Optical Technology Division

Technical Highlights

• Comparison of NIST and NPL Infrared Scales.. The infrared (2 µm to 5.4 µm) spectral responsivity scales of NIST and the UK National Physical Laboratory (NPL) were compared through the mutual calibration of an InSb transfer standard radiometer. The NIST and NPL scales are established through different methodologies but in both cases, they are linked to their respective primary-standard cryogenic radiometers.

  This was the first international comparison of infrared spectral responsivity scales and the excellent agreement found demonstrates the advances made in this spectral region in recent years. The two scales were shown to be equivalent to better than 1 %, which is within the combined uncertainty of the comparison and the individual scale uncertainties. The results will lead to improved international measurement consistency and traceability for a wide range of radiometric applications. (G.P. Eppeldauer)

• The Second Photometry Short Course. The Division hosted the second NIST Photometry Short Course, held September 14 to 17, 1999. There were 18 participants from industry and academia, who were photometry engineers, technicians, and laboratory managers.
  
  
  
  
  Figure 1. Students and Staff of the Second Photometry Short Course.

  The course consisted of eleven lectures, three laboratory sessions, and a half-day laboratory tour. It was extended to three and a half days this year, at the suggestion of last year’s participants, allowing more time for each lecture and laboratory session.

  Dr. Sauter from PTB, Germany, was invited again this year to give three lectures and one laboratory session. Other lectures were given by Division staff members. The subjects of the lectures included basic concepts in photometry, measurements of luminous intensity, luminous flux, illuminance, luminance, goniophotometry, colorimetry, optical properties of materials, quality systems, and uncertainty evaluation. The laboratory sessions were carried out using our 4 m photometry bench, the 2.5 m integrating sphere, and color-measurement equipment. The participants, divided into three groups, had hands-on experiences with our facilities.

  We expect continuing demand for this successful short course, and plan the third one for next year. (Y. Ohno)

• Radiation Thermometry Short Course. In 1997, the Division initiated a four-day, hands-on short course designed to present the fundamental concepts in radiation thermometry through a combination of lectures and experiments. The experiments are held in a facility dedicated to the short course and are focused on the underlying radiometric principles, the characterization of a radiation thermometer, and the measurement of the emittance of several materials. The equipment is lent by leading U.S. manufacturers. Since 1997, up to 16 participants have attended each year and have come from industry, government, and university laboratories. (B.C. Johnson)



• Magneto-Raman Spectroscopy. The interplay of charge, orbital, and spin excitations in manganese oxides was probed using Raman and magneto-Raman spectroscopy. In the half-filled doped layered manganites, La_1/2Sr_3/2MnO₄ and LaSr₂Mn₂O₇, and in Bi_0.26Ca_0.74MnO₃, we observed Raman signatures that suggest the formation of charge, orbital, and spin stripes. These signatures persist in phases that manifest the phenomenon of colossal magnetoresistance. The dynamics of these phases, as revealed by the temperature and magnetic field dependence of their Raman response, demonstrates the importance of electronic phase-segregation in understanding the colossal magnetoresistance phenomenon in the manganites.

  On a more applied front, a major obstacle in integrating non-volatile ferroelectric memories with silicon-based technologies is the loss of electric polarization upon hydrogen anneal of Pb(Zr,Ti)O₃ thin-films. Using Raman spectroscopy, we have shown two effects that could lead to this loss. First is our observation of an (OH)^- vibrational peak that indicates the incorporation of hydrogen at interstitial tetragonal sites. This defect prevents the Ti ion from undergoing polarization switching. Second is the presence of a peak associated with Ti=O₂ vibration. This peak manifests an unusual temperature dependence that we associated to localized oxygen-vacancy defects. The presence of oxygen vacancies can lead to the formation of antiphase ferroelectric domains that could result in the loss of the remnant polarization. (D.B. Romero and B.E. Nachumi)

• Electrical Substitution Bolometer. An electrical substitution bolometer (ESB) has been developed to serve as a portable transfer-standard detector over the full wavelength range of 200 nm to 20 µm. The ESB is designed to transfer the optical power scale from the HACR to the Spectral Comparator Facility in the spectral region where silicon photodiode trap-detectors cannot be used and where the noise of pyroelectric detectors limits their utility.

