Quantum Physics Division

1995/1996 Technical Highlights

• Characterizing the Bose-Einstein Condensate. The world's first observation of Bose-Einstein condensation (BEC) in a dilute gas was made at JILA in June of 1995. Since then the JILA BEC group has performed a variety of experiments with the goal of characterizing the basic properties of this novel quantum fluid. One study produced a relatively precise measurement of the critical temperature and a preliminary measure of the specific heat near the transition. It turned out that the basic value predicted in 1925 was correct. A second round of observations determined the frequencies of the lowest-order standing-wave acoustic modes in the sample. These phonon-like excitations are providing an important handle on the fluid properties of the condensate. The rate at which the excitations damp out, for example, is a measure of the viscosity of the condensate fluid. Most recently, it has been discovered that as the temperature of the sample is decreased further and further below the transition temperature, the excitations take longer and longer to damp out, indicating that the viscosity of the sample gets very small at low temperatures. (Cornell [Wieman, CU], and Jin)



Figure 1. Widths of each component, condensate and non-condensate, of the freely oscillating cloud are fit by an exponentially damped sine wave. For each pair of points (condensate and non-condensate widths) a fresh cloud of atoms is cooled, excited, and allowed to evolve a time t before a single destructive measurement.

• Mechanical measurements. The expanding need is being addressed for accurate and less costly methods of measurement in the production of, for example, microprocessors, which require the accurate positioning of up to 25 successive patterning masks over a period of about two weeks. Optical interferometry with the ubiquitous HeNe laser is commonly used to sense and control for any minor flexures, but the limited accuracy of this measurement is a growing problem. The heat of the HeNe laser can distort the measuring apparatus by thermal expansion, and the speed of light which scales frequency into length depends importantly upon the ambient temperature, pressure, moisture and other compositional variations of the ambient air.
  
  A small and low-cost method has been developed for reading the actual index of refraction in situ, based on video camera capture and computer processing of the interference rings of a simple stable interferometer designed with an air flow channel between its mirrors. This system has been patented. In future technology, it is clear that a semiconductor diode laser will be the laser of choice, due to its long life, higher light output and vastly lower heat generation. Frequency-comparison of a 633 nm semiconductor diode laser with the HeNe laser standard based on iodine molecular absorption shows very attractive stability: drift below 2 × 10^-8 in one week in the very first trials. Thus when the cavity is exposed to ambient air, there is confidence that a stabilized wavelength source is achieved for precision interferometry whose wavelength is constant in the laboratory. Industrial design of this system can be packaged in the volume of a box of the current 3.5 inch minifloppy disks. One can foresee wide application of this stabilized laser, in either its constant wavelength or constant frequency modes in many practical tasks in engineering and science. (Hall)
• Optical Frequency Measurement Techniques. Preliminary results have been obtained on the use of a strongly-driven electro-optic modulator situated within a low-loss and resonant cavity. World-wide interest in these devices is exploding for their use as an "optical comb generator." Better parameters have been achieved than have been so far reported from other laboratories. A novel frequency-selective cavity mirror allows effective in-coupling of the monochromatic light to be modulated, and out-coupling of the selected frequency-shifted "comb" component. Observations of useful sidebands shifted by >1.5 THz have been made. (Hall)
• Super Spring. Work continues on developing and understanding the limitations of simple spring systems designed to achieve long periods with compact-structured springs. This development should have enormous practical consequences. The basic pretensioned spring system has been modified through the addition of spring-constant-canceling auxiliary spring elements to overcome the slow scaling improvements beyond that initially achieved from extreme pre-stressing. The improvement is not rapid enough to justify the required increase in the number of elements. Using the auxiliary springs, one can in principle get an infinite period with just a single element -10 s is the JILA record to date. Research is focusing on ways to eliminate or at least greatly reduce the complications of internal modes. (Faller)
• Measurement of Newtonian Gravitational Constant. An FG5 absolute gravimeter is being used, together with a moveable 1200 kg tungsten mass surrounding the dropping chamber, to measure big G, the Newtonian constant of gravitation. This work, with a large cast of collaborators from the National Geological Society, Micro-g solutions, and JILA is motivated by the present nearly 1 % discrepancy between the recent Physikalisch Techniche Budesanstalt (PTB) measurement and the "accepted" value. The fact that the PTB measurement appears to have been competently and thoughtfully done makes the discrepancy even more intriguing. A proof-of-concept experiment was carried out using an existing 100 kg bronze mass and the data analysis is nearing completion. It appears that a preliminary result of 1 % or better will be obtained and that optimization of the mass geometry together with an increase of its mass should yield an order of magnitude better result. (Faller)



Figure 2. Graduate student Sam Richman with the isolation chamber.

