In the ten years since the traditional assignment of its degenerate stretching fundamental became controversial, a great deal of work - both theoretical and experimental - has been done on the NO₃ molecule. A brief review of these developments will be given, and results of very high-level calculations of the dispersed fluorescence and negative ion photoelectron spectra of this molecule will be presented together with the corresponding experimental results. In addition, the question of "what is next to do" on the ground state - from a theoretical point of view - will be addressed. Time permitting, some discussion will also be devoted to the strongly Jahn-Teller active ²E^′′ (first excited) electronic state, where the level of understanding and agreement thus far obtained from experiment and theory is still at a rather primitive stage.

Experimentalists and theorists alike have been intrigued by the infrared spectra of the methoxy radical; due to the presence of a conical intersection at the C_3v molecular geometry, methoxy's IR spectrum is strongly influenced by Jahn-Teller vibronic coupling which leads to large amplitude vibrations and extensive mixing of the two lowest electronic states. This radical's complex IR spectra, which also contains moderate mixing from spin-orbit and Fermi couplings, serves as an important test for models which seek to understand complex molecular vibrations. The assignment of the IR spectra in methoxy, and its partially and fully deuterated analogues, is considered. All vibronic states below 2575 cm⁻¹ in CH₃O and 2035 cm⁻¹ in CD₃O are assigned. The mixing between the zero-order normal modes complicates the assignment using this representation. Alternative zero-order representations, that include specific Jahn-Teller couplings, are explored and used to create definitive assignments for the low lying vibronic states. In many instances it is possible to plot the wavefunctions on which the assignments are based. The plots also enable one to visualize the conical seam and its effect on the wavefunctions. The first and second order Jahn-Teller coupling in the rocking motion dominates the spectral features in CH₃O, while first order and modulated first order couplings dominate the spectral features in CD₃O. The methods described here are general and can be applied to other Jahn-Teller systems.

We present a computer code (DVR3DUV) for calculations of high-accuracy ro-vibronic spectra of triatomic molecules. The current implementation is an extension to the DVR3D suite [1], which operates with the exact kinetic energy operator, a single potential energy surface and a single dipole moment surface (ro-vibrational transitions only). The main function of the new code is calculation of transition intensities between different electronic states in the rotational-vibrational resolution. As a case study, two electronic states of SO₂ molecule are considered. Ro-vibrational wavefunctions and energy levels for the ground X̃¹A₁ state of SO₂ are calculated using Ames PES [2], while energy levels and wavefunctions of the C̃¹B₂ state are calculated using ab initio PES (MRCI-F12-AVTZ). Transition intensities are computed using a) Franck-Condon approximation; b) ab initio dipole moment surface between the two electronic states. Results are compared to the latest theoretical and experimental works. Future applications of the DVR3DUV code will focus on highly accurate electronic spectra for atmospherically important species, such as ozone molecule.

Cyclopentadienyl (Cp) radical has been subject to numerous studies for the greater part of half a century. Experimental work has involved photo-electron spectroscopy, T. ICHINO, et al. J. Chem. Phys. 129, 084310 (2008)aser induced fluorescence excitation L. YU, S. C. FOSTER, J. M. WILLIAMSON, M. C. HEAVEN AND T. A. MILLER J. Phys. Chem. 92, 4263 (1988)nd emission, B. E. APPLEGATE, A. J. BEZANT AND T. A. MILLER J. Chem. Phys 114, 4869 (2001)nfrared absorption spectroscopy, D. LEICHT, M. KAUFMANN, G. SCHWAAB, AND M. HAVENITH J. Chem. Phys. 145, 7 (2016), 074304.nd recently rotationally resolved spectra in the CH stretch region taken at JILA. Even more theoretical works appear in the literature, but substantial advances in computation have occurred since their completion. Cp's highly symmetric (D_5h) structure and doubly degenerate electronic ground (X̃²E₁^′′), which is subject to linear Jahn-Teller distortion, have been a great motivation for work on it. We have commenced new computational work to obtain a broad understanding of the electronic, vibrational, and rotational, i.e. rovibronic, structure of the Cp radical as revealed by its spectra, with particular emphasis on the new infrared spectra. The goal is to guide experiments and their analyses and reconcile results from spectroscopy and quantum chemistry calculations.

