Atoms whose nuclei have an exotic number of nucleons can have a `core nucleus' surrounded by a `halo' formed by a nucleon orbiting the core nucleus. For example, due to the two halo neutrons orbiting the core nucleus of ¹¹Li, its nucleus has a cross section that is roughly the same size as that of ²⁰⁸Pb. Halo nucleic atoms have been studied extensively both in theory and in experiments, however halo nucleic molecules have not been studied in either. We first show, using HeH⁺, BeH, and MgH as examples, that with measurements of any two isotopologues of a molecule, we can determine crucial properties of a third isotopologue well within spectroscopic accuracy. We then use the extremely precise empirical information available^1,2,3,4 for the low-lying states of ^6,6Li₂, ^6,7Li₂ and ^7,7Li₂ to predict potentials and various properties of the halo nucleic molecule ^11,11Li₂, along with isotopologues containing ³Li, ⁴Li, ⁵Li, ⁸Li, ⁹Li, ¹⁰Li, and ¹²Li. We believe that our predictions of the ro-vibrational energies are reliable for experiments for the first detection of a halo nucleic molecule.
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¹R. J. Le Roy, N. S. Dattani, J. A. Coxon, A. J. Ross, P. Crozet, C. Linton, J. Chem. Phys. 131, 204309 (2009). ²N. S. Dattani, R. J. Le Roy, J. Mol. Spec. 268, 199-210 (2011). ³M. Semczuk, X. Li, W. Gunton, M. Haw, N. S. Dattani, J. Witz, A. Mills, D. J. Jones, K. W. Madison, Phys. Rev. A 87, 052505 (2013) ⁴W. Gunton, M. Semczuk, N. S. Dattani, K. W. Madison, Phys. Rev. A 88, 062510 (2013)

A quantum electrodynamics (QED) correction surface for the simplest polyatomic and polyelectronic system H₃⁺ is computed using an approximate procedure. This surface is used to calculate the shifts to vibration-rotation energy levels due to QED; such shifts have a magnitude of up to 0.25 cm⁻¹ for vibrational levels up to 15 000 cm⁻¹ and are expected to have an accuracy of about 0.02 ⁻¹. Combining the new H₃⁺ QED correction surface with existing highly accurate Born-Oppenheimer (BO), relativistic and adiabatic components suggests that deviations of the resulting ab initio energy levels from observed ones are largely due to non-adiabatic effects.

Quantum Monte Carlo (QMC) is a computational technique that can be applied to the electronic Schrödinger equation for molecules. QMC methods such as Variational Monte Carlo (VMC) and Diffusion Monte Carlo (DMC) have demonstrated the capability of capturing large fractions of the correlation energy, thus suggesting their possible use for high-accuracy quantum chemistry calculations. QMC methods scale particularly well with respect to parallelization making them an attractive consideration in anticipation of next-generation computing architectures which will involve massive parallelization with millions of cores. Due to the statistical nature of the approach, in contrast to standard quantum chemistry methods, uncertainties (error-bars) are associated with each calculated energy. This study focuses on the cost, feasibility and practical application of calculating potential energy curves for small molecules with QMC methods. Trial wave functions were constructed with the multi-configurational self-consistent field (MCSCF) method from GAMESS-US.[1] The CASINO Monte Carlo quantum chemistry package [2] was used for all of the DMC calculations. An overview of our progress in this direction will be given. References: M. W. Schmidt et al. J. Comput. Chem. 14, 1347 (1993). R. J. Needs et al. J. Phys.: Condensed Matter 22, 023201 (2010).

