An ab initio study of the rovibronic spectrum of sulphur monoxide (SO): diabatic vs. adiabatic representation.

We present an ab initio study of the rovibronic spectra of sulphur monoxide (32S16O) using internally contracted multireference configuration interaction (ic-MRCI) method and aug-cc-pV5Z basis sets. It covers 13 electronic states X3Σ-, a1Δ, b1Σ+, c1Σ-, A''3Σ+, A'3Δ, A3Π, B3Σ-, C3Π, d1Π, e1Π, C'3Π, and (3)1Π ranging up to 66 800 cm-1. The ab initio spectroscopic model includes 13 potential energy curves, 23 dipole and transition dipole moment curves, 23 spin-orbit curves, and 14 electronic angular momentum curves. A diabatic representation is built by removing the avoided crossings between the spatially degenerate pairs C3Π-C'3Π and e1Π-(3)1Π through a property-based diabatisation method. We also present non-adiabatic couplings and diabatic couplings for these avoided crossing systems. All phases for our coupling curves are defined, and consistent, providing the first fully reproducible spectroscopic model of SO covering the wavelength range longer than 147 nm. Finally, an ab initio rovibronic spectrum of SO is computed.

Angle-Resolved Electron Scattering from H_{2}O near 0°.

An electron beam, characterized by a high-angular discrimination (≃0.7°), has been used to measure the total (elastic plus inelastic) cross section of H_{2}O in the energy range 3-100 eV. Broad coincidence is found with recent experiments, including a pronounced shoulder in the 6-12 eV region. However, at energies ≲6 eV, the present cross sections are ≃30% higher. Furthermore, forward scattering has been probed in the angular range 0°-3.5° and measures of the average (rotationally and vibrationally summed) differential elastic cross sections for incident energies ≤12 eV are obtained at a scattering angle ≃1^{∘}. The measurements, which provide the first test of theoretical predictions in an angular region experimentally unexplored until now, are found to be within 1 standard deviation of corresponding ab initio R-matrix calculations.

Interaction of molecular nitrogen with free-electron-laser radiation.

We compute molecular continuum orbitals in the single center expansion scheme. We then employ these orbitals to obtain molecular Auger rates and single-photon ionization cross sections to study the interaction of N2 with Free-Electron-Laser (FEL) pulses. The nuclei are kept fixed. We formulate rate equations for the energetically allowed molecular and atomic transitions and we account for dissociation through additional terms in the rate equations. Solving these equations for different parameters of the FEL pulse, allows us to identify the most efficient parameters of the FEL pulse for obtaining the highest contribution of double core hole states (DCH) in the final atomic ion fragments. Finally we identify the contribution of DCH states in the electron spectra and show that the DCH state contribution is more easily identified in the photo-ionization rather than the Auger transitions.

Highly accurate intensity factors of pure CO2 lines near 2 µm.

Line intensities for carbon dioxide are measured with a novel spectroscopic approach, assisted by an optical frequency comb synthesizer for frequency calibration purposes. The main feature of the spectrometer consists in the exploitation of optical feedback from a V-shaped high-finesse optical resonator to effectively narrow a distributed feedback diode laser at the wavelength of 2 µm. Laser-gas interaction takes place inside an isothermal cell, which is placed on the transmission from the cavity. High quality, self-calibrated, absorption spectra are observed in pure CO2 samples at different gas pressures, in coincidence with three lines of the R-branch of the ν1 + 2ν2 + ν3 band. Line intensities are determined using a global fitting approach in which a manifold of spectra are simultaneously analyzed across the range of pressures between 5 and 100 Torr, sharing a restricted number of unknown parameters. Various sources of uncertainty have been identified and carefully quantified, thus leading to an overall uncertainty ranging between 0.17% and 0.23%. The measured values are in a very good agreement with recent ab initio predictions.

Stark coefficients for highly excited rovibrational states of H2O.

Quantum beat spectroscopy is combined with triple-resonance vibrational overtone excitation to measure the Stark coefficients (SCs) of the water molecule for 28 rovibrational levels lying from 27,600 to 41,000 cm(-1). These data provide a stringent test for assessing the accuracy of the available potential energy surfaces (PESs) and dipole moment surfaces (DMSs) of this benchmark molecule in this energy region, which is inaccessible by direct absorption. SCs, calculated using the combination of a high accuracy, spectroscopically determined PES and a recent ab initio DMS, are within the 1% accuracy of available experimental data for levels below 25,000 cm(-1), and within 4.5% for coefficients associated with levels up to 35,000 cm(-1). However, the error in the computed coefficients is over 60% for the very high rovibrational states lying just below the lowest dissociation threshold, due, it seems, to lack of a high accuracy PES in this region. The comparative analysis suggests further steps, which may bring the theoretical predictions closer to the experimental accuracy.

Water on the sun: line assignments based on variational calculations.