  The ESB, similar to other liquid-helium-cooled bolometers, uses a gold-black absorber film and silicon temperature sensor on a sapphire substrate. The novel aspect of the ESB is that the gold-black optical absorber film is also used as an electrical heater for chopper-synchronized electrical substitution of optical power at 15 Hz. This operating mode is similar to that used in the commercially-available electrically-calibrated pyroelectric radiometer (ECPR). However, the advantage of the ESB over the ECPR is that its noise floor is nearly 1000 times better for a similarly sized device. The noise floor of the 8 mm diameter active-area ESB approaches 10 pW/Hz^1/2, corresponding to a detectivity D* of the order 10¹¹ cm Hz^1/2/W.

  The advantage of the ESB over traditional helium-cooled bolometers is that, by virtue of the electrical substitution, it is inherently linear over a much wider dynamic range. The ESB is linear from the noise floor to 1 mW (the power range of the HACR), whereas older helium-cooled bolometers are non-linear above 10 µW.

  Because of its low noise, linearity over a wide dynamic range, and spectrally and spatially flat response, the ESB may find additional applications. These include use as a reference detector in the infrared region for the SIRCUS facility discussed under Future Directions. (J.P. Rice)

• Ultraviolet Detectors. Ultraviolet irradiation alters the chemical bonds in many materials, including photodetectors meant to measure UV and visible irradiation. The aggressiveness of the radiation makes it difficult to employ detectors that are stable during prolonged use. This poses a problem, for example, for the metrology of UV radiation at 193 nm, which is a rapidly growing need in such applications as semiconductor lithography, micromachining, and medicine.

  We have compared the stability of different types of photodetectors subject to irradiation from an ArF excimer laser operating at 193 nm. A purge housing contained a Xe arc lamp, and a 0.25 m monochromator that is capable of measuring the spectral responsivity and the reflectivity of the detectors from 180 nm to 500 nm. The simultaneous measurement of the spectral responsivity and the reflectivity yields the internal quantum efficiency of the detectors, which is an important parameter that is used to characterize their behavior. Measurements were made for a large range of total accumulated UV doses.

  As shown in Fig. 2, different detector types had markedly different resistance to damage. Their stability was a function of both the total accumulated UV dose and the peak powers of the laser pulses. Such tests assist in the design of instruments needed in the various applications.

  Similarly, the transmission of a polymer-based photochromic film was also measured as a function of the dose at 193 nm. The transmission of photochromic film in the visible region changes as a function of the dose that is delivered to the film in the ultraviolet. The material has potential for application in UV metrology as a consumable that is not subject to the long-term degradation of traditional photodetectors. (R. Gupta and P.-S. Shaw)

• Detector-Based Spectral Radiance. The spectral radiance scale is based on the absolute radiance-temperature determination of a gold-freezing-point blackbody, and it is disseminated using tungsten-strip lamps. However, the current procedures do not make the best use of the HACR and the reduction in uncertainties that can be gained from a detector-based spectral radiance scale.

  In a recent experiment, the radiance temperature of a high-temperature blackbody (HTBB) was determined using filtered radiometers calibrated for spectral power responsivity, as derived from the HACR. The HTBB was used to determine the spectral radiance of a tungsten-strip lamp, using a double monochromator. Its spectral radiance was also determined using a scale derived from the gold-freezing-point blackbody, and the two independent results were compared.

  

  Figure 3. Comparison of spectral radiances determined from filtered radiometers (FR) and FASCAL. The error bars correspond to an uncertainty in radiance temperature of 0.5 K. The lines indicate the expanded uncertainty (k=2) of the FASCAL measurements.