S₂Cl₂ is a candidate molecule for the detection of ortho-para transition, because the Cl atoms on the skewed position from the rotational principle axes give large off-diagonal terms in the quadrupole interaction, which can mix ortho and para states. In order to estimate the ortho-para mixing in a hyperfine-resolved rotational state, pure rotational transitions were measured by millimeter-wave spectroscopy using two different experimental set-ups. The transitions from the term value around 20 K was measured with a supersonic jet and those around 200 K were measured with a dry ice cooled gas cell. Hundreds of peaks were assigned for the naturally abundant S₂³⁵Cl₂ and S₂³⁵Cl³⁷Cl isotopic species, and the rotational molecular constants including the fourth-order and sixth-order centrifugal distortion constants were determined. The hyperfine structures were partly resolved in some Q-branch transitions and those spectral patterns were well reproduced with the hyperfine constants determined by the previous FTMW spectroscopy in the cm-wave region¹. With the new molecular constants determined in this study and the previous hyperfine constants, it becomes possible to predict a more reliable ortho-para mixing ratio and to narrow down the possible candidate transitions in the mm-wave region for the detection of ortho-para transition.
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¹Mizoguchi et al., J. Mol. Spectrosc. 250,86-97(2008)

Carbon-cage molecules have generated a considerable interest from both experimental and theoretical point of views. We recently performed a high-resolution study of adamantane (C₁₀H₁₆), the smallest hydrocarbon cage belonging to the diamandoid family. There exist another family of hydrocarbon cages with additional interesting chemical properties: the so-called Platonic hydrocarbons that comprise dodecahedrane (C₂₀H₂₀) and cubane (C₈H₈). Both possess C-C bond angles that deviate from the tetrahedral angle (109.8^°) of the sp³ hybridized form of carbon. This generates a considerable strain in the molecule. Cubane itself has the highest density of all hydrocarbons (1.29 g/cm³). This makes it able to store larges amounts of energy, although the molecule is fully stable. Up to now, only one high-resolution study of cubane has been performed on a few bands [2]. We report here a new wide-range high-resolution study of the infrared spectrum of cubane. The sample was synthesized in Bari upon decarboxylation of 1,4-cubanedicarboxylic acid thanks to the improved synthesis of literature [3]; its ¹H and ¹³C NMR, FTIR, and mass spectrometry agreed with reported data [4]. Several spectra have been recorded at the AILES beamline of the SOLEIL French synchrotron facility. They cover the 800 to 3100 cm⁻¹ region. Besides the three infrared-active fundamentals (ν₁₀, ν₁₁ and ν₁₂), we could record many combination bands, all of them displaying a well-resolved octahedral rotational structure. We present here a preliminary analysis of some of the recorded bands, performed thanks the SPVIEW and XTDS software, based on the tensrorial formalism developed in the Dijon group [5].  

Nitrous acid (HONO) is an important member of the atmospheric nitrogen cycle whose chemistry involves a variety of gas-phase, photochemical, and heterogeneous processes. Among its formation pathways in the atmosphere is the ternary association of hydroxyl (OH) with nitric oxide (NO), but the formation of the isonitrosyl hydroxide (HOON) isomer has largely been ignored owing to early theoretical studies that questioned its stability. Guided by new high-level ab initio calculations, we have detected the rotational spectrum of trans-HOON in an electrical discharge of a dilute mixture of NO and water vapor by a combination of Fourier transform microwave spectroscopy and double resonance methods. No evidence for the cis isomer was found in any of our spectroscopic surveys between 15.4-17.0 GHz. A semi-experimental equilibrium structure for trans-HOON has been derived to high precision from isotopic substitution (DOON, H¹⁸OON, HO¹⁸ON, HOO¹⁵N) along with zero-point vibrational corrections calculated at the CCSD(T)/aug-cc-pVTZ level of theory. Most notably, the central O-O bond in trans-HOON is found to be 1.9149 ± 0.0005 Å in length, which is the longest known O-O bond in a molecule (nearly 20% longer than the analogous bond in the HOOO radical).

