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Technical Highlights

• Coherent Phenomena in Semiconductors. Many-body phenomena in direct gap semiconductors influence the performance of optoelectronic devices such as laser diodes, where the gain spectrum cannot be calculated without the inclusion of many-body effects. Because the low-temperature coherent response of direct gap semiconductors is also very sensitive to many-body effects, it provides an excellent probe for improving our understanding of these phenomena. Our recent work has revealed new many-body effects that couple excitonic states with free electron-hole pairs. These effects come into play when broadband coherence is studied.

• Ultrahigh Resolution Spectroscopy using Femtosecond Laser. We have explored the theoretical aspects of novel high-resolution spectroscopy that employs a femtosecond laser. A phase-coherent wide-bandwidth optical comb is shown to induce the desired multi-path quantum interference effect for the resonantly enhanced two-photon transition rate. The analysis is carried out in both the frequency and the time domains, providing a fundamental connection between the two physical pictures. Our calculation has provided a solid link between the time-domain carrier-envelope phase and the frequency-domain carrier frequency offset. We showed the consequence of these results in terms of absolute control of both degrees of freedom for the fs comb, namely the comb spacing and the carrier offset frequency. The multi-pulse interference in the time domain gives an interesting variation and generalization of the two-pulse-based temporal coherent control of the excited-state wavepacket. (J. Ye)

• Epitaxial Growth of Quantum Dot Structures. The epitaxial growth of self-assembled quantum dots is an important technique to prepare novel structures for optical and semiconductor devices. The spontaneous formation of germanium dots on silicon under certain growth conditions occurs because of the large difference (4 %) in lattice spacing between the two elements. In new studies the kinetics of the growth of germanium dots on silicon is explored under varying flux and temperature conditions, and with arsenic surfactant overlayers, by the post-growth technique of atomic force microscopy. Three types of islands are observed at different growth stages, hut clusters, domes, and large superdomes. The transition from the small hut clusters to the larger dome islands has been studied in detail. Bimodal distributions have been characterized and the energetic barrier for this transition has been obtained. The transition involves an upward diffusion flux of germanium material on the island facets, perhaps requiring an energetic transition over steps. The larger superdomes, once formed, grow rapidly, most likely because these structures already have dislocations that reduce the strain in their lattice so that atoms can readily join to the growing island. With the introduction of arsenic, both the free energy of the surface adatoms is lowered and the diffusion of germanium atoms on the wetting layer of germanium on silicon is decreased, resulting in the formation of much smaller islands. New work involves the use of patterned substrates to produce a localization of the quantum dots for specific placement of one or more dots during the growth. (S.R. Leone)

• Development of a Novel Cavity Ringdown Heterodyne Spectroscopy Capable of 1 x 10^-10 Sensitivity with a Microwatt Level of Light Power. Traditional cavity ringdown spectroscopy (CRDS) has two shortcomings. First, because CRDS, basically a d.c. method, is limited by residual fluctuations and drifts, the resultant sensitivity is orders of magnitude lower than that limited by the fundamental noise. Second, because CRDS needs an enormous dynamic range to record the whole decay curve, it usually requires a large optical power. In our new approach, both of these issues are addressed and shot-noise-limited sensitivity in CRDS is demonstrated for the first time. The scheme utilizes asynchronous switching of two cavity modes and heterodyne detection between the two dynamic waveforms transmitted by the cavity with one of the modes tuned to a molecular resonance. This technique yields a quick comparison of on-resonance and off-resonance information and thus avoids technical noise associated with both light and the cavity. The method also relieves us from the necessity of recording a ringdown waveform that spans a few decades of dynamical range. This technique provides an attractive complement to another ultrasensitive method, developed by the JILA/NIST team in the high-resolution (sub-Doppler) regime. (J. Ye)

