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We use resonant soft X-ray holography to image the insulator−metal phase transition in vanadium dioxide with element and polarization specificity and nanometer spatial resolution. We observe that nanoscale inhomogeneity in the film results in spatial-dependent transition pathways between the insulating and metallic states. Additional nanoscale phases form in the vicinity of defects which are not apparent in the initial or final states of the system, which would be missed in area-integrated X-ray absorption measurements. These intermediate phases are vital to understand the phase transition in VO2, and our results demonstrate how resonant imaging can be used to understand the electronic properties of phase-separated correlated materials obtained by X-ray absorption., Peer Reviewed, Postprint (author's final draft)

Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO2) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M2,3 edge is used to track the insulator-to-metal phase transition in VO2 This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO2 absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d2 character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott-Hubbard-type mechanism is favored, as the observed timescales and d2 nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.

Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO2) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M2,3 edge is used to track the insulator-to-metal phase transition in VO2 This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO2 absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d2 character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott-Hubbard-type mechanism is favored, as the observed timescales and d2 nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.

This proceedings contains papers on the following topics: Laser processing, pulsed laser deposition, Surface and thin film treatments, Laser ablation, Laser-matter interaction, Machining and microstructuring, Microsystems and microdevices.

The object of this research program is to carry out experimental and theoretical studies of the detailed microscopic mechanisms by which electronic energy is absorbed, transported and dissipated in ionic solids. The ultimate aim of this program is identify and characterize essential constituent elements of comprehensive models which will quantitatively describe radiation-induced electronic phenomena. The theoretical and experimental aspects of the project has been carried out concurrently and interactively in order to realize the greatest scientific benefit from the collaboration. Throughout this project we have employed (1) experimental techniques already developed to monitor bulk and surface properties before, during, and after electron and photon irradiation and to characterize time and energy-dependent desorption phenomena, and (2) ab initio quantum-theoretical approaches to develop and refine computational models for determining properties of excited electronic states of relevant localized species. This research program bears directly on a broad spectrum of questions germane to the long-term operation of platforms in space, including structural, optical and electronic degradation of materials in the ambient near-earth environment, survivability under and hardening against irradiation from directed-energy weapons, vulnerability in disturbed nuclear atmospheres, and discrimination and sensing techniques based on radiation (glow) signatures.

The mechanism leading to photon-stimulated desorption (PSD) of excited atoms from the surfaces of alkali halides has been a subject of controversy since it was first discussed a decade ago. We have recently carried out a systematic experimental study of PSD from alkali halides under both valence-band and core-level excitation. Our data demonstrate that the desorption yields for excited alkali atoms track the excitonic optical response of these crystals even at photon energies below the bulk band gap. Secondary electron measurements, taken simultaneously, implicate the formation of excess metal in the time and dose dependence of PSD yields. These results suggest a unified picture of PSD in which the ion motion initiating desorption is pictured as the decay of a localized, highly deformed vibrational mode of the crystal. Plausible mechanisms for creation of the excited atomic state include relaxation of excited F-centers and resonant neutralization into the excited state on metallic microclusters., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series. Volume 3. The Microphysics of Surfaces: Beam-Induced Processes. Held in Santa Fe, New Mexico on 11-13 Feb 1991,' AD-A254 135,p157-160.

The widespread application of lasers in semiconductor processing has produced a voluminous literature on the mechanisms of laser desorption and ablation. There are significant disagreements, however, about the extraordinarily complex physical mechanisms underlying laser desorption and ablation. Here we discuss recent experiments and a model of a particularly simple case: desorption of Ga+ and GaO by selective excitation of a surface electronic state on GaP (110). By choosing the electronic channel leading to desorption, and by monitoring the condition of the surface, one can determine accurately the threshold fluence above which the surface electronic structure is irreversibly altered by laser ablation. Below this threshold, desorption occurs at isolated defect sites and at perfect three-fold coordinated surface sites. This two-hole laser-induced desorption and ablation can be described by a bond-orbital model of electronic structure. Pure crystalline n-type (S-doped) GaP (110) surfaces l0xl5 mm2 were polished and etched according to a standard protocol, then sputter-cleaned and annealed to 500 C in ultrahigh vacuum (10-10 torr). The existence of a contamination-free lxl surface was verified by LEED and Auger spectroscopies. This surface of GaP, shown schematically in Fig. 1, has an unoccupied surface state about 2 eV above the valence-band edge. This surface state was excited by an excimer-pumped dye laser tuned to 600 nm., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series. Volume 3. The Microphysics of Surfaces: Beam-Induced Processes. Held in Santa Fe, New Mexico on 11-13 Feb 1991,' AD-A254 135,p39-42.

