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Intermediate pressure matrix-assisted laser desorption/ ionization (MALDI) 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 biomolecules.

Studies are performed to determine an upper limit on the optical damage threshold of a soft-x-ray molybdenum-silicon multilayer reflective coating by the use of a 308-nm, 15-ns pulse from a Xe-Cl excimer laser in order to simulate the potential damage induced by the x-ray flux from a pulsed laser-produced plasma. Experimental results yield a value of 0.26 J/cm squared to produce visible signs of damage as determined by optical microscopy. Experiments are conducted first on silicon, as a reference point of a bulk material, and then applied to molybdenum-silicon in an effort to facilitate a theoretical comparison between a simple and a more complicated material. Theoretical predictions are in reasonable agreement with experimental results, but suggest that a lower value of 0.085 J/cm squared might cause significant thermal-induced damage.

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]

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]

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 study uses femtosecond pulse-pair excitation to explore ultrafast electron transfer dynamics of low-coordinated MgO surface sites. Results indicate that ultraviolet excitation of high surface area MgO films using 4.7 eV femtosecond pulses results in neutral Mg-atom desorption with hyperthermal kinetic energies in the range 0.1-0.4 eV.

The alpha-[Cr.sub.2][O.sub.3]/[alpha-[Fe.sub.2][O.sub.3] core-shell polycrystalline nanostructures are synthesized by using alpha-[Cr.sub.2][O.sub.3] nanoparticles as seed crystals during aqueous nucleation. The femtosecond transient absorption spectroscopy results have shown the that carrier lifetimes decay within a few picoseconds regardless of sample, which is due to fast recombination or trapping of carriers to defects and iron d-states.

The mechanisms of laser control of photodesorption from alkali halides using frequency selected laser pulses are discussed. The results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.

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]