• Active Low-Frequency Isolation System. Excellent progress has been made by Division researchers on this isolation system planned for use on the Laser Interferometer Gravitational Observatory (LIGO). The now operating preliminary single stage provides more isolation at 1 Hz than any other research or commercial isolation system. Preliminary tests with the single stage operating in all 6 degrees of freedom and the next stage with only the three vertical loops locked have also been carried out and the initial results are very encouraging. In addition to the LIGO (Caltech and MIT) group there is a possibility of working with the British-German GEO-600 gravity wave detector group. Since the GEO-600 involves smaller sized mirrors, a cooperative venture could provide an excellent proof of concept test for the larger versions of the JILA isolators that will be required for LIGO. (Faller [Bender, CU])



Figure 3.

• Atom Guiding in Hollow Fibers. JILA researchers have been using the force that light exerts on atoms to guide atoms through hollow glass fibers. Glass fibers can guide light, and light can guide atoms, preventing them from touching the inside of the glass. The result is a flexible atom guide, a useful building block for many atom optics experiments, including atom interferometry. In recent work, atoms have been guided using the evanescent light field from laser light confined in the annular region surrounding the hollow core. Researchers have been successful in demonstrating the cooling of the atoms inside the fiber. Current research directions include loading atoms with a laser cooled, intense atomic beam and guiding through ever smaller diameter fibers. (Cornell [Anderson and Wieman, CU])
• Tests of QED. A new and potentially important test of Quantum Electrodynamics (QED) has become of interest due to the abrupt availability of powerful dipole magnets originally developed for the Superconductor Super Collider (SSC). QED theory predicts that in a "light-by-light" scattering process, even an ideal vacuum becomes slightly birefringent due to a powerful transverse magnetic field. For the experiment at Fermilab, the readout methods are based on use of a Direct Digital Synthesis to produce a test frequency of adequately high resolution and low noise and, of course, on the use of "super mirrors" and a long interaction region (50 m). Some new modulation methods that optimally isolate the desired birefringence signal from the influence of residual laser frequency noise will also be used. In view of the importance of the subject and the general utility of new laser locking techniques, a research phase is now underway, mainly at JILA, to prove the new techniques and identify limiting factors. (Hall)
• Kinetic-Energy-Enhanced Etching of Silicon. Potentially damaging effects of the dry process plasma etching steps will become a critical issue when gate oxides of semiconductor devices shrink to 5 nm thicknesses. Thus there is considerable recent interest in the possibility of supplying energy for the etching process in the form of neutral-species kinetic energy. New work in this area by the Division involves the development of a more general source of kinetic-energy-enhanced neutrals by extraction and charge neutralization of ions from a plasma source. The kinetic energy distributions of ion and neutral species emanating from those plasmas containing rare gases, chlorine, or nitrogen have now been characterized. Laser single photon ionization has also been used to measure the SiCl and SiCl₂ products of thermal chlorine etching of Si(100). (Leone)
• Time-Resolved Near Field Optical Microscopy (NSOM). Ultraminiature devices, such as recording heads for hard disk storage based on the magnetoresistive effect, already exceed the limits of measurement capability required to analyze the size and quality of the layered structures. When the devices fail or are fabricated improperly, there is no way to determine what went wrong in the process. New kinds of measurement capabilities are eventually needed that have both element-specific sensitivity and 1 nm to 10 nm resolution. Such measurement capabilities will ultimately also be invaluable to the broader microelectronics and photonics industry.
  
