Benzyl radical is a prototypical aromatic free radical and has been the subject of numerous spectroscopic studies. On the other hand, ring-substituted benzyl radicals, benzyl-type radicals, have received less attention due to the difficulties associated with production in corona discharge and analysis of spectra. We report vibronic emission spectra of hetero halogen multi-substituted benzyl radicals generated by corona discharge of corresponding toluene derivatives using a pinhole-type glass nozzle, from which visible vibronic emission spectra were recorded using a long-path monochromator. The spectra show nice features of strongest origin band and a series of vibronic bands in the lower energies originating from the vibrationless D₁ state. From the analysis of the spectra observed, we determined the energies of the D₁ → D₀ electronic transition and vibrational mode frequencies in the ground electronic state.¹ On the other hand, all substituted benzyl radicals show the origin bands shifted to red region with respect to the parental benzyl radical at 22002 cm⁻¹. The shifts of multi-substituted benzyl radicals can be well estimated using the method developed from mono-substituted benzyl radicals as well as the positions of nodal point and mutual orientation of substituents, which could be useful for scientists to set a proper scanning range of their spectrometers for the spectroscopic observation of transient molecules. In this presentation, we will discuss the substituent effect²³on electronic transition energy and the experimental technique developed in this laboratory.
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¹Y. W. Yoon and S. K. Lee, J. Phys. Chem. A 117, 2485 (2013). ²Y. W. Yoon, S. Y. Chae, and S. K. Lee, Chem. Phys. Lett. 584, 37 (2013). ³Y. W. Yoon and S. K. Lee, Chem. Phys. Lett. 570, 29 (2013).

The D₀ ← D₁ transitions of many aromatic resonance stabilised radicals (RSRs) have been observed in the gas-phase in recent years. This work has been primarily motivated by the suggestion that such molecules may be carriers of the diffuse interstellar bands (DIBs). Most gas- phase studies of these molecules have focused on the D₀ ← D₁ electronic transitions, primarily due to experimental limitations. These transitions are generally weak, a feature of odd- alternate hydrocarbon radicals, with intensity instead going to an electronically similar higher energy transition. This presentation will focus on higher electronic transitions with calculated intensity f > 10⁻². Experimental data will be presented for observed strong transitions of three benzilic polycyclic aromatic hydrocarbons (PAHs) radicals’ 1-naphthylmethyl, 2-naphthylmethyl and 9-anthracenylmethyl. Experimental data will also be presented of a strong state of the aromatic/aliphatic RSR 1-phenylpropargyl. Trends in this experimental and theoretical data will be used to predict the spectroscopic properties of larger RSR molecules, and the relevance of these higher electronic states to astronomical observations will be discusses.

Infrared spectra in the C-H stretch region are reported for the allyl (CH₂CHCH₂) and allyl peroxy (CH₂=CH-CH₂OO) radicals solvated in superfluid helium nanodroplets. Nine bands in the spectrum of the allyl radical have resolved rotational substructure. We have assigned three of these to the ν₁ (a₁), ν₃ (a₁), and ν₁₃ (b₂) C-H stretch bands and four others to the ν₁₄/(ν₁₅+2ν₁₁) (b₂) and ν₂/(ν₄+2ν₁₁) (a₁) Fermi dyads, and an unassigned resonant polyad is observed in the vicinity of the ν₁ band. Experimental coupling constants associated with Fermi dyads are consistent with quartic force constants obtained from density functional theory computations. The peroxy radical was formed within the He droplet via the reaction between allyl and O₂ following the sequential pick-up of the reactants. Five stable conformers are predicted for the allyl peroxy radical, and a computed two-dimensional potential surface for rotation about the CC-OO and CC-CO bonds reveals multiple isomerization barriers greater than 300 cm⁻¹. Nevertheless, the C-H stretch infrared spectrum is consistent with the presence of a single conformation following the allyl + O₂ reaction within helium droplets.