  The data are shown in Fig. 3. In the wavelength region from 260 nm to 1050 nm, the results did not differ by more than 0.5 %, which was within the combined uncertainties of the two methods. This study lays the groundwork for a future detector-based spectral radiance scale. The gains in accuracy will become even greater with improvement in the filter radiometer characterizations. (H.W. Yoon)

• Correlated Photon Metrology. Absolute metrology techniques based on correlated pairs of photons have advanced to the point where other laboratories have chosen to pursue them. This step forward is made possible by a more thorough understanding of the method, as well as technological advances that greatly simplify the production of correlated photons pairs. Having their own absolute standard may be advantageous to end-users if the calibration chain can be shortened in comparison to one relying on a high-accuracy primary standard in a distant laboratory.

  A EUROMET working group, consisting of members of at least five international laboratories, has been formed to coordinate metrological research in this field. NIST is collaborating with this group with the goal of transferring our techniques so that they may be used by others to full advantage. (A.L. Migdall)

• AC/DC Technique for Luminous Flux Measurements. The Division is collaborating with the BIPM (International Bureau of Weights and Measures, Sèvres, France) to improve the means by which they maintain the unit of luminous flux, the lumen. The lumen is perhaps the most important unit of measurement in the international lighting industry since it is used to describe the total useful output of a lamp.
  
  

  Division staff visited the BIPM in the summer of 1998 to initiate the work. We are investigating the Absolute Integrating Sphere Method, developed by NIST, as a better means to measure and maintain the lumen at BIPM. Currently, they rely on traditional methods employing standard lamps. The sphere method has been used to realize the lumen at NIST since 1995. However, in the early stage of the project at BIPM, a large discrepancy was observed between the BIPM-maintained unit and that of the new apparatus. To investigate the causes of the discrepancy, a new technique employing a chopper for the external source (the AC/DC Technique) has been developed. This technique allows for continuously monitoring the integrating sphere responsivity while an internal lamp is turned on or off. It allows detecting and correcting for effects that were not considered previously. The results revealed a larger-than-expected effect of the heat from the internal lamp on the BIPM sphere coating, which explained the discrepancy.

  Figure 4. The NIST integrating-sphere facility for realizing the lumen.

  Preliminary realization of the lumen at BIPM showed an agreement with the NIST lumen to within a few tenths of a percent. Further work is in progress using a recoated BIPM integrating sphere. (Y. Ohno)

LED Photometric Standards. LEDs are rapidly finding new applications, driven by the recent development of high-intensity LEDs in a wide range of colors. Low-maintenance traffic signals are a widely noted example; others include roadway signs (barricade lights), airport lighting, and color displays. "White" LEDs (sometimes consisting of a phosphor stimulated by a blue or UV device) are being developed to replace traditional lamps in even more applications. As LED applications develop, accurate specifications of LED characteristics are increasingly important. However, large discrepancies in the photometric measurements of LEDs have been reported among their manufacturers and users. LEDs are very different from traditional lamps in terms of physical size, flux level, spectrum, and spatial intensity distribution, and traditional photometric instrumentation is often not designed or calibrated for these differences. In order to improve the situation, the Division has begun a project to develop standard LEDs for luminous intensity, luminous flux, and color, to be supplied to industry. We are also investigating improved measurement methods of these quantities, in collaboration with three technical committees of the CIE. NIST standard LEDs and calibration services are expected to be established within three years. (Y. Ohno)

Figure 5. High-intensity LEDs are now available in a wide range of colors.

• Colorimetry of Display Devices. There is a growing need among high-end users of computer and video displays to calibrate their devices for consistent color between work sites, suppliers, and customers. In response, there is a proliferation of measurement devices being sold for this purpose. Many of these colorimeters consist of three broadband filtered detectors that take advantage of the fact that CRTs and other self-luminous displays emit spectra that are linear combinations of specific phosphors.

  We have developed a new calibration facility to meet the special needs of this community. A key component of the facility, a reference spectroradiometer, has been developed, and its uncertainty for display measurements has been estimated using a series of computer simulations. The simulations predict that the NIST reference spectroradiometer, corrected for wavelength error and variable band pass, can measure any color of a CRT or liquid crystal display (LCD) with an expanded uncertainty (k=2) of 0.002 in chromaticity (x, y) and 2 % in luminance (Y).