---

Footnotes:

T. ICHINO, et al. J. Chem. Phys. 129, 084310 (2008)l L. YU, S. C. FOSTER, J. M. WILLIAMSON, M. C. HEAVEN AND T. A. MILLER J. Phys. Chem. 92, 4263 (1988)a B. E. APPLEGATE, A. J. BEZANT AND T. A. MILLER J. Chem. Phys 114, 4869 (2001)i D. LEICHT, M. KAUFMANN, G. SCHWAAB, AND M. HAVENITH J. Chem. Phys. 145, 7 (2016), 074304.a

The structure in rotational spectra of many free radical molecules is complicated by Jahn-Teller distortions. Understanding the magnitudes of these distortions is vital to determining the equilibrium geometric structure and details of potential energy surfaces predicted from electronic structure calculations. For example, in the recently studied òE^′′ state of the NO₃ radical, the magnitudes of distortions are yet to be well understood as results from experimental spectroscopic studies of its vibrational and rotational structure disagree with results from electronic structure calculations of the potential energy surface. By fitting either vibrationally resolved spectra or vibronic levels determined by a calculated potential energy surface, we obtain vibronic eigenfunctions for the system as linear combinations of basis functions from products of harmonic oscillators and the degenerate components of the electronic state. Using these vibronic eigenfunctions we are able to predict parameters in the rotational Hamiltonian such as the Watson Jahn-Teller distortion term, h₁, and compare with the results from the analysis of rotational experiments.

The bound states of electrons on the surface of superfluid helium have been a research topic for several decades. One of the first systems treated was an electron bound to an ionized helium cluster.A. Golov and S. Sekatskii, Physica B, 1994, 194, 555-556ere, a similar system is considered, which consists of a helium droplet with an ionized dopant inside and an orbiting electron on the outside. In our theoretical investigation we select alkali metal atoms (AK) as central ions, stimulated by recent experimental studies of Rydberg states for Na,E. Loginov, C. Callegari, F. Ancilotto, and M. Drabbels, J. Phys. Chem. A, 2011, 115, 6779-6788b,F. Lackner, G. Krois, M. Koch, and W. E. Ernst, J. Phys. Chem. Lett., 2012, 3, 1404-1408nd CsF. Lackner, G. Krois, M. Theisen, M. Koch, and W. E. Ernst, Phys. Chem. Chem. Phys., 2011, 13, 18781-18788ttached to superfluid helium nanodroplets. Experimental spectra , obtained by electronic excitation and subsequent ionization, showed blueshifts for low lying electronic states and redshifts for Rydberg states. In our theoretical treatment the diatomic AK⁺-He potential energy curves are first computed with ab initio methods. These potentials are then used to calculate the solvation energy of the ion in a helium droplet as a function of the number of atoms. Additional potential terms, derived from the obtained helium density distribution, are added to the undisturbed atomic pseudopotential in order to simulate a 'modified' potential felt by the outermost electron. This allows us to compute a new set of eigenstates and eigenenergies, which we compare to the experimentally observed energy shifts for highly excited alkali metal atoms on helium nanodroplets.

---

Footnotes:

A. Golov and S. Sekatskii, Physica B, 1994, 194, 555-556H E. Loginov, C. Callegari, F. Ancilotto, and M. Drabbels, J. Phys. Chem. A, 2011, 115, 6779-6788R F. Lackner, G. Krois, M. Koch, and W. E. Ernst, J. Phys. Chem. Lett., 2012, 3, 1404-1408a F. Lackner, G. Krois, M. Theisen, M. Koch, and W. E. Ernst, Phys. Chem. Chem. Phys., 2011, 13, 18781-18788a

The binding energies of the hydrogen atom are given by the Rydberg formula

En = − \fracR∞ μ/me(n−δ)2,

where the quantum defect δ vanishes in the case of a pure Coulomb potential. Heavy Rydberg systems can be realized when the electron is replaced by an anion, which leads in the case of H⁺H⁻ to an almost 1000 times larger Rydberg constant and to an infinite number of vibrational states. In the diabatic molecular basis, these ion-pair states are described by long-range Coulomb potentials with ¹Σ_g⁺ and ¹Σ_u⁺ symmetry. In this basis, the level energies are described by an almost energy-independent, nonzero quantum defect, reflecting the finite size of H⁻. Strong interactions at small internuclear distances lead to strong variation of δ with n. Gerade [2] and ungerade [3] ion-pair states have been observed in H₂ with principal quantum numbers up to n=240. The quantum defects in this range were found to vary with energy, indicating the inadequacy of a pure diabatic picture. Spectra of ungerade heavy Rydberg states of H₂ with n=160−520 showing that the quantum defect only becomes energy independent for n > 350 will be presented, supporting the description using a diabatic basis. I will also present first observations of ion-pair states in HD, showing two series of heavy Rydberg states, H⁺D⁻ and H⁻D⁺, which have different series limits. The experimental results will be discussed and compared with calculations using both an adiabatic and a diabatic basis. [1] S. Pan, and F. H. Mies, J. Chem. Phys. 89, 3096 (1988). [2] M. O. Vieitez, T. I. Ivanov, E. Reinhold, C. A. de Lange, and W. Ubachs, Phys. Rev. Lett. 101, 163001 (2008). [3] R. C. Ekey, and E. F. McCormack, Phys. Rev. A 84, 020501(R) (2011).

