Time and Frequency Division

Technical Highlights

• Evaluation of the NIST Cesium-Fountain Frequency Standard, NIST-F1. D. Meekhof and S. Jefferts of the Division have now completed four preliminary evaluations of NIST’s newest atomic frequency standard, a cesium-fountain frequency standard. The new standard, NIST-F1, uses laser-cooled atoms that are tossed vertically through the microwave cavity and return under the influence of gravity to a detector below the level of the cavity. Because the atoms move at much lower speed, this standard suffers much smaller systematic frequency shifts than are found in atomic beam standards. The linewidth of the central Ramsey fringe can be as narrow as 1 Hz (see Fig. 1). This is to be compared with a linewidth of approximately 60 Hz for NIST-7.

  

  Figure 1. Central Ramsey fringe for NIST-F1. The central fringe in this pattern has a width just greater than 1 Hz. The narrow linewidth results in a smaller error from the servo system that locks the local oscillator to the center of the fringe. The large number of fringes reflects the narrow range of velocities of the tossed atoms. The low value of the velocity and the narrow velocity range dramatically reduces errors associated with the first-order and second-order Doppler shifts.

  The lowest uncertainty for these evaluations was 2.9 parts in 10¹⁵, about half that of NIST’s atomic beam standard, NIST-7. This result is dominated by measurement noise, which should be reduced through improvement of the signal-to-noise performance of the standard. The uncertainty associated with systematic effects, particularly the collision shift, is estimated to be 1.1 parts in 10¹⁵. It is these effects that are expected to limit the performance of the device.
  
  The agreement in frequency between NIST-7 and NIST-F1 is within the stated uncertainties of the two standards. D. Lee of the Division will continue to operate NIST-7 for at least another six months to provide an overlap of operation with the new standard, thus assuring a higher level of confidence as the Division makes the transition from one standard to the next. (S. Jefferts)
• Decoherence Studies of Motional States of Trapped Ions. Quantum-state-engineering of trapped atomic ions relies on using motional states entangled with the internal states of the ions. Since trapped ions form a charged oscillator, the motion of this oscillator is very susceptible to external fluctuating electric fields. Currently, such fields limit the fidelity of engineered states; this has prompted a study of various forms of motional decoherence.
  
  Certain superpositions of motional states, commonly called "Schrödinger cats," are especially sensitive to the strength and nature of the fluctuating fields. C. Myatt, Q. Turchette, C. Sackett, B. King, D. Kielpinski, W. Itano, C. Monroe and D. Wineland have studied the decoherence of these cat states under different kinds of impressed noise as well as that from ambient fields. These studies indicate the ambient fields are uniform and stochastic with a relatively large bandwidth. Additional studies correlating the magnitude of these fields with the trap electrode dimensions indicate that the source of this noise is not due to thermal electronic noise (e.g., Johnson noise) and is consistent with fluctuating patch fields on the electrode surfaces. Such studies will be used to identify and eliminate this source of decoherence. On a fundamental level, for the first time in any system, the group has shown experimentally that the rate of decoherence scales exponentially with the "size" of the cat state. (D. Wineland and C. Monroe).
• Optical Frequency Standard Based on ¹⁹⁹Hg⁺. In a continuing effort toward development of an optical frequency standard based on mercury ions, B. Young, R. Rafac and J. Bergquist have improved both the laser local oscillator and the ion-trap system, bringing them very close to a full demonstration of a prototype of a new generation of frequency standards. The goal of this project is to lock the frequency of a narrow-linewidth laser to the S-D resonance in ¹⁹⁹Hg⁺ (wavelength of 282 nm and natural linewidth of 2 Hz). The optical output of this standard will be frequency divided (see the following highlight) to the microwave region where comparisons can be made with other microwave frequency standards.
  
