vations of a discontinuity near 1150 km in subduction arcas [(3, 6), for example] indicates that this may be a global feature o t yet unknou.n nature.T h e observation of reflectors at mid-1ua11tle iiepths comparable to those in subduction zones, hut in a different environment, indicates that a search for yet unidentitied nlineral asscnlblagcs o t global significance may he ~\.orthn.hilc. Recent cxperiments have shown the possihle existence of phase tra~lsitions at lolvcr mantle conditions: orthorhomhic-to-c~113ic silicate pcrovskite (3d) a i d rutile SiOz to CaClz structure ( 3 1 ). T h e role ancl proportions of volatiles such as water or carbon dioxide in the nlantle remain largely unknown and co~lld he o t importance (32). Additional seismic observations with the great resolving power of BB arrays s110~11ci help answer the yucstio~n o t the global character of our observations.
The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum-ultraviolet laser-induced fluorescence was used to probe the CO fragment, and multiphoton ionization time-of-flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.
Inspired by the discovery of bismuth metal used as a pigment in a fifteenth century illuminated manuscript, the identification of bismuth compounds by Raman spectroscopy is investigated. Raman spectra of bismuth metal powder, bismuth (III) sulfide (Bi 2 S 3 ) and bismuth (III) oxide (α-Bi 2 O 3 ) are presented. The influence of excitation laser wavelength is explored, along with the laser-induced oxidation of bismuth to α-Bi 2 O 3 and β-Bi 2 O 3 . The implication of these results with respect to the selection of appropriate analytical protocols in the investigation of bismuth pigments in works of art is discussed.
Naturally occurring arsenic sulfide minerals are brightly colored and consequently have been used as artists' pigments since ancient times. Orpiment, As 2 S 3 , is yellow and is often found associated with realgar, As 4 S 4 , which is red to red-orange. When exposed to sunlight, unprotected realgar develops a surface coating of friable yellow material that, until recently, had generally been assumed to be orpiment. However, it has been determined that this material is pararealgar, a light-induced polymorph of realgar. We have identified pararealgar in a major work by the Renaissance master, Tintoretto. The accurate identification of pigments in artistic and historic works is of great importance for determining the construction, history, and future preservation of these works. We report here the characterization by Raman microspectroscopy of pararealgar and the various intermediate species involved in the light-induced transformation of realgar to pararealgar. The relative merits of Raman microspectroscopy and X-ray diffraction for the accurate and efficient characterization of these arsenic sulfide compounds are discussed.The identification and characterization of pigments found in artistic and historic works is an important aspect of the care and preservation of museum collections, yielding information on the artists' materials and techniques, authenticity, previous restoration history, and present condition of works of art. The Dreams of Men 1 (ca. 1547), a painting by the Venetian artist Jacopo Robusti (1518-1594), known as Tintoretto, recently underwent extensive study and conservation 2 in the Conservation Services Laboratory at the Detroit Institute of Arts. The large ceiling painting, which is over 12 ft long and 7 ft wide, depicts the interaction of human dreams, fortune, and the cycles governing heaven and earth through complicated mythological and astrological symbols. Extremely small paint samples, with surface areas of 0.5 mm 2 or less, were removed from a few carefully selected sites on the painting. Upon sectioning, between 5 and 15 discernable layers of varnish, glaze, paint, and glue were exposed in the resulting cross section. Analysis of the materials in each layer revealed the sequence in which paint layers of different colors and opacity were applied to create the desired overall tonal effect.During our study of one small fragment removed from The Dreams of Men, elemental analysis 3 showed the presence of arsenic and sulfur in the top yellow paint layer. We initially interpreted this information as indicating that the artist had used orpiment, As 2 S 3 , a yellow pigment known to have been used by Venetian painters during the Renaissance. The few yellow pigment particles in the layer were carefully isolated and subjected to X-ray powder diffraction analysis, which revealed that the material was not orpiment but rather was pararealgar, a naturally occurring yellow mineral characterized only recently. Pararealgar is a light-induced polymorph of the red mineral realgar, As 4 S 4 , that readily fo...
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