Most of this division was absorbed into the
Optical Technology Division

Molecular Physics Division

Technical Highlights

• Oxygenated Hydrocarbon Analysis for the Automotive Industry. In an effort to comply with future emissions regulations the automotive industry is seeking new analytical techniques that will allow real-time measurements of 18 different oxygenated hydrocarbons. These species consist of a number of aldehydes, ketones, alcohols, and ethers. The ethers are methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE) and tertiary amyl methyl ether (TAME), which are the oxygenated additives that are added to gasoline to lower emissions. In the past, only total quantification of these compounds was required. The new regulations require the quantification of each individual species.

  

  Figure 1. Spectral scan of ETBE showing the simplicity of the spectrum at 1K. The arrow indicates the transition that will be used for monitoring purposes.

  Fourier-transform Microwave Spectroscopy (FTMS) lends itself toward the solution of this problem. Initial efforts have been aimed at developing the required database of rotational spectra necessary for this study. Of the eighteen required species only eleven had been previously analyzed. We have now completed the analysis of four of the remaining species and work on the final three is in progress. The construction of a transportable FTMS instrument is 75 % complete and should be completed by the summer of 1995. A CRADA has been signed with the American Industry/Government Emissions Research (AIGER) Consortium, in part to develop the FTMS technique for use as a Fast Oxygenated Hydrocarbon Analyzer. The new transportable instrument will first be used at Ford Motor Co. in a tested environment beginning in the summer of 1995. (R.D. Suenram, F.J. Lovas, and A.R. Hight Walker)
• Fourier-Transform Microwave Spectroscopy Applied to Measurement of EPA's Title III Compounds. The high sensitivity, 100 % species selectivity, and real-time capability of NIST's FTMS provide analytical chemists with a new technique for the analysis of trace gas species in a wide variety of industrial situations ranging from process monitoring and control to verification of compliance with EPA regulatory procedures.

  An initial analysis of the microwave literature indicates that, of the 189 compounds on the EPA's Clean Air Act amendment of 1990, 55 have well characterized rotational spectra and approximately 45 additional compounds lend themselves to this technique. To develop a spectral database of rotational transitions for the 55 species with known spectra, a comparison of experimentally-determined verses theoretically-calculated lower detection limits for a number of these compounds has been carried out. This work indicates that a qualitative relationship exists between theory and experiment and that good initial estimates of minimum detection limits can be obtained from theory. Once specific compounds are identified for any given application however, experimental lower limits of detection will be needed in order to precisely quantify the results.

  Experimentally, lower detection limits have been established for a dozen compounds. These were chosen to represent a wide variety of chemical classes, including halogenated and sulfur containing species, hydrocarbons, nitriles, alcohols, aldehydes, ketones, ethers, and epoxides. The minimum detectable signal for these species was recorded under identical conditions for ease of cross comparison. The results indicated that the lower detection limits fall in the low parts-per-billion range (50 s integration/ neon carrier) for most species. In some of the best cases, detection limits were in the sub-ppb regime. These tests indicate that the FTMS technique should have a wide range of applicability in trace gas analysis situations. (F.J. Lovas, R.D. Suenram, G.T. Fraser, and A.R. Hight Walker)
• Spectroscopy of "Non-Polar" Molecular Complexes. We have enhanced the sensitivity of the Fourier-transform microwave spectrometer for weakly polar molecules. We increased the polarization pulse power to levels up to 1 Watt with a new solid state amplifier, and we incorporated a fast single-pole double-throw PIN diode switch to protect the low-noise heterodyne detector from direct reflection and to subsequently transmit the free-induction decay radiation. This has enabled us to study the Ne-Ar complex, whose dipole moment is measured to be 7 × 10^-33 C · m (ν 0.002 D), one of the smallest yet observed. Transitions of the most abundant isotopomer, ²⁰Ne-⁴⁰Ar, could be observed with an excellent signal-to-noise ratio with a single polarization pulse. Isotopic substitution provided an estimate for the equilibrium separation at the well minimum of R_e = 348.05(7) pm. The enhanced sensitivity may enable us to observe pure rotational transitions for highly symmetric molecules with centrifugally-induced moments, such as CF₄ or (HF)₃ or isotopically-induced moments beyond H/D substitution. (J.-U. Grabow, A.S. Pine, G.T. Fraser, F.J. Lovas, and R.D. Suenram)
• Spectroscopic Characterization of Unstable Atmospheric Molecules. Chlorine nitrate (ClNO₃) and dinitrogen pentoxide (N₂O₅) play an important role in the ozone chemistry of the polar regions by furnishing night-time traps for ozone-depleting odd-oxygen containing free radicals. Under the influence of sunlight, ClNO₃ and N₂O₅ are photodissociated to release ClO and NO₃ which catalytically destroy ozone (O₃). To furnish improved spectroscopic constants for modeling the atmospheric infrared band profiles of chlorine nitrate and dinitrogen pentoxide used to infer atmospheric abundances and height profiles, we are investigating the microwave and infrared spectra of chlorine nitrate and dinitrogen pentoxide. Infrared spectra of the 1293 cm^-1 band of chlorine nitrate have been measured at approximately 25 K using a lead-salt diode-laser spectrometer coupled to a pulsed slit-nozzle molecular-beam apparatus. The analysis of the stronger ³⁵Cl isotopic bands allow the determination of spectroscopic constants useful for the modeling of the intense Q-branch feature observed in the spectra of the earth's atmosphere.
  
