Electronic spectra of polyatomic molecules with resolved individual rotational transitions

The density of rotational transitions for a polyatomic molecule is so large that in general many such transitions are hidden under the Doppler profile, this being a fundamental limit of conventional high resolution electronic spectroscopy. We present here the first Doppler-free cw two-photon spectrum of a polyatomic molecule. In the case of benzene, 400 lines are observed of which 300 are due to single rotational transitions, their spacing being weil below the Doppler profile. The resolution so achieved is 1.5 X 10'. Benzene is a prototype planar molecule taken to have D •• symmetry in the ground as weil as in the first excited state. From our ultra-high resolution results it is found that benzene in the excited SI state i8 a symmetrical rotor to a high degree. A negative inertial defect is found for the excited state. The origin of this inertial defect is discused

Back to the roots of "channel three"

Rotationally resolved fluorescence excitation and resonance enhanced multiphoton ionization (MPI) spectra of the 610130 one-photon band of benzene at the onset of ``channel three'' are reported. The fluorescence decay is monitored after rotationally selected excitation and a large variation of the nonradiative decay time (<1 to 6.8 ns) is found for the different rotational states at the vibrational excess energy of 3287 cm−1 in S1. The rotational structure of the fluorescence excitation spectrum and the MPI spectrum measured with delayed laser pulses differ considerably. All observed lines of the MPI spectrum were assigned and the rotational line structure can only be understood with a model which incorporates interference between different decay channels. Due to this interference, particular rotational states decay fairly slowly and give rise to lines in the spectrum while states with neighboring rotational quantum numbers decay rapidly and are therefore not found in the spectrum. The previously reported drastic increase of the electronic, nonradiative decay of benzene in this region of excess energy, which led to the postulation of ``channel three,'' cannot be confirmed. Instead, the optically excited rovibronic states are thought to be coupled to background states within S1 which are themselves broadened due to strong coupling to the highly excited S0 electronic state rather than due to an unknown (``channel three'') or isomerization process. The Journal of Chemical Physics is copyrighted by The American Institute of Physics

Parity-Affected Superconductivity in Ultrasmall Metallic Grains

We investigate the breakdown of BCS superconductivity in {\em ultra}\/small metallic grains as a function of particle size (characterized by the mean spacing $d$ between discrete electronic eigenstates), and the parity ($P$ = even/odd) of the number of electrons on the island. Assuming equally spaced levels, we solve the parity-dependent BCS gap equation for the order parameter $\Delta_P (d,T)$. Both the $T=0$ critical level spacing $d_{c,P}$ and the critical temperature $T_{c,P} (d)$ at which $\Delta_P = 0$ are parity dependent, and both are so much smaller in the odd than the even case that these differences should be measurable in current experiments.Comment: 4 pages RevTeX, 1 encapsulated postscript figure, submitted to Physical Review Letter

Diversity of animal communities on southwestern rangelands: Species patterns, habitat relationships, and land management

