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Band-to-band photoexcitation of potassium iodide single crystals with UV photons produces thermal and hyperthermal I-atom emission in both the ground I(2P3/2) and spin-orbit excited I*(2P1/2) states. Thermal halogen atom emission is preceded by H-center diffusion from bulk to surface and excited atom emission indicates that the excited hole spin state relaxation is incomplete before H-center diffusion to the surface. The hyperthermal I-atom kinetic energy distribution is inverted in the sense that the electronically excited I*(2P1/2) atoms are more energetic than the ground state I(2P3/2) atoms. The mechanism for hyperthermal emission of halogen atoms and their kinetic energy distributions are discussed in terms of recent calculations and a simple curve crossing model for the dynamical desorption process. [ABSTRACT FROM AUTHOR]

We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range from 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following electronic energy transfer from higher coordinated surface sites. [ABSTRACT FROM AUTHOR]

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin–orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl([sup 2]P[sub 1/2]), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibrational relaxation of SiF4 and C6F5H in an Ar bath was measured as a function of initial energy using the method of time-resolved optoacoustics. SiF4 was found to relax nonexponentially, with a rate constant which varied approximately as the vibrational energy raised to the 3/2 power. This behavior is similar to that observed earlier for SF6+Ar below the bottleneck. In contrast, C6F5H was observed to decay exponentially, as expected for a large molecule in the quasicontinuum. These two types of behavior are discussed in the context of previously proposed propensity rules for the vibrational relaxation of highly excited molecules. [ABSTRACT FROM AUTHOR]

The acoustic waves generated by a relaxing gas have a shape which depends on the ratio of the relaxation rate to the acoustic propagation rate. It is shown how the relaxation time constant can be extracted from this waveform for a variety of relaxation rate laws, using a Green’s function to solve the linearized wave equation. The accuracy of the method is confirmed by measuring the relaxation rate constants for OCS diluted in Ar and He. These, plus a previous measurement of the relaxation rates of highly excited SF6 and C6F5H in Ar, are in good agreement with the literature. [ABSTRACT FROM AUTHOR]

The method of time-resolved optoacoustics was used to measure the time constant τ for the vibrational relaxation of SF6 by Ar as a function of the average initial excitation 〈〈E〉〉0. It was found that pτ=210±30 μs Torr for <>0 between 4000 and 19 000 cm-1, implying nearly pure exponential decay. The average amount of energy change per collision varies linearly with <>, having a value of -8 cm-1/collision at 19 000 cm-1. Below 4000 cm-1 pτ increases, approaching the low energy value obtained from IR fluorescence measurements. [ABSTRACT FROM AUTHOR]

When an ionic metal oxide crystal is cleaved, inhomogeneous electrical charging of the surface can be a result. Such an effect has been well documented in magnesium oxide (100). For example, recent rigorous AFM studies indicate that nanoscale charged clusters of MgO are created during cleavage, with high concentrations often located at terrace step edges (Barth and Henry in J. Phys. Chem. C 113:247, ). In addition, ablation processes of freshly cleaved magnesium oxide crystals may be effected by remnant surface charging and microstructures (Stoneham et al. in Appl. Phys. A 69:S81, ). We report here that such surface charging strongly impacts neutral atom desorption, even under conditions of extremely mild excitation of surface terrace features. In our experiments, single-crystal MgO (100) is cleaved in air and placed in an ultra-high vacuum chamber (UHV). We irradiate the crystal at 6.4 eV, photon energy resonant with five-coordinated (5-C) terrace sites and probe desorbing neutral oxygen atoms. It is found that a significant fraction of desorbed neutral oxygen atoms from the charged surface possess kinetic energies in excess of 0.7 eV. This is in contrast to uncharged samples (discharged in vacuo over 24 hours) that display a near-thermal oxygen atom distribution. [ABSTRACT FROM AUTHOR]

Abstract: Indium selenide thin films are important due to their applications in non-volatile memory and solar cells. In this work, we present an initial study of a new application of deposition-site selective laser back ablation (LBA) for making thin films of In2Se3. In vacuo annealing and subsequent characterization of the films by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicate that control of substrate temperature during deposition and post-deposition annealing temperature is critical in determining the phase and composition of the films. The initial laser fluence and target film thickness determine the amount of material deposited onto the substrate. [Copyright &y& Elsevier]

