We report a new semi-classical method to compute highly excited rotational energy levels of an asymmetric-top molecule. The method forgoes the idea of a full quantum mechanical treatment of the ro-vibrational motion of the molecule. Instead, it employs a semi-classical Green’s function approach to describe the rotational motion, while retaining a quantum mechanical description of the vibrations. Similar approaches have existed for some time, but the method proposed here has two novel features. First, inspired by the path integral method, periodic orbits in the phase space and tunneling paths are naturally obtained by means of molecular symmetry analysis. Second, the rigorous variational method is employed for the first time to describe the molecular vibrations. In addition, we present a new robust approach to generating rotational energy surfaces for vibrationally excited states; this is done in a fully quantum-mechanical, variational manner. The semi-classical approach of the present work is applied to calculating the energies of very highly excited rotational states and it reduces dramatically the computing time as well as the storage and memory requirements when compared to the fully quantum-mechanical variational approach. Test calculations for excited states of SO₂ yield semi-classical energies in very good agreement with the available experimental data and the results of fully quantum-mechanical calculations. We hope to be able to present at the meeting also semi-classical calculations of transition intensities. See also the open-access paper Phys. Chem. Chem. Phys. 19, 1847–1856 (2017). DOI: 10.1039/C6CP05589C

Electronic states whose potential energy functions have 'shelf' or double-minimum shapes have always presented special challenges because, as functions of vibrational quantum number, the vibrational energies/spacings and inertial rotational constants either have an abrupt change of character with discontinuous slope, or past a given point, become completely chaotic. The present work shows that a `traditional' methodology developed for deep `regular' single-well potentials can also provide accurate `parameter-fit' descriptions of the v-dependence of the vibrational energies and rotational constants of shelf-state potentials that allow a conventional RKR calculation of their Potential energy functions. It is also shown that a merging of Pashov's uniquely flexible 'spline point-wise' potential function representation with Le Roy's `Morse/Long-Range' (MLR) analytic functional form which automatically incorporates the correct theoretically known long-range form, yields an analytic function that incorporates most of the advantages of both approaches. An illustrative application of this method to data to a double-minimum state of Na₂ will be described.

RuO₄ is a heavy tetrahedral molecule which has practical uses for several industrial fields. Due to its chemical toxicity and the radiological impact of its 103 and 106 isotopologues, the possible remote sensing of this compound in the atmosphere has renewed interest in its spectroscopic properties. We investigate here for the first time at high resolution the bending dyad region in the far IR and the line intensities in the ν₃ stretching region. Firstly, new high resolution FTIR spectra of the bending modes region in the far infrared have been recorded at room temperature, using a specially constructed cell and an isotopically pure sample of ¹⁰²RuO₄. New assignments and effective Hamiltonian parameter fits for this main isotopologue have been performed, treating the whole ν₂/ν₄ bending mode dyad. We provide precise effective Hamiltonian parameters, including band centers and Coriolis interaction parameters. Secondly, we investigate the line intensities for the strongly infrared active stretching mode ν₃, in the mid infrared window near 10 μm. New high resolution FTIR spectra have also been recorded at room temperature, using the same cell and sample. Using assignments and effective Hamiltonian parameter for ¹⁰²RuO₄, line intensities have been retrieved and the dipole moment parameters fitted for the ν₃ fundamental. A frequency and intensity line list is proposed.

The resolution of any spectroscopic experiment is limited by the coherent interaction time between the probe radiation and the particle that is being studied. The introduction of cooling techniques for atoms and ions has resulted in a dramatic increase of interaction times and accuracy, it is hoped that molecular cooling techniques will lead to a similar increase. Here we demonstrate the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiment, beams of ammonia molecules are decelerated, trapped and cooled using inhomogeneous electric fields and subsequently launched. Using a combination of quadrupole lenses and buncher elements, the beam is shaped such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature of less than 10μK and a longitudinal temperature of less than 1μK) while the molecules are in free fall, but strongly focused at the detection region. The molecules are in free fall for up to 266 milliseconds, making it possible, in principle, to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.

