D. M. Riffe

Femtosecond thermionic emission from metals in the space-charge-limited regime

We study femtosecond-laser-pulse-induced electron emission from W(100), Al(110), and Ag(111) in the subdamage regime (1–44 mJ/cm2 fluence) by simultaneously measuring the incident-light reflectivity, total electron yield, and electron-energy distribution curves of the emitted electrons. The total-yield results are compared with a space-charge-limited extension of the Richardson–Dushman equation for short-time-scale thermionic emission and with particle-in-a-cell computer simulations of femtosecond-pulsed-induced thermionic emission. Quantitative agreement between the experimental results and two calculated temperature-dependent yields is obtained and shows that the yield varies linearly with temperature beginning at a threshold electron temperature of ~0.25 eV The particle-in-a-cell simulations also reproduce the experimental electron-energy distribution curves. Taken together, the experimental results, the theoretical calculations, and the results of the simulations indicate that thermionic emission from nonequilibrium electron heating provides the dominant source of the emitted electrons. Furthermore, the results demonstrate that a quantitative theory of space-charge-limited femtosecond-pulse-induced electron emission is possible.

Measurement of silicon surface recombination velocity using ultrafast pump–probe reflectivity in the near infrared

We demonstrate that ultrafast pump–probe reflectivity measurements from bulk Si samples using a Ti:sapphire femtosecond oscillator (λ=800 nm) can be used to measure the Si surface recombination velocity. The technique is sensitive to recombination velocities greater than ∼104 cm s−1.

Temperature dependence of silicon carrier effective masses with application to femtosecond reflectivity measurements

The conductivity effective masses of electrons and holes in Si are calculated for carrier temperatures from 1 to 3000 K. The temperature dependence of the electron mass is calculated by use of a phenomenological model of conduction-band nonparabolicity that has been fitted to experimental measurements of the dependence of the electron conductivity effective mass on carrier concentration. The hole mass is investigated by tight-binding calculations of the valence bands, which have been adjusted to match experimental values of the valence-band curvature parameters at the top of the valence band. The calculations are in excellent agreement with femtosecond-laser reflectivity measurements of the change in optical effective mass as hot carriers cool from 1550 to 300 K.

Submonolayer Oxidation of W(110): A High-Resolution Core-Level Photoemission Study

We have measured high-resolution 4f72 core-level photoemission spectra from W(110) in the presence of submonolayer O coverages. Coverages ≤0.5 monolayer (ML) were obtained via room-temperature adsorption of O2. Higher coverages (<1.0 ML) were facilitated by a substrate temperature of 1500 K or a predose of alkali metal (K or Cs). Core-level shifts associated with the chemically distinct types of surface W atoms that occur for three-fold-coordinated O adsorption have been identified. The core-level shifts can be characterized by an oxygen-induced change in the average surface-atom core-level binding energy of ∼ 1.0eV per ML of adsorbed O. We show that the intensities of the surface-atom photoemission lines can be used to determine the absolute oxygen coverage.

A compact rotating-mirror autocorrelator design for femtosecond and picosecond laser pulses

An interferometric rapid-scanning autocorrelator employing two antiparallel rotating mirrors in a variable arm is optimized for maximum optical path difference as a function of the separation of the two rotating mirrors. A very compact design (mirror separation≈mirror diameter) is possible without a reduction in the maximum pulse width that can be measured.

Hemispherical Emissivity of V, Nb, Ta, Mo, and W from 300 to 1000 K

The hemispherical emissivities of five transition elements, V, Nb, Ta, Mo, and W, have been measured from 300 to 1000 K, complementing earlier higher-temperature results. These low-temperature data, which are similar, are fitted to a Drude model in which the room-temperature parameters have been obtained from optical measurements and the temperature dependence of the dc resistivity is used as input to calculate the temperature dependence of the emissivity. A frequency-dependent free-carrier relaxation rate is found to have a similar magnitude for all these elements. For temperatures larger than 1200 K the calculated emissivity is always greater than the measured value, indicating that the high-temperature interband features of transition elements are much weaker than those determined from room-temperature measurements.

Linewidth of H Chemisorbed on W(100): An Infrared Study

Abstract The symmetruc stretch vibration of hydrigen on W(100) at saturation is measured with high resolution by infrared spectroscopy and found to be broad (≈ 100 cm−. This unusually large width is thought to reflect large H-H dynamical interactions.

An embedded-atom-method model for alkali-metal vibrations

We present an embedded-atom-method (EAM) model that accurately describes the vibrational dynamics in the alkali metals Li, Na, K, Rb and Cs. The bulk dispersion curves, frequency-moment Debye temperatures and temperature-dependent entropy Debye temperatures are all in excellent agreement with experimental results. The model is also well suited for studying surface vibrational dynamics in these materials, as illustrated by calculations for the Na(110) surface.

Infrared reflection-adsorption spectroscopy of W(100)-H at 100 K

Abstract Using infrared reflection-absorption spectroscopy in conjunction with the broad-band technique of Fourier-transform interferometry, changes in the absorption spectrum of W(100) have been investigated as a function of H coverage at 100 K from 900 to 3900 cm -1 . The changes measured upon initial adsorption indicate the removal of intrinsic-surface states 0.36 eV below the Fermi surface as well as the establishment of the unreconstructed-surface electronic absorption at ≈0.15 eV. The appearance of the 0.15 eV mode upon initial adsorption implies that the surface largely switches to an unreconstructed state at low coverages, consistent with H-atom immobility at low temperatures. For higher coverages [0.7 monolayer (ML) (2.0 ML¬saturation)] the measured reflectivity changes show that free-carrier surface scattering dominates the coverage induced variations. It is seen that infrared measurements of these surface-scattering variations can lead to determination of the effective plasma frequency ω P of the metal.

Core-hole lifetime and screening are different in the surface of W(110)

High resolution 4f photoemission spectra from clean W(110) show that the natural lifetime width and the singularity index, which characterizes the conduction electron screening, are both larger in the first atomic layer than in the bulk. The phonon broadening of the surface and bulk components are smaller than earlier theoretical estimates. The excess broadening at the surface is compatible with a simple Debye model. These findings are very different from the interpretation previously given surface-atom core-level line shapes and have implications extending to other systems.