Extinction coefficients k(lambda) for water at 25 degrees C were determined through a broad spectral region by manually smoothing a point by point graph of k(lambda) vs wavelength lambda that was plotted for data obtained from a review of the scientific literature on the optical constants of water. Absorption bands representing k(lambda) were postulated where data were not available in the vacuum uv and soft x-ray regions. A subtractive Kramers-Kronig analysis of the combined postulated and smoothed portions of the k(lambda) spectrum provided the index of refraction n(lambda) for the spectral region 200 nm </= lambda </= 200 microm.

Optical Constants of Water in the 200-nm to 200-microm Wavelength Region.

The demand for compact ultraviolet laser devices is increasing, as they are essential in applications such as optical storage, photocatalysis, sterilization, ophthalmic surgery and nanosurgery. Many researchers are devoting considerable effort to finding materials with larger bandgaps than that of GaN. Here we show that hexagonal boron nitride (hBN) is a promising material for such laser devices because it has a direct bandgap in the ultraviolet region. We obtained a pure hBN single crystal under high-pressure and high-temperature conditions, which shows a dominant luminescence peak and a series of s-like exciton absorption bands around 215 nm, proving it to be a direct-bandgap material. Evidence for room-temperature ultraviolet lasing at 215 nm by accelerated electron excitation is provided by the enhancement and narrowing of the longitudinal mode, threshold behaviour of the excitation current dependence of the emission intensity, and a far-field pattern of the transverse mode.

Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal.

A compilation of the optical constants of ice Ih is made for temperatures within 60 K of the melting point. The imaginary part mim of the complex index of refraction m is obtained from measurements of spectral absorption coefficient; the real part mre is computed to be consistent with mim by use of known dispersion relations. The compilation of mim requires subjective interpolation in the near-ultraviolet and microwave, a temperature correction in the far-infrared, and a choice between two conflicting sources in the near-infrared. New measurements of the spectral absorption coefficient of pure ice are needed, at temperatures near the melting point, for 185–400-nm, 1.4–2.8-μm, 3.5–4.3-μm, 33–600-μm, and 1–100-mm wavelengths.

Optical constants of ice from the ultraviolet to the microwave

Optical constants from the far infrared to the x-ray region: Mg, Al, Cu, Ag, Au, Bi, C, and Al 2 O 3

Charge density waves (CDWs) are periodic modulations of the density of conduction electrons in solids. They are collective states that arise from intrinsic instabilities often present in low-dimensional electronic systems. The most well-studied examples are the layered dichalcogenides–an example of which is TiSe2, one of the first CDW-bearing materials to be discovered. At low temperatures, a widely held belief is that the CDW competes with another collective electronic state, superconductivity. But despite much exploration, a detailed study of this competition is lacking. Here we report how, on controlled intercalation of TiSe2 with Cu to yield CuxTiSe2, the CDW transition can be continuously suppressed, and a new superconducting state emerges near x=0.04, with a maximum transition temperature Tc of 4.15 K at x=0.08. CuxTiSe2 thus provides the first opportunity to study the CDW to superconductivity transition in detail through an easily controllable chemical parameter, and will provide fundamental insight into the behaviour of correlated electron systems.

Superconductivity in CuxTiSe2

An investigation was carried out on the excited state of the $F$ center in various alkali halide crystals as a function of temperature. Results are presented on the time dependence of photoconductivity and luminescence following excitation by a very short, ${10}^{\ensuremath{-}8}$ sec, pulse of light. The data can be interpreted in terms of a simple energy-level diagram of the $F$ center consisting of a ground state and an excited state. Estimates are given of the depth of the excited state below the conduction band $\ensuremath{\Delta}E$, the frequency factor for thermal ionization $\frac{1}{{\ensuremath{\tau}}_{0}}$, and the radiative lifetime ${\ensuremath{\tau}}_{R}$, for KCl, KBr, KI, and NaCl. The radiative lifetime ${\ensuremath{\tau}}_{R}$ depends upon the alkali halide, but may be as long as ${10}^{\ensuremath{-}6}$ sec. Suggestions are made for understanding this long lifetime in view of the large oscillator strength of the $F$ center. The effect of $F$-center concentration and also of an admixture of $M$ centers is discussed.

Lifetime of the Excited F Center

The SSRL ultrahigh vacuum grazing incidence monochromator: Design considerations and operating experience

The effect of strain on thallous-halide films has been investigated, and a technique developed for producing essentially strain-free films. The optical absorption of strain-reduced TlBr and TlCl films has been measured down to 4.2\ifmmode^\circ\else\textdegree\fi{}K, and the results analyzed for the optical constants. Relatively narrow (2-meV) firstexciton lines were found in the near ultraviolet accompanied by longitudinal-optical-phonon sidebands. The Faraday rotation pattern was measured through the first peak in TlBr and found to be linear in $H$. An analysis of the rotation pattern together with the known $n$ and $\ensuremath{\kappa}$ yields an exciton effective $g$ value of 0.7 for TlBr. Oscillatory magnetoabsorption has been observed in both TlBr and TlCl originating from the edge of the sideband. The exciton binding energies are 6.5\ifmmode\pm\else\textpm\fi{}1 meV in TlBr and 11\ifmmode\pm\else\textpm\fi{}2 meV in TlCl. The exciton-optical transition is discussed and the importance of lattice and polaron effects emphasized.

Exciton-Optical Properties of TlBr and TlCl

The electronic structure of high-quality Ti${\mathrm{Se}}_{2}$ and Ti${\mathrm{S}}_{2}$ crystals has been investigated using angle-resolved photoemission with HeI, HeII, and NeI resonance radiation. Results compare well with recent self-consistent energy-band calculations although differences occur which may be due to the three- rather than two-dimensional nature of specific bands. Occupied $d$ states at the zone edge are observed in both materials. A small overlap with the $\frac{s}{p}$ valence band at $\ensuremath{\Gamma}$ is observed in the case of Ti${\mathrm{Se}}_{2}$ in approximate agreement with other workers. Ti${\mathrm{S}}_{2}$ appears to be a defect semiconductor with a band gap of 0.3 \ifmmode\pm\else\textpm\fi{} 0.2 eV.

Angle-resolved photoemission studies of the band structure of Ti Se 2 and Ti S 2

High-resolution ${L}_{\mathrm{I}\mathrm{I}\mathrm{I},\phantom{\rule{0ex}{0ex}}\mathrm{I}\mathrm{I}}$ absorption spectra are presented for well-characterized films of amorphous and crystalline silicon. The crystalline spectrum shows features which are related to the detailed conduction-band structure of silicon. These features are not seen for the amorphous material presumably because of the lack of long-range order. The amorphous spectrum rises less steeply than the crystalline and has a broad peak near threshold.

Frederick C. Brown

Papers

Lifetime of the Excited F Center

The SSRL ultrahigh vacuum grazing incidence monochromator: Design considerations and operating experience

Exciton-Optical Properties of TlBr and TlCl

Angle-resolved photoemission studies of the band structure of Ti Se 2 and Ti S 2

Extreme Ultraviolet Transmission of Crystalline and Amorphous Silicon