Sulfur mass independent fractionation (S-MIF) describes anomalous sulfur isotope ratios commonly found in sedimentary rocks older than 2.45 billion years. These anomalies likely originate from photochemistry of small, sulfur-containing molecules in the atmosphere, and their sudden disappearance from rock samples younger than 2.45 years is thought to be correlated with a sharp rise in atmospheric oxygen levels. The emergence of atmospheric oxygen is an important milestone in the development of life on Earth, but the mechanism for sulfur MIF in an anoxic atmosphere is not well understood. In this context, we present an analysis of the B-X UV spectrum of S₂, an extension of work presented last year. The B state of S₂ is strongly perturbed by the nearby B” state, as originally described by Green and Western (1996). Our analysis suggests that a doorway-mediated transfer mechanism shifts excited state population from the short-lifetime B state to the longer-lifetime B” state. Furthermore, access to the perturbed doorway states is strongly dependent on the population distribution in the ground state. This suggests that the temperature of the Achaean atmosphere may have played a significant role in determining the extent of S-MIF.

The ~C¹B₂ state of SO₂ has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. The asymmetry in the potential energy surface is expressed as a staggering in the energy levels of the ν₃′ progression. We have recently made the first observation of low-lying levels with odd quanta of ν₃′, which allows us to characterize the origins of the level staggering. Our work demonstrates the usefulness of low-lying vibrational level structure, where the character of the wavefunctions can be relatively easily understood, to extract information about dynamically important potential energy surface crossings that occur at much higher energy. The measured staggering pattern is consistent with a vibronic coupling model for the double-minimum, which involves direct coupling to the bound 2¹A₁ state and indirect coupling with the repulsive 3¹A₁ state. The degree of staggering in the ν₃′ levels increases with quanta of bending excitation, which is consistent with the approach along the ~C-state potential energy surface to a conical intersection with the 2¹A₁ surface at a bond angle of  ∼ 145^°.

3-phenyl-2-propynenitrile (PPN) is a potentially important component of Titan’s atmosphere. This molecule exhibits intriguing patterns in high-resolved absorption and fluorescence spectra. To better understand PPN’s photochemistry, we employ computational tools to examine its electronic structure and excited states. The presence of vibronic coupling is evaluated by mapping potential energy surfaces of the first four electronic excitations along different vibrational modes. The parameters that describe the interactions between vibrational and electronic states are used to build the vibronic Hamiltonian and predict the absorption and emission spectra of PPN with the multi configuration time dependent Hartree (MCTDH) algorithm.

r0.2 Carbon monoxide is after molecular hydrogen the second most abundant molecule in the universe and an important molecule for processes occurring in the atmosphere, hydrocarbon combustion and the interstellar medium. The rate coefficients of CO in collision with dominant species like H, H₂, He, etc are necessary to understand the CO emission spectrum or to model combustion chemistry processes. The inelastic scattering of CO with H has been intensively studied theoretically in the past decades,¹ mostly using the so-called WKS PES⁶ developed by Werner et al. or recently a modified version by Song et al.² Though the spectroscopic agreement of the WKS surface with experiment is quite good, so far the studies of scattering dynamics have neglected coupling to an electronic excited state. We present new results on a set of HCO surfaces of the ground and the excited Renner-Teller coupled electronic states³ with the principal objective of studying the influence of the Renner-Teller coupling on the inelastic scattering of CO+H. Our calculations done using the MCTDH⁴ algorithm in the 0-2 eV energy range allow evaluation of the contribution of the Renner-Teller coupling on the rovibrationally inelastic scattering and discuss the relevance and reliability of the calculations. References:
1. N. Balakrishnan, M. Yan and A. Dalgarno, Astrophys. J. 568, 443 (2002); B.C. Shepler et al, Astron. & Astroph. 475, L15 (2007); L. Song et al, J. Chem. Phys. 142, 204303 (2015); K.M. Walker et al, Astroph. J. 811, 27 (2015).
2. L. Song et al, Astrophys. J. 813, 96 (2015).
3. H.-M. Keller et al, J. Chem. Phys. 105, 4983 (1996).
4. S. Ndengue, R. Dawes and H. Guo, J. Chem. Phys. 144, 244301 (2016).
5. M.H. Beck et al., Phys. Rep. 324, 1 (2000).