  Significant improvement has been made in the spectral purity of the laser oscillator and they have verified that light from the laser can be sent through 100 meters of optical fiber without compromising its frequency stability. In a recent experiment (see Fig. 2), they have measured the beat frequency between the outputs of two independent laser systems to be 0.2 Hz, a factor of four smaller than the world record 0.8 Hz reported last year by this group. To verify that the additive noise introduced by light transmission through a fiber could be stripped away, the radiation sent through the fiber was heterodyned with some of the light introduced into the fiber. In the absence of feedback and with the fiber in a noisy environment, the heterodyne signal revealed that the frequency of the light transmitted through the fiber was broadened to about 20 kHz. However, when the feedback servo was enabled, the beat signal was only a few millihertz wide indicating near complete elimination of the noise introduced by the fiber. Since the frequency instability of the laser is about 200 mHz, the residual contamination by the fiber will not limit the performance of the clock.

  

  Figure 2. Beatnote observed in mixing the outputs of two 563 nm sources. The inset shows the simple experimental arrangement. The data is for a measurement time of 20 s.

  In order to operate the standard with a single ion, the ion must be laser cooled and tightly confined so the amplitude of its motion is less than the wavelength of the light probing the optical transition (the so-called Lamb-Dicke regime). While the group had previously demonstrated Lamb-Dicke confinement of a single ion in a room temperature trap, chemical reactions with the background gas prevented long storage times. They have just recently observed Lamb-Dicke confinement in a linear Paul trap in a cryogenic environment where the storage time has been shown to be at least several days. Since all known systematic shifts for the mercury optical transition are expected to be very small, this new standard should perform better than all previous frequency standards. The frequency-synthesis system described below should allow translation of the performance of this system to a broad (including the microwave) region of the spectrum. (J. Bergquist)
• Femtosecond Lasers for Optical-Frequency Measurements. In collaboration with S. Cundiff, S. Diddams and J. Hall of the Quantum Physics Division, L. Hollberg and K. Vogel have initiated a program to use frequency combs generated by femtosecond lasers to do precision frequency comparisons over a broad range of the spectrum. The comb concept is depicted in Fig. 3. Preliminary results with a new high-repetition-rate laser system show that it is possible to stabilize the pulse repetition rate of the laser to the frequency of a hydrogen maser, the frequency of which is in turn known relative to the cesium primary standard. Frequency combs extending over a range greater than 100 THz have already been generated, and it is now clear that the spectral range can be substantially increased.

  

  Figure 3. Concept for frequency synthesis using a mode-locked femtosecond laser. The repetition rate of the laser pulses is phase locked to a well-known microwave reference (for example, the output of the cesium primary standard), and the frequency spectrum of the output pulse string is a comb of narrow lines separated exactly by the microwave reference frequency. Optical frequencies can be easily measured in terms of the microwave frequency by comparing the unknown optical frequency with one of the high-order lines in the comb.

  One particular objective is to phase coherently connect the output of the cesium primary frequency standard near 9.2 GHz to optical frequency references such as the mercury-ion optical transition at 286 nm and the neutral calcium intercombination line at 657 nm. This will allow the very high stability and reproducibility of the frequency of these optical standards to be compared to the frequencies of standards operating in the microwave region. There are fundamental reasons to believe that the performances of such optical frequency standards will surpass the performances of frequency standards operating at lower frequencies.

  As part of this program, optical fiber is now being installed to connect laboratories of the Time and Frequency Division at the NIST-Boulder site to those of the Quantum Physics Division situated some 2 km away on the University of Colorado campus. This fiber connection is being made as a part of a larger program to network various city and federal agency programs with optical-fiber communication systems. Two strands of the fiber in this network will be dedicated to this measurement program. Since the laboratories engaged in this work are so widely separated, a natural part of this work will be the testing of new concepts for frequency-and-time transfer through the connecting fiber. (L. Hollberg)
• Cold-Calcium Optical Frequency Reference. During this last year C. Oates of the Division and A. Curtis, a graduate student at the University of Colorado have improved the performance and reliability of the NIST calcium frequency standard, which operates on the 657 nm intercombination line. Their efforts have focused on stability and accuracy limiting effects. In particular, they have studied the key effects of residual motion of the cold atoms, optical-geometry effects, and the conversion of laser-frequency instability into instability of the calcium-reference output. The results of this study have allowed them to alter system parameters to improve performance. The inferred frequency stability of this reference is now ~1 × 10^-14 -1/2.

