Almost no physically interesting physico/chemical parameter is determined directly from a measurement. Rather, they are determined by performing a least-squares fit of some model to a set of data. Unfortunately, there seems to be no commonly accepted set of `best practices' for determining how to round off such fitted parameter values to a minimum number of significant digits while ensuring that they retain the ability to reproduce the experimental data within their uncertainties. This sometimes results in lists of fitted parameters with no quoted uncertainties that have 2-3 times as many significant digits as the data being fitted, or to the results of an analysis being defined by parameters that are (unnecessarily?) quoted to more digits than normal computer double precision, which makes those results difficult or impossible to apply. Alternatively, it may also lead to fitted parameters being `over-rounded' so that the model no longer accurately represents the experimental data. This presentation describes a `best practice' to address these problems, offers a general-purpose least-squares fitting program that applies it, and provides an illustrative application of this approach in a study of the A ¹Σ_u⁺ − X ¹Σ_g⁺ system of Mg₂.

CH⁺ has been a species of interest since the dawn of molecular astrophysics,¹ and it is an important intermediate in combustion processes. In the domain of `conventional' spectroscopy there have been a number of studies of low v′ and v" portions of the A <sup>1</sup>Π−X <sup>1</sup>Σ<sup>+</sup> band system of various isotopologues, and Amano recently reported microwave measurements of the ground-state R(0) lines of <sup>12</sup>CH<sup>+</sup>, <sup>13</sup>CH<sup>+</sup> and <sup>12</sup>CD<sup>+</sup>.<sup>2</sup>  In addition, Helm et al.<sup>3</sup> used photodissociation spectroscopy to observe transitions to very high-J′ tunneling-predissociation levels (shape resonances) involving v(A)=0−10, for many of which they also measured the photo-fragment kinetic energy release. More recently Hechtfischer et al. used photodissociation spectroscopy of `Feschbach resonance' levels at very high v′(A) and low J′ to obtain the first direct measurement of the ¹²CH⁺ dissociation energy with near-spectroscopic accuracy (±1.1 cm⁻¹).⁴ However, to date, all analyses of the data for this system had been performed using traditional band-constant or Dunham-expansion fits to data for the lowest vibrational levels,⁵ and there have been no attempts to combine the `conventional' low-v data with the high-J′ and high-v′ photodissociation data in a single treatment. The present work has addressed this problem by performing a Direct-Potential-Fit (DPF) analysis that obtains full analytic potential energy functions for the X ¹Σ⁺ and A ¹Π states of CH⁺ that are able to account for all of the available data (on average) within their uncertainties.
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¹ A.E. Douglas and G. Herzberg, Astrophys. J. 94, 381 (1941). ² T. Amano, Astrophys. J. Lett. 716, L1 (2010) ³ H. Helm, P.C. Crosby, M.M. Graff and J.T. Mosley, Phys. Rev. A 25, 304 (1982) ⁴ U. Hechtfischer and C. J. Williams, M. Lange, J. Linkemann, D. Schwalm, R. Wester, A. Wolf and D. Zajfman, J.Chem.Phys. 117, 8754 (2002). ⁵H.S.P. Müller, Astron. Astrophys. 514, L7 (2010)

