Most of this division was absorbed into the
Optical Technology Division

Molecular Physics Division

Technical Highlights

• FTMW Spectrometer Development for Analytical Applications. A number of recent improvements in Fourier-transform microwave spectroscopy (FTMS) have increased the detectability limits for gas phase species down to the 10 ppb to 100 ppb range. This high sensitivity coupled with 100 % species selectivity, millisecond response times and complete automation of the instrument provides analytical chemists with a new technique for the determination of trace gas species that are present in industrial chemical processes. The technique can be used either in-lieu-of or as a complementary detection technique to gas chromatography mass spectrometric techniques (GCMS) for the detection of gas phase species.

  

  Figure 1: Comparison of the signal-to-noise ratio for 1 gas pulse and 100 gas pulses of a 120 ppb sample of acrolein in Neon carrier gas showing a minimum detectable signal of approximately 2 ppb for a 100 pulse average.

  Sensitivity tests of the new spectrometer were carried out using 800 ppm samples of acrolein (CH₂ = CHCHO) and propionaldehyde (CH₃CH₂CHO). Using these samples and the known isotopic abundances of ¹³C = 1.1 %, ¹⁸O = 0.2 %, and ²H = 0.015 %, single shot (one gas pulse from the pulsed nozzle) detection limits of approximately 40 ppb were attained. Averaging for 100 nozzle pulses yielded a signal-to-noise ratio of 35/1 on transitions of the monodeuterated isotopes in natural abundance. This corresponds to detection limits of approximately 3 ppb. Additional improvements in the instrument's detection system and software modifications should yield another order of magnitude improvement in sensitivity.
  
  Advantages of the new technique in monitoring trace gas constituents include the 100 % certainty of the identity of the species being monitored. Using a pulsed molecular beam nozzle in conjunction with a specially designed flow nozzle which operates at up to 30 Hz repetition rates allows rapid response to changes in industrial process flow streams. The cooling provided by the pulsed molecular beam permits larger molecular species to be monitored with the same ease as smaller species since only the lowest rotational energy levels will of any species will be populated at the 1 K temperature of the molecular beam. (R.D. Suenram and F.J. Lovas)
• Large Amplitude Internal Motions - Cooperative Effects. Cooperative effects involving large numbers of molecules play an important role in condensed phase chemical and biochemical reactions, but such cooperative effects are thought to be driven by a succession of nearest neighbor interactions.

  

  Figure 2: Spectral trace of the J = 2-1 region of the methanol dimer showing the complex spectrum that results from the facile tunneling and internal rotations of the methyl groups. Under non-tunneling conditions only three peaks would be present.

  Since many cooperative effects are equivalent to a collection of internal rotations about C-C single bonds, and since we noted several years ago that a substantial reduction in barrier height to methyl group internal rotation was produced in methanol when the methanol was hydrogen bonded to other molecular species, we have investigated a larger class of methanol dimers with a view to barrier determination and elucidation of any large changes observed. These efforts provide a wide variety of benchmark measurements for use by those involved in developing new algorithms for treating weak interactions in these systems (Biosym, a current ATP Awardee). The species that have been studied, include methanol dimer, methanol-water, methanol-formamide, methanol-carbon monoxide and methanol-hydrogen cyanide. One of the general structural features common to these species is that the hydroxyl hydrogen of methanol is not involved in hydrogen bonding. The two exceptions are the methanol dimer itself in which the OH group of one monomer binds to the O atom of the other and methanol-formamide which has a doubly hydrogen bonded structure. The methanol dimer, as well as methanol-water, are rather complicated systems and required the development of extensive new theory in order to achieve an adequate description of all internal motions. A simplified model employing the internal axis method could be used to extract internal rotation barriers from all the molecules studied, however, leading to the following V₃ barriers: (CH₃OH)₂ V₃(donor) = 191 cm^-1, V₃ (acceptor) = 136 cm^-1 CH₃OH·CO V₃ = 183 cm^-1; CH₃OH·HCN V₃ = 137 cm^-1; and NH₂COH·CH₃OH V₃ = 231 cm^-1. Comparison of these effective barrier heights with free methanol (373 cm^-1) indicates a substantial reduction upon dimer formation. These barrier heights are unexpectedly sensitive to deuteration of the hydroxyl hydrogen not involved in the hydrogen bond, suggesting that the librational motion of the OH group contributes to this effective lowering of the barrier, and modeling this motion can explain the large discrepancy. This work has been carried out in collaboration with M. Tretyakov and S. Belov, Institute of Applied Physics, Russia, W. Stahl, University of Kiel, Germany, P. Stockman and G. Blake, California Institute of Technology, J. Sobhanadri, Indian Institute of Technology, Madras, India, N. Ohashi, Kanazawa University, Japan, and guest worker J. Ortigoso, Instituto de Estructura de la Materia, Madrid, Spain. (F.J. Lovas, J.T. Hougen, R.D. Suenram and G.T. Fraser)
• High Resolution IR Studies of Atmospheric Molecules. The burgeoning importance of remote-sensing Fourier-transform infrared spectroscopy as a new technique for detecting and monitoring fugitive emissions at chemical plant boundaries has sparked renewed interest in this area. This is in addition to the on-going NASA sponsored work in the division. Much of the frequency and intensity data resulting from this effort winds up in the HITRAN tables, an exhaustive compilation of the spectrum of the atmosphere produced by the Air Force Geophysics Laboratory.

