Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session C6: Spectroscopy of Molecules and Molecular Clusters |
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Chair: N. Moazzen-Ahmadi, University of Calgary Room: TELUS Convention Centre Olde Scotch Room |
Wednesday, June 6, 2007 1:30PM - 1:42PM |
C6.00001: Observation of the `missing' polar OCS dimer. Mahin Afshari, Mehdi Dehghani, Ziad Abusara, Nasser Moazzen-Ahmadi, Robert McKellar The lowest energy, non-polar isomer of (OCS)$_{2}$ has long been known from infrared spectroscopy, while the polar form has only been deduced from qualitative beam ``refocusing'' experiment. We have observed a new infrared band at 2069.3 cm$^{-1}$ and assigned to the long-anticipated polar isomer of OCS dimer, helping to explain apparent discrepancies among earlier studies. Reported data which have been confirmed by direct observation of the rotational spectrum of polar OCS dimer, should motivate new theoretical work on the energies of OCS dimer isomers and interconversion energy barriers. OCS dimer is now one of the rare weakly bound clusters to have more than one isomer observed in high resolution spectroscopy. [Preview Abstract] |
Wednesday, June 6, 2007 1:42PM - 1:54PM |
C6.00002: The structure of OCS tetramer from infrared laser spectroscopy Mehdi Dehghany, Mahin Afshari, Ziad Abusara, Nasser Moazzen-Ahmadi, A.R.W. McKellar The motivation for studying weakly bound complexes is building a bridge between the gas and condensed phase. There are many condensation pathways that can be taken, owing to exponentially increasing number of isomers with cluster size. The factors that determine which of these are preferred and why are not well understood. High resolution infrared laser spectroscopy has been used for the first time to determine the structure of OCS tetramer. We observe a non-planar structure in which all four monomers are approximately parallel. The spectra are best described as an asymmetric top with an accidental spherical top structure. The spectrometer and analysis of data will be presented. [Preview Abstract] |
Wednesday, June 6, 2007 1:54PM - 2:06PM |
C6.00003: The Pure Inversion-Tunneling Transition of Ammonia in Helium Droplets Rudolf Lehnig, Wolfgang J\"ager The pure inversion tunneling transition of ammonia embedded in superfluid helium droplets has been measured for the two isotopomers $^{14}$NH$_3$ and $^{15}$NH$_3$. The transitions were found at 21.72\,GHz for $^{14}$NH$_3$ and at 20.68\,GHz for $^{15}$NH$_3$ and are therefore shifted by 1.97\,GHz and 1.94\,GHz to the red compared to the gas phase values of the two isotopomers, respectively [Pickett {\it et al.}, J. Quant. Spectrosc. Radiat. Transfer {\bf 60}, 883 (1998).]. The measured peaks consist of a broad background with a width at half maximum of 1.5\,GHz and an unexpectedly sharp peak with a width at half maximum of only 15\,MHz sitting on top of the broad background. The difference between gas phase and droplet spectra will be discussed and a interpretation of the complex line shape will be given. [Preview Abstract] |
Wednesday, June 6, 2007 2:06PM - 2:18PM |
C6.00004: Cyanoacetylene-doped \textit{ortho}-hydrogen and \textit{para}-hydrogen clusters studied using rotational spectroscopy Julie Michaud, Wolfgang J\"{a}ger A high-resolution Fourier transform microwave spectrometer was used to measure the rotational spectra of the \textit{ortho}H$_{2}$ -- HCCCN and \textit{para}H$_{2}$ -- HCCCN dimers as well as larger (\textit{ortho}H$_{2})_{N}$ -- HCCCN and (\textit{para}H$_{2})_{N}$ -- HCCCN van der Waals clusters ($N$ = 2 and greater). To generate the clusters, low concentrations of HCCCN and H$_{2}$ (normal or enriched \textit{para}H$_{2})$ in helium are used at pressures up to 70 atm. The helium supersonic jet expansion into the spectrometer cavity/sample cell produces a collision free environment where the clusters are stabilized and studied. Varying the backing pressure, sample concentrations and/or nozzle temperature can control cluster size. The doped \textit{para}H$_{2}$ clusters are of particular interest because \textit{para}H$_{2}$ is suspected to exhibit the bulk property of superfluidity, similar to $^{4}$He. The study of the successive solvation of HCCCN with H$_{2}$ molecules may provide evidence for superfluidity in doped H$_{2}$ clusters. [Preview Abstract] |