• High-Accuracy International Optical Frequency Inter-comparisons. Recent developments involving the merging of ultrafast laser technology and high-precision optical frequency metrology have resulted in tremendous breakthroughs in both fields. In ultrafast physics, control of both the absolute phase of the optical carrier and the pulse envelope is now possible. In high-precision measurement science, optical frequency metrology can now enjoy the status of a general laboratory tool, thanks to the great simplification in absolute frequency determination and generation due to a femtosecond frequency comb. However, over the course of measuring several pre-existing optical standards, we found frequency offsets of a few kHz between the values measured using the comb technique and those measured previously with traditional frequency synthesis chains. To check the relative accuracy between these two approaches, and especially to establish confidence in the use of the novel, straightforward, and yet powerful femto-comb technique, it is important that an optical frequency standard be measured and compared using these two different methods simultaneously. Recently we carried out just such a measurement, with a collaborating effort from JILA, NRC (Canada), and BIPMBIPM, all leading institutes in the field of optical frequency metrology. The NRC group has a traditional Cs-based harmonic-generation frequency-synthesis chain and they have achieved the absolute measurement of a frequency standard based on a single ion, which they now use to measure the absolute frequency of the international length standard, an iodine-stabilized HeNe laser maintained at BIPM. A frequency intercomparison among the three groups allows the first-ever accuracy check between the traditional frequency chain and the newly developed technique of femtosecond comb system at JILA. The most accurately measured frequency to date of the international length standard is reported as a result of this research. (J. Ye)

• Development of Highly Stable Solid-State Lasers. Diode pumped Yb:YAG lasers provide an excellent candidate for high-power and high-efficiency laser operation. Yb:YAG also offers a broad emission spectrum that supports femtosecond pulse generation. A high-resolution (sub-Doppler molecular-vibration overtone transition) frequency reference for the Yb:YAG laser around 1.03 µm was demonstrated. Using a high finesse cavity and the established molecular transition, an all-external-frequency stabilization system has been achieved for this important future laser of choice. (J. Ye)

• Shaped Molecular Wave Packets for Computational Decision Problems. Quantum information is a subject of recent intense interest because of the possibility of achieving remarkable gains in computational horsepower. While quantum computing, per se, is challenging and requires the important formal quantum property of entanglement, other quantum processing steps may already be implemented in somewhat straightforward ways to obtain a gain in computational efficiency. In recent studies, the quantum interferences inherent in shaped molecular wave packets are used to solve a computational decision problem involving graphs. Points of a graph that are connected represent important constructs for such problems as airline route scheduling. In the studies developed here, the quantum interferences of wave packets, with specific phases encoded into the wave packet elements, are used to solve a graph problem to determine whether there are any points on a graph that are not connected to any other point (yes/no decision). Such points are called isolated vertices. Wave packet states are chosen to represent the vertices of the graph, and the relative phases between the states are chosen to represent the connections, or edges, of the graph. A single measurement, with sufficient signal-to-noise, of the quantum interferences of the entire wave packet is then used to determine whether any of the vertices are isolated by sampling the amplitude of the interference signal at a chosen time delay. Depending on how many edges are involved in the assignment of the graph encoding, the gain in efficiency in the evaluation step that determines whether there are any isolated vertices goes from one (when the number of edges is much greater than the number of vertices) to N (when all the vertices are isolated). Demonstration graphs have been encoded into wave packets in lithium dimer molecules and the analysis for isolated vertices successfully performed with six vertices. Extrapolation to larger numbers of vertices can also be performed. This problem may be categorized as coherent information processing, rather than quantum computing; it can also be applied using any other coherent superpositions, such as are available with classical elements such as electromagnetic waves. (S.R. Leone)