The Criegee intermediates are carbonyl oxides that play key roles in ozonolysis of unsaturated organic compounds. This mechanism was first proposed by Criegee in 1949, but the first direct observation of the simplest Criegee intermediate CH₂OO in the gaseous phase has been reported only recently using photoionization mass spectrometry.¹ Our group has reported the low-resolution IR spectra of CH₂OO, produced from the reaction of CH₂I + O₂, with a second-generation step-scan Fourier-transfom IR absorption spectrometer.² The spectral assignments were based on comparison of observed vibrational wavenumbers and rotational contours with theoretical predictions. Here, we report the IR absorption spectra of CH₂OO at a resolution of 0.32 cm⁻¹, showing partially rotationally-resolved structures. The origins of the ν₃, ν₄, ν₆, and ν₈ vibrational modes of CH₂OO are determined to be 1434.1, 1285.7, 909.2, and 847.3 cm⁻¹, respectively. With the analysis of the vibration-rotational spectra, we provide a definitive assignment of these bands to CH₂OO. The observed vibrational wavenumbers indicate a zwitterionic contribution to this singlet biradical showing a strengthened C-O bond and a weakened O-O bond. This zwitterionic character results to an extremely rapid self reaction via a cyclic dimer to form 2H₂CO + O₂ (¹∆_g). Another group of weak transient IR bands centered at 1231.5, 1213.3, and 899.8 cm⁻¹are also observed. These bands might be contributed from dioxirane, which was postulated to be another important intermediate that might be isomerized from the Criegee intermediate in the reaction of O₃ with 1-alkenes.
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¹O. Welz, J. D. Savee, D. L. Osborn, S. S.Vasu, C. J. Percival, D. E. Shallcross, and C. A. Taatjes, Science 335, 204 (2012). ²Y.-T. Su, Y.-H. Huang, H. A.Witek, and Y.-P. Lee, Science 340, 174 (2013).

The ~A ²B₁(Π_u)−~X ²A₁ linear-bent electronic transition of the BH₂ free radical from 11700 - 14600 cm⁻¹ was previously studied by Herzberg and Johns in 1967.¹ In the current work, we have performed extensive LIF and emission studies of the electronic spectrum of jet-cooled ¹¹BH₂, ¹⁰BH₂, ¹¹BD₂, and ¹⁰BD₂ up to 21100 cm⁻¹. High resolution studies of ¹¹BD₂ have allowed us to refine the ground state geometry. We have also used ab initio calculations and variational methods to predict the ro-vibronic energy levels of this Renner-Teller system. The results agree very well with the experimental data, confirming and extending our assignments.
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¹G. Herzberg  and J. W. C. Johns Proc. R. Soc. Lond. A 298(142), 1967.

The electronic spectra of the X₂BO and X₂BS free radicals have been studied by a combination of experimental and theoretical techniques. Experimentally, we have succeeded in preparing some of these species in a pulsed discharge jet and detecting them by laser-induced fluorescence and emission spectroscopy through the ~B ²A₁−~X²B₂ transition. The radicals exhibit emission transitions down to the ground state and the low-lying ~A ²B₁ electronic state. We have also used high level ab initio theory [CCSD(T)/aug-cc-pV5Z] to calculate the properties of the ground and excited states and simulate the observed spectra. Experiment and theory agree that the radicals are planar, C_2v symmetry species in all the three combining states, with only small changes in geometry on electronic excitation.

Pure rotational spectra of SiCCN ( ~X ²Π_3/2) have been observed using Fourier-transform microwave (FTMW) spectroscopy in the frequency region 13 to 35 GHz. The SiCCN radical was produced in a supersonic jet by discharging a mixture gas, 0.2% SiCl₄ and 0.2% CH₃CN diluted in Ar. The effective rotational constant B_eff,3/2, the centrifugal distortion constant D, and the hyperfine coupling constants, a + (b + c)/2 and eQq₀ , were determined with a standard deviation of the fit to be 6 kHz. Determined B and eQq₀ are consistent with those derived from ab initio calculations. Λ-type doublings were not resolved for the observed spectra.