Wednesday, June 6, 2007 2:18PM - 2:30PM |
C6.00005: High resolution infrared spectra of helium clusters doped with carbon dioxide or nitrous oxide Robert McKellar Infrared laser spectroscopy is used to study low temperature ($\sim $0.2 K) helium clusters in a pulsed supersonic jet expansion. The clusters are doped with a probe molecule (CO$_{2}$ or N$_{2}$O) whose vibration-rotation spectrum is observed as a function of cluster size. Our previous work on smaller clusters (N $<$ 20) is extended to larger clusters (N $\sim $ 70) using a new apparatus with a skimmed jet nozzle. Various isotopes are studied in order to make the assignment of cluster size more secure, including the asymmetric species $^{16}$O$^{13}$C$^{18}$O. The cluster rotational parameters exhibit broad oscillations, similar to those recently observed in He$_{N}$ - OCS, which are related to the formation of superfluid helium solvation shells around the probe molecule. [Preview Abstract] |
Wednesday, June 6, 2007 2:30PM - 2:42PM |
C6.00006: Spectroscopic and \textit{ab initio} study of the open-shell Xe-O$_{2}$ van der Waals Complex Qing Wen, Wolfgang J\"ager Rotational spectra of the open-shell Xe-O$_{2}$ van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. Magnetic hyperfine structure arising from the earth's magnetic field was observed and reduced by mounting Helmholtz coils on to the sample chamber. Several spectral lines measured in the in the 4-18 GHz region were assigned to transitions within the $\Omega $ = 0 spin component of the $^{3}\Sigma ^{- }$electronic ground state. Transitions within the $\Omega =\pm $1 spin components were not observed because of the small population in these levels. The hyperfine structures due to Fermi contact coupling between the electron spin of O$_{2 }(S$ = l) and the $^{129}$Xe nuclear spin ($I $= 1/2) were detected and analyzed. The determined spectroscopic constants, including the Fermi contact coupling constant, were used to derive information about the electronic and geometric structure of the complex. The experimental data were complemented by the construction of an \textit{ab initio} potential energy surface at the RCCSD(T) level of theory. The results provide detailed insight into the intermolecular interaction between Xe and paramagnetic oxygen. This may help to understand the relaxation mechanism of hyperpolarized $^{129}$Xe in human blood in \textit{in vivo} magnetic resonance imaging experiments. [Preview Abstract] |
Wednesday, June 6, 2007 2:42PM - 2:54PM |
C6.00007: Intermolecular Hydrogen Bonding Effect on the VA and VCD Spectra of Lactic Acid in Water and in Methanol: Experimental and DFT Studies Martin Losada, Ha Tran, Yunjie Xu Understanding the structure, stability and formation of intermolecular H-bonded complexes involving chiral molecules on the molecular level is of crucial importance in life sciences. Carboxilic acids are well known for their strong intermolecular associations. Lactic acid (LA) is studied using vibrational absorption (VA) and vibrational circular dichroism (VCD) spectroscopic techniques to examine the effect of dimerization. Experimental results indicate that for LA in CDCl$_{3}$ solution, a complex equilibrium exists between the monomers and dimers. Furthermore, VCD spectroscopy was used to probe the solute-solute and solute-solvent H-bonding interactions of LA-water and LA-methanol complexes in solution phase. Geometry optimizations were carried out for LA monomer, LA dimer and both LA-(H$_{2}$O)$_{n}$ and LA-(CH$_{3}$OH)$_{n}$ complexes, with $n = 1, 2, 3$, using both B3PW91/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels of theory. Some of these clusters were also investigated at the B3LYP/cc-pVTZ level. Detailed spectral simulations were performed in order to understand the dependence of the VA and VCD spectra on the specific binding characters. Three solvent models were exploited to evaluate solvent effects; a pure implicit continuum approach, a pure explicit model with up to three solvent molecules, and a combined approach. [Preview Abstract] |