  As a result of the recent volcanic activity in the Philippines and Japan, and the emission of great quantities of SO₂ into the atmosphere, it was discovered that the spectral data for SO₂ and SO₃ in the HITRAN tables was inadequate. As a result, an extensive study of the fundamental and a number of overtone and combination bands of SO₂ is being carried out in collaboration with J.-M. Flaud of LPMA in Paris. A global fitting of the (010), (020), (100) and (001) levels has been made resulting in much improved spectroscopic parameters. In addition the ν₁ + ν₃, 2ν₁ + ν₃, 2ν₃ and 3ν₃ bands have been studied and an excellent set of rotational and vibrational anharmonic constants and intensity parameters has been obtained.

  The HOCl molecule is one of the species involved in the stratospheric ozone reactions. In collaboration with James Burkholder of NOAA, we have just finished a study of the a-,b-type hybrid ν₁ band of this molecule. Since this band falls in the intense water region in the 3 µm region, it is not of great importance directly for remote sensing purposes except in the far wings of the band; however, the improved ground state rotational constants derived permit calculation of the far IR spectrum with a uncertainty of better than ±0.001 cm^-1.

  Molecular beam spectra of several bands of N₂O₃ and N₂O₄ have been obtained and completely analyzed. The cooling obtained in the molecular beam experiments greatly simplifies the spectra, and facilitates the line assignment. Also in a collaborative effort with DuPont, the beam spectrum of chlorine nitrate has been obtained, and a band analysis is in progress.

  At the request of NASA, we have also been studying the collision induced absorption in the fundamental bands of N₂ and O₂ in order to permit modeling of the background continuum produced by these molecules in the 1600 cm^-1 and 2340 cm^-1 regions. Laboratory measurement requires long paths (80 m) and high pressure [1.2 MPa (12 atm.)]. Initial measurements were plagued by ppb amounts of impurities. This problem has recently been solved, and we expect to finish this project in the next three months. (W.J. Lafferty and A. Andrews)

• Collisional Studies of Atmospheric Molecules. There is currently an increased level of activity in remote-sensing Fourier transform infrared spectroscopy for use in identifying and quantifying multiple toxic gases simultaneously. In efforts aimed at developing the requisite models for treating the effects of line broadening in these complex systems we are continuing our work on collisional lineshapes with emphasis on the phenomenon of line mixing in heavily overlapped spectra. We have recorded self-, N₂- and Ar-broadened combination band Q branches in HCN and HCCH from the well-resolved Doppler limit at low pressures to the blended contours at atmospheric pressures using a tunable difference-frequency laser spectrometer. At lower pressures where the lines are not overlapped, the broadening coefficients as a function of rotational quantum number J are independent of vibrational level and can be modelled successfully using an energy-corrected-sudden (ECS) approximation scaling law for self-broadened HCN and with simpler empirical energy-gap fitting laws for the other collision partners. No empirical collision dynamics model was found suitable for all cases. At higher pressures where overlap is severe, line mixing, manifest as a non-additive superposition of Lorentzian lines, strongly affects the blended contours, and indicates that the coupling between lines varies from 30 % to 64 % of the collisional broadening. The decoupling presumably arises from collisional energy transfer to the other member of the l-type doublet not observed in the Q-branch transitions of these II-Σ bands. A vibrational angular momentum coupling model recently proposed by a French group to account for line mixing in infrared Q branches of CO₂ does not seem to work for the similar molecules studied here, so we have utilized empirical coupling factors to fit our observed contours.
  