High resolution jet-cooled absorption spectra of the formic acid dimer (HCOOH)₂ have been measured in the C-O stretching region at 1215-1240 cm⁻¹using a rapid-scan tunable diode laser spectrometer. Three vibrational bands of (HCOOH)₂ have been assigned unambiguously. They were interpreted as the Fermi-triad system consisting of the ν₂₂ fundamental band and two combination bands in a previous low-resolution study [F. Ito, Chem. Phys. Lett. 447, 202(2007)]. The spectral coverage in the high-resolution study of the middle band [M. Ortlieb and M. Havenith, J. Phys. Chem. A. 111, 7355(2007)] were extended. These three vibrational bands were analyzed together using a standard rigid rotor Watson A-reduced Hamiltonian without explicit consideration of the perturbation among three vibrationally excited states. The perturbed energies for three vibrationally excited states are 1219.71637(20), 1225.34666(15), and 1233.95863(17) cm⁻¹, respectively.

We report on a comb-resolved, broadband, direct-comb spectroscopy system in the mid-IR and its application to the detection of trace gases and molecular line shape analysis. By coupling an optical parametric oscillator (OPO), a 100 m multipass cell, and a high-resolution Fourier transform spectrometer (FTS), sensitive, comb-resolved broadband spectroscopy of dilute gases is possible. The OPO has radiation output at 3.1-3.7 and 4.5-5.5 μm. The laser repetition rate is scanned to arbitrary values with 1 Hz accuracy around 417 MHz. The comb-resolved spectrum is produced with an absolute frequency axis depending only on the RF reference (in this case a GPS disciplined oscillator), stable to 1 part in 10⁹. The minimum detectable absorption is 1.6x10⁻⁶ cm⁻¹Hz^−1/2. The operating range of the experimental setup enables access to strong fundamental transitions of numerous molecular species for applications based on trace gas detection such as environmental monitoring, industrial gas calibration or medical application of human breath analysis. In addition to these capabilities, we show the application for careful line shape analysis of argon-broadened CO band spectra around 4.7 μm. Fits of the obtained spectra clearly illustrate the discrepancy between the measured spectra and the Voigt profile (VP), indicating the need to include effects such as Dicke narrowing and the speed-dependence of the collisional width and shift in the line shape model, as was shown in previous cw-laser studies.¹ In contrast to cw-laser based experiments, in this case the entire spectrum ( ∼  250 cm⁻¹) covering the whole P and R branches can be measured in 16 s with 417 MHz resolution, decreasing the acquisition time by orders of magnitude. The parallel acquisition allows collection of multiple lines simultaneously, removing the correlation of possible temperature and pressure drifts. While cw-systems are capable of measuring spectra with higher precision,² this demonstration opens the door for fast, massively parallel line shape parameters retrieval combined with analysis reaching beyond the VP and with absolute frequency calibration delivered by frequency combs.
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¹R. Wehr et al. J. Mol. Spec. 235 54-68 (2003) ²A. Cygan, et al. Eur. Phys. J. Special Topics 222 2119-2142 (2013)

Pressure effects on surface-enhanced Raman scattering spectra of Rhdoamine 6G adsorbed on silver nanoparticle surfaces was studied using a confocal Raman microscope. Colloidal silver nanoparticles were treated with Rhodamine 6G (R6G) and its isotopically substituted partner, R6G-d4. Mixed isotopomers let us identify single-molecule spectra, since multiple-molecule spectra would show vibrational transitions from both species. The nanoparticles were embedded into a poly vinyl alcohol film, and loaded into a diamond anvil cell for the high-pressure Raman scattering measurement. Argon was the pressure medium. Ambient pressure Raman scattering spectra showed few single-molecule spectra. At moderately high pressure (  1GPa), a surprising effect was observed. The number of sites with observable spectra decreased dramatically, and most of the spectra that could be observed were due to single molecules. The effects of high pressure suppressed the multiple-molecule Raman sites, leaving only the single-molecule sites to be observed.

A comprehensive spectroscopic analysis of recently synthesized novel emissive materials, such as perylene diimide nanostructures, pincer complexes, and newly developed dyes, provides insight into how to modify these materials to be better suited for applications in photovoltaics and photodiodes. Properties of interest in this study include fluorescence emission, fluorescence lifetime, and quantum yield. Tracking how the photophysics of a compound change as different environments are introduced to the system helps to develop a better understanding of the fundamental photophysical properties of the material. Both solid phase and samples in solution are examined on the bulk and single molecule level.