Wednesday, June 6, 2007 2:54PM - 3:06PM |
C6.00008: Microwave spectroscopic studies of molecular recognition: Analysis of diastereomeric interactions between ethanol and oxirane derivatives Nicole Borho, Yunjie Xu High resolution microwave spectroscopy complimented by ab initio calculations has been used to elucidate the diastereomeric interactions in a set of small model complexes. Ethanol, a transient chiral alcohol, was combined with oxirane (achiral), methyl-oxirane\footnotemark[1] (1 stereocenter) and trans-2,3-dimethyloxirane (2 stereocenters) to form hydrogen-bonded 1:1 complexes. The rotational constants of two conformers of ethanol$\cdots$oxirane, six conformers of ethanol$\cdots$methyl-oxirane and three conformers of ethanol$\cdots$trans-2,3-dimethyloxirane have been determined and the relative stability order of the conformers has been established. The dependence of the observed intensity on pressure, nozzle temperature and different carrier gases has been investigated for the case of ethanol$\cdots$trans-2,3-dimethyloxirane to give a first insight into the kinetical and thermodynamical influence on the formation of different conformers. The step-by-step methyl addition to oxirane helps to unravel the diastereomeric interactions at play via analysis of the subtle energy differences between each set of conformers, allowing for a detailed understanding of molecular recognition in this benchmark system.\\ \footnotemark[1]{Nicole Borho, Yunjie Xu, \textit{Angew. Chem.}, \textbf{2006}, (VIP paper, Published Online: 17 Nov 2006, DOI: 10.1002/anie.200603809).} [Preview Abstract] |
Wednesday, June 6, 2007 3:06PM - 3:18PM |
C6.00009: A New VISTA on the Infrared Spectrum of Ammonia in the 1.5 $\mu $m Region: Assignment of Combination Bands of $^{14}$NH$_{3}$ and $^{15}$NH$_{3}$ by Isotopic Shift Fingerprinting Ronald Lees, Li Li, Li-Hong Xu The infrared spectrum of ammonia in the 1.5 $\mu $m region contains a complex mixture of vibrational combination and overtone bands. By comparing spectra of $^{14}$NH$_{3}$ and $^{15}$NH$_{3}$ recorded with a tunable diode laser spectrometer, we have been able to implement a Vibrational Isotopic Shift Technique for Assignment (VISTA) in which isotopic shift fingerprinting (ISF) is used to classify spectral lines into their respective absorption bands. We have thereby been able to identify numerous lines belonging to the strong $\nu _{3}$+2$\nu _{4}$ band for $^{14}$NH$_{3}$, extend assignments for previously known $^{14}$NH$_{3 }$bands, and simultaneously assign the corresponding bands for $^{15}$NH$_{3}$. [Preview Abstract] |
Wednesday, June 6, 2007 3:18PM - 3:30PM |
C6.00010: A global frequency analysis of data in the four lowest vibrational states of ethane Leila Borvayeh, Nasser Moazzen-Ahmadi The $\nu _{9}$ fundamental band of ethane occurs in the 12 $\mu $m region. It is the strongest band of ethane in a terrestrial window and is commonly used to determine ethane's abundance in the atmospheres of the Jovian planets and comets, and to determine their temperature. The $\nu _{9}+\nu _{4}-\nu _{4 }$ band occurs in the same region; neither can be analyzed as an isolated band, since both are embedded in the torsional bath of the ground vibrational state. Precise and accurate absolute intensities of these bands are crucial for correct interpretation of recent Cassini observations of ethane spectra in the atmospheres of Saturn and Titan. Although, our group has carried out a satisfactory frequency analysis of the $\nu _{9}$ fundamental, a complete analysis of $\nu _{9}+\nu _{4}-\nu _{4}$ is hampered due to an interaction with the $\nu _{12}$ fundamental. This fundamental vibration is infrared inactive. It is also very weakly Raman active. To access this vibrational state, we have obtained a high resolution Fourier transform spectrum of the weak $\nu _{12}-\nu _{9}$ band using a Bruker IFS120HR. In this talk, I will describe a global frequency analysis of data including the four lowest vibrational states of ethane. [Preview Abstract] |
Wednesday, June 6, 2007 3:30PM - 3:42PM |