  This system will be used as the reference frequency for a preliminary measurement of the frequency of the mercury-ion optical-frequency standard being developed in the Division by Young, Rafac and Bergquist. The frequency of the calcium reference is known relative to that of the cesium frequency standard through measurements made at PTB in Germany. The NIST measurement will involve mixing of signals (in special nonlinear crystals fabricated at NIST by graduate student J. Mitchell, in a collaboration between L. Hollberg and N. Sanford of EEEL) from the calcium reference, the mercury standard, diode lasers, and a CO overtone laser operating at 3.9 µm. These systems, operating in three separated laboratories, will be connected using fiber-optic links. (C. Oates)

• PARCS Progresses. The Primary Atomic Reference Clock in Space (PARCS), a NASA-funded program to put a laser-cooled cesium atomic clock in space has passed the first major NASA review, and system development has started. This collaborative program involves NIST, the Jet Propulsion Laboratory (JPL), the University of Colorado, the Harvard-Smithsonian Center for Astrophysics and the University of Torino. The Science Concept Review by an external panel was satisfactorily completed in January of 1999 allowing the program to move to the next stage of development where prototypes of the various system components are constructed and tested to demonstrate the feasibility of the concept. The majority of the space hardware will be developed and tested at JPL, but the entire team will collaboratively direct the development work. D. Sullivan of NIST and N. Ashby of the University of Colorado are the principal investigators for the program.

  Work within the Division that contributes to PARCS includes: 1) the development by F. Walls and his collaborators of a space-qualified microwave frequency synthesizer; 2) an experimental study by T. Heavner and D. Meekhof of the laser-power requirements and the atom-trapping parameters; 3) design of the microwave cavity by S. Jefferts; 4) simulations of clock and microwave-cavity performance by H. Robinson and 5) theoretical work on transverse laser-cooling schemes by guest researchers A. Taichenaclev and V. Udin of Novosibirsk State University in Russia. In addition, S. Rolston, W. Phillips and L. Lising of the Atomic Physics Division have been doing experimental work on transverse laser cooling. One significant result is the quantitative verification that the spin-exchange shift, a serious problem for fountain standards on earth, decreases with increasing Ramsey time (see Fig. 4). This provides one of the motivations for operating such clocks in a microgravity environment.

  As part of the PARCS program, co-investigator J. Kitching of NIST recently spent four months in Paris where he collaborated with the team working on ACES, a European effort to put a laser-cooled atomic clock into space. During this visit, he worked with staff from l’Ecole Normale Superieure at the Laboratoire Primaire du Temps et des Frequences on the laser systems required for such space clocks. This visit continues the exchange of information between these U.S. and European projects. (D. Sullivan).

• International Frequency Comparisons Using GPS Carrier Phase. J. Levine and L. Nelson, in collaboration with K. Larson of the University of Colorado, have completed cross-country frequency comparisons using GPS carrier-phase methods and have now initiated comparisons of NIST-7 and ’ PTBs CS-1. These feasibility tests have involved baselines from NIST to USNO and from USNO to Colorado Springs. These test have shown a time stability of 200 ps and a frequency stability at one day of 2.5 × 10^-15. Preliminary comparisons with PTB look promising, but final results will not be available for about one month. This appears to be the most cost-effective way for decreasing the uncertainty in the comparison of primary frequency standards, a step that must be taken as the accuracy of these standards continues to improve. It is becoming increasingly difficult to effect these comparisons using the traditional method of GPS common-view time transfer. (J. Levine).
• Improvements in Time Transfer. Following the development last year of a model of multipath effects on time transfer with pseudo-random phase codes, F. Ascarrunz, M. Weiss and T. Parker have implemented improvements (suggested by the model) in the NIST two-way satellite-time-transfer (TWSTT) and GPS common-view systems that have measurably improved performance. Similar improvements of the TWSTT system have been made at NPL in Teddington, and recent comparisons between NIST and NPL now exhibit a time transfer noise of only 600 ps, the lowest noise yet achieved using two-way time transfer over this particular trans-Atlantic path.
  