  The microwave spectrum of dinitrogen pentoxide has been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. The observed spectrum is complicated by the large-amplitude, nearly free internal rotation of the two NO₂ groups about their twofold axes. The ¹⁴N nuclear quadruple hyperfine structure definitively establishes the origin of the spectra as due to dinitrogen pentoxide and shows the dynamical equivalence of the two NO₂ groups. The observed spectrum is not well characterized by spectroscopic constants obtained by electron diffraction studies suggesting that these constants are contaminated by unaccounted for contributions from nitric acid formed by the hydrolysis of the highly reactive dinitrogen pentoxide with residual water. The spectroscopic analysis also suggests that a previous identification of the microwave spectrum of dinitrogen pentoxide is incorrect. Attempts are presently being made to determine the large-amplitude potential energy surface characterizing the NO₂ internal rotation. An accurate internal rotation potential is necessary for the reliable calculation of thermodynamics properties of dinitrogen pentoxide. Future efforts are directed at observing the high-resolution molecular-beam infrared spectrum of N₂O₅. The above research is supported in part by NASA and NATO grants. (A.M. Andrews, J.L. Domenech, G.T. Fraser, R.D. Suenram, W.J. Lafferty, and P.Watson (DuPont)
• Spectroscopy of Atmospheric Molecules. Doppler-limited and sub-Doppler spectra have been recorded of several heavy atmospheric species including acids, alcohols, aromatics and fluorocarbons in order to interpret their spectra and predict their atmospheric properties. A Doppler-limited spectrum of the 2ν₃ band of CF₄ was recorded at 77 K to reduce spectral congestion and was analyzed by collaborators in Dijon, France using a model incorporating interactions between the F₂, E and A₁ tetrahedral vibrational sub-levels of this overtone. Further work is needed to treat the ν₃/2ν2₄ Fermi resonance. We have also recorded the molecular beam optothermal spectra of several Q branches of ethane using a color-center laser to obtain tunneling splittings and K-doublings, unresolvable at the Doppler-limit, for modeling atmospheric spectra of C₂H₆.
  