  We have also studied self-, N₂-, O₂-, H₂-, Ar- and He-broadening of the ν₁ band of ammonia with the difference-frequency laser. A systematic J and K dependence of the broadening coefficients is observed, with striking similarities for N₂, O₂ and Ar and for H₂ and He buffer gases. Self broadening is adequately explained by Anderson-Tsao-Curnutte theory which, however, fails in the cases of the non-polar buffer gases studied. Dicke narrowing is evident at intermediate pressures, yielding an average narrowing coefficient and an optical diffusion constant for each buffer gas. The broadening coefficients are in good agreement with ground-state inversion measurements except for H₂. We are currently studying Ar broadening of HF for comparison with recent "exact" quantum mechanical calculations from a very realistic intermolecular potential obtained from high resolution microwave and infrared spectra of the Ar-HF van der Waals species. Our initial results appear to be in excellent agreement, with an as yet unexplained lineshape asymmetry observed. (A. Pine)
• Free Internal Rotors. Most of the metalorganic molecules used in semiconductor MOCVD and MOMBE processing and fabrication are di- or trimethyl metals exhibiting facile internal rotation of the methyl groups. This internal rotation leads to extremely congested and complicated spectra which have not been adequately studied in the infrared. To test the feasibility of such studies for the NIST Semiconductor Initiative, we have recorded a low temperature spectrum of a prototype species, dimethylacetylene (a.k.a. DMA, C₄H₆, H₃C-C ≡ C-CH₃, 2-butyne), using a tunable-laser difference-frequency spectrometer and a White cell cooled to approximately 195 K. Most of the free internal rotor structure of the molecule has been resolved and the low energy torsional hot bands have been suppressed. The spectrum is under analysis by P.R. Bunker of the National Research Council of Canada, Ottawa, Canada and C. di Lauro of the University of Naples, Italy. (A. Pine)
• Submillimeter Molecular-Beam Spectrometer for the Investigation of Weakly Bound Complexes Based on Phase-Locked Backward-Wave Oscillators. Phase-locked Russian-made backward wave oscillators have been coupled to an electric-resonance optothermal molecular-beam spectrometer to allow the study of the submillimeter and far-infrared spectra of weakly bound complexes and stable molecules. The rapid scanning and double-resonance capabilities of the spectrometer simplify the quantum-state assignment of complex spectra. Initial studies were undertaken on isotopic water dimer to unravel the complex tunneling dynamics of the various isotopomers. The spectra will be used to more fully characterize the pair potential of water. The goal is to achieve a reliable intermolecular potential for water to allow for the accurate computer modeling of the properties of water. Spectra have also obtained and analyzed for deuterated ammonia dimer and the ammonia hydrogen-sulfide complex.

  

  Figure 3: Q-branch spectrum of water dimer obtained using NIST's EROS molecular beam instrument using a computer-driven, phase-locked Russian BWO source.