  The key changes made (suggested by the model) involve measures that minimize signal reflections within cables in the system. This was achieved by replacing key cables with high-phase-stability cables and by more carefully matching the impedance of all circuits to the system cables. While multipath effects at the antennas remain a concern, it is variations in these effects that give rise to time transfer noise, so such effects can be minimized by maintaining careful control over the geometry of peripheral objects that scatter signals at the antenna locations. (T. Parker)
• Two-Way Time Transfer Link to Australia. T. Parker is collaborating with P. Fisk of the Commonwealth Scientific and Industrial Organization (CSIRO) in Australia in developing a satellite link between NIST and CSIRO to compare time scales using two-way satellite time transfer. This C-band link between these widely separated laboratories provides a fully reciprocal path for comparisons. This means that receive and transmit footprints of the satellite cover both sites and that the phase-delay through the satellite is fixed and stable. In principle, this is an ideal type of link, since it is not subject to variation in delays associated with conversion from one spot beam to another, a difficulty that has been encountered using Intelsat for the two-way time transfer link between Boulder and Europe. The NIST satellite ground station for this link is located at the WWV radio-station site, so the signals must still be linked to the time scale in Boulder. This very short link is accomplished with very high precision using GPS common-view time transfer. The system has only recently been brought into operation, so present results are only preliminary. (T. Parker).
• Time-Scale Improvements. J. Gray, J. Levine and T. Parker have initiated an upgrade program for the NIST time scale. The first improvement is the addition of a low-noise synthesizer to be used for generating an improved real-time output of UTC(NIST). The synthesizer, controlled by the time-scale computer, transforms the output of a selected hydrogen maser to UTC(NIST) with substantially less short-term noise (50 times less) than the previous system. This synthesizer system is now undergoing tests to assure reliability and tracking precision. The changeover to the new synthesizer will be made in the near future.
  
  The next phase of improvement involves the replacement of the time-scale measurement system with a more-reliable, lower-noise system. The current measurement system is so old that replacement parts are hard to find, so long-term reliability is becoming a concern. Furthermore, measurement noise is starting to become a factor in the performance of the time scale. A careful study of upgrade options is the first step in this process, since the desire is not only to improve performance and reliability, but also to assure that the selected system can be readily maintained for at least 20 years. (T. Parker).
• Blue-Light Generation Using Rubidium Vapor and Low-Power Diode Lasers. Using two diode laser beams in the near infrared (780 nm and 795 nm), L. Hollberg and guest researcher A. Zibrov of the Lebedev Institute in Moscow discovered that it was possible to convert the diode laser light into coherent blue light at 420 nm in a rubidium vapor cell. Coherent multi-wave mixing near resonance in the atomic vapor generates this blue light. With a single pass of the laser beams through the vapor cell, the output power in the blue is still fairly low, however the actual conversion efficiency from the infrared to the blue is relatively high compared to that obtainable in nonlinear optical crystals. For example, this system produces 10 µW of blue light for an input of two infrared beams of 10 mW each. This unexpected result could prove useful for generating shorter wavelengths of blue or ultraviolet light using low-power diode-laser sources and resonant atomic or molecular systems. (L. Hollberg)
• Compact Rubidium Raman Oscillator. Following further development of the rubidium Raman oscillator reported last year, several companies have expressed interest in further developing the device for commercial applications. The high stability, small size, low-power requirement and simplicity of this oscillator make it an attractive candidate for meeting requirements for applications in areas such as telecommunications, electronic instrumentation, and position-determination. The oscillator operates on the 6.835 GHz hyperfine transition of Rubidium (see Fig. 5).

