We present the high-resolution spectrum of C₃ produced in a supersonically expanding propyne plasma, which is recorded around 3 μm using continuous wave cavity ring-down spectroscopy (cw-CRDS). Fifteen fully resolved ro-vibrational bands are observed, which have been assigned to vibrationally excited nν₁+mν₃ combination bands of C₃; fourteen of which are reported for the first time. This work is a significant extension of the known electronic ground state vibrational energy levels, with the observed number of quanta being: n ≤ 7 and m ≤ 3. Furthermore, with the new observations of highly excited vibrational modes, up to the (7,0,1) energy level, we are able to test the fundamental understanding of this "floppy" benchmark molecule. A detailed analysis of the experimental spectra is supported by ro-vibrational calculations based on an accurate local ab initio potential energy surface (PES) for C₃ (~X¹Σ⁺_g).B. Schröder and P. Sebald, J. Chem. Phys. 144, 044307 (2016)he presented variational calculations give remarkable agreement compared to experimental values with typical accuracies of  ∼ 0.01% for the vibrational frequencies and  ∼ 0.001% for the rotational parameters, even for high energy levels around 10000 cm⁻¹.B. Schröder et al., J. Chem. Phys. 149, 014302 (2018)html:<hr /><h3>Footnotes: B. Schröder and P. Sebald, J. Chem. Phys. 144, 044307 (2016)T B. Schröder et al., J. Chem. Phys. 149, 014302 (2018)

Striking variations in excited state band intensities were observed in the photoelectron spectra of Sm₂O⁻ collected using eight different photon energies. The spectra exhibit a pronounced overall increase in excited state band intensities relative to the transition to the ground neutral state as the photon energy decreases. This anomaly opposes that which would be expected to arise from threshold effects. The photoelectron spectra of several homonuclear Sm and heteronuclear Sm-Ce oxides collected previously with the second and third harmonic outputs of a Nd:YAG (2.33 eV and 3.49 eV) reveal a similar relationship, making the likelihood of coincidental resonance seem remote. Moreover, the absence of this phenomenon in homonuclear Ce-based clusters implicates the exceptionally high density of accessible spin states originating from the partially-filled 4f subshell of Sm. In addition, a broad oscillation in plots of the relative band intensities versus electron kinetic energy may map onto quasibound states of the anion. The results presented bear significance in the study of other electron-rich systems and models the interaction of the photoelectron and remnant neutral-like species.

Photoelectron spectroscopy is a powerful technique for investigating the structure and reactivity of metal oxide clusters, which can serve as models of surface defect sites. Assigning photoelectron spectra typically requires corroborating computational simulations. Motivated by the complicated electronic structure often exhibited by these species that can challenge the quality of computational results using widely available quantum chemistry methods, our group has explored the development of efficient electronic structure models to describe photodetachment. This talk will describe these efforts and our lab’s recent applications of such models in investigations of various metal oxide clusters.

We report first-principles variational simulation of the non-resonant Raman spectrum for methyl radical (¹²CH₃) in the electronic ground state. Calculations are based on a high level ab initio potential energy and polarizability tensor surfaces of CH₃ and employ accurate variational treatment of the ro-vibrational dynamics implemented in the general code TROVE [S. N. Yurchenko, W. Thiel, and P. Jensen, J. Mol. Spectrosc. 245, 126-140 (2007); A. Yachmenev and S. N. Yurchenko, J. Chem. Phys. 143, 014105 (2015)]. We extend the capabilities of TROVE towards simulations of the Raman spectra, which can in be applied to arbitrary molecule of moderate size. The simulations for CH₃ are found to be in a good agreement with the available experimental data.

