Isolation and Infrared Spectroscopic Characterization of Hemibonded Water Dimer Cation in Superfluid Helium Nanodroplets.

The structure of the minimum unit of the radical cationic water clusters, the (HO) dimer, has attracted much attention because of its importance for the radiation chemistry of water. Previous spectroscopic studies indicated that the dimers have a proton-transferred structure (HO·OH), though the alternate metastable hemibonded structure (HO·OH) was also predicted based on theoretical calculations. Here, we produce (HO) dimers in superfluid helium nanodroplets and study their infrared spectra in the range of OH stretching vibrations.

State-Selective Observation of Radiative Cooling of Vibrationally Excited C.

We have observed radiative cooling of vibrationally excited C in the XΣ electronic ground state via electronic transitions to near-degenerate low-lying vibrational levels of the AΠ electronic excited state. Combining an ion storage technique with high-resolution detachment spectroscopy, we were able to assign rovibronic transitions to the resulting complex spectra. The time evolution of the population at specific vibrational states was measured up to 60 ms, providing the first quantitative experimental support for the long-standing theoretical predictions.

Rotationally Resolved Excitation Spectra Measured by Slow Electron Detachment from Si.

Laser-induced delayed electron detachment from Si stored in an electrostatic ion storage ring was observed on the 10 microsecond time scale. The excitation spectra for photon energies near threshold show well-resolved multipeak structures, which are attributed to rovibronic transitions to the electronic excited state. This structure appears only in the signal measured with the delay.

Detection of Recurrent Fluorescence Photons.

We have detected visible photons emitted from the thermally populated electronic excited state, namely recurrent fluorescence (RF), of C_{6}^{-} stored in an electrostatic ion storage ring. Clear evidence is provided to distinguish RF from normal fluorescence, based on the temporal profile of detected photons synchronized with the revolution of C_{6}^{-} in the ring, for which the time scale is far longer than the lifetime of the intact photoexcited state. The relaxation (cooling) process via RF is likely to be commonplace for isolated molecular systems and crucial to the stabilization of molecules in interstellar environments.

Inverse internal conversion in C4(-) below the electron detachment threshold.

Inverse internal conversion followed by recurrent fluorescence was observed as a fast decay (10 μs range) in the time profile of neutral yields from photo-excited C4(-) molecular ions. We also elucidated the contribution of such electronic radiative cooling to the C4(-) ions with internal energy far below the detachment threshold by an alternative novel approach, observing the laser wavelength and storage time dependence (ms range) of the total yield of the photo-induced neutrals.

Development of a Kingdon ion trap system for trapping externally injected highly charged ions.

We have developed a Kingdon ion trap system for the purpose of the laboratory observation of the x-ray forbidden transitions of highly charged ions (HCIs). Externally injected Ar(q+) (q = 5-7) with kinetic energies of 6q keV were successfully trapped in the ion trap. The energy distribution of trapped ions is discussed in detail on the basis of numerical simulations.