  

  Figure 5. Energy-level diagram for the Rubidium Raman Oscillator. The drive laser at 795 nm couples the ground S (F = 2) state to the excited P state, and stimulates Raman transitions to the F = 1 level of the ground state. The beatnote between the drive signal and the Raman signal generates the oscillator output at 6.835 GHz.

  J. Kitching and L. Hollberg of NIST performed this work in collaboration with guest researcher N. Vukicevic from France and R. Wynands and S. Knappe from the University of Bonn. Recent work on this project, supported by the Army Communication Electronics Command, involved the study of four possible configurations, each with unique advantages (and disadvantages). The oscillator stability, now at a level of –1 × 10^-11, is already commercially interesting, but there is potential for further improvement. (L. Hollberg)
• Pulsed Microwave PM and AM Noise Measurement. F. Walls and C. Nelson, along with guest researcher F. Garcia of the Centro Nacional de Metrologia in Mexico, have developed a new approach to the measurement of PM and AM noise in pulsed amplifiers. There has long been a difficulty in characterizing the noise performance of high-power amplifiers used in systems such as radars, because such amplifiers cannot remain on for very long or they will burn up. The new system dramatically improves the resolution, noise floor and time required for making pulsed measurements of noise close to the carrier frequency. A significant aspect of this work is the reduction in measurement time by two orders of magnitude, but the order-of-magnitude improvement in resolution and three order-of-magnitude reduction in the noise floor are also noteworthy. The new system will allow manufacturers to directly evaluate the performance of pulsed amplifiers rather than to be forced to rely on inferring amplifier performance from the overall performance of the system.
  
  The measurement system, based on a two-channel cross-correlation concept, uses special filters in the intermediate-frequency amplifiers to substantially reduce noise in the measurement process. Another important feature is the rapid (few seconds) in-situ calibration of the gain of the phase or amplitude detectors as a function of frequency offset from the carrier. (F. Walls)
• PM and AM Noise Measurement at 100 GHz. With funding provided by the Office of Naval Research, F. Walls has started development of a system for ultra-low noise measurement of PM and AM noise in amplifiers and oscillators at 100 GHz. The objective is to provide the measurement technology needed to support the development of high-speed gallium-arsenide amplifiers and oscillators to be used in digital and signal-processing applications. Such measurement technology is not now available. It is clear that, as signal processing moves to still higher frequencies, there will be a need to develop still higher-frequency noise-measurement systems.
  