  In addition, a new matrix correction technique (the Four-Color Method) has been developed as a means to transfer the calibration from the reference instrument to a test instrument, including both spectroradiometers and few-channel, broadband colorimeters. Using the Four-Color Method, the errors for any color of one type of display are corrected to within 0.001 in (x, y) (or 1 Δ E*_{a b}) with respect to the reference instrument.

  Further work is in progress to reduce the measurement uncertainty and to experimentally verify the uncertainty. A NIST calibration service for display color-measuring instruments will be available in 2000. (Y. Ohno and S.W. Brown)

• Index of Refraction Modeling. The Division is continuing its optical absorption modeling by examining the index of refraction in semiconductors and insulators. Quantities of interest include the value of the index n, its dispersion dn/dλ, and the "zero-dispersion λ," where one has d²n/dλ²=0. The latter is important for pulse transmission.

  The index of refraction rises and falls at the short-λ (band edge) and long-λ (optical phonon) ends of the transparent region. Dispersion at short λ, because of inter-band transitions, is correctly modeled only when many-body electron-hole interactions are treated. Dispersion at long λ can be treated accurately by first-principles calculations of a crystal lattice’s dynamical matrix and Born effective charges. All of the above effects are treated in a first-principles-based approach using state-of-the-art electronic structure calculations. The modeling can also be used in conjunction with laboratory measurements in order to validate or supplement them. Sample results are shown in Fig. 6. (E.L. Shirley)

• Reference Goniophotometer and Gloss Standards. Specular gloss is the perception by an observer of a mirror-like appearance of a surface. While perceptions cannot be measured, the specific reflectance characteristics of a surface can be described and quantified. They are used by industry to represent gloss in models found appropriate by consensus. Gloss is a commercially important attribute of many materials, such as paints, papers, plastics, and textiles, and it is affected by the production, storage, and use of these materials. For many products, the specular gloss appearance attribute is key to their value and acceptance.

  The Division has recently completed initial development of a new reference goniophotometer. It was developed for the characterization of specular gloss at the 20°, 60°, and 85° geometries that are required by important ASTM and ISO standards. These standards describe the measurement procedure that best correlates with visual ranking of specular gloss for nonmetallic samples. In addition, this instrument is capable of performing bidirectional luminous reflectance and transmittance measurements at angles from 0° to 85° for illumination and viewing, in compliance with ASTM standards. A new specular gloss primary standard with several advantages over previous standards has been developed and characterized. This new standard and the reference goniophotometer provide a resource for specular gloss measurements of the highest possible accuracy. (M.E. Nadal)

• Ultraviolet Optical Properties of Materials. For various industrial applications, such as semiconductor photolithography and UV laser development, accurate measurement of the optical characteristics of various transmissive materials is essential. The data often are required not only at specific laser wavelengths but also in the neighborhood of those wavelengths in order to design optical systems with a minimum of aberrations and pulse dispersion.

  We have made preliminary measurements towards a complete characterization of the optical characteristics of calcium fluoride in the UV from 125 nm to 300 nm. This includes the neighborhoods of 157 nm and 193 nm, which are of particular interest for projection optics in semiconductor lithography. Broadband radiation from SURF is filtered by a 2 m normal-incidence monochromator. The transmittance and the reflectance of samples of different thickness are measured. Additionally, measurements are made to distinguish surface scatter and surface absorption from bulk scattering and bulk absorption.

  In a collaborative effort with researchers in the Atomic Physics Division, we have also made high-accuracy measurements of the index of refraction (standard uncertainty of 1 x 10^-5), dispersion, and their temperature dependence of various grades of calcium fluoride near 157 nm. These measurements are in good agreement with our reflectance measurements. (P.-S. Shaw and R. Gupta)