  The isotopic ammonia dimer will allow us to address the apparent disagreement between ab initio theory and experiment on the equilibrium structure of the (NH₃)₂. Part of this effort has been carried out to provide experimentally derived benchmarks for the new ab initio and molecular mechanics algorithms being developed by Biosym, an ATP Participant in their quest to develop a better understanding of the complex interactions possible between drugs and biological molecules. (G.T. Fraser, R.D. Suenram, E. Karyakin, and G. Hilpert)
• An Evaluation of Theoretical Compressibility Factors for the CH₄/H₂O System. Accurate intermolecular potentials are required by the natural gas and chemical industry for the calculation of second-viral coefficients and compressibility factors for gases of varying composition. Potentials of interest for natural gas transport include those for CH₄·CH₄, CH₄·H₂O, and CH₄·H₂S. In an effort to test and improve the available intermolecular potentials for the CH₄·H₂O system we have examined the microwave spectrum of the C₄H·H₂O complex. The rotational spectrum is extremely complex due to the weak anisotropy of the potential which allows hindered rotation of the CH₄ and H₂O subunits. The spectroscopic constants obtained from isotopic studies determine the minimum energy configuration for the potential, which has the H₂O subunit proton donating to the CH₄. This disagrees with the results of a recent ab initio quantum mechanical calculation of the potential energy surface which has a minimum energy geometry in which the CH₄ is hydrogen bonding to the oxygen of the H₂O unit. The large amplitude motions sample large regions of the intermolecular potential, allowing a quantitative characterization of the anisotropy of the interaction. The present results suggest a revaluation of compressibility factors and second-viral coefficients calculated from the recent ab initio potential surface. (R.D. Suenram, G.T. Fraser, F.J. Lovas, and Y. Kawashima)
• Spectra of Molecular Reaction Intermediates in Chemical Vapor Deposition and Plasma Processing. A special effort has recently been started to obtain previously unavailable information on the molecular energy levels of free radicals and molecular ions which may be formed during the course of chemical vapor deposition and plasma processing. The interaction of neon atoms excited to 16.6 eV to 16.8 eV with precursor molecules is exceptionally well suited to that task. Their interaction with BF₃ has yielded infrared absorptions of all three vibrational fundamentals and one combination band (ν₁ + ν₃) of the BF₂ free radical, as well as absorptions which have been assigned to the stretching fundamentals of BF₃⁺, the ground state structure of which is distorted as a result of interaction with a low-lying degenerate excited electronic state. Photodecomposition of BF₃⁺ in the red spectral region leads to growth in the ν₃ infrared absorption of BF₂⁺. Before these studies, the positions of two of the vibrational fundamentals of BF₂ had been determined, one of them with 30 cm^-1 uncertainty. No spectroscopic data for BF₂⁺ and no quantitative vibrational data for BF₃⁺ were available. The infrared spectral data for BF₂ and for BF₂⁺ are in excellent agreement with recent ab initio calculations of the molecular energy levels of these species performed by Drs. Karl Irikura and Jeffrey Hudgens, of the Chemical Kinetics and Thermodynamics Division. The analogous experiments on NF₃ have led to the assignment of the ν ₃ fundamental and the ν₁ + ν₃ combination band of NF₂⁺, in good agreement with the positions obtained in a recent ab initio calculation, and to the tentative spectroscopic identification of NF₃⁺. (M.E. Jacox)
• Vibrational and Electronic Energy Levels of Small Polyatomic Transient Molecules. There have been two major outputs in this ongoing project, supported in part by the Standard Reference Data Program. A 461-page monograph presenting gas-phase and matrix-isolation spectroscopic data for 1582 species and their fully deuterium-substituted counterparts [M.E. Jacox, Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules, J. Phys. Chem. Ref. Data, Monograph 3 (1994)] has been completed and proofread. The computer-searchable version of these data (NIST VEEL-Standard Reference Database #26, Version 3) was also prepared, and will be ready for distribution in early 1994. Users may search by molecule, by wavenumber range, or by wavelength range. (M.E. Jacox)
• Substrate to Adsorbate Energy Transfer. In a joint research effort involving scientists in the Molecular Physics Division and the Surface and Microanalysis Science Division, picosecond lasers were used to determine the rates and mechanisms for energy to flow from a metal substrate to the vibrational modes of chemisorbed molecules. Such information is critically important to understanding chemical reactivity at surfaces since sticking, desorption, surface mobility and chemical reactions are activated by vibrational excitation. The coupling of optical radiation to surface reactions is receiving attention in the fields of catalysis, semiconductor processing, and solar energy conversion.

