I was a graduate student in the Klemperer group in 1970 when Jon Hougen's "The Calculation of Rotational Energy Levels and Rotational Line Intensities in Diatomic Molecules" appeared. Physical Chemists like to break stuff. My favorite topic, spectroscopic perturbations, presents a rich variety of broken patterns. I began as a collector and NBS 115 both gave me tools to add to my collection and a challenge to look beyond molecular constants. My perturbations were more than molecules behaving badly. The 49 pages of NBS 115 became the foundation of my career as a spectroscopist. Jon Hougen wrote defining guides for many areas of spectroscopy, thereby providing the foundations for many careers. Each such guide echoed the elegant simplicity of NBS 115.

The high-resolution vacuum-uv spectrographic data¹ for the C - X emission and C,(D,E),H,h,I - X(v = 0) absorption transitions of F₂, in combination with pure rotation² and vibration-rotation³ Raman data, have been employed in a least-squares analysis. Attention was given to the extensive blending in the absorption data and to account for plate-to-plate shifts in the emission data. The C - X data, with an estimated uncertainty of 0.05 cm⁻¹, sample X state vibrational levels v = 1 - 22,for which the potential energy function was fitted using the extended-MLR model⁴. 3549 line positions in the weighted fit provided estimates of 1303 term values of excited electronic states and 17 parameters for the ground state. The highest observed v = 22 level of the ground state, which lies only 114 cm⁻¹below the F(²P_3/2) + F(²P_3/2) dissociation limit, is found to be perturbed; all rotational levels (J = 0 - 19) lie at energies 5 - 13 cm⁻¹below their expected positions. A deperturbation model was employed within the direct potential fit; in this novel approach, the eigenvalue of each J-level in v = 22 was determined from a 2 x 2 matrix, with the diagonal level of the perturbing state represented by E_p + B_pJ(J+1), and the off-diagonal element by a + b(J + 1/2). However, the b-parameter was indeterminate; a successful fit of the entire data set with inclusion of the deperturbation model for v = 22 provided the estimates E_p = -70.5(3.7) cm⁻¹, B_p = 0.226(6) cm⁻¹, a = 16.2(8) cm⁻¹and R_e = 1.412555(4)Å. There is much interest in an identification of the perturbing state. The results indicate a J-independent spin-orbit interaction with a weakly-bound perturbing state (R_e = 2.8Å), lying 40 - 50 cm⁻¹above v = 22. The absence of a J-dependent b(J + 1/2) Coriolis interaction implies a perturber with 0_g⁺ symmetry. A plausible candidate is the a'(0_g⁺) state which dissociates to the same atomic limit and which is repulsive at short-R. 1. E.A. Colbourn, M. Dagenais, A.E. Douglas, J.W. Raymonda, Can. J. Phys. 54 (13) (1976) 1343-1359. 2. H.G.M. Edwards, E.A.M. Good, D.A. Long, J. Chem. Soc. Faraday Trans. 272 (1976) 984-987. 3. R.Z. Martinez, D. Bermejo, J. Santos, P. Cancio, J. Mol. Spectrosc. 168 (1994) 343-349. 4. R.J. Le Roy, N.S. Dattani, J.A. Coxon, A.J. Ross, P. Crozet, C. Linton, J. Chem. Phys. 131 (2009) 204309.

Using diborane, B₂H₆, as a reactant gas and ruthenium as the target metal in the UNB laser ablation molecular jet apparatus, ruthenium monoboride molecules have been detected using laser-induced fluorescence spectroscopy. The [18.4]2.5-X²∆_5/2 electronic transition of RuB has been observed previously at pulsed-dye laser resolutionWang, N., Ng, Y. W. and Cheung, A. S. Chem. Phys. Lett. 547, 21-23 (2012). Three vibronic bands were rotationally analyzed and designated as the (1-0), (0-0) and (0-1) bands, but only the Ru¹¹B and Ru¹⁰B isotopologues were resolved. Ruthenium has 7 naturally occurring isotopes ranging from 1.87% to 31.55% abundance, giving a total combination of 14 isotopologues for RuB. Using our cw-ring dye laser, the three vibronic bands were recorded at high resolution and 12 of the isotopologues have been rotationally analyzed. Both of the odd isotopes of Ru have a nuclear spin I=5/2 and their respective isotopologues had resolved hyperfine structure which was analyzed to extract the hyperfine parameters. It was determined that the hyperfine interaction arises from the nuclear spin of the ¹⁰¹Ru and ⁹⁹Ru atoms and not from the boron nucleus.

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Wang, N., Ng, Y. W. and Cheung, A. S. Chem. Phys. Lett. 547, 21-23 (2012)..

