Ion Coincidence Detection Research Articles

A Reaction Microscope for AMO Science at Shanghai Soft X-ray Free-Electron Laser Facility

We report on the design and capabilities of a reaction microscope (REMI) end-station at the Shanghai Soft X-ray Free-Electron Laser Facility (SXFEL). This apparatus allows high-resolution and 4π solid-angle coincidence detection of ions and electrons. The components of REMI, including a supersonic gas injection system, spectrometer, detectors and data acquisition system, are described in detail. By measuring the time of flight and the impact positions of ions and electrons on the corresponding detectors, three-dimensional momentum vectors can be reconstructed to study specific reaction processes. Momentum resolutions of ions and electrons with 0.11 a.u. are achieved, which have been measured from a single ionization experiment of oxygen molecules in an infrared (IR), femtosecond laser field, under vacuum at 1.2×10−10 torr, in a reaction chamber. As a demonstration, a Coulomb explosion experiment of oxygen molecules in the IR field is presented. These results demonstrate the performance of this setup, which provides a basic tool for the study of atomic and molecular reactions at SXFEL.

Direct observation of a Feshbach resonance by coincidence detection of ions and electrons in Penning ionization collisions

Observation of molecular dynamics with quantum state resolution is one of the major challenges in chemical physics. Complete characterization of collision dynamics leads to the microscopic understanding and unraveling of different quantum phenomena such as scattering resonances. Here we present an experimental approach for observing molecular dynamics involving neutral particles and ions that is capable of providing state-to-state mapping of the dynamics. We use Penning ionization reaction between argon and metastable helium to generate argon ion and ground state helium atom pairs at separation of several angstroms. The energy of an ejected electron carries the information about the initial electronic state of an ion. The coincidence detection of ionic products provides a state resolved description of the post-ionization ion-neutral dynamics. We demonstrate that correlation between the electron and ion energy spectra enables us to directly observe the spin-orbit excited Feshbach resonance state of HeAr+. We measure the lifetime of the quasi-bound HeAr+A2 state and discuss possible applications of our method.

Streaking strong-field double ionization

Double ionization in intense laser fields can comprise electron correlations, which manifest in the non-independent emission of two electrons from an atom or molecule. However, experimental methods that directly access the electron emission times have been scarce. Here, we explore the application of an all-optical streaking technique to strong-field double ionization both theoretically and experimentally. We show that both sequential and non-sequential double ionization processes lead to streaking delays that are distinct from each other and single ionization. Moreover, coincidence detection of ions and electrons provides access to the emission time difference, which is encoded in the two-electron momentum distributions. The experimental data agree very well with simulations of sequential double ionization. We further test and discuss the application of this method to non-sequential double ionization, which is strongly affected by the presence of the streaking field.

Absolute ion detection efficiencies of microchannel plates and funnel microchannel plates for multi-coincidence detection.

Modern momentum imaging techniques allow for the investigation of complex molecules in the gas phase by detection of several fragment ions in coincidence. For these studies, it is of great importance that the single-particle detection efficiency ε is as high as possible, as the overall efficiency scales with εn, i.e., the power of the number of detected particles. Here we present measured absolute detection efficiencies for protons of several micro-channel plates (MCPs), including efficiency enhanced "funnel MCPs." Furthermore, the relative detection efficiency for two-, three-, four-, and five-body fragmentation of CHBrClF has been examined. The "funnel" MCPs exhibit an efficiency of approximately 90%, gaining a factor of 24 (as compared to "normal" MCPs) in the case of a five-fold ion coincidence detection.

Pathway-resolved photoelectron emission in dissociative ionization of molecules

Electron–ion coincidence detection allows one to visualize the ultrafast chemical reactions of molecules in strong laser fields. Here, by measuring the photoelectron angular distribution (PAD) of H2 in strong laser fields correlated to different pathways, i.e., direct ionization when the internuclear distance is small, or ionization after the molecular bond stretches to a large internuclear distance, we uncover the roles of the molecular orientation and internuclear distance in the dissociative ionization of H2. As compared to the first dissociation pathway, the regular nodal structures on the concentric above-threshold ionization circles vanish for the second pathway, which are numerically validated by the quantum simulations. Pathway-resolved PADs assisted by electron–ion coincidence detection open new possibilities to probe the rich dynamics of molecules in strong laser fields, in particular to image the instantaneous geometry of molecules.

Strong field ionization of small hydrocarbon chains with full 3D momentum analysis

SynopsisStrong field ionization of small hydrocarbon chains is studied in a kinematic complete experiment using a reaction microscope. By coincidence detection of ions and electrons different ionization continua populated during the ionization process are identified. In addition, photoelectron momentum distributions from laser-aligned molecules allow to characterize the electron wavepackets emerging from different Dyson orbitals.

High-Lying Rydberg States from Strong Field Interaction

SynopsisWe experimentally observed high-lying (n>238) Rydberg states which are formed through recapture of electrons after strong field interaction in atoms and small molecules using electron and ion coincidence detection. The high-lying Rydberg states are identified through weak DC field-induced ionization.