The room temperature absorption spectrum of water vapour highly enriched in ¹⁷O has been recorded by Cavity Ring Down Spectroscopy (CRDS) between 7443 and 7921 cm⁻¹. Three series of recordings were performed with pressure values around 0.1, 1 and 10 Torr. The frequency calibration of the present spectra benefited of the combination of the CRDS spectrometer to a self-referenced frequency comb. The resulting CRD spectrometer combines excellent frequency accuracy over a broad spectral region with a high sensitivity (Noise Equivalent Absorption, α_min ∼ 10⁻¹¹−10⁻¹⁰ cm⁻¹). The investigated spectral region corresponds to the high energy range of the first hexade. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Partridge and Schwenke. Overall about 4150 lines were measured and assigned to 4670 transitions of six water isotopologues (H₂¹⁶O, H₂¹⁷O, H₂¹⁸O, HD¹⁶O, HD¹⁷O and HD¹⁸O). Their intensities span six orders of magnitude from 10⁻²⁸ to 10⁻²² cm/molecule. Most of the new results concern the H₂¹⁷O and HD¹⁷O isotopologues for which about 1600 and 400 transitions were assigned leading to the determination of 329 and 207 new energy levels, respectively. For comparison only about 300 and four transitions of H₂¹⁷O and HD¹⁷O were previously known in the region, respectively. By comparison to highly accurate H₂¹⁶O line positions available in the literature, the average accuracy on our line centers is checked to be on the order of 3 MHz (10⁻⁴ cm⁻¹) or better for unblended lines. This small uncertainty represents a significant improvement of the line center determination of many H₂¹⁶O lines in the considered region.

The pure rotational spectrum of MgI in its ground electronic state (~X²Σ⁺) has been measured using millimeter/submillimeter wave direct-absorption techniques. Rotational transitions arising from the v = 0, 1, and 2 vibrational states of ²⁴MgI, as well as the v = 0 state for the isotopologues (²⁵MgI and ²⁶MgI), have been measured in their natural abundance in the region of 200 – 300 GHz. Rotational, centrifugal distortion, and spin-rotation constants were determined for each isotopologue and the excited vibrational states of ²⁴MgI. Equilibrium parameters B_e, α_e, D_e, and γ_e were ascertained and used to calculate an equilibrium bond length (r_e) for MgI. The spin-rotation coupling constants of several magnesium monohalides were examined and the contribution to the spin-rotation parameter appears to be dominated by second order spin-orbit coupling of the nearby excited A²Π electronic state.

A quantified characterization of a spacecraft’s thruster plume is achievable through measurements of fluorescence from the plume. Fluorescence is present in a spacecraft’s plume due to electronic excitation from solar photons, primarily Lyman-alpha (121.6 nm). Excitation of water with Lyman-alpha leads to photodissociation through four possible channels, one of which produces fluorescent hydroxyl radicals (OH(A)). Dependent on the rovbirational state, this species is either predissociative or fluorescent. Here, dispersed fluorescence from water photolysis at Lyman-alpha has been recorded. Comparing our current florescent data with previous H-atom Rydberg tagging results, the ratio of predissociation to fluorescence of OH(A) is quantified.

ThF⁺ is a promising candidate for a second-generation molecular ion-based measurement of the permanent electric dipole moment of the electron (eEDM). Compared to the current HfF⁺ eEDM experiment, ThF⁺ has several advantages: (i) the eEDM-sensitive ³∆₁ electronic state is the ground state, which facilitates a long measurement coherence time; (ii) its effective electric field (38 GV/cm) is 50% larger than that of HfF+, which promises a direct increase of the eEDM sensitivity; and (iii) the ionization energy of neutral ThF is lower than its dissociation energy, which introduces a greater flexibility for rotational state-selective photoionization via core-nonpenetrating Rydberg states. We use laser-induced fluorescence (LIF) spectroscopy to find suitable intermediate states required for the state selective ionization process. We present the results of our LIF spectroscopy of ThF, and our current progress on efficient ThF ionization and on ThF⁺ dissociation.

This work extends the electronic spectroscopy of RuF, and reports on what we believe is the first observation of RuCl. Both molecules have been created in a laser-ablation molecular beam apparatus at UNB, and their spectra have been detected by laser-induced fluorescence. In the low-resolution survey of RuF from 400 to 770 nm, five bands were detected in the blue, green and infrared regions of the electromagnetic spectrum. Four of them were rotationally analyzed from high-resolution data. The three bands in the green region are associated with the ⁴Γ_11/2−X⁴Φ_9/2 system first observed by Steimle et al.T. C. Steimle, W. Virgo and T. Ma, J. Chem. Phys. 124 024309 (2006). new ⁴∆_7/2−X⁴Φ_9/2 transition in the blue region was also detected. Two high-resolution bands of RuCl were rotationally analyzed, and the ground state was also found to be X⁴Φ_9/2. The data provide detailed structural information about the molecules, such as bond lengths, vibrational frequencies, isotopic structure, spin-orbit interactions and hyperfine interactions.