Diatomic germanium oxide, GeO, and germanium sulfide, GeS, have been studied by Fourier transform microwave spectroscopy using laser ablation from a germanium target rod in the presence of H₂S or O₂ diluted in neon. Extensive isotopic spectroscopy has been performed; measurements include the ground vibrational state as well as states exceeding v=10 for some species. Ground state rotational spectra of several GeS isotopologs have also been studied around 300 GHz through a DC discharge of GeCl₄ and H₂S in a free-space absorption cell. A global isotope invariant fit has been performed combining all previously published and new high-resolution spectroscopic data. From the analysis of the ⁷³Ge data combined with highly accurate quantum chemical values for the germanium electric-field gradient, a revised ⁷³Ge quadrupole moment has been derived.

Calcium monohydride CaH is a simple diatomic molecule found in the Sun and other stars. The A-X electronic spectrum of CaH has been a useful probe for classifying stars. This has lead to nearly a century of spectroscopic work on CaH, with the first laboratory spectrum being the C²Σ⁺ - X²Σ⁺ transition in the near-UV region reported by Mulliken.R. S. Mulliken, Phys. Rev. 25, 509 (1925).We have recently been working on this molecule in the visible and ultraviolet regions and have identified many new vibrational levels of the B/B' state using Laser Induced Fluorescence (LIF). Our primary interest has been the detailed investigation of the B/B' state which has a double−minimum potential energy function. Our previous LIF work was on vibrational levels in the energy regime lying above the potential energy barrier between the two wells.K. Watanabe, N. Yoneyama, K. Uchida, K. Kobayashi, F. Matsushima, Y. Moriwaki, S. C. Ross, Chem. Phys. Lett. 657, 1 (2016).^, K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross, Chem. Phys. Lett. 710, 11 (2018). We were able to confirm the strong irregularity in the vibrational energy spacings that had been predicted by the quantum chemical study of Carlsund−Levin et al.C. Carlsund-Levin, N. Elander, A. Nunez, A. Scrinzi, Phys. Scripta 65, 306 (2002). This irregularity is due to interaction between the vibrational levels of the B/B′ and D states. We have also investigated the energy regime which starts from just below the minimum of the higher lying outer potential well and continues to just above the potential energy barrier between the two wells. There we identified the A-X(4, 0) and B/B′-X(3 or 5, 0) bands and also new vibronic levels around 18,400-20,000 cm⁻¹ which do not belong to any of the A, B, or E states. We conclude that these new levels belong to the previously unobserved lowest lying ²∆ state. We are investigating evidence for possible interactions with other electronic states. We also outline the current status of our work and future prospects as we continue our program of delineating the vibrational levels of the B state over their full energy range: starting at the energy of the minimum of the inner well, progressing through the energy of the minimum of the outer well, the energy of the barrier, and on towards the dissociation limit. R. S. Mulliken, Phys. Rev. 25, 509 (1925).\end K. Watanabe, N. Yoneyama, K. Uchida, K. Kobayashi, F. Matsushima, Y. Moriwaki, S. C. Ross, Chem. Phys. Lett. 657, 1 (2016). K. Watanabe, I. Tani, K. Kobayashi, Y. Moriwaki, S. C. Ross, Chem. Phys. Lett. 710, 11 (2018).\end C. Carlsund−Levin, N. Elander, A. Nunez, A. Scrinzi, Phys. Scripta 65, 306 (2002).

Oxazole and isoxazole (C₃H₃NO) are isomers of five membered ring molecules with two hetero-atoms, nitrogen and oxygen. Some amino acids contains five membered ring structure. Previous microwave spectroscopic studies were carried out in the low frequency region W. C. Mackrodt, A. Wardley, P. A. Curnuck, N. L. Owen, J. Sheridan, Chem. Commun. (London), 692 (1966). O.L. Stievater, P. Nösberger and J. Sheridan, Chem. Phys. Lett. 710, 11 (2018). O. L. Stiefvater, J. Chem. Phys. 63, 2560 (1975). A. Kumar, J. Sheridan, O. L. Stiefvater, Z. Naturforsch. 33a, 145 (1978). A. Kumar, J. Sheridan, O. L. Stiefvater, Z. Naturforsch. 33a, 549 (1978). U. Spoerel, H. Dreizler, and W. Stahl, E. Kraka, D. Cremer, J. Phys. Chem. 100, 14298 (1996). and it is desirable to have information for future interstellar detection. In this study, pure rotational spectroscopy was carried out by using conventional microwave spectroscopy and chirp-pulse Fourier-transform spectroscopy with a waveguide cell. Up to 340 GHz was observed at room temperature. Previous molecular constants made assignment straightforward and detailed analysis using Watson's Hamiltonian will be reported.

