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Abstract Iron-titanium (Fe-Ti) charge transfer is mentioned in numerous articles as the source of the coloration of many natural minerals and some man-made materials, but no global review of this phenomenon has been provided so far. Iron and titanium are ubiquitous in nature and are often found in the same material as Fe2+ and Fe3+, and Ti4+ (more rarely Ti3+). When Fe and Ti ions are in close geometric proximity in an oxide or (alumino)silicate structure, charge transfer can occur between the two ions, even though their concentration might be below 100 ppm. This results in a variety of absorption features that contributes to the color of minerals. Adebate remains on the exact nature of Fe/Ti electronic transition, i.e. Fe2+ + Ti4+ → Fe3+ + Ti3+ or the reverse, but solving this issue is not within the scope of the present work. Ascertaining a metal-metal charge transfer is often not straightforward. This review compiles existing knowledge on Fe-Ti charge transfer in both crystalline and amorphous materials and identifies several key characteristics in more than 40 different materials. A charge transfer is associated with broad, intense, optical absorption bands that decrease in intensity at elevated temperatures. It is also strongly pleochroic in non-isotropic materials. Until now, Fe-Ti charge transfer transitions have been primarily described in the 2.25 to 3.1 eV range, corresponding to yellow to orange to brown colors, with notable exceptions such as blue sapphire or kyanite, and green andalusite. This review suggests that Fe-Ti charge transfer can occur across the entire visible spectrum, and the position of the absorption band correlates with the Fe-Ti nteratomic distance. This correlation highlights the presence of multiple crystallographic sites for both Fe and Ti in many oxides, leading to multiple Fe-Ti bands within these materials (e.g. sapphire, ilmenite, pseudobrookite). Finally, the use of metal-metal distances is suggested to differentiate this heteronuclear Fe-Ti charge transfer from the common homonuclear charge transfer Fe2+-Fe3+. 

Abstract The absorption of light by Fe/Ti and Fe/Fe intervalence charge transfer (IVCT) bands has previously been found in aluminum oxide and Al2SiO5 aluminosilicate minerals to decrease markedly at elevated temperatures. Given the abundance of iron at depth in the Earth, assessing the generality with which and extent to which IVCT mineral phases become more optically transparent at temperature than they are under ambient conditions has potentially significant implications for the modeling of mantle geophysical processes such as radiative conductivity. A broad experimental survey of the optical absorption spectra at elevated temperatures of various mixed valence iron minerals has been conducted. The minerals considered here are cordierite, chloritoid, lazulite, dumortierite, jeremejevite, beryl, osumilite, biotite (mica), pargasite (amphibole) and aegirine (pyroxene). All samples transiently lose significant Fe/Fe IVCT feature intensity at temperature. In beryl, osumilite, biotite, pargasite and aegirine, spin-allowed Fe2+d-d features also decrease in integral intensity at temperature; in all but beryl, the intensity loss is significant. This trend is consistent with d-d band enhancement via Fe2+/Fe3+ exchange coupling, which has not previously been identified in the majority of these minerals. It is contrasted against the behavior of ordinary spinallowed Fe2+d-d bands in non-IVCT minerals forsterite (olivine) and elbaite (tourmaline). The depletion of Fe/Fe IVCT and enhanced Fe2+d-d band intensity at elevated temperatures may both be important mechanisms by which iron-bearing mineral phases become more optically transparent under conditions at depth. 

Optical photothermal infrared spectroscopy (O-PTIR) was used to characterize a terrestrial rock sample as a demonstration of the technique’s enhanced spatial resolution as it corresponds to minerology and the detection of organics. Traditional reflectance-based infrared techniques are limited by the wavelength of the infrared light interacting with the surface along with additional optical dispersion issues. However, because of the nature in which the infrared spectrum is measured via O-PTIR, these traditional issues are eliminated. This is possible through the recent developments of high intensity quantum cascade-based infrared lasers capable of scanning the mid infrared spectrum (3000–500 cm−1). Individual O-PTIR and diffuse reflectance data were collected on a terrestrial rock sample and compared to a recent discovery of NASA JPL’s Perseverance Rover regarding inclusions of comparable size. In addition, an O-PTIR map of a particularly dense area of proteinaceous material in the terrestrial sample was collected, further exemplifying the capability. This technique has significant potential for use regarding future returned Mars samples and in situ planetary surface science when considering the spatial resolution, sensitivity, and negligible sample preparation. 

