T. Majima

Photon-trap spectroscopy of mass-selected ions in an ion trap: Optical absorption and magneto-optical effects

A novel experimental technique has been developed to observe a trace of optical absorption of free mass-selected ions. The technique combines a linear radio-frequency ion trap with a high-finesse optical cavity to perform cavity ring-down spectroscopy (photon-trap spectroscopy for generality), where the storage lifetime of photons in the cavity provides a sensitivity high enough to probe the trapped ions. Absorption spectra of the manganese ion Mn(+) are presented, showing hyperfine structures for the (7)P(2,3,4)<--(7)S(3) transitions in the ultraviolet range. Implementation of a solenoidal magnet allows us to observe the Zeeman splitting and the Faraday rotation as well.

Damage and polymerization of C60 films irradiated by fast light and heavy ions

Abstract C 60 films have been irradiated with various fast ions (H, Li, C, O and Si) in the energy range from 0.75 to 6.0 MeV. Structural changes of C 60 molecules were studied by a time-of-flight (TOF) mass spectrometry and Raman spectroscopy. The TOF yields of secondary fullerene ions can be described fairly well by S mod 3 with a modified energy deposition S mod developed in this work. In the Raman study for 1 MeV H + irradiation, it is found that about 40% polymerization is attained at 6×10 15 cm −2 doses and a damage cross-section of 2.5×10 −17 cm 2 is obtained.

Ionization cross sections of C60 by fast electron impact

By means of a time-of-flight technique, we have measured cross sections for production of and ions from a gas-phase C60 target bombarded by 0.4-5.0 keV electrons. The results were in fairly good agreement with other data at overlapping energies below 1 keV. Semiclassical calculations of the single-ionization cross sections were also performed up to 10 keV using the Deutsch-Mark formula proposed for the molecule. A fairly good agreement between the experimental and theoretical cross sections was obtained both in magnitude and in energy dependence. It was found that the double and triple ionization cross sections both decrease monotonically and exhibit no hump structure, indicating that the inner-shell ionization does not play an important role in electron-impact multiple ionization of C60 .

Photoelectron diffraction from laser-aligned molecules with X-ray free-electron laser pulses

We report on the measurement of deep inner-shell 2p X-ray photoelectron diffraction (XPD) patterns from laser-aligned I2 molecules using X-ray free-electron laser (XFEL) pulses. The XPD patterns of the I2 molecules, aligned parallel to the polarization vector of the XFEL, were well matched with our theoretical calculations. Further, we propose a criterion for applying our molecular-structure-determination methodology to the experimental XPD data. In turn, we have demonstrated that this approach is a significant step toward the time-resolved imaging of molecular structures.

Secondary electron emission from Au by medium energy atomic and molecular ions

Number distributions of secondary electrons emitted from a Au metal surface have been measured for atomic and molecular ions of H þ ,H e þ ,C þ ,N þ ,O þ ,H þ ,H þ , HeH þ ,C O þ and O þ in the energy range 0.3-2.0 MeV. The emission statistics obtained are described fairly well by a Pfunction. The P� o parameter b, determining the distribution shape, is found to decrease monotonously with increasing emission yield c, revealing a surprising relationship of bc � 1 over the different projectile species and impact energies. This finding supports certainly the electron cascading model. Also we find a strong negative molecular effect for heavier molecular ions, showing a significant reduction of c com- pared to the estimated values using constituent atomic projectile data. 2002 Elsevier Science B.V. All rights reserved.

A cryogenic electrostatic storage ring project at RIKEN

A new, compact electrostatic storage ring for atomic and molecular physics is planned to be built at RIKEN, Japan. The new ring will be cooled down below 10 K to store rotationally cold molecular ions for a long period.

Equilibrium charge distributions of lithium ions emerging from a carbon foil

Abstract Equilibrium charge distributions of lithium ions passing through a carbon foil have been measured in the energy range 1–6 MeV. Average equilibrium charges q are found to connect smoothly with previous high energy data above 6 MeV. It was found that the present results of both charge fractions and q can be well reproduced by the empirical calculations based on the independent electron model. We also found that available semiempirical formulae for q cannot reproduce the present experimental data at all. A new semiempirical formula is presented for Li ions in the energy range 0.8–30 MeV.

Structure determination of molecules in an alignment laser field by femtosecond photoelectron diffraction using an X-ray free-electron laser

We have successfully determined the internuclear distance of I2 molecules in an alignment laser field by applying our molecular structure determination methodology to an I 2p X-ray photoelectron diffraction profile observed with femtosecond X-ray free electron laser pulses. Using this methodology, we have found that the internuclear distance of the sample I2 molecules in an alignment Nd:YAG laser field of 6 × 1011 W/cm2 is elongated by from 0.18 to 0.30 Å “in average” relatively to the equilibrium internuclear distance of 2.666 Å. Thus, the present experiment constitutes a critical step towards the goal of femtosecond imaging of chemical reactions and opens a new direction for the study of ultrafast chemical reaction in the gas phase.

Spatial distribution of ions in a linear octopole radio-frequency ion trap in the space-charge limit

We have explored the spatial distribution of an ion cloud trapped in a linear octopole radio-frequency (rf) ion trap. The two-dimensional distribution of the column density of stored silver dimer cations was measured via photofragment-ion yields as a function of the position of the incident laser beam over the transverse cross section of the trap. The profile of the ion distribution was found to be dependent on the number of loaded ions. Under high ion-loading conditions with a significant space-charge effect, ions form a ring profile with a maximum at the outer region of the trap, whereas they are localized near the center axis region at low loading of the ions. These results are explained quantitatively by a model calculation based on equilibrium between the space-charge-induced potential and the effective potential of the multipole rf field. The maximum adiabaticity parameter \eta_max is estimated to be about 0.13 for the high ion-density condition in the present octopole ion trap, which is lower than typical values reported for low ion densities; this is probably due to additional instability caused by the space charge.

Dissociation of biomolecules in liquid environments during fast heavy-ion irradiation

The effect of aqueous environment on fast heavy-ion radiation damage of biomolecules was studied by comparative experiments using liquid- and gas-phase amino acid targets. Three types of amino acids with different chemical structures were used: glycine, proline, and hydroxyproline. Ion-induced reaction products were analyzed by time-of-flight secondary-ion mass spectrometry. The results showed that fragments from the amino acids resulting from the C-Cα bond cleavage were the major products for both types of targets. For liquid-phase targets, specific products originating from chemical reactions in solutions were observed. Interestingly, multiple dissociated atomic fragments were negligible for the liquid-phase targets. We found that the ratio of multifragment to total fragment ion yields was approximately half of that for gas-phase targets. This finding agreed with the results of other studies on biomolecular cluster targets. It is concluded that the suppression of molecular multifragmentation is caused by the energy dispersion to numerous water molecules surrounding the biomolecular solutes.