Quantum Physics Division

Technical Highlights

• Bose-Einstein Condensation: A New State of Matter: as early as 1925, Albert Einstein predicted that if a gas of atoms was cooled to extraordinarily low temperatures, the gas would undergo an unusual phase transition in which a large percentage of the atoms in the sample would all take on exactly the same quantum wave function, becoming completely indistinguishable one from another. The required temperature is so low that realization of this so-called Bose-Einstein condensation (BEC) in a gas was delayed until this spring, when JILA scientists observed the transition in a very dilute cloud of rubidium atoms.

  Using both laser cooling techniques and a more advanced procedure called evaporative cooling, the JILA group cooled the gas to temperatures as low as 10 nK. At this point the atoms travel at a speed of about 1 mm/sec, which can be compared with room temperature velocities of hundreds of meters/sec. The atoms are isolated from the room temperature wall of the test chamber by a bowl-shaped magnetic potential. As the condensate is formed, it spatially separates from the remaining noncondensate atoms, collecting at the bottom of the magnetic bowl (see Figure 1). A photograph of the condensate was taken in the momentary illumination of a flash of laser light.

  

  Figure 1. In a conventional magnetic trap (left), the field falls to zero, allowing atoms to leak out. The TOP trap (right) rotates the field, plugging the leak.

  The behavior of the condensate atoms is dominated by the laws of quantum mechanics, even though the cloud is a thousand times larger than the distance over which quantum mechanics typically applies. This rare opportunity to see the physics of the very small expanded out to a more convenient scale may lead to a better understanding of the fundamental rules that circumscribe any effort to practice technology at the nanoscale.

  The recently attained capabilities to generate condensed samples of gas open the door to a whole generation of experiments. Preliminary studies on low-lying phonon-like excitations will develop into measurements on vortices and viscosity. Thermal behavior near the critical temperature and the kinetics of condensate formation are also promising topics for study. The implications of condensate formation for precision metrology must be explored as well, including theoretically-predicted coherent atom beams, dubbed bosers.
• Mechanical Measurements. The Division is addressing the expanding need 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 long life, higher light output and vastly lower heat generation. By frequency-comparing a 633 nm semiconductor diode laser with the HeNe laser standard based on iodine molecular absorption, a very attractive stability results: 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 ten 3.5" micro floppy 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.
• 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. As far as is known, no one has 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.
• Quantum-State-Resolved Sublimation Dynamics of Thin Molecular Films. The dynamics of how molecules collide with, stick to or 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 above the temperature-controlled surface. Such direct absorption methods have been developed to study CO₂ sublimation dynamics from thin CO₂ films with < 1 monolayer/s detection sensitivities. The 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. Between 90 K to 120 K, 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 µm 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.
• Kinetic-Energy-Enhanced Growth of Cobalt Disilicide. There is considerable evidence to suggest that laser-deposited materials can attain better film properties at lower processing temperatures than materials deposited by normal thermal means. A possible reason for this result is that high kinetic energy species are formed in the laser vaporization process. A goal is to study the role of enhanced kinetic energy in epitaxial growth. A new apparatus has been developed to deposit translationally fast atoms on a semiconductor substrate and to study the crystallinity and ordering on the surface as a function of kinetic energy. In the past year, the program has focused on the deposition of kinetic-energy-enhanced neutral cobalt atoms on Si(100).
  
  Experiments were performed with the new apparatus to produce kinetic-energy-enhanced metal species for studies of epitaxy. A laser vaporization source was perfected which can produce 1 to 10 eV Co neutral atoms, and these atoms are impinged on a Si(100) substrate for studies of epitaxy with variable kinetic energy. These moderate energy atoms are produced by vaporization of thermally deposited thin films. Cobalt beams have also been produced by direct ablation of Co metal, which according to the measurements yields a large fraction of ions in the beam and high kinetic energies up to 100 eV. After deposition of 1 eV to 10 eV Co atoms on Si(100), the observed Auger spectrum is similar to that which results only if a substrate with thermally deposited Co is then heated to 500 K. The results suggest that the hyperthermal Co atoms insert into subsurface sites. Work is in progress to measure the crystallinity of the growing film as a function of kinetic energy.
• Direct Writing with STM Tips. Aluminum features have been deposited on silicon surfaces using the electron beam and electric field produced at the tip of a scanning tunneling microscope (STM), in both tunneling and field emission modes. Lines as small as 3 nm in width have been produced on crystal Si (001). Aluminum deposition is accomplished by exposing the tip-sample junction to trimethylaluminum (TMA) gas while tunneling. The electron beam of the STM tip dissociates the TMA molecules adsorbed on the surface, resulting in the production of surface-bound Al and hydrocarbon gas. Independent auger electron spectroscopy studies of the electron-induced deposition of Al from TMA shows nearly pure Al deposits (see Figure 2).

  
  
  Figure 2. Pad deposited on x-Si(001) at -10 V and 8.8 L of trimethylaluminum (TMA).

• 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 new 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 picosecond pulsed laser will be used to prepare and probe optically induced changes in transmission, which will introduce time-contrast mechanisms into the images.
• Atom Guiding in Hollow Fibers. Using the force that light exerts on atoms, Division scientists have been able to guide atoms through hollow glass fibers. Glass fibers guide light, and light guides 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.
  