Abstract Unconscious neural activity has been shown to precede both motor and cognitive acts. In the present study, we investigated the neural antecedents of overt attention during visual search, where subjects make voluntary saccadic eye movements to search a cluttered stimulus array for a target item. Building on studies of both overt self-generated motor actions (Lau et al., 2004, Soon et al., 2008) and self-generated cognitive actions (Bengson et al., 2014, Soon et al., 2013), we hypothesized that brain activity prior to the onset of a search array would predict the direction of the first saccade during unguided visual search. Because both spatial attention and gaze are coordinated during visual search, both cognition and motor actions are coupled during visual search. A well-established finding in fMRI studies of willed action is that neural antecedents of the intention to make a motor act (e.g., reaching) can be identified seconds before the action occurs. Studies of the volitional control ofcovertspatial attention in EEG have shown that predictive brain activity is limited to only a few hundred milliseconds before a voluntary shift of covert spatial attention. In the present study, the visual search task and stimuli were designed so that subjects could not predict the onset of the search array. Perceptual task difficulty was high, such that they could not locate the target using covert attention alone, thus requiring overt shifts of attention (saccades) to carry out the visual search. If the first saccade to the array onset in unguided visual search shares mechanisms with willed shifts of covert attention, we expected predictive EEG alpha-band activity (8-12 Hz) immediately prior to the array onset (within 1 sec) (Bengson et al., 2014; Nadra et al., 2023). Alternatively, if they follow the principles of willed motor actions, predictive neural signals should be reflected in broadband EEG activity (Libet et al., 1983) and would likely emerge earlier (Soon et al., 2008). Applying support vector machine decoding, we found that the direction of the first saccade in an unguided visual search could be predicted up to two seconds preceding the search array’s onset in the broadband but not alpha-band EEG. These findings suggest that self-directed eye movements in visual search emerge from early preparatory neural activity more akin to willed motor actions than to covert willed attention. This highlights a distinct role for unconscious neural dynamics in shaping visual search behavior. 

In models of visual spatial attention control, it is commonly held that top–down control signals originate in the dorsal attention network, propagating to the visual cortex to modulate baseline neural activity and bias sensory processing. However, the precise distribution of these top–down influences across different levels of the visual hierarchy is debated. In addition, it is unclear whether these baseline neural activity changes translate into improved performance. We analyzed attention-related baseline activity during the anticipatory period of a voluntary spatial attention task, using two independent functional magnetic resonance imaging datasets and two analytic approaches. First, as in prior studies, univariate analysis showed that covert attention significantly enhanced baseline neural activity in higher-order visual areas contralateral to the attended visual hemifield, while effects in lower-order visual areas (e.g., V1) were weaker and more variable. Second, in contrast, multivariate pattern analysis (MVPA) revealed significant decoding of attention conditions across all visual cortical areas, with lower-order visual areas exhibiting higher decoding accuracies than higher-order areas. Third, decoding accuracy, rather than the magnitude of univariate activation, was a better predictor of a subject's stimulus discrimination performance. Finally, the MVPA results were replicated across two experimental conditions, where the direction of spatial attention was either externally instructed by a cue or based on the participants’ free choice decision about where to attend. Together, these findings offer new insights into the extent of attentional biases in the visual hierarchy under top–down control and how these biases influence both sensory processing and behavioral performance. 

Abstract Single-crystal optical spectra of corundum (Al2O3) and the Al2SiO5 polymorphs andalusite, kyanite, and sillimanite, containing both Fe2+-Fe3+ and Fe2+-Ti4+ intervalence charge transfer (IVCT) absorption bands were measured at temperatures up to 1000 °C. Upon heating, thermally equilibrated IVCT bands significantly decreased in intensity and recovered fully on cooling. These trends contrast with the behavior of crystal field bands at temperature for Fe, Cr, and V in corundum, kyanite, and spinel. The effects of cation diffusion and aggregation, as well as the redistribution of band intensity at temperature, are also discussed. The loss of absorption intensity in the visible and near-infrared regions of the spectrum of these phases may point to a more general behavior of IVCT in minerals at temperatures within the Earth with implications for radiative conductivity within the Earth. 

Assisted by predictions from density functional theory, we used infrared spectroscopy to observe hydride ions introduced into SrTiO3 crystals. 

Abstract Very few detections have been made of optical flashes contemporaneous with prompt high-energy emission from a gamma-ray burst (GRB). In this work, we present and analyze light curves of GRB-associated optical flashes and afterglows from the Transiting Exoplanet Survey Satellite (TESS). Our sample consists of eight GRBs with arcsecond-level localizations from the X-Ray Telescope on board the Neil Gehrels Swift Observatory (Swift). For each burst, we characterize the prompt optical emission and any observed afterglow, and constrain physical parameters for four of these bursts using their TESS light curves. This work also presents a straightforward method to correct for TESS's cosmic-ray mitigation strategy on 20 s timescales, which allows us to estimate the “true” brightness of optical flashes associated with prompt GRB emission. We also highlight TESS’s continuous wide-field monitoring capability, which provides an efficient means of identifying optical emission from GRBs and characterizing early time afterglow light curves. Based on empirical detection rates from Swift and the Fermi Gamma-ray Space Telescope, up to 10 GRBs per year may fall within the contemporaneous TESS field of view.