  The study of collisional lineshapes with a difference-frequency laser spectrometer has continued with measurements of air broadening in ethane. Several strong, sharp Q branches in the ν₇ perpendicular band of ethane were recorded as a function of air pressure at temperatures of 296 K and 160 K. These Q branches are in relatively transparent "windows" in the atmospheric spectrum and are used by NASA groups in monitoring atmospheric hydrocarbons. These Q branches are so sharp that they cannot be resolved under Doppler-limited conditions, so we obtained their fine structure components by sub-Doppler molecular beam spectroscopy. We have also studied Ar broadening of HF for comparison with recent "exact" quantum mechanical calculations from a very realistic intermolecular potential based on high resolution microwave and infrared spectra of the Ar-HF van der Waals species. The measured broadening and shift coefficients are in excellent agreement with the calculations once proper account is taken of Dicke narrowing due to velocity-changing collisions. A significant lineshape asymmetry is observed, which we have attributed to partial correlation between velocity and state-changing collisions. The asymmetry parameter in our model is, with a few exceptions, proportional to the measured pressure shifts. This work is supported by contract to the NASA Upper Atmosphere Research Program. (A.S. Pine)
• High-Resolution Infrared Spectra of Alternative Refrigerants. A number of the fluorinated ethanes have been proposed as replacements for the ozone-depleting chlorofluorocarbons (CFCs) used by the refrigeration, air-conditioning, electronics, and chemical industry. To evaluate the utility of these compounds as alternatives to CFCs it is necessary to obtain reliable spectral, kinetic, and thermodynamic data to assess their refrigeration properties and solvent properties, as well as their effect on the chemistry and radiation transport of the atmosphere. To furnish accurate spectroscopic data on these compounds we have examined the infrared spectra of 1,2-difluoroethane (HFC152), 1,1-difluoroethane (HFC152a), 1,1,2-trifluoroethane (HFC143), 1,1,1-trifluoroethane (HFC143a), 1,1,1,2-tetrafluoroethane (HFC134a), and 1,1,2,2-tetrafluoroethane (HFC134). Infrared spectra for these compounds have been recorded at 2 MHz resolution between 900 cm^-1 to 1100 cm^-1 using an electric-resonance optothermal spectrometer developed at NIST. The observations test proposed assignments of the normal mode frequencies of the molecules and furnish extremely accurate values for the band-origins of the normal modes studied. The spectra reveal complex vibrational dynamics arising from the coupling of the normal modes to the large amplitude internal rotation coordinate. The analysis of these spectra provide information on the large-amplitude internal-rotation potential important to the calculation of the low-temperature thermodynamic properties of these molecules. Efforts have been made to fit the observed spectra to simple model Hamiltonians to extract spectroscopic constants for thermodynamic calculations and for simulation of the band profiles as a function of temperature. The above research is supported in part by NASA. (A.M. Andrews, J.L. Domenech, G.T. Fraser, C.C. Miller, B.H. Pate, L.A. Phillips, A.S. Pine, S. Stone, and L.-H. Xu)
• Spectra of Molecular Reaction Intermediates in Chemical Vapor Deposition and Plasma Processing. Emphasis has been placed on identifying the infrared spectra of molecular ions which are formed near the ionization thresholds of small MF_n species used as etchants or dopants in plasma-assisted microcircuit fabrication. These spectra give information on the ground-state chemical bonding properties and can be used for diagnostic development. Molecules studied (and new identifications) include CF₄ (ν₂ of CF₃⁺; CF₃^-), SiF₄ (SiF₃⁺; SiF₃^-), NF₃ (NF₃⁺; NF₂^-; NF₂⁺), and BF₃ (ν₁ and ν₂ of BF₂; BF₃⁺; BF₂⁺; BF_3-). Several of the assignments are supported by ab initio calculations performed by Dr. Karl K. Irikura. Papers were prepared describing the results for each of these systems, as was a review of matrix shifts, with evidence that the neon-matrix absorptions should lie within about 1 % of the gas-phase band centers. In other experiments, all three infrared-active fundamentals of NH₃⁺ appeared close to the gas-phase band centers, and the first identification of fundamentals of ND₃⁺ was achieved. Further studies on CO₂ resulted in a complete vibrational assignment for CO₂^- and in infrared evidence for the stabilization of two different structures of (CO₂)₂^-. Prominent impurity absorptions in deposits of BF₃ in a neon matrix were assigned to BF₂OH, formed by the reaction of BF₃ with traces of water in the system. A complete vibrational assignment was possible. Although the microwave spectrum and structure of that product had previously been reported, no infrared spectroscopic data for it are in the literature. (M.E. Jacox and W.E. Thompson)
• Vibrational and Electronic Energy Levels of Small Polyatomic Transient Molecules. A book entitled "Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules," by Marilyn E. Jacox, was published as Monograph No. 3 of the Journal of Physical and Chemical Reference Data. This book, the culmination of several years of effort, reports critically evaluated vibrational and electronic energy levels, radiative lifetimes, and principal rotational constants for approximately 1580 free radicals, molecular ions, and other chemical reaction intermediates observed in the gas phase and/or in rare gas or nitrogen matrices. (M.E. Jacox)
• Ultrafast Lasers Determine Mechanism of the O + CH₄ Reaction. Because of its importance in combustion and atmospheric chemistry, the reaction between oxygen atoms and methane has been studied by many techniques. Despite dozens of recent experiments, the reaction is not understood. The reaction channel O(¹D) + CH₄ → CH₃ + OH was thought to occur by one of two different mechanisms: 1) a direct reaction in which the CH₄O intermediate, if any, lasted for a very short time (femtoseconds); 2) a reaction in which the O(¹D) atom inserts in a CH bond forming a highly energized methanol molecule, CH₃OH^*, which has a long lifetime (picoseconds), the available energy (reaction exothermicity plus initial kinetic energy) is randomized in the CH₃OH^* before it dissociates to CH₃ and OH, and the CH₃OH^* dissociation rate can be understood by a statistical theory like RRKM theory. There was indirect evidence to support both mechanism 1) and 2), but the issue could be definitively decided by a time-resolved experiment.
  