Spectra of IBr in various wavelength regions have been measured by a number of researchers using traditional diffraction grating and microwave methods, as well as using high-resolution laser techniques combined with a Fourier transform spectrometer.^1,2,3,4,5 In a previous paper at this meeting, we reported a preliminary determination of analytic potential energy functions for the A ³Π₁ and X ¹Σ⁺ states of IBr from a direct-potential-fit (DPF) analysis of all of the data available at that time.⁶ That study also confirmed the presence of anomalous fluctuations in the v-dependence of the first differences of the inertial rotational constant, ∆B_v=B_v+1−B_v in the A ³Π₁ state for vibrational levels with v′(A) in the mid 20's. However, our previous experience in a recent study of the analogous A ³Π₁−X ¹Σ_g⁺ system of Br₂ suggested that the effect of such fluctuations may be overcome if sufficient data are available.⁷ The present work therefore reports new measurements of transitions to levels in the  v′(A)=23−26  region, together with a new global DPF analysis that uses "robust" least-squares fits⁸ to average properly over the effect of such fluctuations in order to provide an optimum delineation of the underlying potential energy curve(s).
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¹ L.E.Selin,Ark. Fys. 21,479(1962). ² E. Tiemann and Th. Moeller, Z. Naturforsch. A 30,986 (1975). ³ E.M. Weinstock and A. Preston, J. Mol. Spectrosc. 70, 188 (1978). ⁴ D.R.T. Appadoo, P.F. Bernath, and R.J. Le Roy, Can. J. Phys. 72, 1265 (1994). ⁵ N. Nishimiya, T. Yukiya and M. Suzuki, J. Mol. Spectrosc. 173, 8 (1995). ⁶ T. Yukiya, N. Nishimiya, and R.J. Le Roy, Paper MF12 at the 65^th Ohio State University International Symposium on Molecular Spectroscopy, Columbus, Ohio, June 20-24, 2011. ⁷ T. Yukiya, N. Nishimiya, Y. Samajima, K. Yamaguchi, M. Suzuki, C.D. Boone, I. Ozier and R.J. Le Roy, J. Mol. Spectrosc. 283, 32 (2013) ⁸ J.K.G. Watson, J. Mol. Spectrosc. 219, 326 (2003).

In the last symposium,¹ we reported that a global fit simultaneously for the Phillips band system (A¹Π_u−X¹Σ_g⁺) and the Ballik-Ramsay band system (b³Σ_g⁻−a³Π_u) was carried out to deperturb the spin-orbit interaction between the X¹Σ_g⁺ state and the b³Σ_g⁻ state of C₂. As the result, the energy gap between the a³Π_u state and the X¹Σ_g⁺ state was obtained as 720.0 cm⁻¹, which is quite larger than the previous value² of about 716.7 cm⁻¹ (converted from 718.3 cm⁻¹ after the definition of the Hamiltonian for the ³Π state is corrected by adding one B value on all the diagonal elements as the one³ we use widely today). This newly determined singlet-triplet energy gap showed that the X¹Σ_g⁺ (v=6) level and the b³Σ_g⁻ (v=3) level cross at J=2 with the energy difference of only 0.07 cm⁻¹ before the spin-orbit interaction is considered, which makes the singlet-triplet mixing nearly 50-50%. Therefore, we thought that the forbidden transitions related to this mixing should be observable. When rechecking the previously observed FTIR emission spectrum in the study of the CH radical,⁴ where the emission spectrum of C₂ appeared to be very strong, we found that the allowed A¹Π_u (v=4)−X¹Σ_g⁺ (v=6) transitions of C₂ around 3950 cm⁻¹ were accompanied by the forbidden A¹Π_u (v=4)−b³Σ_g⁻ (v=3) transitions with J"=2, and two such forbidden transitions were identified clearly with the similar intensities as the corresponding allowed transitions. The observation of the forbidden transitions exactly at the predicted positions means that our deperturbation analysis was successful.
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¹W. Chen, J. Tang, and K. Kawaguchi, 68th OSU International Symposium on Molecular Spectroscopy, WJ11 (2013). ²C. Amiot, J. Chauville, and J. -P. Maillard, J. Mol. Spectrosc. 75, 19 (1979). ³J. M. Brown and A. J. Merer, J. Mol. Spectrosc. 74, 488 (1979). ⁴P. N. Ghosh, M. N. Deo, and K. Kawaguchi, Astrophys. J. 525, 539 (1999).

