Combining high mass resolution and velocity imaging in a time-of-flight ion spectrometer using pulsed fields and an electrostatic lens

Design and characterization of a recoil ion momentum spectrometer for investigating molecular fragmentation dynamics upon MeV energy ion impact ionization

We present the development and performance of a newly built recoil ion momentum spectrometer to study the fragmentation dynamics of ionized molecules. The spectrometer is based on the two-stage Wiley-McLaren geometry and satisfies both time and velocity focusing conditions. An electrostatic lens has been introduced in the drift region to achieve velocity imaging and higher angular collection. The spectrometer is equipped with a 2D position-sensitive detector with multi-hit coincidence electronics. Ionic fragments with kinetic energy ∼8 eV can be detected with 4π collection. The overall performance of the spectrometer has been tested by carrying out three-dimensional ion imaging measurements for diatomic (N₂) and polyatomic (CH₂Cl₂) molecules under the impact of 1 MeV protons. Three-dimensional momentum and kinetic energy release distributions were derived from the measured position and time-of-flight spectra. The observed features of the various fragmentation channels as well as the measured kinetic energy release distributions are in complete agreement with the available data.

A recoil ion momentum spectrometer for probing ionization, e-capture, and capture-ionization induced molecular fragmentation dynamics

The development of a recoil ion momentum spectrometer (RIMS) along with a post-collision projectile charge state analyzer (CSA) and its performance for carrying out studies of molecular fragmentation following direct ionization, electron-capture, and capture-ionization have been demonstrated here. This is a two-stage Wiley-McLaren type spectrometer with slight modification introduced by adding a lens to achieve higher momentum resolution as well as larger angular acceptance. Along with the time and position sensitive detector, it can measure all the three momentum components of singly charged recoil ions of energy up to 10 eV emitted in all directions. The CSA assembly is designed for separating out any neutral or singly or doubly charged post-collision projectiles typically of keV energy. The RIMS with initial trigger ("start") from CSA or an ionized electron can uniquely determine the dynamics of molecular fragmentation following different electron-capture or direct ionization events, respectively. To check the performance of the setup, we carried out an experimental study of the fragmentation of N₂ molecules under the impact of 250 keV protons. Apart from the single-electron-capture channel, we could clearly identify three more capture-ionization channels, which lead to fragmentation. The essential features of the momentum distributions and the kinetic energy release distributions of all three fragmentation channels are discussed in detail. These results are compared with the findings from the ionization induced fragmentation experiments and with the available results from theoretical calculations as well as high resolution experiments. The branching ratios of these fragmentation channels are determined.

Coincident velocity map image reconstruction illustrated by the single-photon valence photoionisation of CF3SF5

Photoion–photoelectron kinetic energy and angular anisotropy correlation maps reveal new details about the ionization mechanism.

The Role of Spin-Orbit Coupling in the Double-Ionization Photoelectron Spectra of XCN(2+) (X = Cl, Br, and I)

The photoelectron spectra of XCN(2+) (X = Cl, Br, and I) were calculated employing ab initio electronic structure methods with high-level electron correlation and explicit treatment of spin-orbit coupling. Twelve scalar-relativistic excited states of the dicationic systems, calculated from state-averaged CASSCF/MRCI calculations, were used as the electronic basis to evaluate spin-orbit eigenstates. While the spin-orbit effects in ClCN(2+) are found to be negligible, the electronic spectroscopy of BrCN(2+) and ICN(2+) is significantly influenced by interstate spin-orbit coupling. Several electronic degeneracies are lifted, and many unexpected accidental degeneracies occurred due to the spin-orbit coupling. In particular, the spin-orbit interactions between X̃ (3)Σ(-)-b̃ (1)Σ(+), Ã (3)Π-c̃ (1)Π, B̃ (3)Δ-ã (1)Δ, and C̃ (3)Σ(+)-d̃ (1)Σ(-) are found to be strong in BrCN(2+) and ICN(2+). By careful analysis of the effect of spin-orbit coupling parameters and the spin-orbit eigenstate composition, an assignment of the hitherto unidentified experimental photoelectron bands of BrCN(2+) and ICN(2+) is presented.

