Ultrafast relaxation of photoexcited superfluid He nanodroplets

The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, we study helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free- electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental pho- toelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He ) within 1 ps. Subsequently, the bubble collapses and releases metastable He at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses

Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li1–x_{1–x}Fex_x(OH)Fe1–y_{1–y}Se

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li$_{1–x}$Fex(OH)Fe$_{1–y}$Se (x$\sim$0.2; 0.02 < $y$ < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low ($y$ < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li$_{1–x}$Fe$_x$(OH) reservoir layer to fill vacancies in the selenide layer

Center for By-Products Utilization USE OF COAL COMBUSTION PRODUCTS IN PERMEABLE ROADWAY BASE CONSTRUCTION USE OF COAL COMBUSTION PRODUCTS IN PERMEABLE ROADWAY BASE CONSTRUCTION

SYNOPSIS This paper presents the results of an investigation carried out to develop permeable base course materials using coal combustion products (CCPs) for roadways, highways, and airfield pavements. Three sources of CCPs were selected for this investigation. These include two sources of high-carbon/sulfate-bearing CCPs, which did not meet ASTM C 618 requirements for coal fly ash for use as mineral admixture in concrete, and one source of variable carbon fly ash. These CCPs were used for no-fines/low-fines concrete as a permeable base material. Two types of mixtures were developed using each of these by-products for base course materials. In these mixtures, the amount of fines was varied for the permeable base, one with open-graded and one with an intermediate-graded structure. Tests were performed for fresh concrete properties as well as for compressive strength, splitting tensile strength, flexural strength, etc. The performance of the permeable base mixtures containing CCPs was also compared with a reference mixture without any ash. Test results up to 181 days of testing indicate that CCPs materials can be effectively used as a permeable base course material

Center for By-Products Utilization USE OF COAL COMBUSTION PRODUCTS IN PERMEABLE ROADWAY BASE CONSTRUCTION Use of Coal Combustion Products in Permeable Roadway Base Construction

Abstract This paper presents the results of an investigation carried out to develop permeable base course materials using coal combustion products (CCPs) for roadways, highways, and airfield pavements. Three sources of CCPs were selected for this investigation. These include two sources of high-carbon/sulfate-bearing CCPs, which did not meet ASTM C 618 requirements for coal fly ash for use as mineral admixture in concrete, and one source of variable carbon fly ash. These CCPs were used for no-fines/low-fines concrete as a permeable base material. Two types of mixtures were developed using each of these by-products for base course materials. In these mixtures, the amount of fines was varied for the permeable base, one with open-graded and one with an intermediate-graded structure. Tests were performed for fresh concrete properties as well as for compressive strength, splitting tensile strength, flexural strength, etc. The performance of the permeable base mixtures containing CCPs was also compared with a reference mixture without any ash. Test results up to 181 days of testing indicate that CCPs materials can be effectively used as a permeable base course material

Ultrafast relaxation of photoexcited superfluid He nanodroplets

The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, helium nanodroplets are resonantly excited by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He*) within 1 ps. Subsequently, the bubble collapses and releases metastable He* at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses

HIGH RESOLUTION SPECTROSCOPY AND DYNAMICS: FROM JET COOLED RADICALS TO GAS-LIQUID INTERFACES

Author Institution: JILA, University of Colorado and National Institute of Standards and Technology, Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0440; Los Gatos Research, 67 E. Evelyn Ave. Suite 3, Mountain View, CA 94041; Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave Building 6-026, Cambridge, MA 02139This talk will attempt to reflect recent work in our group involving two quite different but complementary applications of high resolution molecular spectroscopy for detailed study of intramolecular as well as intermolecular dynamics in small molecules. The first is based on direct infrared absorption spectroscopy in a 100 KHz slit supersonic discharge, which provides a remarkably versatile and yet highly sensitive probe for study of important chemical transients such as open shell combustion species and molecular ions under jet cooled (10-20K), sub-Doppler conditions. For this talk will focus on gas phase spectroscopic results for a series of unsaturated hydrocarbon radical species (ethynyl, vinyl, and phenyl) reputed to be critical intermediates in soot formation. Secondly, we will discuss recent applications of high resolution IR and velocity map imaging spectroscopy toward quantum state resolved collision dynamics of jet cooled molecules from gas-room temperature ionic liquid (RTIL) and gas-self assembled monolayer (SAM) interfaces. Time permitting, we will also present new results on hyperthermal scattering of jet cooled NO radical from liquid Ga, which offer a novel window into non-adiabatic energy transfer and electron-hole pair dynamics at the gas-molten metal interface

Empirical Evaluations with Domain Experts

Over the past thirty years, the visualization community has developed theories and models to explain visualization as a technology that augments human cognition by enabling the efficient, accurate, and timely discovery of meaningful information in data. Along the way, practitioners have also debated theories and practices for visualization evaluation: How do we generate durable, reliable evidence that a visualization is effective? Interestingly, there is still no consensus in the visualization research community how to evaluate visualization methods. The goal of this chapter is to rise awareness of still open issues in the visualization evaluation and to discuss appropriate evaluations suitable for different visualization approaches. This includes user studies and best practices to conduct them but also other approaches for suitable evaluation of visualization. The chapter is structured as a moderated dialog of two visualization experts