• Ultrafast Soft X-Ray Probing of Dissociating Molecules. Photoelectron spectroscopy (PES) of valence orbitals and x-ray photoelectron spectroscopy (XPS) of inner shell, core level orbitals are extremely important techniques to characterize chemical compounds and materials. These spectroscopies, however, have been almost completely restricted to stable ground states of atoms and molecules. In new work here, a femtosecond pump-probe apparatus has been developed to obtain PES and XPS spectra of transient and dissociating states of molecules for the first time. The new measurement tool consists of a high-energy, high-repetition-rate Ti:sapphire ultrafast laser that generates both the exciting pulse in the visible or ultraviolet and the soft x-ray probe pulse, the latter by the method of high-order harmonic generation in rare gases. Every odd harmonic up to the 65^th harmonic at approximately 100 eV is generated routinely, with photon fluxes of 10¹⁰ to 10¹² per second depending on the harmonic. Photoelectron spectra are obtained with a magnetic bottle time-of-flight electron spectrometer, and routine stable molecule spectra are obtained with excellent signal-to-noise. A key breakthrough is the achievement of the time and spatial overlap of the soft x-ray pulses with the pump pulses, using the signatures of above-threshold ionization peaks, or sidebands, on traditional photoelectron peaks. Following immediately after this success, the first time-resolved photoelectron spectrum of dissociating bromine molecules was recorded, showing clearly the appearance of the Br atom features in time. The 17^th harmonic at 47 nm was used for the probe and the pump pulse was 400 nm, which accesses the dissociative state of bromine molecules. The first experiments were limited to 500 fs time resolution; however, with a new dual-grating system, partially compressed pulses of 100 fs duration are expected, which will reveal the first details of photoelectron spectra of a dissociative state. (S.R. Leone)

Direct Optical Frequency Synthesis using Modelocked Lasers. Several members of the QPD formed a team combining expertise in frequency-domain laser stabilization and ultrafast modelocked lasers to develop techniques for stabilizing the pulse-to-pulse phase of the ultrafast pulses emitted by a modelocked laser. In the frequency domain, this results in a comb of known absolute frequencies, where the only input is derived from a cesium clock. This represents a revolutionary new technique for connecting optical frequencies to the microwave frequency primary cesium standard. (Fig. 1) This work was published in the April 28 issue of Science. At JILA it has been used to measure the frequency of the two-photon Rb transition near 778 nm, the frequency of an iodine-stabilized Nd:YAG laser, and an iodine-stabilized HeNe laser. This technique is being used by the Time and Frequency Division where it was recently employed to measure the absolute frequency of calcium and a single mercury ion; both are optical clock candidates. The link between optical and rf frequencies can also be used to generate rf from an optical standard, an important step that has been heretofore missing for optical clocks. (S.T. Cundiff)
		Figure 1. Frequency metrology using the self-referenced frequency comb. (a) Experimental setup to measure the 5S1/2(F=3) → 5D5/2(F=5) two-photon transition in 85Rb with self-stabilized frequency comb. (b) Histogram of one set of measurements relative to the recommended CIPM (1997) value of 385 285 142 378 ± 5.0 kHz for the Rb transition. The standard deviation at 1 s gate time is 5 kHz, which is an absolute uncertainty of 1 part in 1011.

• Imploding Bose-Einstein Condensates Observed. Under normal conditions, there is a positive pressure inside a sample of Bose-Einstein condensate (BEC). The residual interactions between the atoms are repulsive, and as more and more atoms are added to the condensate, the sample becomes larger and larger. At certain very particular values of the ambient magnetic field, however, the situation can be radically different. The interactions between the atoms arise from a two-body scattering process, and if the scattering events can be made resonant with a bound-state, the interactions as well undergo sharp resonant behavior. The ambient magnetic field then plays the role of tuning the molecular levels with respect to the free particles. In rubidium-85, there is a magic value of the field around 0.0166 T. In the presence of ambient magnetic fields just smaller than 0.0166 T the atoms find each other repulsive, but once the field is made even slightly larger than 0.0166 T, the sign of the interactions reverses and the cloud of atoms begins to self-attract. The effect on the condensate can be profound. The JILA BEC collaboration created a relatively large condensate at 0.0164 T, waited for the system to equilibrate under the effects of its own repulsive interactions, and then suddenly increased the magnetic field past the threshold value. The condensate then rapidly caves in on itself, accelerating inward ever faster as the density, and thus the strength of the self-attraction, increases. The condensate reaches some as-yet poorly understood hard-core density and then instantly ejects a plume of super-thermal atoms similar to the case when an imploding star becomes a supernova. The experiment should eventually yield considerable insight into the nature of shock waves at ultra-low temperatures. (E.A. Cornell)