The F. T. Spectrum of CS molecule was recorded with Bruker IFS 120 HR spectrometer at a spectral resolution of 0.03 cm⁻¹ using liquid nitrogen cooled InSb detector in the region 10500 - 1800 cm⁻¹. Intense spectrum of CS radical was excited by DC discharge of mixture of CS₂ (120 mTorr) and He (2 Torr) in flowing condition. Two hours integration time was used for obtaining a good S/N ratio. For the first time seven bands of a’ ³Σ - a ³Π transition of CS molecule are observed lying between 8000 - 4800 cm⁻¹ region. Rotational analysis of these bands, viz. 7-1, 6-1, 5-0, 4-0, 3-0, 2-0, and 3-1 will be presented.

The F. T. Spectrum of CS molecule was recorded with Bruker IFS 120 HR spectrometer at a spectral resolution of 0.03 cm⁻¹ using liquid nitrogen cooled InSb detector in the region 10500 - 1800 cm⁻¹. Intense spectrum of CS radical was excited by DC discharge of mixture of CS₂ (120 mTorr) and He (2 Torr) in flowing condition. Two hours integration time was used for obtaining a good S/N ratio. The recorded spectrum is more intense compared to previous studies, therefore, it has been possible to excite lower values of v’ and v’’ for the d ³∆_i - a ³Π_r transition. The bands of three sub-systems occur with varying intensity. The following bands have been rotationally analysed, viz. 1-0, 2-1, 3-2 ( d ³∆₃ - a ³Π₂); 2-0, 3-1, 4-2 ( d ³∆₂ - a ³Π₁); and 1-1, 3-3 ( d ³∆₁ - a ³Π₀). The d ³∆_i is state is highly perturbed. Using a deperturbation program PGOPHER (C. M. Western, Univ. of Bristol) the molecular constants of the two states have been derived.

Intense emission in the near-ultraviolet and visible from the Comet Tail and Swan bands of CO⁺ and C₂, respectively, has been observed from glow discharges produced in CO₂ at atmospheric pressure. Generated within 200-500 μm microchannels fabricated in nanoporous alumina, the microchannel plasmas are spatially homogeneous, diffuse glows. As the CO₂ flow rate through the microchannels is varied, the visible/UV spectra change dramatically and the chemical kinetics of this fascinating spectrum will be discussed.

A high resolution spectrum of the [18.6]3.5 - X(1)4.5 band of UF was obtained using the laser ablation specrometer at Arizona State University (ASU). The rotational structure showed significant perturbations in the upper state. Examination of the Stark and Zeeman splittings of the lowest J lines in electric fields up to 3.4 kV/cm and a magnetic field of 1650 Gauss yielded permanent electric dipole moments, μ_e, of 1.99 and 1.87 Debye and magnetic g-factors, g_e of 3.28 and 3.26 for the ground and excited states respectively. The above experimental results will be discussed in terms of the configurational composition of the ground and excited states and compared with recent theoretical calculations¹.
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¹Ivan O. Antonov and Michael C. Heaven, J. Phys. Chem. A 2013, 117, 9684-9694

Tritium is a radioactive isotope of hydrogen. Tritium released into natural enviroment is said to be converted into mostly HTO. The detection of HTO is important from the viewpoint of basic science as well as its radioactivity. Spectroscopy is a good tool for detection, however, high-resolution spectroscopy studies are still limited. The microwave study were carried out and the molecular constants of the ground state were determined. ^1

We have generated MgOH in supersonic free jet expansions, and observed the laser induced fluorescence ( LIF ) of the ~A ²Π - ~X ²Σ⁺ transition. We have reported rotational analyses of the bending vibronic bands, 2₀ⁿ, n = 0, 2, and 4,¹ and it has been found that the spin-orbit constant of the bending vibronic levels, ~A ²Π (0n⁰0) μ ²Π, increases with increasing the bending vibrational quantum number, n. This observation is interpreted as the lower level possessing primarily bent character and the higher level possessing linear character. The bending potential surface of the ~A ²Π state is thought to be the reason for the present observation and interpretation. This potential surface is thought to be fairly flat and anharmonic with bent and linear geometries at the r_e and r₀ structures, respectively. We also observed the other two vibronic bands at almost the middle between the 2₀² and 2₀⁴ bands. One of the two has a similar rotational structure with that of the origin band, 0₀⁰, and is assigned to be the Mg-OH stretching vibronic band, ~A ²Π (00⁰1) - ~X ²Σ⁺ (00⁰0). The rotational structure of another vibronic band is very complex, and it seems to overlap, at least, two vibronic bands, ²Π - and ²Σ⁽⁺⁾ - ²Σ⁺. More precise analysis is underway.
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¹M. Fukushima and T. Ishiwata, The 21st Colloquium on High-Resolution Molecular Spectroscopy, F8 (2009).