Imaging an aligned polyatomic molecule with laser-induced electron diffraction.

Laser-induced electron diffraction is an evolving tabletop method that aims to image ultrafast structural changes in gas-phase polyatomic molecules with sub-Ångström spatial and femtosecond temporal resolutions. Here we demonstrate the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C–C and C–H bond lengths in aligned acetylene. Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based on the combination of a 160 kHz mid-infrared few-cycle laser source with full three-dimensional electron–ion coincidence detection. Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas-phase molecules with atto- to femto-second temporal resolution.

Electron ionisation of sulfur dioxide

Relative precursor-specific partial ionisation cross sections for the fragment ions formed following electron ionisation of sulfur dioxide (SO2) have been measured for the first time, from 30 to 200 eV, using time-of-flight mass spectrometry coupled with two-dimensional ion coincidence detection. These data quantify the yields of O(2+), O(+), SO(2+), S(+), O2(+), and SO(+) ions, relative to the formation of SO2(+), via single, double, and triple electron ionisation of SO2. Formation of O(2+), following electron-SO2 collisions, has been quantified for the first time. The data allow a first experimental estimate of the triple ionisation potential of SO2 (69.0 ± 3.6 eV), an energy in good agreement with a value derived in this study via computational chemistry. The triple ion combination S(+) + O(+) + O(+) is clearly detected following electron collisions with SO2 at electron energies markedly below the vertical energy for forming SO2(3 +). This observation is accounted for by the operation of a stepwise pathway to the formation of S(+) + 2O(+) which does not involve the formation of a molecular trication.

Time-of-flight system with a movable ion detector for absolute measurement of cyclotron beam energy

A time-of-flight system for determining the absolute energy of ion beams has been developed for the azimuthally varying field (AVF) cyclotron at Japan Atomic Energy Agency (JAEA). Ion detectors, a microchannel-plate detector and a plastic scintillation detector, were applied to the system in order to achieve high time resolution measurement of the flight time and to cover a wide range of beam intensity in combination with a beam attenuator. The change of the flight length, performed by moving the plastic scintillation detector with flexible bellows, allows determination of the mean beam energy only from the relative measurement of the flight time and the flight length without knowing their absolute values. A maximum movable distance of 2m yields the difference in the time of flight from 16to94ns for the energy range of ion beams accelerated by the JAEA AVF cyclotron. The time-of-flight system even with the 2m change in the flight length achieves accurate energy determination of the order of 0.1%, since the relative measurement has an advantage of elimination of the uncertainties in determination of the time zero and the length zero. The time-of-flight measurement with the relative measurement also allows estimation of the beam energy spread without conventional expensive systems such as a magnetic spectrometer. By comparing the widths of the time-of-flight spectra of ions accumulated at different flight lengths, we have estimated the energy spread. In order to reduce the labor process of the coincidence detection of ions in the time-of-flight measurement, we have also accomplished a simple estimation method for mean beam energy determination with a beam bunch, naturally modulated with the acceleration RF of the cyclotron. The mean beam energy has been obtained from the shift of the beam bunch centroids at different flight lengths in the time-of-flight spectrum.

Pulsed field ionization-photoelectron photoion coincidence spectroscopy with synchrotron radiation: the heat of formation of the C2H5+ ion

Pulsed field ionization photoelectron (PFI-PE) spectroscopy combined with ion coincidence detection has been used with multi-bunch synchrotron radiation at the Advance Light Source (ALS) to energy select ions and to measure their breakdown diagram. The resolution for ion state selection achieved with Ar+ (2P3/2, 1/2) employing this PFI-PE-photoion coincidence apparatus is 0.6 meV (full width at half maximum). The production of C2H5+ from C2H5Br was investigated near the dissociative photoionization limit with this pulsed field ionization-threshold photoelectron photoion coincidence (PFI-PEPICO) scheme. Although the PFI-PE spectra of C2H5Br, C2H5I, and benzene show that the production of ions in the Franck-Condon gap regions is quite low, the selectivity for PFI-PE detection and the suppression of prompt electrons is such that we can detect 1 PFI-PE out of 25,000 total electrons s-1. The derived C2H5+ heat of formation from the analysis of the C2H5Br+ breakdown diagram and a critical analysis of other results is 900.5 +/- 2.0 kJ mol-1 at 298 K, or 913.2 +/- 2.0 kJ mol-1 at 0 K. This leads to an ethylene proton affinity at 298 K of 682.0 kJ mol-1. The measured IE of C2H5Br is 10.307 eV.

Internal conversion in highly excited benzene and benzene dimer: femtosecond time-resolved photoelectron spectroscopy

Photoelectron spectra of benzene molecules and dimers excited to the S 2 electronic state by 140 fs laser pulses at a wavelength of 200 nm have been measured by combining the pump–probe technique with the coincidence detection of ions and electrons. Their time dependence allows one to directly follow the evolution of internal conversion processes in different electronic states on a fs time-scale. Analysis of the corresponding electron energy distribution reveals large variations in the geometry of the different electronic states in both benzene and in its dimer.