The rangelands of the southwestern United States comprise a mosaic of biome types, including deserts, grasslands, chaparral, woodlands, forests, subalpine meadows, and alpine tundra. Taken together, these ecosystems support exceptionally high numbers of vertebrate and invertebrate animal species. Biogeographic patterns of mammal, bird, and reptile species across North America show trends of increasing species numbers for these vertebrate groups, and some invertebrate groups, occur in Texas, New Mexico, Arizona, and California, especially in the border region with Mexico. Underlying causes of the region\u27s high biodiversity are related to (1) the elevational variability inherent in the basin-and-range topography, with its concomitant range of climate conditions, (2) the diverse biogeographic history of the region, particularly with respect to the merging of major faunal groups during glacier retreats, and (3) the architectural variations in vegetation structure across the region\u27s component ecosystems. Climate dynamics and disturbance also play major roles in maintaining a habitat mosaic, promoting greater regional faunal diversity. Disturbances affect animal diversity at many scales, from individuals\u27 home ranges to continental species\u27 distributions. Human activities have generated new suites of disturbances (livestock grazing, timber harvesting, mining, agriculture, prescribed fires, construction of roads and buildings), many of which contribute to the habitat patchiness of the landscape. Studies have shown that these disturbances prove beneficial to some species and detrimental to others. Hence, local increases in biodiversity can be orchestrated by creating or maintaining habitat diversity and disturbance regimes. Such management strategies can be scaled up to regional landscapes, in which areas of intensive human land use and disturbance are interspersed with regions of little or no human interference. Historically, this has been accomplished at local or state levels on an ad hoc bases (i.e., crisis management), with little evidence of long-term, large-scale, regional planning or coordination. If faunal biodiversity is to be preserved and enhanced on southwestern rangelands, human activities must be managed in a fashion that integrates faunal biology, resource requirements, and movement patterns with landscape scale attributes. Therefore, the task of the modern land manager will be to balance carefully the various scales and intensities of human activities, for the purpose of promoting sustainable use of natural resources and assuring the maintenance or enhancement of biodiversity. Future regional planning for biodiversity attributes will clearly require extensive communication and close cooperation among concerned citizens, private landowners, scientists, and government land managers

Optical conductivity of filled skutterudites

A simple tight-binding model is constructed for the description of the electronic structure of some Ce-based filled skutterudite compounds showing an energy gap or pseudogap behavior. Assuming band-diagonal electron interactions on this tight-binding model, the optical conductivity spectrum is calculated by applying the second-order self-consistent perturbation theory to treat the electron correlation. The correlation effect is found to be of great importance on the description of the temperature dependence of the optical conductivity. The rapid disappearance of an optical gap with increasing temperature is obtained as observed in the optical experiment for Ce-based filled-skutterudite compounds.Comment: 6 pages, 7 figures, use jpsj2.cls, to appear in J. Phys. Soc. Jpn. Vol.73, No.10 (2004

Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems

The finite size dependent enhancement of pairing in mesoscopic Fermi systems is studied under the assumption that the BCS approach is valid and that the two body force is size independent. Different systems are investigated such as superconducting metallic grains and films as well atomic nuclei. It is shown that the finite size enhancement of pairing in these systems is in part due to the presence of a surface which accounts quite well for the data of nuclei and explains a good fraction of the enhancement in Al grains.Comment: Updated version 17/02/0

Toward mid-infrared, subdiffraction, spectral-mapping of human cells and tissue: SNIM (scanning near-field infrared microscopy) tip fabrication

Scanning near-field infrared microscopy (SNIM) potentially enables subdiffraction, broadband mid-infrared (MIR:3–25-μm wavelength range) spectral-mapping of human cells and tissue for real-time molecular sensing, with prospective use in disease diagnosis. SNIM requires an MIR-transmitting tip of small aperture for photon collection. Here, chalcogenide-glass optical fibers are reproducibly tapered at one end to form a MIR transmitting tip for SNIM. A wet-etching method is used to form the tip. The tapering sides of the tip are Al-coated. These Al-coated tapered-tips exhibit near-field power-confinement when acting either as the launch-end or exit-end of the MIR optical fiber. We report first time optimal cleaving of the end of the tapered tip using focused ion beam milling. A flat aperture is produced at the end of the tip, which is orthogonal to the fiber-axis and of controlled diameter. A FIB-cleaved aperture is used to collect MIR spectra of cells mounted on a transflection plate, under illumination of a synchrotron- generated wideband MIR beam

Physical Electronics

Contains reports on three research projects

Correlation Effects on Optical Conductivity of FeSi

Effects of electron correlation in FeSi are studied in terms of the two-band Hubbard model with the density of states obtained from the band calculation. Using the self-consistent second-order perturbation theory combined with the local approximation, the correlation effects are investigated on the density of states and the optical conductivity spectrum, which are found to reproduce the experiments done by Damascelli et al. semiquantitatively. It is also found that the peak at the gap edge shifts to lower energy region by correlation effects, as is seen in the experiments.Comment: 4 pages, 3 figure