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ABSTRACT: The evaporation rate of D<sub>2</sub>O has been determined by Raman thermometry of a droplet train (12–15 µm diameter) injected into vacuum (~10<sup>-5</sup> torr). The cooling rate measured as a function of time in vacuum was fit to a model that accounts for temperature gradients between the surface and the core of the droplets, yielding an evaporation coefficient (γ<sub> e </sub>) of 0.57±0.06. This is nearly identical to that found for H<sub>2</sub>O (0.62±0.09) using the same experimental method and model, and indicates the existence of a kinetic barrier to evaporation. The application of a recently developed transition-state theory (TST) model suggests that the kinetic barrier is due to librational and hindered translational motions at the liquid surface, and that the lack of an isotope effect is due to competing energetic and entropic factors. The implications of these results for cloud and aerosol particles in the atmosphere are discussed.
Atmospheric Chemistry and Physics Discussions. 01/2008;
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ABSTRACT: The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an "acceptor-only" configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s --> sigma(NH) preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s --> pi(CN) resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the alpha amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.
The Journal of Physical Chemistry B 11/2005; 109(46):21640-6. · 3.70 Impact Factor
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Science 05/2005; 308(5723):793-793. · 31.20 Impact Factor
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ABSTRACT: The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s --> pi*(C=O) transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine, have been reassigned as transitions to sigma* bound states. The observation of these peaks indicates that the amine moiety is in an acceptor-only hydrogen bond configuration at high pH. At low pH, the oxygen 1s --> pi*(C=O) transition exhibits a 0.25-eV red shift due to the protonation of the carboxylic acid terminus. These spectral differences indicate that the variations in electronic structure observed in the NEXAFS spectra are determined by the internal charge state and hydration environment of the molecule in solution.
The Journal of Physical Chemistry B 03/2005; 109(11):5375-82. · 3.70 Impact Factor
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Journal of Physical Chemistry B - J PHYS CHEM B. 01/2005; 109(46):21640-21646.
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11/2003; 42:369-392.
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04/2002;
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04/2002;
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ABSTRACT: The three‐dimensional intermolecular potential energy surface (IPS) for Ar–NH3 has been determined from a least‐squares fit to 61 far infrared and microwave vibration–rotation–tunneling (VRT) measurements and to temperature‐dependent second virial coefficients. The three intermolecular coordinates (R,θ,ϕ) are treated without invoking any approximations regarding their separability, and the NH3 inversion–tunneling motion is included adiabatically. A surface with 13 variable parameters has been optimized to accurately reproduce the spectroscopic observables, using the collocation method to treat the coupled multidimensional dynamics within a scattering formalism. Anisotropy in the IPS is found to significantly mix the free rotor basis functions. The 149.6 cm−1 global minimum on this surface occurs with the NH3 symmetry axis nearly perpendicular to the van der Waals bond axis (θ=96.6°), at a center‐of‐mass separation of 3.57 Å, and with the Ar atom midway between two of the NH3 hydrogen atoms (ϕ=60°). The position of the global minimum is very different from the center‐of‐mass distance extracted from microwave spectroscopic studies. Long‐range (R≳3.8 Å) attractive interactions are greatest when either a N–H bond or the NH3 lone pair is directed toward the argon. Comparisons with ab initio surfaces for this molecule as well as the experimentally determined IPS for Ar–H2O are presented.
The Journal of Chemical Physics 06/1994; 101(1):146-173. · 3.33 Impact Factor
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ABSTRACT: The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laser spectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration-rotation-tunneling spectra of weakly bound complexes, and vibration-rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laser spectrometers is presented.
08/1991;
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ABSTRACT: A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10-100/cm and exhibits a maximum absorption sensitivity near one part in a million. Each laser line can be tuned by + or - 110 GHz with first-order sidebands.
08/1991;
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ABSTRACT: The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laser spectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration‐rotation‐tunneling (VRT) spectra of weakly bound complexes, and vibration‐rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laser spectrometers is presented in the following article.
Review of Scientific Instruments 08/1991; · 1.37 Impact Factor
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ABSTRACT: A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10–100 cm<sup>-1</sup> and exhibits a maximum absorption sensitivity near one part in 10<sup>6</sup>. Each laser line can be tuned by ±110 GHz with first‐order sidebands. Applications of this instrument are detailed in the preceding paper.
