Hydrazoic acid (HN₃) is a near-prolate asymmetric top molecule which we have extensively studied in the millimeter-wave region. Having completed an R_e structure determination based on 14 isotopologues of HN₃, we have moved on to analyze the very complex rotational spectra for the first 7 vibrationally excited states, as well as the higher K levels of the ground vibrational state. The excited states include the 4 lowest (out of 6) fundamental modes (ν₅, ν₆, ν₄, and ν₃) and the 3 lowest combination and overtone states (2ν₅, 2ν₆ and ν₅+ν₆). All of these states are totally symmetric (A’) except for ν₆ and ν₅+ν₆, which are antisymmetric (A”). The ro-vibrational states are substantially more intermingled than in most molecules due to unusually wide rotational spacing in HN₃. This intermingling leads to a tangled web of perturbations connecting the various ro-vibrational states: a-type and b-type Coriolis interactions between ν₅ and ν₆, between ν₄ and ν₆, and between 2ν₆ or 2ν₅ and ν₅+ν₆, local Fermi resonance between ν₃ and 2ν₆, and a strong centrifugal distortion interaction between the ground state and ν₅. Fortunately, we have been able to make extensive use (in both assignment of spectra and fitting of spectroscopic parameters) of previously published high resolution FTIR data for the ν₅, ν₆, ν₄ and ν₃ bands and the pure rotational spectrum of the ground vibrational state.J. Bendtsen, F. Hegelund and F. M. Nicolaisen, J. Mol. Spectrosc. 118, 121 (1986)^,J. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 119, 456 (1986)^,J. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 124, 306 (1987)^, J. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 119, 456 (1986)J. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 152, 101 (1992)F

Hydrazoic acid (HN₃) and DN₃ have qualitatively different rotational spectra, owing in large part to a substantial difference in their A rotational constants (345 GHz for DN₃ vs 611 GHz for HN₃). Like HN₃, DN₃ has six fundamental vibrational modes, of which four are visible in our millimeter-wave spectra at room temperature. Between 240 and 450 GHz, many pure rotational transitions for the ground vibrational state, ν₅ (496 cm⁻¹), ν₆ (586 cm⁻¹), ν₄ (955 cm⁻¹), ν₃ (1197 cm⁻¹), the first overtones of ν₅ and ν₆, and the combination ν₅+ν₆ have been observed and assigned. Because DN₃ is a light molecule, the rotational energy levels are widely spaced, leading to numerous interactions between rotational states of different vibrational modes. We have drawn on a wealth of previously published ro-vibrational data from high resolution FTIR spectraJ. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 125, 14 (1987)^,J. Bendtsen, F. Hegelund and F. M. Nicolaisen, J. Mol. Spectrosc. 128, 309 (1988)^,J. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 145, 123 (1991)^, J. Bendtsen, F. Hegelund and F. M. Nicolaisen, J. Mol. Spectrosc. 128, 309 (1988)C. S. Hansen, J. Bendtsen and F. M. Nicolaisen, J. Mol. Spectrosc. 175, 239 (1996)i

The acetylene emission spectrum from the trans-bent electronically excited <̃span class="roman">A state to the linear ground electronic <̃span class="roman">X state has attracted considerable attention because it grants Franck-Condon access to local bending vibrational levels of the <̃span class="roman">X state with large-amplitude motion along the acetylene \rightleftharpoons vinylidene isomerization coordinate. For emission from the ground vibrational level of the <̃span class="roman">A state, there is a simplifying set of Franck-Condon propensity rules that gives rise to only one zero-order bright state per conserved vibrational polyad of the <̃span class="roman">X state. Unfortunately, when the upper level involves excitation in the highly admixed ungerade bending modes, ν₄′ and ν₆′, the simplifying Franck-Condon propensity rule breaks down-so long as the usual polar basis (with v and l quantum numbers) is used to describe the degenerate bending vibrations of the <̃span class="roman">X state-and the intrapolyad intensities result from complicated interference patterns between many zero-order bright states. We show that when the degenerate bending levels are instead treated in the Cartesian two-dimensional harmonic oscillator basis (with v_x and v_y quantum numbers), the propensity for only one zero-order bright state (in the Cartesian basis) is restored, and the intrapolyad intensities are simple to model, so long as corrections are made for anharmonic interactions. As a result of trans\rightleftharpoons cis isomerization in the <̃span class="roman">A state, intrapolyad emission patterns from overtones of ν₄′ and ν₆′ evolve as quanta of trans bend (ν₃′) are added, so the emission intensities are not only relevant to the ground-state acetylene \rightleftharpoons vinylidene isomerization-they are also a direct reporter of isomerization in the electronically-excited state.

