Cold Target Recoil Ion Momentum Spectroscopy: a &momentum microscope' to view atomic collision dynamics

High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'. Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

/pdf/recoil-ion-momentum-spectroscopy-2fdjshphs1.pdf

Recoil-ion momentum spectroscopy

Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

/pdf/the-ratio-of-cross-sections-for-double-to-single-ionization-56mhdl12v7.pdf

The ratio of cross sections for double to single ionization of helium by high energy photons and charged particles

The removal of two (or more) electrons by a single photon from an atom or molecule is a fundamentally important process as it is a direct probe of electron correlation. Synchrotron radiation provides photons over a wide range of energies from infrared to X-rays to photoexcite or -ionize gaseous samples. With the help of a monochromator the energy of the photons can be tuned to study the photon-energy dependence of the double-photoionization process in which two electrons are emitted by a single photon. In this chapter, I will discuss the photon-energy dependence of the double-photoionization cross sections and the associated double-to-single photoionization ratios for some atoms from threshold to high photon energies. I will also present recent experimental results on the triple-photoionization process, particularly for the text book example of lithium. Furthermore, recent results on multiple ionization by a single photon for selected molecules will be presented. In contrast to atoms, not only ionization but also fragmentation plays an important role in the double-photoionization experiments.

Simultaneous Emission of Multiple Electrons from Atoms and Molecules Using Synchrotron Radiation

An investigation on the asymptotic ratios of double-to-single photoionization cross sections of the helium atom (He) embedded in dense quantum plasmas at very high nonrelativistic photon energies has been made. Exponential cosine-screened Coulomb potential has been used to represent the effective potential of a test charge in a dense quantum plasma. Making use of highly correlated wave functions for the ground states of He, results of the asymptotic ratios (R) of double-to-single photoionization cross sections for the screening parameter in the range [0,0.5] a.u. are reported. The reported value of R for the unscreened case confirms its established value. A comparative study of the behavior of R in weakly coupled plasmas and in dense quantum plasmas has also been made.

Ratios of double-to-single photoionization cross sections of helium atom embedded in dense quantum plasmas at high nonrelativistic photon energies

We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He{sup ++}/He{sup +}=1.6{plus minus}0.3% at {ital h}{nu}=2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Aberg, who predict 1.7%.

/pdf/measurement-of-the-ratio-of-double-to-single-photoionization-1pwya7kd6f.pdf

Measurement of the ratio of double-to-single photoionization of helium at 2.8 keV using synchrotron radiation.

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Measurement of the ratio of double-to-single photoionization of helium at 2.8 keV using synchrotron radiation.