  In these experiments, carbon monoxide, CO, chemisorbed on a Cu(100) crystal initially at a temperature T = 100 K was studied, and the results compared to our previous data for CO on Pt(111) and to theory for CO/Cu (done at AT&T Bell Labs). A short duration (< 1 ps) visible or UV laser pump pulse created hot electrons (characterized by temperature T_e) and hot lattice phonons (characterized by temperature T_lat) in the near-surface region of the Cu substrate. In our experiments the maximum temperatures following the pump were T_e = 300 K and T_lat = 130 K. The increased energy in the electronic and lattice degrees of freedom then caused vibrational excitation of the low frequency (ν = 32 cm^-1) frustrated translation mode of CO. The coupling time of the vibration to the electrons was τ_e = 4.8 ps and the coupling time to the phonons was T_lat = 3.8 ps. Essentially the same values were found for visible and for UV excitation of the Cu, proving that ballistic electrons did not excite the CO vibrations, contrary to many predictions.

  These were the first measurements of τ_e and τ_e, independently, for any system. We compared our results to dozens of recently published theoretical models. Only one (due to Tully at AT&T Bell Labs and Head-Gordon at U.C. Berkeley) correctly predicted significant coupling of this vibrational mode both to electrons and phonons. The coupling (damping) rates are key parameters in understanding surface processes like sticking and diffusion, and these benchmark measurements are very important in guiding the modelling of such processes.

  We developed the experimental methods for studying vibrational dynamics on surfaces, performed the first benchmark measurements, and established the coupling times and mechanisms for different adsorbate modes (i.e., the internal CO stretch and the Cu-CO frustrated translation) on metal surfaces; this expanded our previous studies of molecules on semiconductor and dielectric substrates. Such measurements, built upon earlier NIST work, are now being done elsewhere (e.g., AT&T Bell Labs, IBM, F.O.M. Amsterdam, U. Penn, U. Chicago, Tokyo Inst. Tech.). (T.A. Germer, R.R. Cavanagh, E.J. Heilweil, and J.C. Stephenson)
• Ultrafast Metal-Carbonyl Photochemistry. Femtosecond broadband infrared studies of metal-carbonyl photochemistry were undertaken to identify mechanisms and rates for CO-ligand ejection, solvation, intermediate cooling, and fragment chemical reaction. These initial microscopic processes govern the outcome of many industrial applications of metal-carbonyls: gasification and liquefaction of coal, enhanced oil recovery, and initiation of polymerization reactions. Without high time-resolution spectroscopic methods, detailed pictures of these important reactive species have been primarily speculative and elusive.

  In the photochemistry of M(CO)₆ (M = Cr, Mo, W), for example, the dominant photoproduct following UV excitation in n-hexane at 298 K is M(CO)₅(n-hexane). A significant fraction of all three photoproducts are formed with vibrationally excited CO-stretches which relax in approximately 160 ps; the W and Mo photoproducts are formed with similar amounts of CO-stretch vibrational excitation, but the Cr photoproduct is formed with less. However, experiments which examined metal dicarbonyls such as CpCo(CO)₂ (Cp = η-C₅H₅) found no CO-stretch excitation in the monocarbonyl photoproduct. The reason for these distinctions is unclear, and while we hypothesize the larger organic ligands of the dicarbonyls may act as energy sinks, molecular modelling and theory is needed to clarify these measurements.

  Observations have also been made for UV photolysis of the dimer species [CpFe(CO)₂]₂ in room temperature n-hexane. Our sub-picosecond results determined that only the parent trans isomer is responsible for all observable photochemical reactions. The lack of two distinct CO-stretching bands for the CpFe(CO)₂ radical (formed by Fe-Fe bond cleavage) indicates it is a symmetric species with opposing CO ligands. This structural assignment has never been correctly made until now. Identification of the Cp(CO)Fe(µ-CO)₂FeCp(n-hexane) solvated species has been tentatively made along with its tetrahydrofuran (THF) analog. We find another mechanistic pathway produces the triply-bridged species, [CpFe(µ-CO)₃FeCp], which forms and vibrationally cools in about 70 ps after terminal CO-elimination from the dimer.

