ALBERT

Notes: Time-resolved electron diffraction harbors great promise for resolving the fastest chemical processes with atomic level detail. The main obstacles to achieving this real-time view of a chemical reaction are associated with delivering short electron pulses with sufficient electron density to the sample. In this article, the propagation dynamics of femtosecond electron packets in the drift region of a photoelectron gun are investigated with an N-body numerical simulation and mean-field model. It is found that space-charge effects can broaden the electron pulse to many times its original length and generate many eV of kinetic energy bandwidth in only a few nanoseconds. There is excellent agreement between the N-body simulation and the mean-field model for both space-charge induced temporal and kinetic energy distribution broadening. The numerical simulation also shows that the redistribution of electrons inside the packet results in changes to the pulse envelope and the development of a spatially linear axial velocity distribution. These results are important for (or have the potential to impact on) the interpretation of time-resolved electron diffraction experiments and can be used in the design of photoelectron guns and streak tubes with temporal resolution of several hundred femtoseconds. © 2002 American Institute of Physics.

Notes: The amplitude and frequency of modes driven in the edge region of tokamak high mode (H-mode) discharges [type I edge-localized modes (ELMs)] are shown to depend on the discharge shape. The measured pressure gradient threshold for instability and its scaling with discharge shape are compared with predictions from ideal magnetohydrodynamic theory for low toroidal mode number (n) instabilities driven by pressure gradient and current density and good agreement is found. Reductions in mode amplitude are observed in discharge shapes with either high squareness or low triangularity where the stability threshold in the edge pressure gradient is predicted to be reduced and the most unstable mode is expected to have higher values of n. The importance of access to the ballooning mode second stability regime is demonstrated through the changes in the ELM character that occur when second regime access is not available. An edge stability model is presented that predicts that there is a threshold value of n for second regime access and that the most unstable mode has n near this threshold. © 2000 American Institute of Physics.

Notes: The unique properties of a polymer photonic crystal are examined with respect to applications as a medium for high-density three-dimensional optical data storage media. The nanocomposite material was produced from core-shell latex particles, in which the latex cores contained dye-labeled polymer. Nonfluorescent latex shells were attached to the core particles. Upon annealing, the close-packed core-shell particles formed a nanostructured material with the fluorescent particles periodically embedded into the optically inert matrix in a hexagonal close-packed structure. A two-photon laser scanning microscope was used to write bits of information into the material by photobleaching the optically sensitive particles and, under much lower fluence, read out the resulting image. Relative to conventional homogeneous storage media, the nanostructured periodic material is shown to increase the effective optical storage density by at least a factor of 2 by spatially localizing the optically active region and imposing an optically inactive barrier to cross-talk between bits. This polymer photonic crystal has the potential to dramatically improve performance further through the improved capabilities to optimize the photochemical processes and more fully exploiting the periodic nature of the information domains in the image processing. © 2001 American Institute of Physics.

Notes: By using an atomic-force-microscope-based technique, we image the vibration of high-frequency, bulk-mode, thin-film resonators. Our experimental technique is capable of monitoring the vibration of these devices over a broad frequency range, from 1 MHz to beyond 10 GHz, allowing us to obtain quantitative measurements of the piezoelectric properties of thin-film materials in that frequency range. This technique allows us to map the complex vibration modes of a new generation of high-frequency bulk piezoelectric resonators, revealing the presence of vibration patterns of very different characteristic lengths. © 2000 American Institute of Physics.

Notes: High-resolution infrared laser spectroscopy is used to study hydrogen fluoride solvated in helium nanodroplets. The results clearly show that the vibrationally excited HF (v=1) does not relax on the time scale of the experiments (0.5 ms) and that the large linewidth of the R(0) transition (0.43 cm−1) results from rotational relaxation. A large dc electric field is applied to induce a Q(0) transition, providing an accurate value for the rotational constant of HF in solution (19.47 cm−1), only 2% smaller than in the gas phase. © 2000 American Institute of Physics.

Notes: Photofragment angular and state distributions have been measured following the vibrational predissociation of the OC–HF complex. An F-center laser is used to pump the fundamental H–F stretching vibration of the complex and a second F-center laser is used to probe the rotational states of the HF fragment as a function of recoil angle. The complex dissociates via two different sets of channels, one that produces vCO=1, JHF=6,5,4 (intermolecular V–V transfer) and the other vCO=0, JHF=11 (V–R transfer). Analysis of the data gives correlated final state distributions, as well as an accurate value for the dissociation energy (D0) of the complex, namely 732±2 cm−1. © 2000 American Institute of Physics.