A new spectroscopic scheme was used to gain insight into the predissociation mechanisms of the S₁ electronic state of acetylene in the 47000-47300 cm⁻¹ region. To study this mechanism, H-atom action spectra of predissociative S₁ were recorded. Instead of detecting H-atom via REMPI, an H-atom fluorescence scheme was developed, in which the H-atom was excited to 3s and 3d levels and the fluorescence was detected. The signal-to-noise ratio of H-atom fluorescence-detected action spectra is superior to REMPI detected H-atom spectra. By comparing the LIF and H-atom spectra, there is direct evidence of level-dependent predissociation rates. Some of the line-widths observed in the H-atom spectra are broader than in the LIF spectra, confirming the triplet-mediated nature of S₁ acetylene.

Rovibrational variational calculations on global potential energy surfaces are often essential for investigating large amplitude vibrational motion and isomerization between multiple stable conformers, as well as for understanding the spectroscopic signatures of such dynamics. The efficient and accurate representation of high dimensional potential energy surfaces and the diagonalization of large rovibrational Hamiltonians make these calculations a technically non-trivial task. The first excited singlet electronic state of acetylene (C₂H₂) is an ideal model isomerizing system. The S₁ state supports both a trans conformer and a higher energy cis conformer (T_e^cis−T_e^trans ≈ 2700 cm⁻¹), separated by a planar near-half-linear transition state (T_e^TS−T_e^trans ≈ 5000 cm⁻¹). The low-energy structure of the trans well is complicated by strong Coriolis and Darling-Dennison interactions between the near-resonant torsion and asymmetric bending modes. The resulting polyad patterns are eventually broken as the internal vibrational energy approaches that of the barrier to isomerization. In this region, qualitatively new spectroscopic patterns emerge, such as rotational K-staggering and vibrational effective frequency dips. We examine these effects with an efficient ab initio variational treatment. Our global potential energy surface is constructed as a hybrid of a high-level reduced dimension surface, which excludes the two r_\textCH bond lengths, and a lower-level full dimensional surface incorporating the effects of r_\textCH displacement. Diagonalization of the large, sparse Hamiltonian, which contains an exact internal coordinate rovibrational kinetic energy operator, is achieved with an efficient restarted Lanczos algorithm that generates variational energies and wavefunctions. We discuss how our results elucidate the S₁ state's rich variety of spectroscopic features and the insights they provide into the isomerization process.

Formyl azide (HC(O)N₃) is a highly unstable molecule (t_1/2 ∼ 2 hours at room temperature as a gas) that has only recently been studied spectroscopically by UV, IR, Raman and NMR methods.Banert, K. et al. Angew. Chem. Int. Ed. 2012, 51, 4718-4721Zeng, X. et al. Angew. Chem. Int. Ed. 2013, 52, 3503-3506 We have synthesized formyl azide and obtained its absorption spectrum at room temperature over the range 250-360 GHz. As in the case of carbonyl diazide,Amberger, B.K. et al. J. Mol. Spectrosc. 259, (2014) 15-20wo conformers are expected for HC(O)N₃, with the syn-isomer 2.8 kcal/mol lower in energy than the anti-isomer (CCSD(T)/ANO1). Calculations at the same level of theory and the same basis set predict the dipole moments for the syn-isomer (μ = 1.56 D) and anti-isomer (μ = 2.56 D). These calculations also indicate that b-type transitions should dominate the syn-isomer spectrum, while a-type transitions become more significant in the case of the anti-isomer. Despite the anti-isomer having a larger dipole moment, the syn-isomer still gives rise to all the dominant features of the spectrum. Thus far, five vibrational states (ν₉, ν₁₂, 2ν₉, ν₉ + ν₁₂, ν₁₁) have been studied for the syn-isomer, with the highest energy state ν₁₁ = 582.6 cm⁻¹. Searches for the spectra of the anti-isomer are ongoing.