Abstract: Neutral magnesium atom emission from nanostructured MgO thin films is induced using two-color nanosecond laser excitation. We find that combined vis/UV excitation, for single-color pulse energies below the desorption threshold, induces neutral Mg-atom emission with hyperthermal kinetic energies in the range of 0.1–0.2eV. The observed metal atom emission is consistent with a mechanism involving rapid electron transfer to three-coordinated Mg surface sites. The two-color Mg-atom signal is significant only for parallel laser polarizations and temporally overlapped laser pulses indicating that intermediate excited states are short-lived compared to the 5ns laser pulse duration. [Copyright &y& Elsevier]

Abstract: We excite low-coordinated surface sites of nanostructured MgO samples using 4.7eV UV laser pulses and observe dominant hyperthermal O-atom emission. Excitation of the same samples with 7.9eV photons results in thermal O-atom desorption. These results are analyzed in the context of laser desorption models developed previously for alkali halide crystals and MgO. We detail two multi-step mechanisms for hyperthermal O-atom desorption, under surface selective excitation, based on exciton and hole trapping at three-coordinated (corner) O-atom sites, and evaluate the validity of each based on available experimental data and calculated results. The proposed models are significantly different from and more complex than the surface exciton desorption model established for alkali halides. Nonetheless, the principles of site-specific photo-excitation and exciton and hole localization induced atomic desorption are clearly extendable to a prototypical metal oxide. [Copyright &y& Elsevier]

Abstract: The surface electronic properties of metal oxides critically depend on low-coordinated sites, such as kinks, corners and steps. In order to characterize such surface sites, we prepare defect enriched surfaces by growing thin MgO films and measure X-ray photoelectron spectra of films and vacuum-cleaved MgO single crystals. In the case of thin films, we observe O1s spectra with a significant shoulder feature at 2.3eV higher binding energy (HBE) than the corresponding peak at 530.0eV representing regular bulk lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. Under excitation with 266-nm-laser pulses, known to be resonant with low-coordinated surface anions, we observe preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. The results from XPS studies are in line with 266nm laser stimulated desorption experiments of Mg- and O-atoms. Both results indicate new surface specific excitation regimes, which might eventually allow site-specific manipulation of surface morphology. [Copyright &y& Elsevier]

Intermediate pressure matrix-assisted laser desorption/ ionization (MAIDI) source was constructed and interfaced with a 6-T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for surface-induced dissociation (SID) studies. First MALDI- SID results in FT-ICR are presented, demonstrating unique advantages of SID over conventional FT-ICR MS ion activation techniques for structural characterization of singly protonated peptide ions. Specifically, we demonstrate that SID on a diamond surface results in a significantly better sequence coverage for singly protonated peptides than SORI-CID. A combination of two effects contributes to the improved sequence coverage: shattering of peptide ions on surfaces opens up a variety of dissociation channels at collision energies above 40 eV, and second, wide internal energy distribution deposited by collision with a stiff diamond surface provides an efficient mixing between the primary reaction channels that are dominant at low internal energies and extensive fragmentation at high internal excitation that results from shattering. Activation of MALDI-generated ions by collisions with surfaces in FT-ICR MS is a new powerful method for characterization and identification of bio-molecules. [ABSTRACT FROM AUTHOR]

Pt/TiO2 nanocomposite films were prepared using 2-beam pulsed laser deposition (2B-PLD) technique, where sequential irradiation of ArF excimer laser for ionization after Nd:YAG laser for ablation of the target was utilized. The effect of the time delay of the ArF laser after the ablation laser irradiation on the structures of Pt/TiO2 nanocomposite films was studied. It was inferred from X-ray photoelectron spectroscopy (XPS) of the as-deposited films that the coordination structures around Ti atoms in the films were affected by the time delays. Single phase TiO2 in anatase form was obtained in the annealed Pt/TiO2 nanocomposite films prepared at 200–250 ns of the time delays, which suggests that the effective photoionization of the ablated species could take place by the ArF laser irradiation in this time range and affect the structure of the deposits. [Copyright &y& Elsevier]

We recently demonstrated that sub-band gap photoexcitation of KBr near 6.4 eV leads to desorption of hyper-thermal neutral bromine atoms without a significant thermal velocity component. In contrast, a considerable thermal velocity component is observed at higher excitation energies and is attributed to formation and subsequent diffusion of transient centers from within the bulk crystal to the surface. Excitation with a nanosecond laser at 3.5 eV also produces neutral bromine emission with a seemingly thermal velocity profile, but a near three-photon power dependence. Furthermore, the relative Br&ast; yield is greatly enhanced. We propose an explanation for these results based on formation and secondary excitation of transient defect centers in KBr by individual nanosecond pulses. Photodecomposition of newly formed transient centers leads to emission of Br and Br&ast; in a “near thermal” velocity distribution. These results are likely general for alkali halides and are consistent with a recently described theoretical model. [Copyright &y& Elsevier]