• New, Small, Absolute, and Highly Portable Gravity Instrument. Here work is focused on a new, cam-based absolute gravimeter. This small, fast, portable, and simple-to-use instrument uses a rotating cam to create the following: a release, a two-centimeter free-fall drop, a soft catch, and then a return to the starting position. This instrument inherently yields a high measurement rate (3 drops per second). Furthermore, by employing a second co-rotating cam that drives an auxiliary mass it is possible to keep the instrumental center of mass fixed. In this way recoil effects having to do with both the release of the mass and the accelerations of the mass-carrying carts can be eliminated. The results obtained to date are very encouraging. In the instrument, use is made of a simple 1.2 s period spring (exactly four measurement cycles) to isolate the reference mirror. Magnets are used to damp all unwanted motions of the spring-supported mass without affecting the vertical isolation provided by this spring. (J.E. Faller)

• Newtonian Constant of Gravitation. We are beginning to take data with a new and dedicated G apparatus. To measure this fundamental constant, this instrument measures small length changes by sensing frequency changes in a laser that is locked to a Fabry-Perot cavity whose mirrors are located in two freely hanging masses. This laser is beat with another laser that is locked to a (fixed) cavity, which monitors the separation between the suspension points of the two hanging Fabry-Perot mirrors. When external masses (500 kg of tungsten) are moved nearby, the free-hanging (70 cm long) pendulums deflect. G can then be determined by combining the measured small frequency-sensed changes in this cavity’s length with careful measurements of the experiment’s geometry. The present uncertainty assigned to the value of this fundamental constant is 1.5 parts in 10³. With this new apparatus, we expect to be able to reduce this uncertainty to below 5 parts in 10⁵. (J.E. Faller and H. Parks)

• Particle Growth in Thin-Film Deposition Discharges. Particles grow in the discharges used to deposit many types of thin films; of particular interest here are semiconducting films for electronic devices. Some of these particles escape to the films, causing a deterioration in the electronic properties of a device. In other instances, crystalline particles can be used to initiate thin-film crystallization thus enhancing the properties. To understand the causes and to control the behavior of these particles, their growth and transport to the substrate has been studied for the types of discharges used to make amorphous-silicon, thin-film transistors and photovoltaics. Measurements of light scattering by the suspended particles had previously yielded particle growth rates and densities for the very small particles (<30 nm diameter) that escape to the films. Now a model for the particle growth chemistry in the plasma environment has been developed and used to calculate the dependence of particle densities on discharge conditions. This extensive chemical and plasma model describes the growth of neutral and charged molecules into clusters and then particles, in competition with escape to the electrodes. This unique model has yielded many insights into the causes of, and potential methods for altering, the particle fluxes to the growing device. (A.C. Gallagher).