The infrared spectrum of hot water observed in a sunspot has been assigned. The high temperature of the sunspot (3200 K) gave rise to a highly congested pure rotational spectrum in the 10-micrometer region that involved energy levels at least halfway to dissociation. Traditional spectroscopy, based on perturbation theory, is inadequate for this problem. Instead, accurate variational solutions of the vibration-rotation Schrödinger equation were used to make assignments, revealing unexpected features, including rotational difference bands and fewer degeneracies than anticipated. These results indicate that a shift away from perturbation theory to first principles calculations is necessary in order to assign spectra of hot polyatomic molecules such as water.

State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions.

We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and nonreactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2 = 1, 2), while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ toward the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2 = 2 might be significant in the photon-dominated region of NGC 7027.

Spectroscopy of H3+ and its impact on astrophysics.

Since the original laboratory detection of an H3+ spectrum 20 years ago, the search has been on for astronomical observations of this important and fundamental molecular ion. Successful detection of H3+ in gas-giant planets, supernova ejecta and the interstellar medium as well as the prospects for future observations are discussed. The role H3+ has in determining the atmospheric structure of both the gas giants and cool metal-free planets is explored.

An adiabatic model for calculating overtone spectra of dimers such as (H(2)O)(2).

The near-infrared and visible wavelength spectrum of the water dimer is considered to be the major contributor to the so-called water continuum at these wavelengths. However, theoretical models of this spectrum require the simultaneous treatment of both monomer and dimer excitations. A model for treating this problem is proposed which is based upon a Franck-Condon-like separation between the monomer and dimer vibrational motions. In this model, one of the monomers is treated as the chromophore and its absorption is assumed to be given by its, possibly perturbed, vibrational band intensity. The main computational issue is the treatment of separate monomer and dimer motions. Various approaches for obtaining dimer vibration-rotation tunnelling spectra that allow for monomer motion are explored. These approaches include ways of treating the adiabatic separation of dimer vibrational modes from monomer vibrational modes. We classify the adiabatic separation methods under four main approaches: namely fixed-geometry, free-monomer, perturbed-monomer and coupled-monomer methods. The latter being the most computationally expensive as the monomer wave functions are dependent on the dimer coordinates. For each of these approaches, expectation values over the full potential are calculated for the given monomer vibrational wave functions. Various full (named VAP 2pD in the text) and partial (VAP (+p)D) averaging techniques are outlined to calculate the vibrationally averaged, monomer state-dependent, dimer interaction potentials. The computational costs associated with application of these techniques to the water dimer are estimated and the prospects for full calculations based on this approach are assessed.

Sympathetic cooling by collisions with ultracold rare gas atoms, and recent progress in optical Stark deceleration.

We propose a general scheme for sympathetic cooling of molecules to microK temperatures on a timescale of seconds. Experimental parameters have been estimated from theory, which indicate the viability of the scheme. This method, which is particularly suited to optical Stark deceleration, utilises ultracold, laser cooled metastable rare gas atoms quenched to their ground state as collision partners to co-trapped molecular species within a deep optical trap (150 mK). We also describe the measurement of the role of laser-induced molecular alignment on the dipole force in optical Stark deceleration and outline progress towards the realisation of chirped optical Stark deceleration for producing slow molecular beams with mK energy spreads.

Rotational cooling of HD+ molecular ions by superelastic collisions with electrons.

Merging an HD+ beam with velocity matched electrons in a heavy ion storage ring we observed rapid cooling of the rotational excitations of the HD+ ions by superelastic collisions (SEC) with the electrons. The cooling process is well described using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclei rotation approximation. We verify the DeltaJ=-2 SEC rate coefficients, which are predicted to be dominant as opposed to the DeltaJ=-1 rates and to amount to (1-2)x10;{-6} cm;{3} s;{-1} for initial angular momentum states with J< or =7, to within 30%.

Fluorescence studies of cytochrome b5 topography. Incorporation of cytochrome b5 into brominated phosphatidylcholine vesicles by deoxycholate.

Cytochrome b5 was incorporated into large vesicles of 1-palmitoyl-2-dibromostearoylphosphatidylcholine by mixing lipid, protein, and deoxycholate followed by removal of the detergent by gel filtration. The tryptophan fluorescence emanating from the hydrophobic membrane-binding domain was quenched more effectively when the bromine atoms were in the 6,7-positions than when they were in the 15,16-positions of the acyl chain. To more precisely define the position of the quenchable tryptophan, the experiment was repeated with lipids with the bromine atoms at the 4,5-, 6,7- or 9,10-positions. Again the 6,7 species was the most efficient quencher. The cytochrome b5 bound to these vesicles would not transfer to small unilamellar sonicated vesicles and so was in the "tight" configuration. If the cytochrome were added to the vesicles after the detergent was removed, the same order of quenching was seen but the cytochrome would transfer to other vesicles. These data indicate that the quenching of the tryptophan fluorescence is greatest when the bromines are at the 6,7-positions whether the vesicles are large or small and whether the cytochrome is in the tight or "loose" configuration and so place the tryptophan 0.7 nm below the vesicle surface in all of these membranes.