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Footnotes:

T. C. Steimle, W. Virgo and T. Ma, J. Chem. Phys. 124 024309 (2006).A

Resolved laser induced fluorescence spectra of NiD, recorded at Doppler resolution between 11500 and 18000 cm⁻¹, have defined some 200 term energies in two of the three strongly-interacting, low-lying (X ²∆, W ²Π and V ²Σ⁺) states of NiD associated with an Ni⁺(3d⁹)-D⁻ configuration. Our observations span v = 0 - 5 in the lowest spin-orbit component of the ground state, X₁ ²∆_5/2 , v = 0 - 3 in X₂ ²∆_3/2 and v = 0 - 1 in W₁ ²Π_3/2 , the lower component of the W ²Π state. Spin-orbit and rotation-electronic interactions are strong in NiD. Large parity splittings are seen, due to interactions with the unobserved ²Σ⁺ state. We have attempted a global, multi-isotope fit to reproduce observed term energies up to 6000 cm⁻¹ in NiD and ^58,60,62NiH, in an extension of the `Supermultiplet' model proposed by Gray and co-workers Gray, Li, Nelis, and Field, J.  Chem.  Phys. 95, 7164 (1991) because fits with NiD term energies alone failed to converge to sensible solutions. Dunham-type parameters have been used to represent the unperturbed X ²∆ , W ²Π and V ²Σ⁺ states, with off-diagonal matrix elements (treating spin-orbit, L- and S-uncoupling effects) based on Ni⁺ atomic properties. Some electronic Born-Oppenheimer breakdown terms were included in the model. The spectra show emission from several excited states close to the unique level populated by the single-mode laser. Bands of collisionally-induced fluorescence identify three levels (A (Ω = 5/2) v = 1, E ( Ω = 3/2) v = 1 and I (Ω = 3/2) v = 0) that have not been reported before. Gray, Li, Nelis, and Field, J.  Chem.  Phys. 95, 7164 (1991),

[13]r0pt Figure We investigate collisionally-induced A ¹Σ⁺ → X ¹ Σ⁺ fluorescence in NaK, observed following ro-vibrationally selective excitation of the B ¹Π state. NaK molecules are formed in a heatpipe oven, and excited with a single-mode dye laser operating around 17000 cm⁻¹. Direct B  ¹Π→ X  ¹ Σ⁺ fluorescence is dominated by ∆J = 0 or ±1 transitions, with rotational satellites whose intensities vary according to collisional partner (rare gas or K atoms). The B→X emission also shows even weaker ∆v ±1 vibrational relaxation bands, with a more even spread of rotational population. Some 5000 cm⁻¹ to lower wavenumber, we observe apparently non-selective A  ¹Σ⁺ → X  ¹ Σ⁺ emission, with an oscillating bound-free c  ³Σ⁺ → a  ³ Σ⁺ contribution to the baseline that becomes increasingly important as higher vibrational levels of B  ¹Π are populated by the laser. Earlier work defining analytical functions describing the A  ¹Σ⁺ and b  ³Π states of NaK, and the spin-orbit functions coupling themHarker et al., Phys. Rev. A 92 (1), 012506 (2015) allowed us to assign this dense and irregular A→X spectrum, and to see how upper state level populations change as a function of B state excitation and heatpipe conditions (He, Ar or N₂ as buffer gas). Our spectra reveal a propensity to conserve vibrational quantum number, at least from low v in the B state. This is in contradiction with A-X emission in Na₂ observedAstill et al., Chem. Phys. Lett. 125 33 (1986); Camacho et al., J. Phys B. 39 2665 (2006)ollowing laser excitation of the (corresponding) B   ¹Π_u state. The non-Boltzmann populations in the A  ¹ Σ_u⁺ state of Na₂ were explainedHussein et al., J. Mol Spectrosc. 114 105 (1985)y near-resonant transfer from B   ¹Π_u to 2  ¹Σ_g⁺ (with no counterpart in heteronuclear NaK), followed by spontaneous emission to the A  ¹Σ_u⁺ state. Harker et al., Phys. Rev. A 92 (1), 012506 (2015), Astill et al., Chem. Phys. Lett. 125 33 (1986); Camacho et al., J. Phys B. 39 2665 (2006)f Hussein et al., J. Mol Spectrosc. 114 105 (1985)b

Laser-assisted nuclear fusion is a potential means for providing short, well-controlled particle bursts in the lab, such as neutron or alpha particle pulses. I will discuss computational results of how coherent control by shaped, amplified 800 nm laser pulses can be used to enhance the nuclear fusion cross section of diatomic molecules such as BH or DT. Quantum dynamics simulations show that a strong laser pulse can simultaneously field-bind the diatomic molecule after electron ejection, and increase the amplitude of the vibrational wave function at small internuclear distances. When VUV shaped laser pulses become available, coherent laser control may also be extended to muonic molecules such as D-mu-T, held together by muons instead of electrons. Muonic fusion has been extensively investigated for many decades, but without coherent laser control it falls slightly short of the break-evne point.