  Our approach to directly measure the lifetime of the CH₃OH^* intermediate was to form a percussor species, the van der Waals molecule O₃ · CH₄ in a supersonic molecular beam. The O₃ molecule of the complex was photolyzed by an ultrafast laser pulse at a wavelength of 266 nm. Following absorption, the O₃ precursor falls apart in about 15 fs to form O₂ and O(¹D). The nascent O(¹D) then reacts with the adjacent CH₄ to form CH₃ + OH. The rate (time constant) for OH formation was determined by monitoring the OH concentration vs. time, probing the OH by laser induced fluorescence (LIF) using an ultrafast laser pulse tunable in the 300 nm to 310 nm range. We found that following the photolysis pulse the number density of OH in the v = 0 ground vibrational state increased as 1-exp(-t/τ) where τ, the OH (v=0) product formation time (= CH₃OH^* dissociation time) is τ = 3 ps. That is, all the OH (v  ) is formed via this long-lived CH₃OH^* complex and apparently none is formed via a prompt (femtosecond) direct reaction. The value τ = 3 ps is consistent with calculations using RRKM statistical rate theory. This experiment strongly supports mechanism 2) for the reaction. (R. van Zee and J.C. Stephenson)
• Hydrogen Bond Dynamics. Novel studies of hydrogen-bonded systems in room temperature solution were begun to investigate vibrational relaxation pathways and potential H-bond breaking dynamics. Solutions of pyrrole (C₄H₄NH) with proton-accepting bases such as acetonitrile, acetone, and tetrahydrofuran (THF) were mixed in CCl₄ to form 1:1 complexes. Vibrational energy lifetimes of the pyrrole NH-stretch vibration of the complex were found to monotonically decrease with decreasing NH-stretch frequency (increasing hydrogen-bond strength). Examples include dilute pyrrole (50±10 ps), pyrrole-benzene (25 ± 10 ps), pyrrole-acetonitrile (15 ± 8 ps), and pyrrole-THF (4 ps to 8 ps). These new unambiguous IR results conflict with interpretations of earlier experiments which used a picosecond time-resolved Raman probing technique.

  Excitation of the complexed pyrrole NH-stretch (3200-3400 cm^-1) provides sufficient energy to dissociate the H-bond (ca. 1000-2800 cm^-1) if direct transfer to high overtone states of the low frequency -NH ... Base vibration occurs. A spectroscopic signature for dissociation is incomplete recovery of the H-bonded NH-stretching absorption of a complex after excitation. We observed complete absorption recovery for weakly bound complexes, but see evidence for incomplete recovery (1 % to 2 % of the excited species are dissociatal) for more strongly hydrogen-bonded complexes (i.e., pyrrole with THF and triethylamine).