  The measurement system uses the two-channel cross-correlation method to reduce the noise contributed by the reference oscillators and measurement system. The reference oscillators, which must have exceptionally low noise involve two 100 GHz oscillators, the phases of which are controlled by signals multiplied from two 10 GHz cooled-sapphire resonators. (F. Walls)
• Study of Dual-Mode Oscillators. In collaboration with E. Ferre-Pikal of the University of Wyoming, D. Tsarapkin of the Moscow Power Institute and J. Vig of the U.S. Army Communication Electronics Command, F. Walls and H. Ascarrunz of the Division have analytically and experimentally studied the phase noise of quartz resonators that oscillate in more than one mode. They found that if the gain element is common in two oscillation modes, that stable oscillation is possible only for a few types of nonlinearity and that the phase noise is at best 3 dB worse than that of a single oscillator. However, if the gain elements for the two modes of oscillation are separate, they found that the noise could be unchanged from that found in single-mode operation. This latter result could prove to be important, since the second mode of oscillation can be used as an environmental (for example, temperature) sensor that can serve to correct for the environmental sensitivity of the frequency of the primary oscillation mode. For temperature control, an important environmental sensitivity for quartz resonators, the key advantage is that the sensor and primary oscillator are identical, so there are no problems of thermal lag and thermal gradient between the two. Oscillators using such temperature control could easily surpass the frequency stability of traditional quartz oscillators, particularly for periods from a few hours to about 1 day, a region of performance critical to the operation of nodes in cellular-communication systems. This work merely establishes the possibility for improving quartz-oscillator performance using dual-mode devices. Further work is needed to demonstrate that the concept really works. (F. Walls)
• Expansion of the Automated Computer Time Service. M. Lombardi, A. Novick, J. Wessels and J. Levine have doubled the capacity of the Automated Computer Time Service (ACTS), which delivers time setting signals to computer systems over the telephone network. The expansion, from 12 to 24 lines, was needed to meet increasing demands placed upon the system by new commercial systems that are using the service for setting clocks used in stock trading. There is an increasing need to assure the accuracy of time/date stamps that are used to identify every transaction. While demand for such service had been increasing steadily, a sudden jump in activity was stimulated by the adoption by the National Association of Stock Dealers (NASD) of an SEC-approved rule requiring traceability of time/date stamps to NIST. As implementation of this rule proceeded, the Division noted a sharp rise in ACTS calls, particularly during the hour or two just before the Eastern-time opening of the market. (M. Lombardi)
• Growth of the Network Time Service. Use of the Division’s Internet time service has been growing at a compounded rate of 7 % per month. As of October 1999, approximately 11,000,000 time/date codes were being delivered each day. In addition, this service now provides signals for a time service at http://nist.time.gov, a web site jointly provided by NIST and USNO. This web site received a very high volume of calls during its first weeks of introduction in October of 1999. In the near future, J. Levine and J. Wessels, who operate the Network Time Service, plan to install additional servers for the New York City financial district and for the area of San Jose and silicon valley. (J. Levine)
• Completion of the Upgrade of WWVB. The multi-year program to upgrade and increase the output power of WWVB was completed in December 1999. The station has been operating at a broadcast power of 50 kW since October 1999, but complete auto tuning of the transmitters to the antenna systems was implemented only recently. The station staff; M. Deutch, G. Nelson, D. Sutton and W. Yates did a major portion of this upgrade, but Navy staff and Navy contractors did some designs and fabrication of components. The transmitters were obtained several years ago at no cost (excess equipment) from the Navy, which retains the best U.S. expertise in low-frequency (LF) broadcast systems.

  

  Figure 6. Arrangement of WWVB broadcast systems. Two transmitters located in the transmitter building deliver in-phase signals to separate antenna systems through the impedance matching networks located in the buildings at the base of each antenna downlead. The wavelength of the LF (60 kHz) broadcasts is much greater than the separation of the two antennas, so the radiated signal appears to emanate from a single antenna.

  WWVB broadcasts are now delivered at full power using two separate in-phase transmitter-antenna systems (see Fig. 6). Since antenna impedance can vary substantially under windy conditions, the system design involves servo tuning of the antenna-impedance-matching networks to the impedance of the coaxial transmission lines. A third backup transmitter can be switched into service should either of the primary transmitters fail. The final phase of work that brought this upgrade to completion involved rebuilding the networks that match the antennas to the transmitters, installing safeguards to protect staff from high voltages, and implementing auto-tuning of the antenna impedance matching network. A new generator capable of assuring continuity of operation at full power has been delivered to the site, but is not yet installed. Until that installation is complete, the available backup power is not sufficient to allowed operation at full power. (W. Hanson)
• Driver/Modulators for WWV and WWVH. J. Lowe of the Division has designed and constructed new driver/modulator circuits for operation of the short-wave broadcasts from WWV and WWVH. The present systems are quite old and have become very difficult to maintain, so there has been a clear need to replace them. An earlier estimate for commercial replacement with custom-designed circuits was greater than $150,000, a prohibitive figure considering the burden of the current upgrade costs for WWVB. The Division-constructed devices will cost less than $30,000. One of the new units has been in continuous operation for five months supporting the 15 and 20 MHz broadcasts from WWV. Tests have demonstrated a considerable performance improvement. Not only is signal distortion substantially lower, but energy radiated outside of the allocated bandwidths is also reduced. (W. Hanson).
• Egyptian Satellite Time Service. Staff of the Division are working in support of a program of the National Institute of Standards (NIS) in Cairo, Egypt to implement a time-and-frequency dissemination service on the NileSat satellite. S. Samuel and A. Youssef of are leading the NIS effort. W. Hanson, M. Lombardi, A. Novick, and J. Lowe of NIST are supporting this program under a general agreement between the United States and Egypt.
  