• New Measurement of the Absolute Frequency of Iodine-Stabilized 633 nm Laser using fs Laser Comb Techniques: Clarification of International Differences Between Iodine-Stabilized Lasers, via Improvements in the Anti-Dither Technique. From international intercomparisons of stabilized lasers, it has long been noted that significant differences exist between the apparent hyperfine intervals observed with lasers of differing designs. These troubling international frequency differences that are dependent upon design have resurfaced in connection with our recent absolute frequency measurement of the red iodine-stabilized laser, using the femtosecond laser comb system we have developed at NIST/JILA. Basically it happens that a certain iodine hyperfine peak (peak "i") was adopted years ago for the international comparisons as being the most robust line under variation of laser operating conditions. However it is now the case that the HeNe laser tubes currently available no longer are single neon 20 isotope, but rather employ a mixture of neon isotopes 20 and 22. The associated spectral shift of the gain curve makes peak "i" less reliable and has led the community informally to utilize peaks "f" or "d." Therefore, our recent JILA absolute frequency measurements were performed at the sub-kHz level on all the relevant Iodine components. The accuracy of our new fs laser technique was confirmed by intercomparison of a JILA-measured stable transfer laser with the "traditional" frequency chain at the NRC in Ottawa. This international intercomparison also included the BIPM and its two transfer lasers. Thus these JILA frequency measurements were connected to the "as maintained" length standard for the world, the BIPM-4 reference laser in Paris. These results suggest that a (7 ± 1) kHz correction is needed for the CCDM-recommended value for the iodine-based 633 nm standard. Our report will appear in the Physical Review Letters as befits the wide interest in and applicability of this secondary standard for frequency in the optical domain.

  In a separate investigation, we may have found the origin of this frequency discrepancy, associated with the dither locking technique. The accuracy and long term stability of any atomic resonance-based frequency standard depends on the accuracy of the modulation utilized to define the center of the resonance line. JILA has pioneered the use of rf optical phase modulation that provides access to very rapid locking action; however, this method can suffer from accuracy limitation due to subtle changes in the optical spectrum that the modulator produces. The older dither modulation method may have an offset due to distortion of the modulation waveform, but it is stable. Of course dither was undesirable due to the wide FM spectrum it generates, but recently we have shown this dither can be cancelled externally with reasonable accuracy. Unfortunately, there has been no way to confirm the modulation waveform accuracy due to the extreme dynamic range required (>80 dB). One might suppose that Direct Digital Synthesis would be sufficiently accurate, but this is not the case due to "aliasing" effects that produce non-harmonic spurious outputs. A way has been found to transfer the digital accuracy for the main fundamental wave to a carefully compensated Voltage-Tunable Oscillator, which then can test or compensate the laser modulation waveform in question. Thus a new measurement capability arises by differentially comparing the electronically generated wave with one from a heterodyne beat that includes the laser modulation. In this way we characterized the (stable) distortion generated in the laser PZT motion and thereby find a possible natural explanation for the significant differences of apparent hyperfine intervals observed with iodine-stabilized lasers of different designs. Unfortunately, due to the recent death of the French scientist (Y. Milleroux) who was the principal of the BNM/LPTF measurement leading to the CCDM frequency value, it is not easy to confirm if this is a good explanation of the frequency discrepancy. Some additional investigation into the laboratory records will be needed to clarify the situation. (J.L. Hall)

• Apertureless Near-Field Scanning Optical Microscopy. With the ever-decreasing scale of electronic components and chip design, there is an ever-increasing need to develop efficient optical methods to measure properties of nanoscale objects with sizes well below the diffraction limit of light. There have been considerable breakthroughs in this area based on near-field scanning optical microscopy (NSOM), which has conventionally been achieved by using metal-coated optical fiber tips to confine the light in rapidly tapered optical fibers. However, this method is inherently limited by the skin depth of light in the metal cladding (12 nm for Al), which even for optimum cases yields only 20 nm to 30 nm resolution, and under typical operating conditions, more like 100 nm resolution. An alternative we have actively explored has been to develop new methods in apertureless near field scanning optical microscopy, which already have demonstrated resolution improvements down to the 2 nm to 3 nm length scale. This effort is based on a combination of (i) evanescent wave excitation of molecules/nanostructures on a prism surface, (ii) sharp Si or Ag coated Si structures guided by atomic force microscopy (AFM) to condense the evanescent electric fields in the vicinity of the tip, and (iii) sensitive resonant scattering light and/or fluorescence detection of the molecules subsequent to the excitation event. (A.C. Gallagher and D.J. Nesbitt)