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Banert, K. et al. Angew. Chem. Int. Ed. 2012, 51, 4718-4721

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Amberger, B.K. et al. J. Mol. Spectrosc. 259, (2014) 15-20t

Previous studies of the pure rotational spectrum of deuterated nitric acid, DNO₃, have focused on the ground and first excited state, ν₉. This paper focuses on the next lowest energy vibrational states, covering the spectral range from 128-360 GHz. Two of them are unperturbed,ν₇ and ν₈, and two of them, ν₆ and 2ν₉ are highly perturbed. The unperturbed states are fit separately, while the two perturbed states are fit together using both Coriolis and Fermi interaction terms. Each state is fit to within experimental accuracy. We also extend the assignments and update the rotational constants for ν₉.

Degenerate (one-color) and two-color variants of the resonant four-wave mixing (RFWM) have developed into a sensitive and nonintrusive spectroscopic tool to study molecules in different gaseous environments. Yet, the fully non-degenerate (four-color, 4C) RFWM was scrutinized and implemented only for the Coherent AntiStokes Raman Scattering (CARS) excitation schemeB. Attal-Trétout, P. Berlemont, and J. P. Taran, Mol. Phys. 70, 1 (1990).J.P. Kuehner, S.V. Naik, W.D. Kulatilaka, N. Chai, N.M. Laurendeau, R.P. Lucht, M.O. Scully, S. Roy, A.K. Patnaik, and J.R. Gord, J. Chem. Phys. 128, 174308 (2008). Here,  by using the line-space approachA. Kouzov and P. Radi, J. Chem. Phys. 140, 194302 (2014). we analyze other 4C-RFWM schemes potentially interesting for the efficient up- and down-frequency conversion as well as for studies of molecular states. Decoupled expressions of the 4C-RFWM amplitudes are derived which allows to predict their polarization dependence.

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B. Attal-Trétout, P. Berlemont, and J. P. Taran, Mol. Phys. 70, 1 (1990)., J.P. Kuehner, S.V. Naik, W.D. Kulatilaka, N. Chai, N.M. Laurendeau, R.P. Lucht, M.O. Scully, S. Roy, A.K. Patnaik, and J.R. Gord, J. Chem. Phys. 128, 174308 (2008).. A. Kouzov and P. Radi, J. Chem. Phys. 140, 194302 (2014).,

Given its technological importance, the literature abounds with models for plasma enhanced chemical vapor deposition of the SiH₄/O₂/Ar system. In a continuing effort to identify and characterize the optical spectra of Si₃ generated in a SiH₄/Ar pulsed discharge sourceThe electronic spectrum of Si₃ I: the triplet D_3h system” Reilly, N. J.; Kokkin, D. L.; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C., J. Chem. Phys. 136(19), 194307, 2012. we detected, via two dimensional (2D) LIF, a relatively strong electronic transition in the 570-600 nm region that is strongly enhanced by the addition of a small amount of O₂. The excitation spectrum shows resolved band structure at the pulsed laser resolution of 0.5 cm⁻¹ and exhibits a radiative lifetime of 1.97 μs. The dispersed fluorescence exhibits three vibrational progressions and an unusually small splitting of approximately 50 cm⁻¹. Here we report on efforts to identify the molecular carrier of these bands, with particular interest paid to species resulting from oxygen impurities in the silane discharge.

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The electronic spectrum of Si₃ I: the triplet D_3h system” Reilly, N. J.; Kokkin, D. L.; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C., J. Chem. Phys. 136(19), 194307, 2012.,