  Studies of the industrial agent CpCo(CO)₂ (Vollhardt's catalyst) in neat n-hexane, neat 1-hexene and solvent mixtures have revealed rich solvent substitution chemistry under ambient thermal conditions. The -HC=CH₂ end of 1-hexene displaces initially coordinated n-hexane and preferentially associates with the de-liganded metal center to form a stable species. This displacement reaction was observed in real time for moderately high concentrations of 1-hexene (0.1-2.0 Molar). Solvation takes place at nearly the diffusion-controlled rate so that substitution occurs for each encounter of solvent with solute.

  During our examination of the above and other metal-carbonyl photochemistries, we have discovered previously unknown reactive species and identified several misconceptions of the mechanistic pathways leading to final products. Since reactive transients are produced in several picoseconds in these systems, discrepancies in spectral assignments and interpretation of complex long-time dynamics must clearly be rationalized over all observable time-scales. (T. Dougherty and E.J. Heilweil)
• Interactions of laser cooled atoms. Advances in laboratory techniques for laser cooling and trapping of neutral atoms at temperatures below 1 mK offer many new opportunities to science and technology, such as greatly improved atomic clocks, the optical manipulation of atomic beams, and applications of optical lattices, atom cavities and atom optics. The interactions between laser cooled and trapped atoms exhibit new and unusual physics which must be understood in order to make optimal use of this new technology: for example, collisions between cooled atoms strongly affect cold atomic sources or optical lattices at high fill factors or interfere with precise frequency measurements in atomic clocks. Paul Julienne from the Molecular Theory Group, in conjunction with collaborators outside NIST, have done pioneering research in the unique physics of these collisions at temperatures below 1 mK, supported in part by a NIST Competency Program and in part by the Office of Naval Research. The interactions of ground state atoms are relatively short range, and collisions of ground state atoms are strongly dependent on quantum effects associated with the long De Broglie wavelength and are extremely sensitive to the details of the atomic interaction potentials. When a light field is present, excited molecular states have a profound effect on the interactions, due to the very long range of the resonant dipole-dipole potential. Recent experiments at NIST and elsewhere have measured high resolution molecular spectra due to the photoassociation of the two colliding 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. A knowledge of these interactions is necessary for the accurate simulation of the properties of trapped atoms. In collaboration of C.J. Williams of the University of Chicago, we have calculated very good agreement with experimental spectra at NIST, including the effect of molecular hyperfine structure. Refinements in the calculations will permit the very accurate determination of ground and excited state potentials and accurate simulation of the effects of interatomic interactions when the light is detuned more than a few linewidths below atomic resonance for the cooling transition. Cold collisions of laser excited atoms are also strongly influenced by excited state radiative decay during the time of interaction, and these collisions are typical of a large class of open quantum systems, in which a quantum subsystem couples dissipatively to its environment. Such systems must be described by a density matrix rather than a wavefunction. We are developing new computational algorithms, in collaboration of K. Burnett and K.A. Suominen of Oxford University, for describing open collisions using new Monte Carlo wavefunction simulations. Normal wavefunction evolution methods are used, but at each time step a random number is used to determine if photon emission has occurred, and the wavefunction is readjusted accordingly. We have used these methods to calculate the quantum mechanical evolution of typical collisions that result in loss of atoms from magneto-optical traps. For these cases the light is detuned up to a few linewidths to the red of atomic resonance. With Y.B. Band of Ben Gurion University we also have developed quantum methods based on using complex potentials to simulate the effect of decay. This method only treats the case of low laser power, whereas the Monte Carlo method treats the arbitrary power case. Both the complex potential and Monte Carlo quantum methods were compared to semiclassical models at temperatures from 10 µK to 10 mK. We show that a simple Landau-Zener semiclassical model of the dynamics works much better than a semiclassical optical Bloch equation treatment. Simple two state models of the quantum process do not agree very well with measured trap loss rates, and multichannel models that treat hyperfine structure will be necessary for quantitative modeling of the small detuning case. (P. Julienne and F. Mies)

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