Fluorescence quenching of cytochrome b5 in vesicles with an asymmetric transbilayer distribution of brominated phosphatidylcholine.

Several fluorescence techniques have been used to estimate the depth, in the membrane, of the endogenous tryptophans of membrane-bound proteins. We reported recently the use of phosphatidylcholines specifically brominated at different positions of the sn-2 acyl chain for this purpose (Markello, T., Zlotnick, A., Everett, J., Tennyson, J., and Holloway, P. W. (1985) Biochemistry 24, 2895-2901). The membranes made from these brominated lipids will have the brominated lipid in both monolayers, and so the estimated depth of the fluorophore will be relative to either the inner or outer surface of the membrane, but will not distinguish between these two extremes. To differentiate between these two models vesicles have now been made with an asymmetric distribution of brominated lipid, by use of phosphatidylcholine exchange protein. The asymmetric vesicles were isolated by virtue of their density, and their asymmetry was established by addition of an amphipathic fluorescent carbazole compound. With these vesicles it was shown that the tryptophan in the membrane-binding domain of cytochrome b5 which is quenched by bromolipid is located 0.7 nm below the outer surface of the membrane vesicles, rather than 0.7 nm from the inner surface.

The Spectrum of Hot Water: Rotational Transitions and Difference Bands in the (020), (100), and (001) Vibrational States

Analysis of the hot H2 16O spectrum, presented by Polyansky et al. (1996, J. Mol. Spectrosc. 176, 305-315), is extended to higher vibrational states. Three hundred thirty mainly strong lines are assigned to pure rotational transitions in the (100), (001), and (020) vibrational states. These lines, which involve significantly higher rotational energy levels than were known previously, are assigned using high-accuracy variational calculations. Transitions in (020) are assigned up to Ka = 18, compared with the maximum Ka of 10 known previously. Crossings of vibration-rotation energy levels result in the observation of extra intensity-stealing transitions. In particular, this leads to the assignment of (020)-(100) and (100)-(020) rotational difference band transitions in addition to the conventional pure rotational lines in (020) and (100) states. These extra lines increase the number of transitions and they are likely to complicate the pure rotational water spectrum in higher excited vibrational states to an even greater extent. A few lines from our previous work on the pure rotational spectrum of hot water in the (000) and (010) vibrational states are also reassigned and some further assignments are made. Copyright 1997 Academic Press. Copyright 1997Academic Press

Imaging Jupiter's aurorae from H3+ emissions in the 3-4 micrometers band.

Since H3+ was first spectroscopically detected on Jupiter, there has been considerable interest in using this simple molecular ion to probe conditions existing in the planet's auroral regions. Here we present a series of images of Jupiter recorded at wavelengths sensitive to emission by H3+, which reveal the spatial distribution of excited H3+ molecular ions in the jovian ionosphere, as seen from Earth. We believe that they provide high-spatial-resolution images of polar aurorae on Jupiter. They suggest that the intensity of the auroral emission can vary on a timescale of an hour, a shorter period than had previously been noted. We also find that the spatial distribution of H3+ emissions correlates only partially with the loci of auroral activity inferred from ultraviolet and longer-wavelength infrared observations. The H3+ emission may therefore be controlled by auroral processes that are different from those responsible for the ultraviolet and infrared emissions.

Determination of the topography of cytochrome b5 in lipid vesicles by fluorescence quenching.

Cytochrome b5, a protein isolated from the endoplasmic reticulum by detergent extraction, interacts spontaneously with small unilamellar phosphatidylcholine vesicles. When the vesicles are made from 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), the tryptophan fluorescence of the cytochrome is enhanced, and when they are made from 1-palmitoyl-2-(dibromostearoyl) phosphatidylcholine (BRPC), the fluorescence is quenched. A series of BRPC were synthesized with bromine atoms at the 6,7, 9,10, 11,12 or 15,16 positions. The vesicles synthesized from each of these lipids were similar in size to those made from POPC. The relative fluorescence intensities of the cytochrome b5 in POPC and 6,7-, 9,10-, 11,12- and 15,16- BRPC were 100, 19.4, 29.4, 37.1, and 54.0, respectively. These data suggest that the exposed tryptophan(s) is (are) at a depth of 0.7 nm below the surface of the vesicle. Bromine is a collisional quencher; hence, these data may indicate the relative position of the lipid annulus around the protein rather than the depth of the protein below the average vesicle surface. Cytochrome b5 contains three potentially fluorescent tryptophans, and determinations of fluorescent quantum yield indicate all three potentially fluorescent tryptophans, and determinations of fluorescent quantum yield indicate all three are fluorescent with an average quantum yield, when in POPC vesicles, of 0.21. Fluorescence lifetime measurements by the demodulation technique indicated heterogeneity of fluorescence lifetimes in all vesicles. The lifetimes in the BRPC vesicles ranged from 2.0 to 2.4 ns compared to a value of 3.3 ns in POPC.(ABSTRACT TRUNCATED AT 250 WORDS)