  In a competence initiative through NIST, the Division is investigating the application of near field optical microscopy for both time-resolved and spectral characterizations of materials and molecules. NSOM images have been obtained in transmission and fluorescence using conventional transparent fiber optic probes with spatial resolutions of 100 nm or less. Aggregates of dye molecules have been imaged successfully with the home-built arrangement. A project has been initiated to study hybrid NSOM assisted by scanning tunneling methods. The basic idea is to use the atomic scale sharpness of nanocolumns to enhance the electric field of a laser in the near field of an STM probe tip, and thereby achieve dramatic improvement in spatial resolution over conventional NSOM fiber optic methods. In future experiments, two time-delayed pulses from a ps pulsed laser will be used to prepare and probe optically induced changes in transmission, which will introduce time-contrast mechanisms into the images. (Leone, Cornell, Gallagher, Nesbitt)
• STM Images Reveal Flaw Formation in Films for Solar Panels and Large Flat-Panel Displays. Images of particles only a few nanometers wide, which can reduce the efficiency of certain light-sensitive films, are observed in the plasma processing of solar panel films. Large area films are used in making solar energy panels and large flat-panel displays. The efficiency of the film in converting light into electrical current is best for very thin homogeneous films about 500 nm thick.

  A custom built system for both growing thin films and examining them with an ultra-sensitive scanning tunneling microscope is used. The apparatus can grow amorphous (noncrystalline) films of silicon and hydrogen atoms with plasma-enhanced chemical vapor deposition (PECVD). Images of the film are taken at various stages throughout the growth process. The images show particles 3 nm to 5 nm in size, which form in the vapor and bond to the film surface during growth. As a new layer of silicon and hydrogen atoms deposits on the surface, these clumps cause voids within the film.

  Many people have studied the production of larger particles during PECVD, but these particles are suspended in the plasma and do not reach the growing film. No one appears to have realized that small particles can reach the growing film. If these particles can be prevented from forming or reaching the surface, it should be possible to improve the films' ability to convert light into electrical current. New work will develop a laser-scattering system to detect the silicon/hydrogen clumps as they are forming in the plasma. Laser scattering detects larger particles, but provides a method for real-time monitoring of particulate behavior in the plasma. (Gallagher)
• STM Controlled Aluminum Deposition. The scanning tunneling microscope (STM) can be used to measure surface features with atomic resolution and has the potential to produce atomic scale objects. Silicon semiconductor devices generally use aluminum contacts, so it is highly desirable to learn how to "write" nanometer scale aluminum features on silicon. The Division has made such nanoscale aluminum deposits onto silicon by pinning the aluminum from tri-methyl-aluminum (TMA) vapor with the electron beam from the STM. Detailed studies have revealed the various physical and chemical steps involved, and the electrical characteristics of these nanometer size contacts. (Gallagher)



Figure 4. Pad deposited on x-Si(001) at -10 V and 8.8 L of TMA.

• Quantum State Resolved Sublimation Dynamics of Thin Molecular Films. The dynamics of how molecules collide with, stick to, and bounce off a surface is of considerable importance in molecular beam epitaxy applications. By virtue of microscopic reversibility, such dynamics can also be probed by monitoring the nascent quantum state distributions of molecules subliming from thin films. Under sufficiently low vapor pressure conditions, such quantum state resolved studies can be performed by high resolution diode laser direct absorption experiments, 5 mm above a temperature-controlled surface. Such direct absorption methods have been developed by the Division to study CO₂ sublimation dynamics from thin CO₂ films, using frequency-swept diode laser methods to obtain 1 monolayer/s detection sensitivities. The single-mode diode laser provides resolution of all vibration/rotation states in CO₂, as well as a selective probe of higher clusters (n=2,3) in the subliming flux. From 90 K to 120 K, however, all J<40 sublimation populations are indistinguishable (±5 K) from thermal prediction at the surface temperature, indicating no quantum state dependence to the reverse gas-surface sticking event. Translational velocity distributions are obtained from high resolution analysis of the 4.3 mm Doppler profiles, yielding a speed distribution also consistent with the surface temperature. Since absolute fluxes can be readily measured by the direct absorption method, this data can be converted into an absolute sticking coefficient of 1.0 ± 0.1. Modeling of the surface dynamics with CO₂-CO₂ pair potentials predicts a surprisingly "soft" landing for the impinging CO₂, which is most probably responsible for such efficient and quantum state independent sticking behavior. (Nesbitt)
• Electron Collisions. The merged electron-ion beams energy loss (MEIBEL) technique developed at JILA has been used to measure the cross sections for electron-impact excitation of multiply charged ions to spin-forbidden states. Most recently, the process e+Ar⁶⁺(3s^2 1S) → e+(3s3p ³P⁰) was studied using the technique. As expected, resonances dominated the cross section in the threshold region. Comparison with theory shows that there is significant resonance interference. In fact, the interference is so sensitive to the exact location (energy-wise) of the resonances that present computational techniques may not be capable of giving accurate enough energy values to predict the resonance interference adequately.
  
