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 (H2O)2+ 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 (H3O+·OH), though the alternate metastable hemibonded structure (H2O·OH2)+ was also predicted based on theoretical calculations. Here, we produce (H2O)2+ dimers in superfluid helium nanodroplets and study their infrared spectra in the range of OH stretching vibrations. The observed spectra indicate the coexistence of the two structures in the droplets, supported by density functional theory calculations. This is the first spectroscopic identification of the hemibonded isomer of water radical cation dimers. The observation of the higher-energy isomer reveals efficient kinetic trapping for metastable ionic clusters due to the rapid cooling in helium droplets.

Energy Dispersion in Pyridinium-Water Nanodroplets upon Irradiation.

Postirradiation dissociation of molecular clusters has been mainly studied assuming energy redistribution in the entire cluster prior to the dissociation. Here, the evaporation of water molecules from out-of-equilibrium pyridinium-water cluster ions was investigated using the recently developed correlated ion and neutral time-of-flight (COINTOF) mass spectrometry technique in combination with a velocity-map imaging (VMI) device. This special setup enables the measurement of velocity distributions of the evaporated molecules upon high-velocity collisions with an argon atom. The distributions measured for pyridinium-water cluster ions are found to have two distinct components. Besides a low-velocity contribution, which corresponds to the statistical evaporation of water molecules after nearly complete redistribution of the excitation energy within the clusters, a high-velocity contribution is also found in which the molecules are evaporated before the energy redistribution is complete. These two different evaporation modes were previously observed and described for protonated water cluster ions. However, unlike in the case of pure water clusters, the low-velocity part of the distributions for pyridinium-doped water clusters is itself composed of two distinct Maxwell-Boltzmann distributions, indicating that evaporated molecules originate in this case from out-of-equilibrium processes. Statistical molecular dynamics simulations were performed to (i) understand the effects caused in the ensuing evaporation process by the various excitation modes at different initial cluster constituents and to (ii) simulate the distributions resulting from sequential evaporations. The presence of a hydrophobic impurity in water clusters is shown to impact water molecule evaporation due to the energy storage in the internal degrees of freedom of the impurity.

Testing Quantum Electrodynamics with Exotic Atoms.

Precision study of few-electron, high-Z ions is a privileged field for probing high-field, bound-state quantum electrodynamics (BSQED). However, the accuracy of such tests is plagued by nuclear uncertainties, which are often larger than the BSQED effects under investigation. We propose an alternative method with exotic atoms and show that transitions may be found between circular Rydberg states where nuclear contributions are vanishing while BSQED effects remain large. When probed with newly available quantum sensing detectors, these systems offer gains in sensitivity of 1 to 2 orders of magnitude, while the mean electric field largely exceeds the Schwinger limit.

Broadband high-energy resolution hard x-ray spectroscopy using transition edge sensors at SPring-8.

We have succeeded in operating a transition-edge sensor (TES) spectrometer and evaluating its performance at the SPring-8 synchrotron x-ray light source. The TES spectrometer consists of a 240 pixel National Institute of Standards and Technology (NIST) TES system, and 220 pixels are operated simultaneously with an energy resolution of 4 eV at 6 keV at a rate of ∼1 c/s pixel-1. The tolerance for high count rates is evaluated in terms of energy resolution and live time fraction, leading to an empirical compromise of ∼2 × 103 c/s (all pixels) with an energy resolution of 5 eV at 6 keV. By utilizing the TES's wideband spectroscopic capability, simultaneous multi-element analysis is demonstrated for a standard sample. We conducted x-ray absorption near-edge structure (XANES) analysis in fluorescence mode using the TES spectrometer. The excellent energy resolution of the TES enabled us to detect weak fluorescence lines from dilute samples and trace elements that have previously been difficult to resolve due to the nearly overlapping emission lines of other dominant elements. The neighboring lines of As Kα and Pb Lα2 of the standard sample were clearly resolved, and the XANES of Pb Lα2 was obtained. Moreover, the x-ray spectrum from the small amount of Fe in aerosols was distinguished from the spectrum of a blank target, which helps us to understand the targets and the environment. These results are the first important step for the application of high resolution TES-based spectroscopy at hard x-ray synchrotron facilities.

State-Selective Observation of Radiative Cooling of Vibrationally Excited C2.

We have observed radiative cooling of vibrationally excited C2- in the X2Σg+ electronic ground state via electronic transitions to near-degenerate low-lying vibrational levels of the A2Πu 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.

Threshold Photodetachment Spectroscopy of the Positronium Negative Ion.

Threshold photodetachment spectroscopy of the positronium negative ion has been accomplished for the first time employing an efficient source of the ions and photodetachment techniques combined with a tunable optical parametric oscillator and amplifier laser. The photodetachment threshold, corresponding to the electron affinity of positronium (1^{3}S_{1}), was determined to be 326.88±0.09(stat)±0.10(syst) meV by laser photodetachment threshold measurements. This result is consistent with a variational calculation corrected for leading relativistic and quantum electrodynamical effects.