NIST Advanced Radiometer. The Division led the development of an instrument, the Scripps-NISTAR (NIST Advanced Radiometer), that is scheduled to fly aboard NASA’s Triana mission. The Triana spacecraft is planned to be launched from the Space Shuttle to an orbit about L1 (the Lagrange libration, or neutral gravity point between the Earth and the Sun), about 1.5 gigameters from Earth. From there, an imaging camera and NISTAR will have a continuous, nearly full disk, sunlit view of the Earth. The Division worked with Ball Aerospace and Technology Corporation to develop the NISTAR instrument to meet the science requirements of the Scripps Institution of Oceanography. We will assist with the instrument integration aboard the Triana spacecraft at NASA’s Goddard Space Flight Center. NISTAR includes one silicon photodiode and three active-cavity electrical-substitution radiometers for absolute irradiance measurements. Besides broadband measurements of the total Earth irradiance, filtered channels on NISTAR allow separation of reflected-solar from earth-emitted infrared radiation. Our recently developed techniques for improving the signal-to-noise ratio in active-cavity radiometers were the key to meeting the performance requirements. (J.P. Rice and S.R. Lorentz)

Figure 7. Cutaway view of NISTAR.

• Portable Transfer-Standard Radiometer. The first chamber-deployment test of the NIST Thermal-infrared Transfer Radiometer (TXR) was successfully completed in July 1999 at Los Alamos National Laboratory (LANL). This portable radiometer was designed, fabricated, and calibrated by the Division.

  
  
  Figure 8. TXR optical layout.

  The purpose of the TXR is to transfer the scale of infrared radiance from NIST to working standard sources used by the aerospace industry, particularly those involved with EOS projects. These working standards are usually blackbody sources. They are used in various space-simulating vacuum chambers located throughout the aerospace industry to calibrate space-flight instruments used for Earth remote sensing.

  In the deployment test at LANL, the TXR was mounted in a large vacuum chamber in a location where a space-flight instrument had previously been calibrated. The TXR viewed the same blackbody source that the space-flight instrument had viewed, through the same set of mirrors. This in situ capability allows the TXR to accurately determine the radiance levels that are used to calibrate a space-flight instrument.

  The entire TXR instrument was in vacuum and cooled to near liquid-nitrogen temperatures during the eight-day LANL test. No leaks or other TXR equipment failures occurred, and the TXR was able to acquire useful radiometric data. The TXR has been returned to NIST for calibration maintenance and preparation for future deployments at EOS calibration facilities. (J.P. Rice)

• EOS/NIST Radiometric Comparison. Division personnel have been collaborating with the radiometric calibration laboratory at the NASA Ames Research Center in matters concerning the Earth Observing System. The Ames facility is responsible for the flight operations, data analysis, and radiometric calibration of various optical sensors that are deployed on high-altitude ER-2 aircraft. These sensors are used to validate measurements made from satellites.

  Following consultations in 1997 and 1998, a measurement comparison was held in late August and early September 1999. Participating laboratories included NIST, Ames, NASA Goddard Space Flight Center (GSFC), and the University of Arizona’s (UA) Optical Science Center. Its goals were to verify the spectral radiance of the Ames laboratory standard (a diffuse reflectance standard, illuminated by a standard lamp), the Ames integrating sphere source (which is used to calibrate aircraft instruments before flight), and other instruments used by their respective laboratories. The key challenge in this comparison was the operational orientation of the Ames integrating sphere source, which is "uplooking" so that flight sensors can be calibrated after they are installed on the aircraft.

  The Division brought a new portable integrating sphere source, the EOS Visible Transfer Radiometer (VXR), and the Short Wave Infrared Transfer Radiometer (SWIXR), all of which had been developed for NASA projects. We also provided a flat mirror that was used to direct the flux from the uplooking sphere into the horizontal plane. The S and P components of its reflectance were determined prior to the Ames visit.

  Analysis of the preliminary results indicates the Ames standard of spectral radiance is about 2 % too low in the visible and near infrared. The NIST, GSFC, and UA radiometers agreed to within ± 3.5 % for wavelengths between 430 nm and 2300 nm. The Ames radiometer, which is used routinely to transfer the spectral radiance of the Ames standard to the uplooking sphere source, agreed with the NIST portable sphere source to within 2 % in the visible and near infrared, but only to within 5 % in the short-wave infrared (1000 nm to 2400 nm). This (preliminary) discrepancy is a measure of the accuracy of the Ames methods. The final analysis will include a thorough polarization study and careful consideration of the relative spectral responsivities of each instrument. (S.W. Brown and B.C. Johnson)

• Polarized Light Scattering from Dielectric Spheres. The polarization of light scattered by a material can help to identify the cause of that scatter. For a single interface, particles above the surface, defects below the surface, and the roughness of the interface each have unique polarization signatures. The application of polarized light scattering measurements is also useful for characterizing the size of deposited particles.