  The system will be similar in design to the existing NIST service from the NOAA-operated GOES satellites. However, the larger bandwidth allocated on the NileSat satellite will allow for greater flexibility and higher performance. This bandwidth results in a greater bit rate allowing for a higher reference frequency. Other improvements include orbital elements transmitted with the timing signals and the potential for removing Doppler effects due to satellite movement. (W. Hanson)
• Sympathetic Cooling of Positrons. W. Itano, J. Bollinger, and guest researcher B. Jelenkovic have developed a system for cooling positrons and in preliminary experiments have trapped and cooled several thousand of them. Because they lack anything like atomic energy levels, positrons cannot be cooled directly using laser-cooling methods, but they can be cooled through their Coulomb coupling to an atomic species that is itself laser cooled. Such cooling is called sympathetic cooling. This work extends previous sympathetic laser-cooling studies between different ion species to the wide mass difference between Be⁺ ions and positrons. With sufficient coupling, cold dense positron plasmas of 10¹⁰ per cubic centimeter with temperatures less than 1 kelvin should be possible. These plasmas could be useful for the production of antihydrogen and cold positron beams.
  
  The positrons are detected by excitation of their cyclotron resonance and by the centrifugal separation of the positrons to the center of the plasma due to the plasma rotation. This separation implies that the temperature of the positrons along the magnetic field axis of the trap is on the order of a few kelvin, while their density is nearly equal to that of the Be⁺ ions, approximately 5 × 10⁹ per cubic centimeter. (J. Bollinger)
• New High-Frequency Laser-Magnetic-Resonance Spectrometer. M. Allen and K. Evenson are nearing completion of the development of a new laser-magnetic-resonance (LMR) spectrometer that will operate at higher frequency and have higher sensitivity. Improved performance will be derived from use of a more powerful far-infrared laser and a higher sensitivity detector. The new laser, developed earlier this year, uses a zigzag pumping scheme rather than the traditional transverse pumping. It is 20 times more powerful at a wavelength of 119 µm than the previous transversely pumped laser. The higher efficiency of this laser allows it to lase out to 25 µm, doubling the upper operating frequency and spectral range of the spectrometer. One of the molecular observations that should be accessible at these higher frequencies is the q-branch of the magnetic dipole spectrum of ClO. This is a very important molecule in the chemistry of the upper atmosphere, and its spectrum at 31 µm could prove to be very useful for measuring ClO concentrations. LMR spectroscopy, which uses intra-cavity absorption, is one of the most sensitive spectroscopic techniques known, and is thus well suited to study of samples of low concentration or transient species. (K. Evenson)
• FIR Spectroscopy of Bending Transitions in Carbon-Chain Molecules. M. Allen and K. Evenson, in collaboration with D. Gillett and J. Brown of Oxford University have recently been using far-infrared (FIR) laser-magnetic-resonance (LMR) spectroscopy to make observations of bending vibrational spectra of a number of important carbon-chain molecules. They have just reported on the first direct observation of the ν₂ bending fundamental of the CCN radical and have observed and fit the spectra for the ν₅ bending fundamental of the HCCN radical. In addition, they have conducted survey scans in the region of the ν₅ bending fundamental of the DCCN radical and have now made assignments on nearly all of the predicted resonances. The excellent sensitivity of FIR LMR spectroscopy allows for the detection of transitions that are roughly 100 times less intense than a pure rotational or electronic transition and the method is especially useful where the number density of such highly reactive species is so low.
  
  The accurate determination of such bending spectra can provide insight into the structure of these simple, substituted carbenes and test theoretical models. Finally, the group has initiated studies on HC₃, HC₄, HC₅, … HC₁₁, a series of important interstellar molecules. The goal is to extend measurements of the low-lying bending fundamentals in this series out as far as the sensitivity of the spectrometer will allow. (K. Evenson)