Notes: High-resolution infrared spectra are reported for the "free" and "hydrogen bonded" H–F stretches of the hydrogen fluoride dimer solvated in helium nanodroplets. These rotationally resolved spectra provided detailed information concerning the effect of the helium solvent on the vibrational frequencies, rotational constants and tunneling dynamics of the dimer. The end-over-end rotation of the dimer is slowed by a factor of 2.2 by the helium, while the faster rotation about the a axis remains essentially unaffected. The interchange tunneling is reduced significantly (∼40%) in both the ground and vibrationally excited states. The effective tunneling barrier is higher than in the gas phase, making it easier to quench the tunneling motion with a large dc (direct current) electric field. Rapid rotational relaxation is observed from the Ka=1 state, resulting in significant broadening of the corresponding subband. Combination bands are observed for the intermolecular F–F stretch and trans-bend vibrations, providing further insights into the nature of the solvent effects. © 2000 American Institute of Physics.

Notes: Photofragment angular and final rotational state distributions have been measured, corresponding to the vibrational predissociation of HF polymers [(HF)N N=5–7]. The present experiments confirm the theoretical prediction that, for clusters in this size range, the only open dissociation channel corresponds to the "evaporation" of a single HF monomer. The pump–probe experiments reported here indicate that the resulting HF monomer is produced with only modest rotational excitation. Rotational temperatures of 180 and 140 K have been determined for the evaporated HF monomer following dissociation of the HF pentamer and hexamer, respectively. Energy conservation in these photodissociation studies provides upper limits for the dissociation energies (D0), namely, 2941 cm−1 and 2854 cm−1 for the pentamer and hexamer, respectively. © 2000 American Institute of Physics.

Notes: High-resolution infrared laser spectroscopy has been used to study HCN–H2 and HCN–D2 complexes in the gas phase. The experimental results are compared with ab initio calculations that are also reported here. The latter calculations reveal two prominent minima on the potential surface, one corresponding to a "T-shaped" complex with the H2 at the hydrogen end of the HCN and the other a "linear" complex with the H2 H-bonded to the nitrogen. The latter minimum is the global minimum on the surface, in agreement with the fact that this structure is observed experimentally for both o-H2 and p-D2. © 2001 American Institute of Physics.

Notes: This work exploits the passive phase stabilization of diffractive optics to implement heterodyne detection of the complete χ(5) tensor of liquid CS2 as an example of a simple liquid. This approach permits the use of two different colors for the excitation, probe, and detection beam protocols and enables full optimization of the signal with respect to discrimination against lower order cascaded third-order responses. This work extends the previous study of polarization selectivity, in combination with heterodyne detection, to all independent polarization components to provide further insight into the origins of the fifth-order response and its connection to the multitime correlation of the liquid dynamics. The characteristic feature that clearly distinguishes the direct fifth-order response from lower order cascades is the pronounced ridge along the τ4 axis (probe pulse delay) with very rapid decay along the τ2 axis (excitation pulse delay). This observation is in contrast to recent related work using one-color homodyne detection. With the determination of the direct fifth-order and cascaded third-order signal amplitudes made possible by heterodyne detection, this difference can be attributed to cross terms between the direct fifth-order and cascaded third-order terms inherent to homodyne detection under phase matching conditions used to discriminate against cascades. In support of theoretical treatments, the previously predicted enhancement of rephasing pathways for certain polarization components has been observed. However, even for these tensor elements the remarkable feature is the very rapid decay in the nuclear coherence along τ2. The experiment is predicated on the ability of a 2-quantum transition involving the Raman overtone to rephase the nuclear coherence. These findings indicate that the nuclear motions, in the frequency range accessed, are strongly damped and draw into question the validity of the overtone as a viable pathway for rephasing. With the isolation of the direct fifth-order Raman response, new information regarding relaxation and dephasing pathways in liquids can be determined for the highest frequency modes. The results are in very good agreement with a recent finite field molecular dynamics simulation of liquid CS2 with respect to the polarization dependence of signal magnitudes, relative cascade signal amplitudes, and qualitative agreement with respect to the predicted temporal profiles. © 2002 American Institute of Physics.