The potential of four-wave mixing spectroscopy for deperturbation studies has been demonstrated by an analysis of the spin-orbit and L-uncoupling interaction between the d ³Π_g,v=4 and the b ³Σ_g⁻, v=16 states of C₂.¹ The double-resonance method provides unambiguous assignments of perturbed transitions by intermediate level labeling. Furthermore, the sensitivity of the method unveiled extra transitions that originate from the perturbing b ³Σ_g⁻, v=16 state. A following study² has successfully applied the method to deperturb the d ³Π_g,v=6 state of the dicarbon and lead to the discovery of the first high-spin state of C₂. The energetically lowest quintet (⁵Π_g) has been characterized by applying a conventional Hamiltonian. The detailed study unraveled major issues of the so-called high-pressure band of C₂ which were initially observed back in 1910³ and later observed in numerous experimental environments. In this work we take into account our recent studies on tri-carbon⁴ where we used perturbation-facilitated two-color resonant four-wave mixing spectroscopy to access the (dark) triplet manifold of C₃ from the singlet ~X  ¹Σ_g⁺ ground state via "gate-way" levels (i.e. singlet-triplet mixed levels). In a similar way, we performed for this study perturbation-facilitated optical-optical double-resonance experiments to access the first excited quintet state of C₂ via "gate-way states" in the perturbed d ³Π_g,v=6. The newly found ⁵Π_u state is characterized at rotational resolution by performing a least-squares fit of the observed transitions to a ⁵Π_u - ⁵Π_g Hamiltionian. The work represents a rare case of a successful analysis of a quintet manifold of a molecule exhibiting a singlet ground state (¹Σ_g⁺).
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¹P. Bornhauser, G. Knopp, T. Gerber, and P.P. Radi, Journal of Molecular Spectroscopy 262, 69 (2010). ²P. Bornhauser, Y. Sych, G. Knopp, T. Gerber, and P.P. Radi, J. Chem. Phys. 134, 044302 (2011). ³A. Fowler, Monthly Notices of the Royal Astronomical Society 70, 484 (1910). ⁴Y. Sych, P. Bornhauser, G. Knopp, Y. Liu, T. Gerber, R. Marquardt, and P.P. Radi, J. Chem. Phys. 139, 154203 (2013).

Bands of the C₃Ar complex can be observed near almost all the bands of the ùΠ_u - ~X¹Σ⁺_g transition of C₃. The strongest bands of C₃Ar form close-lying pairs. Rotational analyses have been carried out for the bands at 25025 and 25029 cm⁻¹(near the 02⁻0-000 band of C₃) and 25426 and 25430 cm⁻¹(near the 04⁻0-000 band). Each pair consists of a type A and a type C band of an asymmetric top, where the upper states interact by b-axis Coriolis coupling; this represents the lifting of the degeneracy of the Π state in the lower symmetry of the complex. Only K = even lower state levels are found, showing that C₃Ar has the shape of a distorted letter T. The Ar atom lies 3.82 Å  from the centre of mass of the C₃ part. Emission spectra have been recorded and lifetimes measured for several C₃Ar upper state levels. The assignment of the emission bands is complicated by significant intramolecular relaxation in the upper states, which populates mainly the lowest level of each local potential minimum of the upper state; however the variation of the upper state well depth (binding energy) with vibrational quantum number can then be determined.