  We studied a model system of polystyrene-latex (PSL) spheres deposited on silicon wafers. In Fig. 9, the polarization of light scattered by three different sizes of spheres is compared to the predictions of a theoretical model, using the discrete-dipole approximation.

  The semiconductor industry uses PSL spheres to calibrate their wafer surface-inspection systems. An accurate determination of their size is critical for benchmarking different inspection instruments. While the current method for determining the diameters of these calibration standards uses the aerodynamic properties of the spheres suspended in air, the optical method provides a more absolute determination of the size and allows the measurement to be performed in situ on the wafer.

  These measurements and models help the semiconductor industry detect and identify particulate contamination on silicon wafers. While PSL spheres are rarely found to be a contaminant on a semiconductor fabrication line, the results of the experiment-theory comparison indicate that this optical scattering method could be used to estimate the sizes of actual nonspherical contaminants. By understanding the light scattered by particulates on surfaces, inspection tool manufacturers can increase the sensitivity of their production line inspection tools to these defects. These improvements will allow smaller features to be created reliably on semiconductor wafers. (T.A. Germer)

• Solar Cell Electron Transfer Dynamics Revealed. Inexpensive photovoltaic devices employing organometallic dyes adsorbed on nanoparticle substrates of TiO₂ are being explored as alternatives to silicon-based solar cells. These new solar cells attain photon-conversion efficiencies close to their silicon counterparts and could lead to wider, cost-effective utilization of solar energy. Determining the detailed mechanisms and underlying materials properties of such devices is key to understanding their function and improving solar collection and current generating efficiencies.

  Division researchers were the first to apply time-resolved infrared spectroscopy to unambiguously reveal that electron injection from excited electronic states of the dye molecules [e.g., Ru((bpy)₂dicarboxybipyridine)NCS₂] to the TiO₂ semiconductor occurs in <10^-14 second. Groups worldwide have accepted our technique and use our findings in related studies. We also examined detailed back-electron-transfer and electrolyte-quenching dynamics by applying nanosecond ultraviolet and visible transient-absorption spectroscopy in working cells. Our findings indicate that subtle changes in dye-molecular structure affect the electron injection yields and overall recombination rates. An exciting discovery is that substituting SnO₂ for TiO₂ sufficiently changes the acceptor levels and electronic coupling efficiencies to produce cells with adsorbed photon-to-current efficiencies approaching 40 %, constant across the visible spectrum.

  These findings suggest that minor modifications to the adsorbed dye and substrate properties could lead to cells with efficiencies approaching theoretical conversion limits. (T.A. Heimer and E.J. Heilweil)

• Kinetics of Biomimetic (Biosensor) Membrane Formation. Vibrationally resonant sum-frequency generation (VR-SFG) measurements were performed with a novel, broadband system developed at NIST [Opt. Lett. 23, 1594 (1998)] that allows rapid spectral acquisition. VR-SFG involves the nonlinear mixing of an IR photon, resonant with vibrations in the sample, with a visible photon to produce a new photon at the sum frequency. It is uniquely interface-specific, since it is symmetry forbidden in centrosymmetric media such as liquids or many solids (e.g., glass, silicon).

  We used this approach to study the formation kinetics of hybrid bilayer membranes (HBMs). Developed in the NIST Biotechnology Division, HBM’s are models for biological membranes, and are used for biosensors. They consist of a first organic monolayer chemically bound to a solid surface such as gold or glass, to which a second phospholipid monolayer is attached from solution.