We report emission spectra of scandium monoiodide (resolution of 0.7 Å) generated by the photodissociation of of ScI₃ at 248 nm. Emission from a bound-free transition of ScI near 900 nm was recorded, as were bound-bound transitions in the range of 400 nm to 800 nm. Comparison of experimental investigation and simulations of both bound-bound vibrational progressions and bound-free emission will be presented as well.

Sulfur is the element with the largest number of known binary oxides¹ and as such has attracted the curiosity of chemists and physicists for decades. In particular, the simpler ones are of great interest for diverse scientific disciplines like structural and theoretical chemistry, astrochemistry, atmospheric chemistry and molecular physics. Out of those, the simplest sulfur rich oxides and dioxides S₂O and S₂O₂ have been studied spectroscopically to some extent in the past² but still pose challenging problems to future gas-phase investigations. In the present study, the pure rotational spectrum of S₂O₂ has been investigated in the ground and ν₃ states. In addition, ground state transitions of OS³⁴SO were observed. OSSO was produced in a radio frequency discharge through SO₂. Experimental measurements have been supported by high-level CCSD(T) calculations. An extensive set of molecular parameters has been derived.
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¹Steudel, Top. Curr. Chem. 231, 203-230 (2003) ²Thorwirth et al., J. Mol. Struct. 795, 1-3 (2006), and refs. therein

Harmonic and anharmonic vibrational self-consistent field (VSCF) calculations were employed to investigate the fundamental and overtone modes of SF₆ molecules. Determination of the Potential Energy Surface (PES) on a multidimensional grid of more than 65000 nodes was performed, and a system of 1D coupled-equations was solved. Corrections to the harmonic approximation for the frequencies of the fundamental modes and their overtones were obtained. Ab-initio calculations to the interaction potential between two molecules as a function of their position and orientation, and the corresponding energies for dimer formation, have been computed. Finally, the effect of dimerization on the molecular frequencies is investigated.

Methyl nitrate (CH₃NO₃), is a toxic liquid known for it’s explosive properties. It is metabolically expressed in trace amounts in exhaled human breath and is a potential candidate for interstellar detection. Previous microwave studies of methyl nitrate have yielded a handful line transitions in its vibrational ground state in the 8-34 GHz range. This paper discusses the high-resolution spectrum of methyl nitrate in 210-270 GHz range, and extends the spectroscopic assignment of its rotational transitions in the ground and first excited vibrational states.

Vinyl mercaptan (ethenethiol, CH₂=CHSH) exists in the gas phase in two distinct rotameric forms, syn (planar) and anti (quasi-planar in the ground vibrational state). The microwave spectra of these two isomers were investigated previously¹ however not exceeding frequencies of about 65 GHz. In the present investigation, the pure rotational spectra of both species have been investigated at millimeter wavelengths. Vinyl mercaptan was produced in a radiofrequency discharge through a constant flow of ethanedithiol at low pressure. Both syn and anti rotamers were observed and new extensive sets of molecular parameters were obtained. Owing to its close structural relationship to vinyl alcohol and the astronomical abundance of complex sulfur-bearing molecules, vinyl mercaptan is a plausible candidate for future radio astronomical searches.
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¹M. Tanimoto et al. J. Mol. Spectrosc. 78, 95-105 & 106-119 (1979)