Ion-ion coincidence studies on multiple ionizations of N2 and O2 molecules irradiated by extreme ultraviolet free-electron laser pulses

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.

Short-wavelength free-electron laser sources and science: a review

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

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.

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.