  In a separate series of experiments, the uncomplexed pyrrole NH-stretch was excited for pyrrole mixed with various concentrations of acetonitrile. The vibrational lifetime of the "free" NH-stretch was found to decrease with increasing acetonitrile concentration. A bimolecular rate constant extracted from these results (2.4 × 110¹⁰ M^-1s^-1) is significantly larger than the estimated diffusion-limited encounter rate (0.7 × 10¹⁰ M^-1s^-1), suggesting in-cage interactions, long-range attractive H-bonding forces, or vibrational energy transfer may be involved. Further explorations of this unexpected phenomena (using other complexes and solvents) are underway. (T.P. Dougherty, W.T. Grubbs, and E.J. Heilweil)
• Photoassociation Spectroscopy of Trapped Atoms. Recent experiments at NIST and elsewhere have measured high resolution molecular spectra due to the photoassociation of the two colliding ultracold trapped atoms to make excited molecular vibrational states. Such spectra permit the very accurate characterization of the long range interactions between the atoms, both in the ground and excited states. These spectra can be used to make high precision measurements of molecular dissociation energies and to extract precise atomic dipole transition matrix elements. They also contain much detailed information about the ground state scattering wave function. Consequently, fitting the data will permit refinements to be made in the ground state potential curves so that accurate scattering lengths and cross sections can be calculated in the quantum limit near T = 0. These calculations have been carried out in collaboration with C.J. Williams, an IPA from the University of Chicago, R. Napolitano, a graduate student at the University of Maryland, and Eite Tiesinga, a foreign guest worker. Calculating the details of the high resolution NIST spectra requires calculating the excited state energy levels, including the effect of vibration, rotation, and fine and hyperfine structure. Using new grid methods, we generate very large Hamiltonian matrices to calculate the coupled states eigenvalues. It has been necessary to develop new computational methods for obtaining the eigenvalues of such large matrices. We have calculated very good agreement with experimental spectra at NIST, including the effect of molecular hyperfine structure. (P.S. Julienne, C.J. Williams, and E. Tiesinga)
• Complex Quantum Nanostructures. As semiconductor nanotechnology becomes more developed, the nanostructures being fabricated become more complex, with complicated geometries and strong coupling between structures. An accurate theory for the optical properties of these complex nanostructures requires that one account for quantum confinement by nanostructures with complicated shape, coupling between nanostructures, electron-hole correlation in complicated spatial potentials, and the electron/hole asymmetry. A new staff member, G.W. Bryant, was hired using funding from the High Performance Computing and Communication Initiative, to develop theory in these areas. As the initial effort in a new project on Nanoscale Systems and Technologies, we are modeling the optical properties of complex quantum nanostructures, in collaboration with Prof. Y.B. Band of Ben-Gurion University. Different approaches are needed for different nanostructures. We have studied bound exciton states in t-shaped semiconductor 8 nm wide quantum wires which are trapped at intersections of quantum wells formed by cleaved edge overgrowth methods; lasing from these structures originates from such localized states. We use single-particle states, localized to a grid of points that cover the cross section of the wires, to define basis functions for the problem. We also have started studies on quantum dot quantum wells, multilayer structures formed by chemical growth techniques. In this case, the single particles basis states are generated by solving the single-particle radial Schrödinger equation for a radial quantum well potential. In both cases, we use these single-particle states to define electron-hole pair states for large-scale configuration-interaction calculations that account for pair correlation in these complicated geometries. Initial results for the coupled pairs of t-shaped wires show substantial level repulsion. Strong interwire coupling of exciton states is expected in this case. Initial results for quantum dot quantum wells show that including a well in the internal structure of the quantum dot can produce large level shifts and provides effective control of transition energies in these structures. (G.W. Bryant and P.S. Julienne)

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