Metal clusters are of interest in exploring the transition from atomic to bulk properties in metals. Spectroscopic measurements of metal clusters also provide important benchmarks for theoretical investigations of bonding in metallic systems. In particular, theoretical predictions of properties of small Be clusters are highly dependent on the level of theory employed, and experimental data would provide an important test of the accuracy of the theoretical predictions. To obtain experimental data on the electronic structure of Be clusters, we have constructed a pulsed cavity ringdown spectrometer designed to measure gas-phase metal clusters. We generate the clusters via laser ablation of a metal rod followed by supersonic expansion of the ablated material. The supersonic expansion is then probed by cavity ringdown spectroscopy using an excimer-pumped pulsed dye laser. We will present preliminary results using the spectrometer to observe small clusters of Cu and Al in preparation for observing Be clusters. We will also present any data on Be clusters that are available.

The surprisingly high dissociation energy of the He-Be bond in the He-BeO complex was first reported 25 years ago.¹ Following which, a number of theoretical studies have investigated similar closed shell helium containing complexes. However, despite these investigations, a complex containing a strong He-X bond has thus far eluded experimental detection. In this work, potential energy surfaces of electronically excited states of the He-BeO complex have been calculated employing high level CASSCF+MRCI+Q methodologies and utilizing extended basis sets. Several excited states show strong interactions between helium and BeO lying in Franck-Condon accessible windows of electronic transitions arising from the vibrationless electronic ground state. It is hoped that the conclusions of this study will result in the observation an electronic spectrum of this long hypothesized strongly bound complex in the near future.
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¹W. Koch, J. R. Collins and G. Frenking, Chem. Phys. Lett. 1986, 132 330-333.

A method is developed that gives a unified perspective on the nature of a wide range of pairwise interatomic interactions. The approach is applied to the diatomic molecules CO, H₂⁺, H₂, HF, LiH, Li₂, O₂, and Ar₂, and one-dimensional cuts through the potentials of OC-HBr, OC-HF, OC-HCCH, OC-HCN, OC-HCl, OC-HI, OC-BrCl, and OC-Cl₂. Systems selected for investigation illustrate different bound categories varying from van der Waals, halogen bonded, hydrogen bonded to strongly bound covalently bound carbon monoxide with binding energies varying over almost three orders of magnitude, from 99.3 cm⁻¹to 90683 cm⁻¹. Accurate semi-empirically determined Rydberg-Klein-Rees or morphed interatomic potentials are used in transformations for this wide range of species to a reduced potential demonstrating commonality in their fundamental nature.

Rovibrational transitions associated with tunneling states in the vibration of the N₂-H₂O complex have been recorded using a supersonic jet quantum cascade laser spectrometer at 6.2μm. Analysis of the resulting spectra is facilitated by incorporating fits of previously recorded microwave and submillimeter data accounting for Coriolis coupling to obtain the levels of the ground vibrational state. The results are then used to confirm assignment of the ν₃ vibration and explore the nature of tunneling dynamics in associated vibrationally excited states of the complex.

Rotationally-resolved infrared spectra of normal and bideuterated N₂O-diacetylene van der Waals complexes were observed in the N₂O ν₁ region ( ∼ 2224 cm⁻¹) using a Quantum Cascade Laser to probe a supersonic slit-jet expansion. The observed rotational constants show that the structure has C_s symmetry with the constituent monomers in a slipped near-parallel configuration. ab initio calculations at MP2 level indicate that the observed structure is the lowest energy isomer. Our recent investigation on complexes containing CS₂ and C₂D₂ using an Optical Parametric Oscillator has resulted in the observation of several bands, one of which is centered at 2438.16 cm⁻¹. This b-type band was assigned to CS₂−C₂D₂ dimer with parallel monomers and C_2v symmetry, in agreement with ab initio calculations for the lowest energy isomer. In this talk, we discuss our observation and analysis of N₂O-diacetylene, CS₂−C₂D₂ dimer and probable complexes containing CS₂ and C₂D₂ in our future observation.

Spectra of the weakly-bound CO₂-C₂H₂ and CO₂-C₂D₂ complexes are observed in the regions of CO₂ ν₃ ( ∼  2340 cm⁻¹) and C₂D₂ ν₃ ( ∼  2440 cm⁻¹) fundamental vibrations, using an infrared Optical Parametric Oscillator (OPO) to probe a pulsed supersonic slit-jet expansion. Five bands are measured and analyzed: the fundamental asymmetric stretch of the C₂D₂ component, two combination bands involving the out-of-plane torsional vibrations for CO₂-C₂D₂, and two combination bands involving an intermolecular in-plane bending vibration for CO₂-C₂H₂ and CO₂-C₂D₂. The measured intermolecular vibrational frequencies are 61.408(1), 54.5(5), 39.9(5), and 39.961(1) cm⁻¹, respectively. Torsional vibrational frequencies are in good agreement with the value of 41 cm⁻¹ estimated by Muenter¹ using a harmonic oscillator model. The intermolecular vibrations provide clear spectroscopic data against which theory can be benchmarked. These results will be discussed, along with a brief introduction of our pulsed-jet supersonic apparatus which has been retrofitted with an infrared tunable Optical Parametric Oscillator (Lockheed Martin Aculight Argos^TM).
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¹J. S. Muenter, J. Chem. Phys. 90, 2781 (1991)