  Figure 10 shows VR-SFG spectra recorded during the formation of an HBM, as a layer of DPPC molecules (from vesicles in aqueous buffer solution) assembles in situ on top of the initial layer of octadecanethiol (d-ODT) chemically bound to a gold film. The three strong features at 2875 cm-1, 2935 cm-1, and 2965 cm-1 can be attributed to the terminal CH3 groups of the DPPC acyl tails, oriented with the H’s facing the d-ODT substrate. The absence of strong CH2 features at 2850 cm-1 and 2920 cm-1 indicate that the acyl tails have few gauche defects. The uniform increase of the CH3 features indicates that the DPPC monolayer develops via the growth of ordered islands. Quantitative analysis of the time evolution of the features indicates that the island growth can be described by Langmuir adsorption kinetics, suggesting that vesicles physisorbed on the islands do not participate in the adsorption processes. The newly developed capability of in situ VR-SFG should lead to a better understanding of cell membrane dynamics, and to improved designs for HBM based sensors. (K.A. Briggman and J.C. Stephenson; with L.J. Richter, T.P. Petralli-Mallow, and A.L. Plant of the Chemical Science and Technology Laboratory).

  Figure 10. VR-SFG spectra collected at (a) 2 min, (b) 4 min, (c) 6 min, (d) 8 min, and (e) 34 min after the introduction of vesicles to the buffer solution.




• Studies of the Peptide Bond. Experimental model studies of the peptide bond have been undertaken as part of an effort to provide fundamental data for calibrating the accuracy and reliability of molecular mechanics and ab initio theory for modeling peptides and peptide mimetics. In one study we investigated the rotational spectrum of two water molecules bonded to formamide, the simplest molecule to possess a peptide linkage. The spectroscopic data obtained using a Fourier-transform microwave spectrometer are consistent with a triply hydrogen-bonded structure, in which the two water subunits are bonded in a water-dimer-like arrangement. The structural results are in good agreement with recent predictions using ab initio electronic-structure theory. In a second study we have initiated the investigation of the molecular conformations of ethyl acetamidoacetate (CH₃C(O)NHCH₂C(O)OC₂H₅), a simple dipeptide mimetic which has the advantage of having a sufficiently high vapor pressure that it can be investigated in the gas phase environment, free of a perturbing medium. Additionally, the mimetic also possess peptide-like dihedral conformational angles Φ and Ψ responsible for the flexibility of polypeptide and protein chains. (A.R. Hight Walker, R.D. Suenram, and G.T. Fraser)
• Computer-Aided Spectroscopy. The increasing use of molecular spectroscopy by non-experts has motivated the development of new software tools to simplify the process of assigning, fitting, and predicting high-resolution, rotationally resolved spectra in the microwave through ultraviolet spectral regions.

  Two computer programs have been developed and tested with the aid of collaborators at the Univ. of Pittsburgh and in France. The first program is used for analyzing and predicting the spectra of molecules with a large-amplitude CH₃ or CF₃ functional group internal rotation, such as found in methanol (CH₃OH), acetaldehyde (CH₃C(O)H), and trifluoropropene (CF₃CH=CH₂). This program allows the fitting of these spectra to high precision (1 part in 10⁸) and the extraction of molecular parameters characterizing the moments of inertia and internal-rotation potential function. With DoE support, the program has been extensively tested on methanol and acetaldehyde.

  The second program provides a graphical interface and efficient spectral calculation techniques to ease the tedious process of assigning and fitting high-resolution spectra. The program allows synthetic spectra to be rapidly generated and compared with observations. When the level of agreement between the model and observations is high then the program will perform a least-squares-fit optimization of the model parameters. The program can be applied to most molecules characterized by asymmetric-rotor and internal rotation Hamiltonians, and includes options for quadrupole hyperfine effects.

  Efforts are presently underway to disseminate the programs to interested users. We anticipate benefits in such diverse fields as analytical chemistry, plasma diagnosis, and remote sensing. (J.T. Hougen and D.F. Plusquellic)

• Distinguishing Conformational Isomers. Simple hydrocarbons, such as alkanes and alkenes, are ideal model systems for investigating the complex conformational dynamics found in biologically active molecules. Hydrocarbons have the advantage of possessing simple potential energy functions and a relatively small number of electrons. These two features combine to make accurate molecular modeling predictions possible.