• Scattering/Extinction Near-Field Microscopy. As a necessary first effort, scattering/extinction near-field microscopy methods have been used to evaluate the scattering cross sections of AFM tips in the presence of an evanescent wave as a function of tip distance above the surface. This has yielded the first absolute cross sections for near-to-far field scattering with well-characterized probe shapes, which is crucial in providing "benchmark" data for testing quantitative models of near field interactions. This method has next been used to obtain near field images of Au nanospheres at <5 nm resolution by resonant scattering of 543 nm light near the plasmon resonance. These studies determine that the combination of AFM tip + particle leads to a scattering enhancement of over 4000-fold from that of the bare Au nanospheres. Such data again provides important benchmarks against which to test theoretical models, as is being collaboratively attempted by JILA theoretician J. Bohn and NIST colleague G. Bryant (Div. 842). (D.J. Nesbitt)

• Fluorescence Near-Field Microscopy. As a most recent achievement, fluorescence near-field miscroscopy methods have been extended into the domain of near-field fluorescence microscopy, where the Si AFM tips are used to influence the near field excitation of dye doped nanospheres (on the 20 nm scale) or even down to semiconductor CdSe quantum dots (6 nm scale). The resulting fluorescence is imaged with a high numerical aperture microscope and fiber coupled avalanche photodiode combination. The data indicates substantial increase in the fluorescence from these molecules in the near field presence of the laser excited AFM tip, with spatial resolution that appears to be fully limited by the 5 nm tip radius of curvature. A simple electrostatic analysis of these data is consistent with a roughly 30 fold enhancement of electric field at the tip, which translates into a nearly 10³-fold enhancement of the near field laser intensity. (D.J. Nesbitt)

• Near-field Theoretical Modeling. We have initiated a new program in theoretical modeling of the apertureless NSOM data, in collaboration with JILA Fellow J. Bohn and profiting from early assistance from a theory group in NIST Gaithersburg, MD. The results are based on modeling of the AFM tip, prism surface, and evanescent laser field by discrete electrostatic multipoles, and matching boundary conditions at the respective interfaces with least squares methods. Most importantly, these calculations can be converged for realistic elongated tip shapes that incorporate the lightening rod antenna effect and the actual exponential drop off of the evanescent fields. The results indicate (i) a significant enhancement of the fields near the tip, (ii) a strong sensitivity to the length of the tip elongation, and (iii) a limiting value of the field enhancement of κ ≈ 30 for tip lengths greater than the 1/e evanescent decay. This value is in remarkably good agreement with what is observed experimentally. Furthermore, this analysis indicates a significant contribution to near field enhancement from "image dipoles" generated in the prism when the laser-polarized AFM tip approaches within approximately one tip radius (5 nm) of the surface ("Narscissus effect"). (D.J. Nesbitt)

• Single Molecule and Single Quantum Dot Confocal Microscopy. Spawned in conjunction with the NSOM efforts, we have also been developing new capabilities in high sensitivity detection and spectroscopic characterization of single molecules. This is based on laser excitation and fluorescence detection via scanning confocal microscopy, coupled with high sensitivity avalanche detection and/or a CCD array spectrometer. These methods have been used to investigate the photophysical dynamics of individual nanostructures, in particular focusing on ZnS overcoated CdSe and InP quantum dots on fused silica and glass surfaces. Particularly relevant has been the detailed studies of fluorescence intermittency or "blinking" of individual quantum dots, which begin to provide detailed kinetic information on electron hole pair ejection and recombination from single quantum dot structures. The distribution of time scales over which these ejection/recombination events spans an enormous dynamic range (microseconds to minutes), which can be statistically analyzed to show that the kinetics is intrinsically non-exponential, with "rates" varying over 5 to 6 orders of magnitude. (D.J. Nesbitt and A.C. Gallagher)