  A collaboration between JILA and Swedish and German scientists has resulted in measurements of the effects of ambient electric fields on dielectronic recombination cross sections for Si¹¹⁺. It was shown in JILA experiments in the 1980's on Mg⁺ that dielectronic recombination cross sections could be increased by large factors through state mixing by external fields. However, there have been no further definitive measurements despite many efforts by others. The recent work on Si¹¹⁺ on the heavy ion storage ring (CRYRING) in Stockholm has yielded very clear effects that will complement the earlier work on Mg⁺. Hot plasma environments very often have fields large enough to affect this key process significantly so that understanding the effect is essential to modeling and understanding such plasmas. (Dunn)
• Natural Lifetime of Sodium Atom Resonance Level. The Division completed the lifetime measurement of the Na resonant state and resolved a long-standing discrepancy between the best experiment and the best theoretical calculations. The attained lifetime accuracy was 0.22 %, near the best that has ever been obtained. An important aspect of this new method is that it is subject to different systematic effects compared with the traditional atomic beam method, and may be optimum for atoms of even shorter lifetime which are increasingly difficult for traditional methods. Studies of parasitic effects, such as radiative momentum transfer during interrogation, will be of great value for understanding the true possibilities of the several atomic fountain projects being undertaken by NIST. (Hall)
• Noise-Immune, Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy. Division researchers discovered/invented and applied an extremely powerful new principle that allows one to combine the signal-enhancing technique of cavity enhancement (placing the sample within an ultra-finesse cavity gives a signal enhancement of ~30,000 fold) with our earlier-developed method of optical heterodyne detection. This allows one to reach near the shot-noise-limited detection level even with real laser systems. With this method, the small residual laser-frequency noise is not converted into detected noise as in previous work, but rather is suppressed to below the shot-noise level. The key concept is to match the local oscillator sidebands' frequency offset to the enhancement cavity free-spectral range: in this way the noise-induced phase-shifts are made to be common mode. A detection sensitivity of <1 × 10^-12 for absorption has been made, which is better by two orders of magnitude than the best results achieved to date by cavity ring-down or laser/intracavity absorption spectroscopy methods. By use of this new method and C₂HD for stabilization of a 1.064 µm laser, stability has been achieved of 3 × 10^-13 at 1 s, and better than 1 × 10^-14 after 1000 s. This method can be called Noise-Immune, Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy, i.e., NICE OHMS, and is the object of interest by spectroscopic research groups worldwide. (Hall)
• Calcium Rydberg State Alignment Effects. An area of active control in collisions involves preparation and manipulation of orbital directions to study how the rates of processes are affected by this directionality. Division researchers developed a new method to detect aligned Rydberg states of alkali or alkaline earth atoms by a stimulated-emission "dump" pulse with a laser. Rydberg states of atomic Ca were detected from 8 d to 40 d by selectively dumping the state of interest and detecting fluorescence emission from the lower level. The first state-to-state orbital alignment experiments on Rydberg states were performed for an energy transfer process that has an observed alignment dependence, Ca(4s18d ¹D₂) + Xe → Ca(4s17p ¹P₁) + Xe, by using the stimulated-emission dumping method. Through a theoretical collaboration, the state-to-state Rydberg alignment effects were predicted by quantum mechanical scattering calculations. An unanticipated velocity dependence was observed, indicating oscillations in the m-sublevel dependence, which motivates additional velocity-selected experiments in the laboratory.
  