Stimulated by Fajardo's remarkable study of the rovibrational spectra of CO isotopologues trapped in solid para-hydrogen,¹ we have performed quantum Monte Carlo simulations to predict his observed vibrational frequency shifts and inertial rotational constants using 2-body potentials based on the best available models for the pH₂-pH₂ ² and CO-pH₂ ³  potential energy functions. We started by fitting an analytic `Morse/Long-Range' (MLR) function⁴ to the 1D "adiabaic hindered rotor" version of Hinde's 5D pH₂-pH₂ potential developed by Faruk et al.⁵ We then modified it to take account of many-body effects by scaling it until it yielded the correct equilibrium lattice parameters for the fcc and hcp structures of pure solid para-hydrogen. A CO molecule was then placed at different interstitial or substitution sites in large equilibrated fcc or hcp para-hydrogen lattices, and the structural and dynamical behaviors of the micro-solvation environment around CO were simulated with a PIMC algorithm using a 2D effective pH₂-CO potential based on the 5D H₂-CO potential energy surface recently reported by Li et al.,⁶ with a lattice sum of values of the 2D CO vibrational difference potential being use to predict the vibrational frequency shift. The effective rotational constants B_eff for CO in different solid para-hydrogen structures were also calculated and compared with the experimental observations and with predicted B_eff values for CO in large-sized para-hydrogen-CO clusters.⁷ 
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¹ M.\ E. Fajardo, J. Phys. Chem. A 117, 13504 (2013). ²R. Hinde, J. Chem. Phys.  128, 154308 (2008). ³ H. Li, X-L. Zhang, R.J. Le Roy, and P.-N. Roy, J. Chem. Phys. 139, 164315 (2013). ⁴ R.J. Le Roy, C.C. Haugen, J. Tao and Hui Li, Mol. Phys. 109, 435 (2011). ⁵ N. Faruk, R.J. Le Roy, and P.-N. Roy, J. Chem. Phys. (submitted December 2013). ⁶ H. Li, X-L. Zhang, R. J. Le Roy, and P.-N.\ Roy, J. Chem. Phys. 139, 164315 (2013). ⁷ Y. Mizumoto and Y. Ohtsuki, Chem. Phys.\ Lett. 501, 304 (2011).

Infrared spectra of He-C₂H₂ were recorded around 1990 in Roger Miller's lab, but detailed rotational assignment was apparently not possible even with the help of theoretical predictions. So there were no published experimental spectra of helium-acetylene van der Waals complexes until our recent work on He-C₂D₂ in the ν₃ region ( ∼ 2440 cm⁻¹).¹ The problem is that this complex lies close to the free rotor limit, so that most of the intensity in the spectrum piles up in tangles of closely spaced lines located close to the monomer rotational transitions, R(0), P(1), etc. Our previous He-C₂D₂ assignments were limited to the R(0) region, that is, the j = 1 ← 0 subband, where j represents C₂D₂ rotation. Here, we extend the analysis to j = 0 ← 1 and 2 ← 1 transitions with the help of new spectra obtained using a tunable OPO laser probe and a cooled supersonic jet nozzle. These subbands are weaker, not only because of the Boltzmann factor, but also the 2:1 nuclear spin statistics of j" = even:odd C₂D₂ levels. Moreover, the j = 0 ← 1 subband is overlapped by strong (C₂D₂)₂ transitions. We use a term value approach, obtaining a self-consistent set of "experimental" energy levels which can be directly compared with theory or fitted in terms of a Coriolis model. Challenges also arise with Ne-C₂D₂, which is not quite so close to the free rotor limit, but still has many overlapping lines. Insights gained here help in assigning the tricky R(1) region for Ne-C₂D₂.
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¹M. Rezaei, N. Moazzen-Ahmadi, A.R.W. McKellar, B. Fernández, and D. Farrelly, Mol. Phys. 110, 2743 (2012).