• Single Molecule Microscopy and Characterization of Biomolecules. We are exploiting single molecule microscopy in new directions that involve biophysical applications. First of all, we have utilized confocal methods to study green fluorescent protein (GFP), and have initiated studies of intercalation kinetics of fluorescent dye molecules into DNA strands tethered to surfaces by biotin-streptavidin interactions. We are also developing tools based on wide-field illumination and cooled CCD array cameras to image electrophoresis of single biomolecules in agarose gels. Finally, we are building up new time-resolved capabilities for single biomolecule fluorescence detection in liquids utilizing polarized fluorescence resonant energy transfer (FRET) and burst integrated fluorescence (BIFL) methods. Monitoring the frequencies, polarization, and fast time correlations between the fluorescence photons emitted will allow one to watch relative distance and orientation of fluorescent donor/ acceptor "tagged" biomolecules, and thereby probe kinetics of conformational changes in real time. (D.J. Nesbitt)

• High-Resolution IR Laser Spectroscopy in Plasmas. The overwhelming majority of all chemical processes occur via fast bimolecular reactions involving molecular species with unpaired electrons, i.e., free radicals. The reactivity of these species makes them especially hard to detect and study in detail. Of special interest are methods of spectroscopically characterizing these radicals at low temperature and high resolution, which is a crucial prerequisite for laser remote sensing of these species in the much more complicated "real world" environments of plasmas, combustion, etching, and so on. We have been developing methods to combine the advantages of low temperature expansions in slit supersonic jets with pulsed electrical discharges to produce unprecedentedly intense sources of molecular radicals under the low temperature (10 K to 20 K) and long-absorption-path environment ideally suited for direct-absorption laser spectroscopy. We have been exploiting this source for high-resolution spectroscopic investigations of open shell hydrocarbon species (such as ethyl radical), as well as jet-cooled molecular ions (such as protonated H₂, CH₄, and HCCH). As one example, vibrational spectra of H₃⁺, H₂D⁺, HD₂⁺, D₃⁺ in the adiabatic approximation all derive from motion on the same Born Oppenheimer potential surface, which for a two-electron problem can be calculated to near spectroscopic accuracy. Comparison of high-resolution spectra for these different mass combinations therefore provides direct evidence for non-adiabatic, Born Oppenheimer breakdown in the various isotopic mass combinations. (D.J. Nesbitt)

• State-to-state Reaction Dynamics. Chemistry is a discipline of enormous technological and economic importance and yet a detailed understanding of how the simplest of chemical reactions occur still represents a state-of-the-art area of experimental and theoretical chemical physics research. We have been making major advances in this area by exploiting (i) slit discharge technology as an intense source of jet cooled radicals and (ii) shot- noise-limited infrared laser absorption in order to explore quantum state-to-state reactive scattering in crossed supersonic jets. The initial focus has been on hydrogen abstraction reactions X + RH → HX(v,J) + R, where the nascent rovibrational product states of HF can be sensitively detected via direct absorption methods. The crucial advantage of such a high-resolution laser-based approach is that it offers 10^-4 cm^-1 spectral resolution on the product states, which is more than a 10⁵ to 10⁶-fold improvement over previous crossed-beam time-of-flight studies. These high-resolution studies have revealed major dynamical insights even for "simple" atom + diatom systems. For example, F + HD product-state distributions have yielded direct evidence for Feshbach resonances corresponding to fleetingly bound vibrational states of the FHD complex with the H atom bouncing back and forth between the F and D atoms. These so called "transition state resonances" have been theoretically predicted but have proven extremely elusive to detect experimentally without full quantum-state-resolved reactive scattering methods. (D.J. Nesbitt)

• Reaction Dynamics at the Gas-Liquid Interface. A major fraction of chemical reactions of industrial, commercial, and environmental importance occur at the gas-liquid interface, ranging from atmospheric processing of ozone to rapid detonation in internal combustion engines. Despite this importance, the tools for exploring reaction dynamics at the gas-liquid interface are remarkably limited. We have initiated a new program of study based on direct IR laser absorption to investigate quantum-state-resolved dynamics of reactions at the gas-liquid interface. Building on our success in crossed molecular beam reactive scattering, the first test system has been F atom reactive scattering from long-chain liquid hydrocarbons (essentially low- vapor-pressure pump oils), which leads to HF(v,J) formation that is detected via direct IR laser absorption. Since the F atom source is pulsed and the HF detection is time resolved, this permits the dynamics of "direct" reactive scattering vs. slower temporarily trapped interfacial reactions to be distinguished with full-quantum-state resolution of the products. (D.J. Nesbitt)

