18. 4-fluorophenylthiolate-capped nanoparticles of cubic CdS, CdSe, ZnS, and PbS were prepared by a modified literature synthesis (17) and dispersed in acetone to a concentration of È0.05 to 0.5 M. Samples of 2 to 4 mL of the sols in polyethylene vials were treated with 0.1 to 0.2 mL of 3% H 2 O 2 (CdS, ZnS, PbS) or 3% tetranitromethane (CdSe) to induce gelation. Gels were aged for several days to a week, exchanged multiple times with acetone, and dried in a SPI-Dry (Structure Probe, Inc., West Chester, PA) critical-point dryer using CO 2. to 124 m 2 /g (mean 115 m 2 /g); adsorption average pore diameter (BJH), 23 to 28 nm; adsorption cumulative pore volume (BJH), 0.70 to 0.73 cm 3 /g. Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 Â 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude. In recent years, the invention of the femto-second frequency comb laser (1-3) has brought about a revolution in metrology. A frequency comb acts as a bridge between the radio frequency (RF) domain (typically tens of MHz) and the optical frequency domain (typically hundreds of THz). Thus, in precision spectroscopy, the optical cycles of a continuous wave (CW) ultrastable laser can be phase-locked and counted directly with respect to an absolute frequency standard such as an atomic clock (4, 5). The resultant frequency measurements approach a precision of 1 part in 10 15 in certain cases, potentially enabling the detection of possible drift in the fundamental constants (6, 7), among other quantum mechanical applications. Here, we perform precision metrology without the use of a CW laser. Instead, an atomic transition is excited directly with amplified and frequency-converted pulses from a femtosecond frequency comb laser. As a result of quantum interference effects in the atomic excitation process, we can achieve an accuracy that is about six orders of magnitude higher than the optical bandwidth of the individual laser pulses. The method used is related to Ramsey_s principle of separated oscillatory fields (8), which probes the phase evolution of an atom in spatially separated interaction zones. This technique is widely used in the RF domain for atomic fountain clocks (9). By extension, in the optical domain, excitation can be performed by pulses separated in time (rather than in space) to maintain phas...

Using a photoemission spectroscometer that operates close to ambient conditions of pressure and temperature we have determined the Pd-O phase diagram and the kinetic parameters of phase transformations. We found that on the (111) surface oxidation proceeds by formation of stable and metastable structures. As the chemical potential of O2 increases chemisorbed oxygen forms followed by a thin surface oxide. Bulk oxidation is a two-step process that starts with the metastable growth of the surface oxide into the bulk, followed by a first-order transformation to PdO.