Lenzi et al’s comments on the number of photons absorbed by sulfur hexafluoride, and the subsequent transfer of energy to argon atoms are answered. It is suggested that the optoacoustic and the IR absorption measurements should be complimentary to each other.(AIP) [ABSTRACT FROM AUTHOR]

We use multi-photon photoemission electron microscopy (PEEM) to image the enhanced electric fields of silver nanoparticles supported on a silver thin film substrate. Electromagnetic field enhancement is measured by comparing the photoelectron yield of the nanoparticles with respect to the photoelectron yield of the surrounding silver thin film. We investigate the dependence of the photoelectron yield of the nanoparticle as a function of size and shape. Multi-photon PEEM results are presented for three average nanoparticle diameters: 34, 75, and 122 nm. The enhancement in photoelectron yield of single nanoparticles illuminated with femtosecond laser pulses (400 nm, ∼3.1 eV) is found to be a factor of 102 to 103 times greater than that produced by the flat silver thin film. High-resolution, multi-photon PEEM images of single silver nanoparticles reveal that the greatest enhancement in photoelectron yield is localized at distinct regions near the surface of the nanoparticle whose magnitude and spatial extent is dependent on the incident electric field polarization. In conjunction with correlated scanning electron microscopy (SEM), nanoparticles that deviate from nominally spherical shapes are found to exhibit irregular spatial distributions in the multi-photon PEEM images that are correlated with the unique shape and topology of the nanoparticle. [ABSTRACT FROM AUTHOR]

We present results of a combined two-photon photoemission and scanning electron microscopy investigation to determine the electromagnetic enhancement factors of silver-coated spherical nanoparticles deposited on an atomically flat mica substrate. Femtosecond laser excitation of the nanoparticles produces intense photoemission, attributed to near-resonant excitation of localized surface plasmons. Enhancement factors are determined by comparing the respective two-photon photoemission yields measured for single nanoparticles and the surrounding flat surface. For p-polarized, 400 nm (∼3.1 eV) femtosecond radiation, a distribution of enhancement factors is found with a large percentage (67%) of the nanoparticles falling within a median range. A correlated scanning electron microscopy analysis demonstrated that the nanoparticles typifying the median of the distribution are characterized by spherical shapes and relatively smooth silver film morphologies. In contrast, the largest enhancement factors were produced by a small percentage (7%) of particles that displayed silver coating defects that altered the overall particle structure. Comparisons are made between the experimentally measured enhancement factors and previously reported calculations of the localized near-field enhancement for isolated silver nanoparticles. [ABSTRACT FROM AUTHOR]

We have developed a new method for determining surface exciton band energies in alkali halides based on velocity-resolved atomic desorption (VRAD). Using this new method, we predict the surface exciton energies for KI, KBr, KCl, and NaCl within ±0.15 eV. Our data, the measured and calculated shifts of the surface exciton energies with respect to the bulk excitons, combined with the available EELS data for alkali fluorides, demonstrate a universal linear correlation with the inverse inter-atomic distance in these materials. The results suggest that surface excitons exist in all alkali halides and their excitation energies can be predicted from the known bulk exciton energies and the obtained correlation plot. [Copyright &y& Elsevier]

Irradiation of cleaved KI single crystals with photons near the bulk absorption threshold between 5.1 and 6.0 eV produces hyper-thermal I(2P3/2) emission with a minor thermal component, and with little spin orbit excited I(2P1/2) emission. The I-atom kinetic energy distribution may be tuned by choice of photon energy indicating that control of photon energy and pulse power can produce I-atoms in selectable quantities and velocity distributions. [Copyright &y& Elsevier]

We report photoelectron emission microscope observations of the thermal martensitic transformation in a CuZnAl shape memory alloy. The phase transformation appears at 48 °C during heating and at 42 °C upon cooling. The transformation is marked by a sharp change in photoelectron intensity, as well as a significant displacement and reorientation of surface features. The difference in the photoelectron intensity before and after the transformation is attributed to a change in work function of about 0.2 eV. Photoemission electron microscopy provides real-time information on microstructural changes and phase-dependent electronic properties. [ABSTRACT FROM AUTHOR]