  We have recently begun an investigation of the rotational spectra of the 1-alkenes from 1-pentene to 1-dodecane in a molecular beam at a rotational temperature of approximately 2 K using molecular-beam Fourier-transform microwave spectroscopy. The measurements provide data to test the ability of molecular mechanics and ab initio quantum chemistry to correctly predict the geometries and energy ordering of low-energy conformations of simple alkenes. The number of conformers for these systems is expected to grow approximately as 3^n-2 for alkene C_nH_2n for increasing n. For 1-pentene we have been able to record the rotational spectra for 4 of the 5 conformers predicted theoretically. For 1-hexene, 1-heptene, and 1-octene, 7 (out of 13), 7 (out of 42), and 15 (out of 131) conformers, respectively, have been experimentally observed.

  The large number of conformers observed is unprecedented and is particularly noteworthy, since the samples have been cooled in a supersonic expansion to a rotational temperature of 2 K. The conformational degrees of freedom are not cooled in the expansion and one finds a population distribution characteristic of the pre-expansion temperature of the gas (room temperature in the present case). The observation of additional, higher energy conformers was limited by their relatively small populations at room temperature.

  The larger alkenes, for which our studies have only just begun, are expected to display an even richer conformational chemistry. The preliminary results show that molecular-mechanics potential functions and ab initio quantum chemistry provide reliable tools for predicting the conformations of small hydrocarbons. (G.T. Fraser and R.D. Suenram)

• Mid-infrared Cavity-Ring-Down Spectroscopy. Cavity-Ring-Down Spectroscopy (CRDS) is generating much interest as a burgeoning technique for sensitive concentration measurements of trace contaminates in gas samples, and for the absolute measurement of small molecular absorption cross sections. The absorption cell in CRDS consists of a confocal Fabry-Perot optical cavity made of high reflectivity mirrors, which achieves effective optical path lengths on the order of kilometers. Previously, the technique had been limited to the near-infrared and visible spectral regions, where high-reflectivity mirrors and a variety of tunable lasers were available.

  We have developed a new mid-infrared cavity-ring-down spectrometer, using a CO₂ laser with tunable microwave sidebands. The sidebands are produced by nonlinear mixing of CO₂ and microwave radiation in a CdTe crystal. The mid-infrared region of the spectrum offers significant advantages for CRDS since virtually all molecules absorb strongly in this spectral region. We have taken advantage of this to demonstrate nonlinear saturation of transitions. The resultant line widths are on the order of 1 MHz, or less, or approximately 2 % of the full Doppler width. The technique also offers potential analytical chemistry applications, since saturation signals were observed using gas partial pressures as low as 0.004 Pa (30 µ Torr).

  Industry is showing increased interest in the development of commercial CRDS instruments to take advantage of this high sensitivity and high frequency-precision. Potential applications include the absolute measurement of the absorption cross sections for unstable molecules and characterization of the atmospheric water continuum to better improve our understanding of the solar absorption by the atmosphere. (D.F. Plusquellic and G.T. Fraser; with C.R. Bucher of Princeton Univ.)

• Coherent Rayleigh Scattering for Temperature Measurement. A nonlinear variation of frequency-resolved Rayleigh scattering is being investigated as a diagnostic for atom and ion temperatures in weakly ionized gases, such as are found in the shock waves of aircraft at high altitude.

  The instrument consists of two frequency-doubled Nd:YAG laser systems, one running on a single longitudinal mode the other running on several longitudinal modes. The beam from the multimode laser is split into two beams which counter propagate through the sample at an angle of approximately 175º. These interfere, creating a superposition of traveling-wave and stationary fringe patterns. The gas atoms, which are polarizable, tend to be attracted and compressed into regions of high field intensity. This force leads to spatially periodic density perturbations that can Rayleigh scatter the single-mode probe beam. The traveling components introduce Doppler shifts into the scattered light as well. However, the equilibrium velocities within the gas compete against the formation of regular, periodic density fluctuations. By measuring the Doppler profile of the scattering, one can derive the underlying temperature of the gas.

  Initial tests indicate greatly enhanced sensitivity over spontaneous Rayleigh scattering at low pressures. (J.H. Grinstead)

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