The members of the alkoxy radical family play important roles in oxidation processes, both in combustion and the atmosphere, and their spectrocopy is well studied. The simplest species, CH₃O, has a degenerate X̃²E ground electronic state, which has a near-UV transition to a non-degenerate electronic state presently referred to as B̃²A. Larger family members formed by alkyl group substitution of the H atom(s) shift the Ã-X̃ electronic transition to the red as the size of the alkyl group grows. If the H atom(s) substitution is not symmetric, the degeneracy of the X̃ state is resolved into two non-degenerate electronic states, presently referred to as X̃ and Ã. Typically the energy separation, ∆E₀, between these two states, caused by the vibronic quenching mechanismR. Renner, Z. Phys. 92, 172 (1934).^,D. A. Mills, C. M. Western, and B. J. Howard, J. Phys. Chem. 90, 3331 (1986) is small ( 1000cm^-1). Historically, the approach to the spectra involving these three states has been to analyze the rotational structure in the and states separately via a Hamiltonian including an asymmetric top rotational term and a spin−rotation interaction. Recently LiuJ. Liu, J. Chem. Phys. 148, 124112 (2018).as suggested that, as is done with the ^2E state of methoxy, the structure of both the and states, now separated by E_0, and coupled by the spin−orbit and Coriolis interactions, is better considered together. This “coupled two−state model” also allows semi−quantitative prediction of effective spin−rotation constants using calculated molecular geometry and spin−orbit constants, which can be calculated with considerable accuracy. In the present work, we have simulated rotationally and fine−structure resolved laser−induced fluorescence (LIF) spectra of alkoxy radicals with the Liu model and fit the rotational constants, as well as the spin−orbit and Coriolis coupling parameters between the and D. A. Mills, C. M. Western, and B. J. Howard, J. Phys. Chem. 90, 3331 (1986),J. Liu, J. Chem. Phys. 148, 124112 (2018).h

The proposed possibility of laser cooling of alkaline earth monoalkyl (MR) and monoalkoxide (MOR) free radicals, e.g. CaCH₃ and CaOCH₃, has generated significant experimental interest in the spectroscopic analysis of these open-shell molecules with orbitally degenerate or nearly degenerate low-lying electronic states. The analysis of laser induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of such molecules requires an in-depth analysis of couplings between close lying electronic states. The Jahn-Teller and psuedo-Jahn-Teller effect plays an important role in the spectra of these molecules. The molecular interaction picture is even more complex due to the presence of spin-orbit couplings between electronic states. In this talk we present our methodology for treating psuedo Jahn-Teller couplings for calcium methoxide (CaOCH₃), calcium ethoxide (CaOC₂H₅) and calcium iso-propoxide (iso-CaOC₃H₇). A combination of EOM-CCSD and multi-mode Spin-Vibronic calculations have been employed to calculate transition frequencies and intensities for the excitation and emission spectra of these molecules. These calculations are used to understand the LIF and DF spectra, and to predict the feasibility of laser cooling of these molecules.

A Jahn-Teller active molecule demonstrates a characteristic signature in its rotationally resolved spectra due to the distortions from the symmetric configuration. In the past decades, this signature has been used to experimentally access and understand the dynamics around a conical intersection. In this talk we present a method to calculate this rotational signature starting with electronic structure calculations. We derive an Effective Rotational Hamiltonian (ERH) for Jahn-Teller active systems and determine the relationship between the experimentally observable rotational parameters and electronic structure theory. The methodology has been further extended to include molecules with significant spin-orbit interaction. We have calculated both h₁, which manifests from distortions in the plane perpendicular to the highest symmetry rotational axis of the molecule, and h₂, which manifests from out-of-plane distortions of the molecule. We present our findings for cyclopentadienyl (C₅H₅), methoxy (CH₃O) and nitrate (NO₃) radicals and compare them to experimental data available in the literature. These calculations not only help guide experiments but are also a great tool for benchmarking high-level quantum chemistry calculations.

We report the branching ratio for the H + propene tunneling reaction to form n- or i-propyl radicals within a solid para-hydrogen (p-H₂) matrix at 3.2 K. Chlorine and propene are co-deposited in the p-H₂ matrix, and then 365 nm and infrared light sources are used to produce HCl and free H atoms. These free H atoms can tunnel through the matrix until they encounter a propene molecule, at which point they can react to form either an n-propyl or an i-propyl radical. i-Propyl was produced in greater proportion than n-propyl, indicating that for hydrogen addition to the double bond, the rate of addition to the terminal carbon (i-propyl) is faster than the rate of addition to the center carbon (n-propyl). Because the barriers for addition are approximately 700 cm⁻¹ – 1500 cm⁻¹ (1000 K – 2000 K), the only available mechanism for reaction in the p-H₂ matrix (3.2 K) is tunneling. Ab initio calculations were used to compute the tunneling probabilities for the formation of the n-propyl and i-propyl radicals. The rate of addition to the terminal carbon (i-propyl) was calculated to be faster, in agreement with experiment.