It is now eighteen years since John Madey published a paper pointing out that a high-brightness relativistic electron beam traversing a spatially periodic magnetic field could stimulate the emission of photons over a broad range of wavelengths, indeed, from the far infrared to the ultraviolet. In a way, the free-electron laser was the ultimate homage paid by the laser, viewed as an optical device, to its antecedents in radar and electron-beam science and technology dating back into the 1940's. In the intervening years, successful infrared and visible free-electron-laser (FEL) experiments, for example, at Stanford, Orsay, Santa Barbara, and Los Alamos, have shown significant promise for applications based on the unique optical characteristics of the FEL. A variety of accelerators can provide the high-brightness electron beams necessary for the FEL: room-temperature pulsed linear accelerators, superconducting accelerators, storage rings, and Van de Graaff generators have all been successfully used so far for this purpose. The existence of this variegated collection of pumps for the stimulated emission generated in the FEL implies a correspondingly broad range of temporal pulse shapes, interpulse spacings, pulse-repetition frequencies, output powers, and spectral ranges for users. With the increasing maturity of the free-electron laser comes a new phase of scientific opportunity for those who are primarily laser users rather than laser physicists. During the past two years, FEL users' facilities at Stanford University and the University of California at Santa Barbara began to provide significant quantities of time to photon users, particularly in surface and materials science and bio-medical studies.

A central goal has been to establish a multidisciplinary Center of Excellence concentrating on the atomic-scale dynamics of surface reactions in the space environment. The major research focus has been the investigation of the ways in which energy deposited by incident atoms, ions, electrons and short wavelength photons is absorbed and localized to produce bond-making and bond- breaking on surfaces and in the near-surface bulk. Knowledge of these microscopic mechanisms provides detailed clues which lead to an understanding of the macroscopic processes which manifest themselves as surface erosion, modification and damage. This research program bears directly on a broad spectrum of questions germane to the long-term operation of platforms in space, including long-term structural, optical and electronic degradation of materials in the ambient near-earth environment, survivability under and hardening against irradiation from directed-energy weapons, vulnerability in disturbed nuclear atmospheres, and discrimination and sensing techniques based on characteristic radiation (glow) signatures. Significant, and in some cases, startling progress has been made in carrying out the research goals of this effort.

We present recent measurements of excited-atom and ion emission from KCl surfaces illuminated by vacuum-ultraviolet synchrotron radiation (hv = 8-28 eV) and ultraviolet laser light (hv = 4 eV). At low intensities characteristic of the synchrotron experiments, excited atoms are desorbed by simple valence-band excitation process involving the metallization of the KCl surface. At the higher intensities typical of laser desorption and ablation, we observe a strong decrease in K emission as a function of the number of laser shots, but an essentially constant yield of Cl*. K+ and Cl- emission at high intensities show similar behavior. The energetics of these desorption phenomena can be treated in a bond-orbital model which shows that creation of a single valence hole is sufficient to excite an ion to an anti-bonding state. jg p3, Presented at Boulder, CO, Oct 90.

The Medical Free Electron Laser Program was awarded to develop, construct and operate a free-electron laser facility dedicated to biomedical and materials studies, with particular emphases on: fundamental studies of absorption and localization of electromagnetic energy on and near material surfaces, especially through electronic and other selective, non-statistical processes; non-thermal photon-materials interactions (e.g., electronic bond-breaking or vibrational energy transfer) in physical and biological materials as well as in long-wavelength biopolymer dynamics; development of FEL-based methods to study drug action and to characterize biomolecular properties and metabolic processes in biomembranes; clinical applications in otolaryngology, neurosurgery, ophthalmology and radiology stressing the use of the laser for selective laser-tissue, laser-cellular and laser-molecule interactions in both therapeutic and diagnostic modalities.