Infrared spectrum of a slipped near parallel isomer of OCS−C₄H₂ was observed in the region of ν₁ fundamental band of OCS monomer ( ∼ 2062 cm⁻¹) using a diode laser to probe the supersonic slit jet expansion. The ab initio calculations at MP2 level indicate that the observed structure is the lowest energy isomer. The OCS−C₄H₂ band is composed of hybrid a/b-type transitions and was simulated by a conventional asymmetric top Hamiltonian with rotational constants of A=2892.15(10) MHz, B=1244.178(84) MHz, and C=868.692(52) MHz. The spectrum shows a relatively large red-shift of  ∼ 6 cm⁻¹with respect to the OCS monomer band origin. Also, one band for (OCS)₂−C₄H₂ trimer is observed around 2065 cm⁻¹. This band is blue-shifted by 3 cm⁻¹relative to the ν₁ fundamental band of OCS monomer. Our analysis shows that this trimer has C₂ symmetry with rotational constants of A= 855.854(61) MHz, B=733.15(11) MHz, and C=610.10(38) MHz and c-type transitions. This structure is comparable with that of (OCS)₂−C₂H₂ where the OCS dimer unit within the trimer is non-polar.¹ In addition to the normal isotoplogues, OCS−C₄D₂ and (OCS)₂−C₄D₂ were observed. In this talk, we discuss our observations and analysis on OCS−C₄H₂ dimer and (OCS)₂−C₄H₂ trimer.
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¹Mojtaba Rezaei, A. R. W. McKellar, and N. Moazzen-Ahmadi, J. Phys. Chem. A, 115, 10416 (2011).

Structure determination of weakly bound OCS clusters is a challenging problem due to many low energy isomers on the potential energy surface. The premier tool for studying these clusters is high-resolution infrared spectroscopy, as it can be used to analyze non-polar clusters. Following the analysis of high-resolution IR spectra of clusters formed in a molecular beam expansion of OCS there were some outstanding questions about the structures of the observed clusters. The chirped-pulse Fourier transform microwave spectrum in the 3-9 GHz frequency range was measured for a pulsed molecular beam of OCS in neon (1%). All ¹³C, ¹⁸O and ³⁴S isotopologues of the previously detected OCS trimer have been observed in natural abundance in the 3-9 GHz band using chirped-pulse Fourier transform microwave spectroscopy. The structure of this trimer features a barrel-shaped structure with two aligned and one anti-aligned OCS monomers. A new OCS trimer is also observed for the first time, and its structure is consistent with a barrel-shaped structure with 3 aligned monomers. Using the infrared spectrum for guidance, a spectrum corresponding to a polar OCS tetramer has been assigned. This cluster has a similar barrel-like structure but with an additional tilted OCS monomer added to the top of the barrel. All ¹³C and ³⁴S isotopologues have been assigned for the tetramer. However, due to sign ambiguities in Kraitchman's equations, and small rotational constant differences between aligned and anti-aligned combinations of OCS molecules in the trimer barrel, absolute structural assignment is indeterminate without additional constraints. Therefore a combinatoric approach was used to compute the most reasonable tetramer structure using distance and sign constraints between pairs of carbon and sulfur coordinates, assuming the experimental OCS monomer structure. Results of this approach will be presented, as well as a comparison of the experimental results with the most recent ab initio structures for the OCS tetramer.

An OCS trimer was originally observed in the 1990s by microwave spectroscopy.¹ New broadband chirped-pulse microwave spectra (preceding talk) reveal an OCS tetramer and a second distinct trimer isomer. In the present talk, we discuss OCS cluster structures and infrared spectra. Our structure calculations are based on a recent ab initio potential energy surface² and assume pairwise additivity. There are also recent direct ab initio trimer and tetramer calculations, which are (necessarily) at a lower level of theory.³ We find that the observed OCS trimers indeed correspond to the two lowest energy isomers in both calculations, and that there is fairly good agreement of experimental and theoretical structures. For the tetramer the global minimum is at -2773 cm⁻¹ relative to dissociation, and we calculate (at least) twenty different isomers within 100 cm⁻¹ of this minimum (and seven within 20 cm⁻¹). Remarkably, the observed microwave tetramer does correspond to our lowest calculated isomer. However this isomer is not included in the published direct ab initio calculation - it may just have been overlooked due to the large number of isomers! In the mid-infrared region of the OCS ν₁ fundamental (~2060 cm⁻¹), we observe two bands which are clearly due to the same microwave OCS tetramer. But a third band is assigned to a different tetramer not observed in the microwave spectrum. It appears to correspond to our seventh calculated isomer, located about 20 cm⁻¹ above the most stable one, and it is also missing from the direct ab initio calculation. Neither observed tetramer has any symmetry elements.
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¹J.P. Connelly, A. Bauder, A. Chisholm, and B.J. Howard, Mol. Phys. 88, 915 (1996); R.A. Peebles and R.L. Kuczkowski, J. Phys. Chem. A 103, 6344 (1999). ²J. Brown, X.-G. Wang, R. Dawes, and T. Carrington, Jr., J. Chem. Phys. 136, 134306 (2012). ³N. Sahu, G. Singh, and S.R. Gadre, J. Phys. Chem. A 117, 10964 (2013).

Pyrimidine is known to possess a negative electron affinity. Anions created from such molecules, whose energies are higher than those of their neutral counterparts, are unstable with respect to autodetachment. The solvation of pyrimidine with just one water molecule results in a positive electron binding energy. The addition of water molecules stabilizes the excess charge and increase the binding energy. The most interesting feature is the orientation of the hydrated pyrimidine complex to help accommodate an excess electron.