  State-of-the-art experiments in orbital alignment dynamics will be addressed by studying four-vector correlations on a new machine designed to study energy pooling, associative ionization, and Penning ionization processes of alkaline-earth excited states with alignment probing of final states. (Leone)
• Photophysics and Photochemistry in Quantum State Selected Clusters. Division researchers have been developing high resolution tunable, optical parametric oscillators (OPOs) for use in studies of chemical reactions in quantum-state and size-selected clusters. The method is based on cw injection seeding of a 4 mirror, β-barium borate (BBO), ring resonator pumped by a single mode 355 nm laser. The resonator is servo loop locked, and therefore automatically scans with the injection seed laser, delivering up to 10 mJ of Fourier transform limited light (0.005 cm^-1) on both "signal" and "idler" frequencies. This is sufficient to saturate v=3-0 vibrational overtone transitions in OH, CH, FH, and NH chromospheres, and, as a result of vibrational Franck Condon shifts, can be used to switch on/off the subsequent breaking of the excited bond by subsequent photolysis with an excimer laser pulse. This apparatus has been used to study far-off-resonance single UV photon dissociation in HOH, HOD, and DOD, as well as vibrationally mediated photodissociation in rotationally state selected v_OH=3 HOH molecules. The method has recently been used to study vibrationally mediated photophysics in quantum-state selected clusters of Ar with HOH and HOD. The current focus is evolving toward clusters with reactive channels energetically open, such as studies of HO/OD + H₂/D₂ reactions from vibrationally mediated photolysis of isotopically labeled H₂-HOH clusters. (Nesbitt)
• State-to-State Inelastic Collision Dynamics in Crossed Supersonic Jets. High sensitivity, direct IR-laser absorption methods have been developed as a powerfully general, quantum-state selective probe of inelastic collision dynamics in crossed supersonic jets. The approach is as follows. The "target" and "collider" molecules are cooled to their lowest (rotational) quantum states in a pair of supersonic jets, crossed to achieve a reasonably well defined center-of-mass collision energy. These species then interact in the single collision regime in the low density (<10¹¹/cm³) region of the jet. The final states populated by these single collisions are then probed by direct absorption of a tunable IR laser propagating perpendicular to the scattering plane. Information on final-state velocity distributions can also be obtained, by high resolution analysis of the Doppler profiles. This method has been used to investigate state-to-state scattering of CH₄, HF, and H₂O with rare gases. By comparison with full, close-coupled quantum scattering calculations, these studies are now providing new tests for inelastic collision dynamics and refinement of potential-energy surfaces. (Nesbitt)
• IR Laser Studies of Ozone Chemical Chain Reaction Kinetics. In the past decade there has been a steadily growing concern about the chemistry of the ozone layer, and in particular the influence of "anthropogenic" sources of chemicals on the atmosphere. One of the dominant chemical reaction cycles responsible for removal of ozone is the so-called HO_x chain cycle, OH + O₃ → HO₂ + O₂ (a), and HO₂ + O₃ → OH + 2O₂ (b), which cycles OH into HO₂ and back, thereby catalytically converting O₃ into O₂. This has lead to considerable concern with regard to proposed high speed air traffic in the upper troposphere and lower stratosphere, which would release considerable amounts of water vapor into what would otherwise be a quite "dry" region of the atmosphere, thus generating OH and HO₂. Kinetic information on the HO_x chain reaction has therefore assumed particular importance in developing reliable atmospheric models.
  
  New methods have recently been developed to investigate the HO_x chain cycle by monitoring the concentrations of the OH radical with time-resolved IR laser absorption in fast flow cells. The process relies on pulsed excimer laser photolysis to produce OH radicals in a flow mixture of O₃ and buffer gases and thereby initiate the chain reaction. By detecting OH in the near IR, this method circumvents problems associated with LIF/resonance-fluorescence detection of OH radical, specifically, the unavoidable photolysis of O₃ by the UV probe source. This alternative IR method permits operation at more than an order-of-magnitude higher ozone concentrations, and has led to real-time detection and kinetic analysis of the HO_x chemical chain reaction under laboratory conditions. These studies indicate that the room temperature rate of the chain propagation step (a) is significantly faster (20 % to 30 %) than the values currently used in the atmospheric models. Construction of temperature-controlled flow cells will permit kinetic investigations of these chain reaction rates at temperatures relevant to the upper troposphere/lower stratosphere. (Nesbitt)



Figure 5. OH absorption profile.

• Femtosecond Wave-Packet Dynamics in Lithium Dimer. The behavior of molecules prepared by ultrafast lasers and with coherent control represents an important new area of active manipulation of materials; for example, the direction of photocurrents in semiconductor quantum wells can be manipulated with coherent light fields. New experiments at JILA involve two-color preparation and probing, which give preliminary evidence that the ionization probability is dependent on internuclear separation. In addition, compositional control and pulse shaping experiments have been used to demonstrate several new effects: a new form of two-level rotational coherence spectroscopy, compositional control of the wave packet state amplitudes, and specific modification of state amplitudes by pulse shaping. (Leone)



Figure 6. Manipulating wave packets.