We use photoemission electron microscopy (PEEM) to image 3 μm diameter polystyrene spheres supported on a metal thin film illuminated by 400 nm (∼3.1 eV) and 800 nm (∼1.5 eV) femtosecond (fs) laser pulses. Intense photoemission is generated by microspheres even though polystyrene is an insulator and its ionization threshold is well above the photon energies employed. We observe intense photoemission from the far side (the side opposite the incident light) of the illuminated microsphere that is attributed to light focusing within the microsphere. For the case of p-polarized, 800 nm fs laser pulses, we observe photoemission exclusively from the far side of the microsphere and additionally resolve sub-50 nm hot spots in the supporting Pt/Pd thin film that are located only within the focal region of the microsphere. We compare the PEEM images with finite difference time domain (FDTD) electrodynamic simulations to model our experimental results. The FDTD simulations predict light focusing in the microsphere and subsequent interaction with the supporting metal surface that is consistent with the experimental observations. [ABSTRACT FROM AUTHOR]

We describe a solid-state Br atom source for surface etching, kinetics, and reaction dynamics studies. Pulsed laser irradiation of crystalline KBr, near the bulk absorption threshold at 6 eV, produces hyperthermal Br atoms in dense plumes. The Br atom density and velocities may be controlled by choice of laser pulse power and photon energy. Single and multiple pulse excitation of KBr produces Br and Br[sup *] in controllable quantities and velocities, thus providing an attractive ultrahigh vacuum compatible solid-state radical atom source. The solid-state atom source is in principle extendable to other halogens using other alkali halides and perhaps other materials. [ABSTRACT FROM AUTHOR]

We desorb ground state Br and spin-orbit excited Br atoms from KBr single crystals using single pulses and sequential pulse pairs of tunable nanosecond laser radiation. Irradiation of cleaved KBr crystals near the bulk absorption threshold produces hyperthermal Br emission without a significant thermal component, and with little spin-orbit excited Br emission. The Br kinetic energy distribution may be controlled either by choice of photon energy or by excitation of transient defect centers created within the crystal. In this latter scheme, a first laser pulse generates transient centers within the bulk crystal and in the vicinity of the surface, and a second delayed laser pulse then excites the transient centers leading to atomic desorption. The Br to Br yield ratio is significantly enhanced using two-pulse excitation as compared to resonant single-pulse desorption. Single and multiple pulse excitation of KBr produces Br and Br in controllable quantities, velocities, and spin state distributions. [ABSTRACT FROM AUTHOR]

We demonstrate evidence of selective laser-induced desorption of ground state Br([sup 2]P[sub 3/2]) and spin–orbit excited state Br([sup 2]P[sub 1/2]) atoms from KBr single crystals following 6.4 eV irradiation. Laser excitation tuned selectively to a surface resonance below the first bulk absorption band excites surface states preferentially leading to surface specific reactions while inducing relatively insignificant bulk reaction. The experimental results are supported by embedded cluster ab initio calculations that indicate a reduced surface exciton energy compared to that of the bulk exciton with a slight further reduction for steps and kink sites. Low fluence irradiation of cleaved KBr crystals, near the calculated surface exciton energy of 6.2 eV, produces hyperthermal Br([sup 2]P[sub 3/2]) emission without a significant thermal or Br([sup 2]P[sub 1/2]) component. The hyperthermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition while thermal emission is attributed to bulk photoreaction. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A correction to the article "In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy" that was published in the April 28, 2006 issue is presented.

Phase transformation of thin film (∼30 nm)In2Se3/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous In x Se y was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In2Se3 single crystal with its {0001} planes parallel to the Si(111) substrate and $(11\bar{2}0)$ parallel to Si $(1\bar{1}0)$ , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm2) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous In x Se y film. [ABSTRACT FROM AUTHOR]

Photoemission electron microscopy is used to study copper diffusion through a ruthenium thin film. The photoemission electron microscopy images display a large contrast between Cu and Ru due to the differences in work function, making photoemission electron microscopy an ideal methodology to study thin film diffusion in real time. Between 175 and 290 °C, Cu mainly diffuses through defect sites in the thin Ru film. Uniform diffusion of Cu through the Ru film begins at approximately 300 °C. The results are confirmed by x-ray photoemission spectroscopy depth profiling and scanning electron microscopy–energy dispersive x-ray spectroscopy analysis. [ABSTRACT FROM AUTHOR]

β-FeOOH nanorods of dimensions of 15 nm diameter and 200 nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation display three time scales. The first is a subpicosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2 ps. The trapped electrons and holes persist for significantly longer times (at least tens of picoseconds), similar to previous results from α-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states. [ABSTRACT FROM AUTHOR]