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W. C. Mackrodt, A. Wardley, P. A. Curnuck, N. L. Owen, J. Sheridan, Chem. Commun. (London), 692 (1966).\end O.L. Stievater, P. Nösberger and J. Sheridan, Chem. Phys. Lett. 710, 11 (2018). O. L. Stiefvater, J. Chem. Phys. 63, 2560 (1975).\end A. Kumar, J. Sheridan, O. L. Stiefvater, Z. Naturforsch. 33a, 145 (1978). A. Kumar, J. Sheridan, O. L. Stiefvater, Z. Naturforsch. 33a, 549 (1978).\end U. Spoerel, H. Dreizler, and W. Stahl, E. Kraka, D. Cremer, J. Phys. Chem. 100, 14298 (1996).

Nitric Oxide (NO) is a combustion pollutant known for its role in the formation of photochemical smog. Nitric oxide is also interesting from the viewpoint of fundamental spectroscopy since it has two closely spaced ground electronic states. Consequently, in addition to pure-rotational Raman spectrum, NO also exhibits electronic-Raman spectrum near 120 cm⁻¹. We applied a dual-pump combined CARS system (DPCCS) to investigate the spectrum of NO. In a DPCCS, in contrast to a typical pure-rotational CARS system, all beams have different wavelengths. This allows us to preferentially suppress Q or S branch Raman transitions and investigate the polarization character of a target molecule, in this case NO. Theoretical spectrum of NO was calculated by solving the time-dependent Schrodinger wave equation using perturbation theory. By comparing the measured and the computed spectrum we obtain the anisotropy of the polarizability tensor of NO as well as a quantitative estimate of the strength of electronic-Raman transitions. The figure in this abstract shows comparison between CARS data and calculated NO spectrum with no preferential suppression of either the Q or S branch Raman transitions. Notice the spin-splitting of the 2 Π_1/2 and 2 Π_3/2 states of NO, evident, near a Raman shift of 80 cm⁻¹. r0pt Figure

Quantitative measurements of combustion system fueling and hot-Jupiter exoplanets require accurate methane absorption data at elevated temperatures. The ExoMol and HITRAN spectral databases in the near-infrared 6500-9000cm-1 range are based on the 10to10 potential energy surface, and the 80K and 296K empirical WKLMC linelist, respectively, which do not empirically constrain all elevated-temperature behavior. We present spectra of the near-infrared methane overtone band around 1400nm at temperatures from 296 K to 900 K. The spectra are taken using a three-zone tube furnace and a dual-frequency comb spectrometer with 600 cm-1 bandwidth and .00667cm-1 resolution. These measurements are targeted toward providing a compact, accurate methane absorption linelist for 300-900K.

All polyatomic molecules have IR allowed fundamentals, 0 → ν_s, each of which has a corresponding "hot band’’ ν_s → 2ν_s that has an origin shifted by 2X_ss from the corresponding fundamental, where X_ss is the diagonal anharmonic spectroscopic constant for mode s. There will always be Doppler-free two-photon absorption transitions where a P(J) transition of the fundamental will be nearly resonant with a R(J−1) transition of the hot band. For bands with Q branches, there is also the possibility for near-resonant Q followed by R transitions and for P followed by Q branch transitions. For a linear molecule without missing levels, the maximum detuning from exact 2-photon resonance will be less than B, the rotational constant of the molecule. For symmetric and asymmetric tops, the multiple branches increase the probability of a very near resonance. I have derived general expressions for the two-photon absorption cross section for such transitions and have assembled predictions for cases where the necessary data is available in HITRAN. The resonant enhancement combined with the intracavity intensity enhancement leads to cases of strong and selective two-photon absorption, particularly at low pressure, in some cases even stronger than one-photon absorption as the entire population of the lower state can be pumped rather than only molecules with Doppler shifts within a power-broadened homogeneous width. The one and two-photon absorption contributions to the cavity decay can be separately fit, uncoupling the two-photon absorption from other sources of cavity lass. I previously published, App. Phys B 116, 147 (2014), an analysis of the expected sensitivity limit of such a combined fit in both the detector noise and shot noise limited cases. Combined with the approximately two orders of magnitude increase in resolution, and the fact that the two-photon absorption spectrum will be extremely sparse due to the near-resonance requirement, this should provide extremely high sensitivity and unprecedented selectivity for trace detection in low-pressure gases. An experimental apparatus is currently being assembled to experimentally verify these predictions and it is hoped that preliminary data will be available at the time of the meeting.