• Atmospheric and Environmental Chemistry. There has been an ongoing effort in our labs towards studying reaction dynamics relevant to atmospheric chemistry using direct IR laser absorption of highly reactive radicals. One important target has been chemical chain-reaction depletion of ozone in the lower stratosphere (based on the ubiquitous reactions between OH, HO₂ and O₃) that is studied by excimer laser photolytic generation of OH radicals in a fast flow reactor, with time-resolved high-resolution IR laser absorption over a long flow path length as a direct measure of OH, HO₂ radical concentrations and therefore the relevant rate kinetics. Most importantly, these IR based methods completely circumvent problems due to UV photolysis of ozone, which plagues conventional LIF (laser induced fluorescence) detection methods for OH radicals. Consequently, this IR laser apparatus uniquely permits such reactions to be explored down to temperatures crucially relevant to the stratospheric models of the atmosphere. Other projects on this apparatus include studies of formation and energy transfer of highly rotationally excited OH(v,N ≈ 30), whose remarkable and highly non-equilibrium presence has been recently detected via IR emission from the upper atmosphere. (D.J. Nesbitt)

Exploring a Quantum Degenerate Gas of Fermionic Atoms. A year ago we succeeded in producing the world’s first quantum degenerate Fermi gas of atoms and thus introduced a new system for study. By exploiting the exquisite experimental control and unique theoretical accessibility of this ultracold atomic gas system, we plan to explore Fermi-Dirac statistics in a novel regime and enhance understanding of this important class of quantum systems. Since reaching quantum degeneracy in a Fermi gas of potassium-40 atoms, we have put considerable effort into understanding the current limits to reaching even lower temperatures, T, relative to the Fermi temperature of the gas, TF. This is motivated by the fact that all quantum effects in the gas become more prominent as T/TF decreases. With technical improvements to the evaporative cooling we have now reached T/TF = 0.2; this represents a factor of 2 improvement compared to our first results. In addition we have worked with JILA Fellow M. Holland to explore the fundamental limits to cooling which arise from the quantum behavior of fermions. Additionally, we have begun exploring a two-component Fermi gas – a system that opens new possibilities for investigating the interplay of interactions and quantum statistics. Identical fermions (in the same internal state) do not interact at ultralow temperatures. However, as has been seen in BEC studies, interactions play a crucial role in many interesting quantum phenomena, such as superfluidity. Using 40K atoms in two different spin states we can experimentally realize an interacting Fermi gas. With thermodynamic measurements we have shown that we can indeed produce a Fermi gas in which both components are clearly quantum degenerate. Furthermore, we have observed Pauli blocking, which is a dynamical effect of Fermi-Dirac quantum statistics on collisions, in the thermal relaxation of this two-component system. Exploring this effect is important not only in understanding the quantum behavior of the gas but also because it is predicted to play a major role in limiting the efficiency of further evaporative cooling. Ultimately we plan to investigate the possibility of controlling the interactions between atoms with a magnetic-field Feshbach resonance. This would allow us to access a wide range of possible interaction strengths, including both effectively repulsive and attractive interactions. One example of the phenomena we will explore is the prediction of a phase transition to a superfluid state, whose underlying physics is analogous to superconductivity in metals. (D. Jin)

Figure 2. A two-component Fermi gas. In this 50/50 mixture of 40K atoms in two different spinstates, mf = 9/2 and mf = 7/2, both component gases are quantum degenerate; this can be seen in the fact that the average energy per particle E rises above the classical expectation of 3 kT.