C6.00011: \textit{Ab initio} and Rotational Spectroscopic Study of Propylene Oxide -- (Water)$_{N=2/3}$ Complexes Zheng Su, yunjie Xu Water is the principal constituent of the environment for all living organisms. Nearly all biological molecules required for life are chiral. Therefore the studies of the solvation of chiral molecules in water are of fundamental importance to life science. In this work, we report \textit{ab initio} and rotational spectroscopic studies of the hydrogen bonded propylene oxide (PO) -- (H$_{2}$O)$_{N =2/3}$ clusters, which is a continuing study from our success on PO with one water molecule in the gas phase. The sequential complexation of PO with a few water molecules is a significant step towards understanding the solvation process for this simplest cyclic ether chiral molecule. Complete geometry optimizations for the PO-water complexes are carried out at the MP2 level of theory with the 6-311++G(d,p) basis set using the GAUSSIAN03 software package. The calculated rotational constants and dipole moment components are used to aid the initial spectroscopic investigations. By systematically increasing the pressure, attachment of more water molecules to PO can be formed and distinguished from only one. Both experimental and theoretical results are used to extract structural and dynamic information about the complexes. The experimental analysis will in turn be used to judge the quality of the theoretical predictions and then determine the appropriate model for further calculations. [Preview Abstract] |
Wednesday, June 6, 2007 3:42PM - 3:54PM |
C6.00012: Experimental determination of the Herman-Wallis factor of Q-branch Raman transitions Michele Marrocco Vibration-rotation interaction is crucial to the accurate understanding of spectroscopic line intensities of light molecules. The interaction is quantified by the so-called Herman-Wallis (HW) factor F [J. Chem. Phys. 23, 637 (1955)] whose calculation is debatable for Q-branch transitions. Indeed, different authors have suggested different HW factors in manifest disagreement with each other. To clarify this matter, an experiment based on vibrational coherent anti-Stokes Raman spectroscopy (CARS) applied to H2 molecules has been realized. The idea behind this experimental proof is very simple. First of all, the spectra have been acquired from a very stable adiabatic H2/air flame, whose temperature is well known. In this way, the optical nonlinearity and the high temperature have enhanced the dependence of the data on the factor F. Secondly, the determination of the best expression for F has been based on the comparison of the measured line Q(7) with calculated line strengths obtained from the HW factors found in literature. The sole HW factor that justifies the experimental data is given in the work of Tipping and Bouanich [J. Quant. Spectrosc. Radiat. Transfer 71, 99 (2001)]. [Preview Abstract] |
Wednesday, June 6, 2007 3:54PM - 4:06PM |
C6.00013: Off-axis cavity enhanced absorption spectrometer based on a mid-infrared continuous wave quantum cascade laser Yunjie Xu, Raviraj Kulkarni, Waishun Tam, Zheng Su, Igor Leonov We present the design and construction of a mid-infrared off-axis cavity enhanced absorption (OA-CEAS) spectrometer based on a cw quantum cascade laser in combination with a pulsed slit jet molecular beam expansion. A computer program was developed to automate and to synchronize the timing of the CEAS experiments with the pulsed molecular beam. The dominant source of noise in CEAS experiments arises from incomplete averaging of the cavity mode structures. The suppression of noise was accomplished by implementing the following procedures: (1) Optimize off-axis alignment to excite as many higher order transverse modes as possible. (2) Modulate the cavity length using a piezoelectric actuator mounted on one of the cavity mirrors. (3) Introduce mechanical perturbation in the optical cavity to randomize the residual mode structures. (4) Optimize laser frequency speed to even cavity mode intensities. (5) Apply wavelength modulation to the laser. The CEAS experimental result measured with a static NH$_{3}$ gas and with jet-cooled larger organic molecules will also be presented. [Preview Abstract] |
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