• Supersonic Slit Discharges: an Intense New Source of Jet Cooled Molecular Ions and Radicals. The vast majority of chemical reactions taking place in the atmosphere, combustion, flames, plasmas, chemical vapor depositions, semiconductor etching, etc., occur via open-shell "radical" species and/or molecular ions. The reactivity of these open-shell radicals is so much higher than the corresponding closed-shell species that they dominate the reaction kinetics, even though typically present in extremely low concentrations. It is this high reactivity that makes them a challenging species to generate in sufficient density to characterize spectroscopically under controlled gas-phase laboratory conditions.
  
  The Division has developed new methods for generating intense sources of radicals and molecular ions, based on striking a pulsed discharge in the stagnation region behind a slit supersonic jet. This has several key advantages over more common discharge sources. First, the species are formed and then supersonically cooled down into the lowest few quantum states. Second, the molecules have their velocities collimated perpendicular to the slit direction, which is both ideal for long path, direct absorption spectroscopy with tunable lasers and for generating "sub-Doppler" absorption profiles that are as much as 10-fold narrower than under non-supersonic discharge conditions. Third, the closed-shell precursor species move supersonically in the discharge for only a few microseconds, which permits reactive species to be formed faster than they can be removed via secondary chemical reactions. These laser absorption studies have verified number densities on the order of 10¹⁴/cm³ for radicals such as OH and CH₃, and of order 10¹²/cm³ for molecular ions such as H₃⁺ and H₃O⁺. (Nesbitt)



Figure 7. The slit jet discharge method yields high densities of radicals and molecular ions. A pulsed negative voltage is applied to two insulated metal jaws which define the slit expansion orifice. When synchronized with the gas, this pulse strikes a smooth discharge with respect to the grounded valve body, causing electrons to flow upstream in the expansion. The net result is a discharge current wholly contained within the 300 m × 4 cm region upstream of the slit and, therefore, transient species cooled effectively to supersonic jet temperatures.

• Slit Jet IR Laser Spectroscopy of Combustion Radicals. One of the most fundamental organic species relevant to fuel combustion processes is the methyl radical, CH₃. However, as a result of both its high reactivity and planar equilibrium structure (i.e., zero dipole moment), this has been an extremely elusive radical species to characterize in the gas phase. The slit jet discharge method developed in the Division now provides access to sufficient densities of CH₃ radicals under jet cooled, sub-Doppler conditions to monitor them via direct absorption in the CH stretching region. As a consequence of the high resolution and jet cooling, the near IR spectra resolve fine and hyperfine splittings due to the spin interaction between the unpaired electron spin on the C atom and the H atoms. This is the first time the so-called Fermi contact interaction has been unambiguously determined for such a fundamental radical species in the gas phase, and was found to be negative due to spin polarization of the CH bond. These results, both in sign and magnitude, are in good agreement with ab initio theoretical calculations. (Nesbitt)
• Probing the Potential Surface Topology of a Hydrogen Bond. Although weak by comparison to normal chemical bonds, hydrogen bonding is responsible for a truly vast array of chemical, physical, and biochemical processes, ranging from the 3-dimensional folding structure of proteins to the short-range order of liquids. Any predictive understanding of such phenomena requires a detailed knowledge of the potential energy surface as a function of intermolecular geometry, i.e., the relative distances and angles of the bonding subunits. Division researchers have been developing spectroscopic methods for indirect but precise probing of the shapes of these potentials, based on high resolution IR laser absorption of hydrogen-bonded clusters in a slit jet supersonic expansion. The method exploits the high sensitivity of the long path length slit geometry to probe low frequency "hydrogen bond" modes as combination bands built on top of high frequency "intramolecular" HX stretching modes. Such work nicely complements the prospects for studies of these modes in the far-IR, but with the considerable advantage of a much wider single-mode laser scanning range in the near IR.
  
  Recently this method has been successfully applied to one of the simplest hydrogen-bonded molecules, HF-HF and its isotopic equivalent DF-DF. As a result of the method's high resolution and sensitivity, combination band excitation into all 4 hydrogen-bond intermolecular modes has been observed for both HF-HF and DF-DF, corresponding to stretching, bending (both symmetric/antisymmetric), and twisting of the hydrogen bond. In conjunction with full 6D quantum calculations, these high resolution spectroscopic results are providing an unprecedentedly rigorous test of current state-of-the-art ab initio and semiempirical potentials for hydrogen bonding. As one specific example, these results indicate a significantly stronger coupling of stretching and bending degrees of freedom in the hydrogen bond than was previously predicted. (Nesbitt)

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