The strongest IR band of NO₃ appears at 1492 cm⁻¹and has been assigned traditionally to the N-O degenerate stretching ν₃ mode. In 2007 Stanton proposed the ν₃ to be about 500 cm⁻¹lower, i.e. it is located around 1000 cm⁻¹, based on theoretical calculations. Jacox and collaborators supported this proposal, on the basis of their IR spectra observed in Ne matrix, and reassigned the 1492 cm⁻¹band to ν₃ + ν₄. The traditional vibrational assignment is referred to as Assignment I and the Stanton-Jacox one to as Assignment II, and thus the upper state of the 1492 cm⁻¹band Z is ν₃ and ν₃ + ν₄ for Assignment I and II, respectively. Kawaguchi, Ishiwata, and Hirota (KIH) have been making much effort to settle which assignment is correct, by observing and analyzing FTIR spectra. They thought in 2009 that the observation of hot bands from the in-plane ONO degenerate bending ν₄ state to the Z state will make it possible for KIH to select the correct assignment among the two. Namely for Assignment I only one hot band of E - E type (i.e. ν₃ - ν₄) will appear, whereas three bands for Assignment II: E - E, A₁ - E, and A₂ - E. It was straightforward to detect and assign the E - E type hot band, because the upper state is Z in common with that of the 1492 cm⁻¹band. After careful searching for the spectra, KIH arrived at a conclusion that there is only one A - E type hot band present, which is difficult to reconcile with Assignment II, and the observed A - E hot band is reasonably ascribed to 2ν₂ - ν₄ in Assignment I. The NO3 radical has been thought to be subjected to strong vibronic interaction. This view originated from an anomalous ν₄ progression appearing in the NO₃⁻ photoelectron spectra by Neumark et al.; they explained this observation in terms of Herzberg-Teller (H-T) effect with a sizable interaction parameter. However, KIH did not observe any anomalous features in the ν₄ vibration-rotation structure, which would be caused by the huge H-T perturbation as presumed by Neumark. Hirota found that the ν₄ progression of Neumark can be explained by the coupling of the unpaired electron orbital angular momentum with that of the ν₄ mode in the ground electronic state.

A moderate resolution spectrum of the ~A² E^′′ state of the NO₃ radical has been obtained using jet-cooled cavity ringdown spectroscopy. The analysis of the vibronic structure of this spectrum has been undertaken using a quadratic Jahn-Teller Hamiltonian. All observed transitions have been assigned and unperturbed frequencies and Jahn-Teller coupling constants were determined by fitting the experimental spectrum using nonlinear least squares regression. We find evidence of strong Jahn-Teller coupling in this electronic state, particularly in the ν₃ mode. These results are compared to electronic structure calculations including terms up to fourth order. Calculated and experimental spectra are presented and critically compared.

The vibronic structure of the NO₃ radical has been the subject of much recent research in our group.¹ We have also collected several high resolution spectra of transitions to the ~A² E^′′ state. Parallel bands, with a₁^′′ symmetry, have been satisfactorily fit using an oblate symmetric top Hamiltonian with spin rotation. Some lines were seen to be perturbed and it is likely that this is the result of random perturbations from levels originating from the ground electronic state. The perpendicular bands, which have e′ symmetry, are not satisfactorily described using this Hamiltonian. In particular the rotational structure of the e′ levels has many more transitions than in the oblate top model predicts. For this reason we have developed two different rovibronic Hamiltonians for the analysis of the vibronically degenerate levels. Both include spin-orbit, coriolis, spin-rotation, and Jahn-Teller distortion terms. However, they are derived starting from two different limiting cases. In Case 1 the Hamiltonian is built by assuming first a D_{3 h} configuration and then perturbations are added. Case 2 starts at the statically distorted, low symmetry geometry and introduces interactions among the vibronic levels. In the case of Jahn-Teller coupling that is neither very weak nor very strong these models should both adequately describe the observed spectra. These models and preliminary analysis of several e′ bands are presented.
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¹Codd, T. et al. 68^th Int. Symp. Molec. Spec. (2012)