A recoil ion momentum spectrometer for molecular and atomic fragmentation studies

We report the development and performance studies of a newly built recoil ion momentum spectrometer for the study of atomic and molecular fragmentation dynamics in gas phase upon the impact of charged particles and photons. The present design is a two-stage Wiley-McLaren type spectrometer which satisfies both time and velocity focusing conditions and is capable of measuring singly charged ionic fragments up-to 13 eV in all directions. An electrostatic lens has been introduced in order to achieve velocity imaging. Effects of the lens on time-of-flight as well as on the position have been investigated in detail, both, by simulation and in experiment. We have used 120 keV proton beam on molecular nitrogen gas target. Complete momentum distributions and kinetic energy release distributions have been derived from the measured position and time-of-flight spectra. Along with this, the kinetic energy release spectra of fragmentation of doubly ionized nitrogen molecule upon various projectile impacts are presented.

Development and characterization of a multiple-coincidence ion-momentum imaging spectrometer

The design and performance of a high-resolution momentum-imaging spectrometer for ions which is optimized for experiments using synchrotron radiation is presented. High collection efficiency is achieved by a focusing electrostatic lens; a long drift tube improves mass resolution and a position-sensitive detector enables measurement of the transverse momentum of ions. The optimisation of the lens for particle momentum measurement at the highest resolution is described. We discuss the overall performance of the spectrometer and present examples demonstrating the momentum resolution for both kinetics and for angular measurements in molecular fragmentation for carbon monoxide and fullerenes. Examples are presented that confirm that complete space-time focussing is possible for a two-field three-dimensional imaging spectrometer.

High-resolution reflecting time-of-flight momentum and energy mapping system

A new system to map electron momentum and energy is proposed. A reflecting electrode is introduced into a time-of-flight (TOF) system whose decelerating electric field sends all the electrons back to a position-sensitive detector close to but behind the source of the electrons. The longer flying distance that results makes it possible to significantly improve the energy-resolved performance, especially in the higher energy region. The dependence of the new TOF system on its characteristic parameters is analyzed, along with its application to attosecond streak cameras. Experimental results verified a relative energy resolution better than 0.2 eV for 22 eV electrons and also revealed the availability of the improved relative energy resolution smaller than 1.0% for electron energy ranging from 30 to 40 eV.

Cold-target recoil-ion momentum spectroscopy for diagnostics of high harmonics of the extreme-ultraviolet free-electron laser light source at SPring-8

We have developed a cold-target recoil-ion momentum spectroscopy apparatus dedicated to the experiments using the extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan and used it to measure spatial distributions of fundamental, second, and third harmonics at the end station.

A versatile electron-ion coincidence spectrometer for photoelectron momentum imaging and threshold spectroscopy on mass selected ions using synchrotron radiation

We present a photoelectron-photoion coincidence (PEPICO) spectrometer named DELICIOUS II which combines a velocity map imaging apparatus with a Wiley-McLaren time of flight analyzer for the study of gas phase samples in interaction with the synchrotron radiation (SR). This versatile system is capable of providing photoelectron images on mass-selected compounds with kinetic energy resolutions of DeltaE/E=5% and a 17 eV bandwidth, as well as threshold photoelectron spectra with a measured resolution of 0.8 meV, as demonstrated on the 3p(-1) ionization of argon. This instrument is also employed for threshold PEPICO experiments, allowing the selection of the parent ion's internal state with sub-meV resolution for light masses (<40 amu) and with typically 2 meV resolution for a mass of 100 amu and with a mass resolving power above 200. The continuous operation of the extraction fields and the independence from the electron's time of flight are well adapted to the quasicontinuous multibunch mode of the SR. This, together with the high transmission of both the electron and ion detection, allows a high coincidence counting rate and facilitates the subtraction of false coincidences. We illustrate the spectrometer's coincidence principle of operation with examples from the valence photoionization of an Ar+Xe mixture and of CF(4).

Is CO carbon KVV Auger electron emission affected by the photoelectron?

Angular distributions (ADs) of O+ fragments from C 1s photoexcited CO detected in coincidence with carbon KVV Auger electrons emitted in the horizontal direction were measured at photon energies of 298, 305, 320, and 450 eV. At 450 eV, the ADs are polarization-independent and coincide with the molecular-frame Auger electron angular distribution. All measured ADs can be rationalized as a product of the same molecular-frame Auger electron angular distribution and the axial selectivity in the photoionization process. Thus the interaction between the photoelectron and the Auger electron for the normal Auger decay of CO can be neglected, and the two-step model is a good approximation.

A photoelectron-photoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of sigma=18 ps and energy resolution Delta E/E=3.5%

We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5 microm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to sigma=18 ps. We observed that our electron coincidence detector has a timing resolution better than sigma=16 ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of DeltaE(FWHM)/E=3.5% for electrons with kinetic energy near 2 eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about Deltam(FWHM)/m=14150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to sigma=115 microm.