Review of Scientific Instruments 08/1991; · 1.37 Impact Factor
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ABSTRACT: Two new intermolecular vibration–rotation‐tunneling (VRT) bands of Ar–NH3 have been measured using tunable far infrared laser spectroscopy. We have unambiguously assigned these and a previously measured FIR band [Gwo et al., Mol. Phys. 71, 453 (1990)] as Π(10, n=0)←Σ(00, n=0), Σ(10, n=0)←Σ(00, n=0), and Σ(00, n=1)←Σ(00, n=0). The three upper states of these are found to be strongly mixed by anisotropy and Coriolis effects. A simultaneous least squares fit of all transitions has yielded vibrational frequencies, rotational and centrifugal distortion constants, and a Coriolis parameter as well as quadrupole hyperfine coupling constants for the upper states. An effective angular potential energy surface for Ar–NH3 in its lowest stretching state has been determined from these data, after explicitly accounting for the effects of bend stretch interactions. Features of the surface include a global minimum at the near T‐shaped configuration (θ=90°), a 30 cm−1 to 60 cm−1 barrier to rotation at θ=180° (or 0°), and a very low barrier or possibly a secondary minimum at θ=0° (or 180°). Both attractive and repulsive interactions are shown to contribute significantly to the anisotropic forces in the complex. Comparison with ab initio calculations are presented.
The Journal of Chemical Physics 06/1991; 95(1):9-21. · 3.33 Impact Factor
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ABSTRACT: Hyperfine resolved spectra have been measured for the Ar-NH3 complex over the range 21–28 cm-1 using a tunable far-infrared laser in combination with a continuous planar supersonic jet. Twenty-three transitions are assigned to the lowest Σ-bending vibration (v 0 = 26·470633(77)cm-1) from the (asymmetric) inversion level of the Σ, j k c = 00 internal rotor state of NH3 to the Σ, (10) symmetric inversion level. A nearly free-rotor model is used to deduce the zeroth-order intermolecular vibrational energy-level diagram and corresponding selection rules, which are used in assigning the spectra. All evidence obtained in this study supports the contention of Nelson et al. that NH3 is an inverting nearly free rotor in this T-shaped complex.
Molecular Physics. 10/1990; 71(2):453-460.
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ABSTRACT: Seven rovibrational transitions of the (01(1)0) <-- (00(0)0) fundamental bending band of C3 have been measured with high precision with the use of a tunable far-infrared laser spectrometer. The C3 molecules were produced by laser vaporization of a graphite rod and cooled in a supersonic expansion. The astrophysically important nu 2 fundamental frequency is determined to be 63.416529(40) cm-1. These measurements provide the basis for studies of C3 in the interstellar medium with far-infrared astronomy.
Science 08/1990; 249:897-900. · 31.20 Impact Factor
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ABSTRACT: Measurements of the fundamental van der Waals stretching vibration Σ(000,vs=1) ←Σ(000,vs=0) of Ar–H2O [ν0=907 322.08(94) MHz] and a transition from the lowest excited internal rotor state Σ(101,vs=0) to the Σ(101,vs=1) level [ν0=1019 239.4(1.0) MHz] are presented. A simultaneous rotational analysis of the new stretching data with the internal rotor bands observed by us previously [J. Chem. Phys. 89, 4494 (1988)], including the effects of Coriolis interactions, provides experimental evidence for the new assignment of the internal rotor transitions suggested by Hutson in the accompanying paper. Fits to the rotational term values for the vs=0 states are used to derive effective radial potential energy surfaces for each of the Σ internal rotor states. The results show the well depth (153.4 cm−1) of the effective radial potential for the Σ(101,vs=0) level to be approximately 25 cm−1 deeper than that for the Σ(000,vs=0) ground state of the complex, indicating that the former is stabilized considerably more by the anisotropic intermolecular potential energy surface than is the ground state.
The Journal of Chemical Physics 12/1989; 92(1):169-177. · 3.33 Impact Factor
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ABSTRACT: The ground state Ka =0(u)→1(g) b‐type subband of the rotation–tunneling spectrum of the symmetric 35Cl–35Cl, 37Cl–37Cl, and the mixed 35Cl–37Cl hydrogen chloride dimers have been recorded near 26.3 cm−1 with sub‐Doppler resolution in a continuous two‐dimensional supersonic jet with a tunable far‐infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H35Cl)2 center‐of‐mass separation of 3.81 Å is derived for the Ka =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20–25 deg for the hydrogen bonded proton and at least 70–75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15–18 cm−1, and the selection rules observed indicate that the trans tunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°.
The Journal of Chemical Physics 11/1988; 89(11):6577-6587. · 3.33 Impact Factor
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ABSTRACT: Laboratory observations of the pure rotational spectrum of HCO(+) in its
lowest excited bending state are reported. Because of their severe
excitation requirements, such vibrational satellites and the high-J
ground-state lines, also measured here, sample only hot, dense regions
of matter in active molecular cloud cores and circumstellar envelopes.
As the HCO(+) abundance is tied directly to the gas fractional
ionization, it is probable that the vibrationally excited formyl ion
transitions will provide high-contrast observations of shocked molecular
material, rather than the more quiescent, radiatively heated gas
surrounding stellar sources detected with the few vibrationally excited
neutral species observed to date.
The Astrophysical Journal 04/1987; 316:L45-L48. · 6.02 Impact Factor
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