Search

Pump-probe photoelectron spectroscopy is used to study excited states in pristine and photopolymerized C 60 . For the first time in a molecular solid, vibrationally broadened singlet (S 1 ) and triplet (T 1 ) exciton photoemission spectra are temporally isolated and unambiguously assigned. Complex transient behaviors of S 1 and T 1 populations, including non-Markovian decays and a strongly enhanced production of T 1 states at high excitation, provide evidence for interactions among excitons and free carriers. A new rate-equation model reproduces a wide range of both laser- and synchrotron-based data from polymerized films.

Miniature acousto-fluidic devices are described that operate as pumps without valves in channel widths of millimeters and below. These devices can also be configured to produce mixing in low-Reynolds-number flows. The prototypes are based on radio-frequency, ultrasonic piezoelectric transducers that exert a directed body force on the fluid via acoustic attenuation. The process is a type of acoustic streaming termed quartz wind. In microfluidics applications, this mechanism has the advantages of insensitivity to the chemical state of the fluid or walls and greatly reduced crosstalk in a multichannel system. The observed pump flow velocities are on the order of 1 mm/s in 1.6×1.6 mm 2 channels and with a calculated maximum backpressure that can be pumped against of 0.13 Pa. Due to the low backpressure, quartz wind devices are not competitive pumps for open-loop and high-impedance microfluidics systems but could find application in pumping in low-impedance planar and closed-loop systems and for mixing in reservoirs and channels.

For over a decade, laser pump-probe photoelectron spectroscopy (PES) has proven unique in its ability to determine the energy, momentum, and population dynamics of surface and near-surface excited states in solids.1,2 Practitioners most often generate the requisite probe UV pulse with laser upconversion techniques. The broader and continuous tunability of synchrotron radiation (SR) make it also an attractive source, albeit with reduced ultimate time resolution, and, in fact, SR is employed for time resolved spectroscopies spanning the far infrared to the hard x-ray regimes.3 We have developed a laser-pump, SR-probe PES system with nanosecond resolution at the National Synchrotron Light Source, and here we summarize results from GaAs, C60 films, and Si. This work anticipates specific areas where new photon probes, particularly femtosecond high harmonic generation4, UV-FEL’s, and 3rd generation SR sources, will find important applications.

Experimental correlations are established among steady state optical and ESR spectra and transient luminescence and optical absorption. These correlations support the existence of self-trapped holes in the form of substitutional Ag2+ in AgCl and imply that intrinsic electron-hole recombination is the source of a strong emission band around 2.4 eV.

The decay of recombination luminescence in undoped KI and RbI at low temperatures has been measured following pulsed excitation by two-photon absorption and other methods. Several distinct exponential components are observed whose decay times vary with temperature. The most notable case is the principal component of the intrinsic luminescence in KI, whose decay time increases exponentially from 2 μsec at 12 K to 280 μsec at 1.38 K. The data are accounted for in terms of changing populations within a zero-field-split triplet state of the self-trapped excition. Evidence concerning the origin of a second luminescent triplet state in KI, RbI, and CsI is discussed; this state is similar to those of the self-trapped excition, but may involve an extrinsic perturbation.

Several emission bands in the near ultraviolet are observed for KBr:OH and KBr:OD at low temperatures and are identified with OH and OD by excitation spectra, isotope shift and comparison with vibrational frequencies of the molecules. A composite configuration coordinate diagram is proposed to account for the experimental results.

A strong transient optical absorption band on the exciton shoulder has been observed in pure alkali iodine crystals following pulsed electron irradiation. The ultraviolet transition is ascribed to creation of a second exciton in the vicinity of a self-trapped exciton. The perturbation caused by the proximity of the self-trapped exciton lowers the energy needed to form the second exciton by 0.14 eV for KI and 0.12 eV for RbI.