Rotationally Resolved Excitation Spectra Measured by Slow Electron Detachment from Si2.

Laser-induced delayed electron detachment from Si2- 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. The occurrence of delayed detachment on such a long time scale is unusual for diatomic molecules, suggesting that both the autodetachment and fluorescence are slow.

Impact of a hydrophobic ion on the early stage of atmospheric aerosol formation.

Atmospheric aerosols are one of the major factors affecting planetary climate, and the addition of anthropogenic molecules into the atmosphere is known to strongly affect cloud formation. The broad variety of compounds present in such dilute media and their specific underlying thermalization processes at the nanoscale make a complete quantitative description of atmospheric aerosol formation certainly challenging. In particular, it requires fundamental knowledge about the role of impurities in water cluster growth, a crucial step in the early stage of aerosol and cloud formation. Here, we show how a hydrophobic pyridinium ion within a water cluster drastically changes the thermalization properties, which will in turn change the corresponding propensity for water cluster growth. The combination of velocity map imaging with a recently developed mass spectrometry technique allows the direct measurement of the velocity distribution of the water molecules evaporated from excited clusters. In contrast to previous results on pure water clusters, the low-velocity part of the distributions for pyridinium-doped water clusters is composed of 2 distinct Maxwell-Boltzmann distributions, indicating out-of-equilibrium evaporation. More generally, the evaporation of water molecules from excited clusters is found to be much slower when the cluster is doped with a pyridinium ion. Therefore, the presence of a contaminant molecule in the nascent cluster changes the energy storage and disposal in the early stages of gas-to-particle conversion, thereby leading to an increased rate of formation of water clusters and consequently facilitating homogeneous nucleation at the early stages of atmospheric aerosol formation.

Anion photoelectron spectroscopy of free [Au25(SC12H25)18].

Previous theoretical studies have shown that the thiolated gold cluster compound [Au25(SR)18]- can be viewed as a prototypical superatom with a closed electronic structure. The quantized electronic structure of [Au25(SR)18]- has been experimentally demonstrated by optical and electrochemical methods in the dispersed state. Nevertheless, no direct information is available on the energy levels and densities of occupied states. Here, we report the photoelectron spectrum of [Au25(SC12H25)18]- isolated under vacuum for the first time. The spectrum exhibits two distinct peaks, corresponding to electron detachment from the superatomic 1P orbitals and Au 5d orbitals of the Au13 core. The adiabatic electron affinity of [Au25(SC12H25)18]0 was experimentally determined to be 2.2 eV, which is significantly smaller than that of [Au25(SCH3)18]0 predicted theoretically.

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.

Observation of a shape resonance of the positronium negative ion.

When an electron binds to its anti-matter counterpart, the positron, it forms the exotic atom positronium (Ps). Ps can further bind to another electron to form the positronium negative ion, Ps(-) (e(-)e(+)e(-)). Since its constituents are solely point-like particles with the same mass, this system provides an excellent testing ground for the three-body problem in quantum mechanics. While theoretical works on its energy level and dynamics have been performed extensively, experimental investigations of its characteristics have been hampered by the weak ion yield and short annihilation lifetime. Here we report on the laser spectroscopy study of Ps(-), using a source of efficiently produced ions, generated from the bombardment of slow positrons onto a Na-coated W surface. A strong shape resonance of (1)P(o) symmetry has been observed near the Ps (n=2) formation threshold. The resonance energy and width measured are in good agreement with the result of three-body calculations.

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.

Characteristics of Okinawan native agu pig spermatozoa after addition of low-density lipoprotein to freezing extender.

Technical refinement of boar sperm cryopreservation is indispensable for effective breeding of the rare Okinawan native pig, the Agu. The objective of the present study was to determine whether addition of low-density lipoprotein (LDL) extracted from hen egg yolk to the freezing extender improves the characteristics of cryopreserved Agu spermatozoa. Ejaculated Agu sperm frozen in extender supplemented with 2, 4, 6, 8 or 10% LDL instead of egg yolk was thawed, and the post-thaw sperm characteristics were evaluated. Treatment with 4-8% LDL during cooling and freezing significantly increased the intracellular cholesterol content, as compared to that of sperm frozen in extender containing 20% egg yolk (P<0.05). Higher potential resistance to cell damage from cryoinjury was also observed in sperm frozen in extender supplemented with LDL: the integrities of plasmalemma and DNA, mitochondrial activity and proteolytic activity of the acrosomal content in the post-thaw sperm were superior to those of sperm that were not treated with LDL. Moreover, the percentages of total motile sperm and the extent of rapid progressive motility at 1 and 3 h after incubation were markedly higher in sperm treated with 4 or 6% LDL, and these sperm also had more ATP. However, LDL did not inhibit in vitro sperm penetrability, even though the cholesterol content of post-thaw sperm was higher after treatment with LDL. These findings indicate that addition of 4-6% LDL instead of egg yolk to the freezing extender improves the post-thaw characteristics of Agu sperm by protecting sperm against cold shock damage during cryopreservation.