potential published for iso-electronic Se2+ as part of this same research program.

Inner-shell photodetachment from Ni-([Ar] 3d(9)4s(2)) leading to Ni+, Ni2+, andNi(3+) ion production was studied near and above the 3p excitation region, in the 60-90 eV photon energy range, using a merged ion-photon beam technique. The absolute photodetachment cross section of Ni- leading to Ni+ ion production was measured. The 3p -> 3d photoexcitation in Ni- gives rise to a giant Feshbach resonance. In the near-threshold region, a Fano profile, modified by aWigner s-wave (l = 0) threshold law, accurately fits the Ni- single-photodetachment cross section. A lower-order R-matrix calculation shows overall agreement with essential features of the experimental data, confirming the nature of the strong, asymmetric Fano profile of the giant 3p -> 3d photoexcitation-autodetachment resonance in Ni-.

Author(s): Macaluso, DA; Bogolub, K; Johnson, A; Aguilar, A; Kilcoyne, ALD; Bilodeau, RC; Bautista, M; Kerlin, AB; Sterling, NC

Coincidence momentum imaging is a powerful method to study molecular structure and dynamics. Here, we used the Coulomb explosion imaging method together with synchrotron radiation and ultrafast laser pulses in order to distinguish between geometric isomers of C2H2Br2, C2H2Cl2, and difluoroiodobenzene and to investigate their fragmentation dynamics following both strong-field and inner-shell photoionization.

Author(s): Macaluso, DA; Bogolub, K; Johnson, A; Aguilar, A; Kilcoyne, ALD; Bilodeau, RC; Bautista, M; Kerlin, AB; Sterling, NC

In the present paper we describe a newly designed collinear photoelectron spectrometer for angular distribution measurements. We will henceforth refer to this instrument by the acronym PEARLS (PhotoElectron Angle-Resolved Linear Spectrometer). The design was motivated by the desire to collect electrons emitted from an extended linear source consisting of collinear photon and ion beams at a synchrotron radiation site. The electrons could be produced in either photoionization or photodetachment events. The primary advantage of a collinear beams geometry is that the effective interaction volume can be made much larger than that obtainable with a crossed beams geometry, which has been used in many earlier photoelectron spectroscopic studies. The present apparatus is capable of collecting electrons over a beam source length of 22cm. The electrons are detected using Channel Electron Multipliers (CEMs). There are 4 detector planes placed perpendicular to the direction of the beam source, where each plane contains 4 CEMs. The use of all 4 detector planes with a total of 16 CEMs enhances the photoelectron signal, which is important at a synchrotron radiation site where the photon flux is typically low. If photoelectrons of different energies are emitted, the design allows for electrostatic energy analyzers to be placed in front of the CEMs. We have performed a photodetachment experiment to demonstrate the functionality of the PEARLS apparatus using a pulsed laser as the photon source. In particular, we have measured the angular distribution of photoelectrons ejected from Ag- at two different photon energies.

Author(s): Macaluso, DA; Bogolub, K; Johnson, A; Aguilar, A; Kilcoyne, ALD; Bilodeau, RC; Bautista, M; Kerlin, AB; Sterling, NC