  A "fire hose" effect occurs when a mechanical actuator is used to deflect the output tip of the fiber: the direction of the emitted atoms is deflected as well. If the tip were held very close to a substrate, the fiber could be used as a type of air-brush to deposit atoms directly onto a nanostructure. Current research directions include moving toward ever smaller diameter guiding fibers. By combining the fiber-guiding techniques with laser-cooling techniques, the group hopes to confine the atoms to the centermost region of the hollow area. Thus while the fiber may have a comparatively large opening, say 2 µm, the atoms could emerge in a much more tightly confined column.
• Electron-Ion Collisions. A major step forward in the collisions of highly-charged ions by electrons was achieved when the first intercombination excitation of a multiply-charged ion was measured on the JILA MEIBEL (merged electron-ion beam energy loss) apparatus at Oak Ridge National Laboratory. The results for the 4s^2 1S → 4s4p ³P electron-impact excitation of Kr⁶⁺ show strong resonance structure. The process serves as a benchmark for theoretical calculations of this class of excitations.

  Quantum Physics Division participation in the use of heavy-ion storage rings was initiated in collaborations both with Heidelberg (TSR) and Stockholm (CRYRING). Measurements at Heidelberg on Cl⁶⁺, a sodium-like ion, yielded excellent examples of clearly resolved indirect ionization processes REDA and EA (resonant excitation double autoionization and excitation autoionization). Measurements were also made of dielectronic recombination to a host of states. The superb energy resolution provides a testing ground for theoretical calculations of these processes, which contribute substantially to the cross sections (and hence rates) for these processes. Measurements at Stockholm concentrated on measuring the effect of ambient electric fields on dielectronic recombination. Strong and measurable effects were observed in the early work at JILA on Mg⁺, and theoretical calculations subsequently agreed reasonably well, although not perfectly, with the experiment. However, no other experiments have been able to show this field effect in an interpretable way, and what work has been done indicates possible strong disagreement with theory. Hence the work at Stockholm on Si¹¹⁺ (Li-like) was undertaken to provide another good test bed for the theoretical calculations of the field effects, which can be very large. Excellent data were obtained in Stockholm and are being analyzed.

  A crossed-beams apparatus at JILA was modified to study the electron-impact dissociative excitation of molecular ions with emphasis on detecting light fragment ions. There have been only one or two experiments done on these processes since some early work at JILA more than two decades ago, despite the fact that data and understanding are badly needed for modeling environments such as plasma processing generators and the edge plasma in Tokamaks. Detection of light fragments is very difficult, since they carry essentially all of the kinetic energy of dissociation and hence come off in many directions and energies. The new approach has been successful, and results have been obtained for e + CD⁺ → e + C + D⁺, for e + CD₃⁺ → e + D⁺ + products and e + CD₃⁺ → e + D₂⁺ + products.
• Mobility of Cluster Ions. Most ions in the upper atmosphere are found to be clustered with water and other molecules, and their transport through the atmosphere is important. Therefore it is surprising that there have been relatively few investigations of the mobilities of cluster ions. The measurement methodology is largely unexplored and each result can be qualitatively new. In this Division's labs, a method has been established to measure the mobilities of cluster ions.
  
  Ions are produced in a mass-selected ion source and injected into a flow tube. Here the ions are converted to clusters by three body reactions with a solvent molecule. An example is the ion NO⁺ with cluster ligand CH₃CN, acetonitrile. One, two or three acetonitrile molecules can be added to the NO⁺ to form cluster ions of varying size. The mobility of the cluster ions in a buffer gas are measured by the arrival times of ion "packets" at a mass spectrometer detector, following a pulsed depletion of a small fraction of the ions at two places in the flow tube. Similar methods have been applied for benzene dimer ions, water cluster ions, ammonia cluster ions, and mixed clusters. In general the size of the cluster ion determines the mobility in He in a predictable fashion. However, for collisions of cluster ions such as NO⁺(CH₃CN)₃ in acetonitrile itself (obtained by adding a small fraction of CH₃CN to the He buffer gas) a remarkably small mobility is observed, smaller than any reported value of an ionic mobility. Part of the reason is the very strong ion-dipole interaction. However, the extremely small value of the mobility also suggests that internal degrees of freedom of the ion and the collision partner play a role in dissipating the energy gained from the electric field. At present, there is no theory to understand the role of internal energy states in ion mobility.
• 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.

  
  
  Figure3. Nesbitt Figure - OH Absorption Profile.

  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 (see Figure 3). The process relies on pulsed excimer laser photolysis to produce OH radicals in a flow mixture of O₃/buffer gases and thereby initiate the chain reaction. By detecting OH in the near IR, this method circumvents problems associated with laser induced fluorescence (LIF)/resonance fluorescence detection of the 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 region.
• Photophysics and Photochemistry in Quantum State Selected Clusters. High resolution tunable OPO's (optical parametric oscillator) are being developed 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 BBO (barium borate) ring resonator pumped by a single mode at 355 nm. The resonator is servo-loop-locked to (and therefore automatically scans with) the injection seed laser, and delivers up to 10 mJ of Fourier transform limited light (0.005 cm^-1) on both "signal" and "idler" frequencies. This is sufficient to nearly saturate v=3-0 vibrational overtone transitions in OH, CH, FH and NH chromophores, and as a result of vibrational Franck Condon shifts, can be used to switch on/off the subsequent photolysis 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.