Rotationally-resolved high-resolution fluorescence excitation spectra and the Zeeman effects of the 662 nm band, which is called as the 0-0 band of the B ²E′← X ²A₂′ electronic transition, of ¹⁴NO₃ have been observed. ¹ Sub-Doppler excitation spectra were measured by crossing a single-mode laser beam perpendicular to a collimated radical beam, which was formed by the heat decomposition of ¹⁴N₂O₅; ¹⁴N₂O₅ → ¹⁴NO₃ + ¹⁴NO₂. The typical linewidth was 30 MHz and the absolute wavenumber was calibrated with accuracy 0.0001 cm⁻¹ by measurement of the Doppler-free saturation spectrum of iodine molecule and fringe pattern of the stabilized etalon. In the observed spectra, only the rotational line pairs from the X ²A₂′(v"=0, K"=0, N"=1, F₁ and F₂) levels are assigned, but the other rotational lines were not found yet. In this work, we expanded the measurement of the Zeeman splittings for the other rotational lines, which are predicted their position by using the combination differences calculated from the reported molecular constants. ² From the observed Zeeman patterns, we have assigned unambiguously several transition lines for the 0-0 band. Additionally, we have measured the rotationally-resolved high-resolution spectra of vibrational excited levels of the B ²E′ state, which lies 770 cm⁻¹ and 948 cm⁻¹ above the 0-0 band, and found many tiny rotational lines in these vibronic bands.
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¹K. Tada, W. Kashihara, S. Kasahara, M. Baba, T. Ishiwata, and E. Hirota, The 68th OSU Symposium, WJ04 (2013). ²R. Fujimori, N. Shimiza, J. Tang, T. Ishiwata, and K. Kawaguchi, J. Mol. Spectrosc., 283, 10 (2013).

Nitrate radical (NO₃) has two electronic excited states: A ²E" and B ²E′ near the electronic ground state: X ²A₂′. These three electronic states can vibronically interact each other. Therefore, NO₃ is one of the great subjects for understanding intramolecular interactions in polyatomic radicals. High-resolution fluorescence excitation spectrum and its magnetic effect of the 662 nm band, which is assigned as the 0 - 0 band of the B ²E′← X ²A₂′ transition, of ¹⁴NO₃ have been observed.¹ The observed ¹⁴NO₃ spectrum was too complicated to be analyzed rotationally because of less rotational regularity. In this work, we observed high-resolution fluorescence excitation spectrum of the 662 nm band of ¹⁵NO₃. The observed region was 15080 - 15103 cm⁻¹. We also observed the Zeeman splitting of intense rotational lines for unambiguous rotational assignment. A part of the observed rotational lines was successfully assigned by using ground state combination differences calculated from the reported molecular constants² and the observed Zeeman patterns. The effective molecular constants of the excited state were determined under the oblate symmetric-top model.
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¹K. Tada, W. Kashihara, S. Kasahara, M. Baba, T. Ishiwata, and E. Hirota, The 68th OSU Symposium, WJ04 (2013). ²R. Fujimori, N. Shimizu, J. Tang, T. Ishiwata, and K. Kawaguchi, J. Mol. Spectrosc., 283, 10 (2013).

The quasidiabatic model of Köppel, Domcke and Cederabaum has proven to be a powerful tool for computing intensities and level positions in electronic spectra involving strong vibronic coupling effects. The model has been used, as parametrized by very high-level equation-of-motion coupled cluster (EOM-CC) calcaultions, is demonstrated by applications to the NO₃ radical.

In this work we present a quantum-chemical calculation of the electronic spin-orbit splittings for the spatially degenerate low-lying E′ and E^′′ states of the nitrate radical (NO₃). The calculation is based on a degenerate perturbation theory using scalar-relativistic equation of motion ionization potential (EOMIP) coupled-cluster singles and doubles (CCSD) wave functions together with spin-orbit matrix elements constructed in the framework of exact two-component (X2C) theory. The computed SO splittings are discussed in view of recent experiments by Babu et al. for the B²E′ state and Miller et al. for the X²A₂′ state. Calculations of splittings based on vibronic, rather than simply electronic, wave functions are also discussed.

The B̃ ²E′ - X̃ ²A₂′ electronic transition of NO₃ generated in a supersonic free jet expansion was investigated by four wave mixing ( 4WM ) spectroscopy. The degenerated 4WM and laser induced fluorescence ( LIF ) spectra around the 0₀⁰ band region were measured simultaneously. The D4WM spectrum shows broad band features for the 0₀⁰ band similar to that of the LIF spectrum. The broad 0₀⁰ band does not consist of one sub-band, but of several bands. The intensity distribution of the sub-bands of the D4WM spectrum is similar, but not identical to that of the LIF spectrum.