Dynamic Jahn-Teller Research Articles

Dynamic Jahn-Teller effect in the strong spin-orbit coupling regime

Exotic quantum phases, arising from a complex interplay of charge, spin, lattice and orbital degrees of freedom, are of immense interest to a wide research community. A well-known example of such an entangled behavior is the Jahn-Teller effect, where the lifting of orbital degeneracy proceeds through lattice distortions. Here we demonstrate that a highly-symmetrical 5d1 double perovskite Ba2MgReO6, comprising a 3D array of isolated ReO6 octahedra, represents a rare example of a dynamic Jahn-Teller system in the strong spin-orbit coupling regime. Thermodynamic and resonant inelastic x-ray scattering experiments, supported by quantum chemistry calculations, undoubtedly show that the Jahn-Teller instability leads to a ground-state doublet, resolving a long-standing puzzle in this family of compounds. The dynamic state of ReO6 octahedra persists down to the lowest temperatures, where a multipolar order sets in, allowing for investigations of the interplay between a dynamic JT effect and strongly correlated electron behavior.

Electron Spin Polarization in the Triplet State of Methoxy-substituted Phosphorus(V) Tetraphenyl Porphyrins

Photoexcited triplet states of porphyrins are of great relevance in various applications due to their high yield, long lifetime, and strong electron spin polarization. This study delves into properties and spin dynamics of the triplet state of a series of hypervalent phosphorus(V) porphyrins. Transient Electron Paramagnetic Resonance (TrEPR) measurements, supported by quantum chemical calculations as well as by optical absorption/luminescence experiments, reveal that, unlike singlet states, the lowest triplet state does not exhibit charge-transfer (CT) character upon photoexcitation. However, the presence of excited CT singlet states alters the intersystem crossing in phosphorus(V) porphyrins, leading to a sign change in the initial multiplet polarization of the photoexcited triplet state. TrEPR results further demonstrate that significant net polarization develops in the triplet states of the phosphorus(V) porphyrins due to the dynamic Jahn-Teller effect. Yet, this effect remains largely unaffected by differences in their molecular structures.

Molecular Pairing in Twisted Bilayer Graphene Superconductivity.

We propose a theory for how the weak phonon-mediated interaction (J_{A}=1-4 meV) wins over the prohibitive Coulomb repulsion (U=30-60 meV) and leads to a superconductor in magic-angle twisted bilayer graphene (MATBG). We find the pairing mechanism akin to that in the A_{3}C_{60} family of molecular superconductors: Each AA stacking region of MATBG resembles a C_{60} molecule, in that optical phonons can dynamically lift the degeneracy of the moiré orbitals, in analogy to the dynamical Jahn-Teller effect. Such induced J_{A} has the form of an intervalley anti-Hund's coupling and is less suppressed than U by the Kondo screening near a Mott insulator. Additionally, we also considered an intraorbital Hund's coupling J_{H} that originates from the on-site repulsion of a carbon atom. Under a reasonable approximation of the realistic model, we prove that the renormalized local interaction between quasiparticles has a pairing (negative) channel in a doped correlated insulator at ν=±(2+δν), albeit the bare interaction is positive definite. The proof is nonperturbative and based on exact asymptotic behaviors of the vertex function imposed by Ward identities. Existence of an optimal U for superconductivity is predicted. In a large area of the parameter space of J_{A}, J_{H}, the ground state is found to have a nematic d-wave singlet pairing, which, however, can lead to a p-wave-like nodal structure due to the Berry's phase on Fermi surfaces (or Euler obstruction).

Large Orbital Moment and Dynamical Jahn-Teller Effect of AlCl-Phthalocyanine on Cu(100).

Submonolayer amounts of chloroaluminum-phthalocyanine on Cu(100) were studied with scanning tunneling spectroscopy. The molecule can be prepared in a fourfold symmetric state whose conductance spectrum exhibits a zero-bias feature similar to a Kondo resonance. In magnetic fields, however, this resonance splits far more than expected from the spin of a single electron. Density functional theory calculations reveal a charge transfer of 1.3 electrons to the degenerate lowest unoccupied molecular orbitals. These orbitals are mixed by the orbital momentum operator L[over ^]_{z} with a large matrix element corresponding to m_{L}≈2.7. Dehydrogenation of a ligand lifts the degerenracy of the lowest unoccupied molecular orbital, reduces the splitting in magnetic fields, and induces a polarity dependence of the spectra. Using model calculations of the spin, orbital, and vibrational degrees of freedom we show that a dynamical Jahn-Teller effect reproduces the main experimental observations.

Structure, Thermodynamics, Chemomechanics, and Kinetics: The Hidden Complexities of LiNiO2

After being abandoned due to mechanical and thermodynamic instabilities, layered transition metal oxides with ultra-high Ni content currently see a revival as promising cathode active material in the effort of further pushing the energy density of lithium ion batteries. Because such materials are in many aspects closely related to the pure LiNiO2, understanding the properties and mechanisms of this model system on an atomic level is vital for the development of more complex materials. Therefore, we have analyzed the structure, thermodynamics, chemomechanics and kinetics of LiNiO2 and its delithiated states including the presence of native defects by making use of atomistic simulations to grasp the peculiarities of this material.What makes the structure and related properties of LiNiO2 especially interesting and challenging is the presence of dynamic Jahn-Teller distortions of the NiO6 octahedra. This leads to local monoclinic distortions and our simulations revealed why the material still appears to be of rhombohedral symmetry on a global scale.[1]Upon delithiation and the concurrent oxidation to Ni4+, the Jahn-Teller distortions start to vanish and particularly stable Li orderings may be observed. In this context, we used a cluster expansion approach and were able to reproduce the experimental phase diagram. Moreover, we included native defects in the analysis because LiNiO2 can hardly be prepared stoichiometric. Instead, many synthesis approaches lead to additional Ni ions occupying the Li layers (NiLi). The introduction of such defects as well as other substitutionals in a surrogate model shows that single phase regions tend to be suppressed and solid-solution behavior is favored. This smooths the voltage curves and gives good agreement with experiments.[2]We then focused on more extended defects in layered transition metal oxides: Stacking faults (observed at low Li content and responsible for stacking changes from O3 to O1-type layering) and dislocations (needed to drive stacking changes through the material). We found that the energetics and gliding barriers of stacking faults in stoichiometric LiNiO2 are similar to LiCoO2 for all degrees of lithiation. However, by acknowledging the presence of NiLi we quantified for the first time how this defect hinders layer gliding: In the fully delithiated material, 2% of NiLi in the Li layer has a similar effect as 11%-15% Li, effectively increasing the gliding barriers. In terms of dislocations, their excess energies in LiNiO2 are much lower than in LiCoO2, which we mostly attribute to the Jahn-Teller distortions that are able to collinearly orient in the vicinity of a dislocation in LiNiO2, effectively taking a part of the strain. Moreover, dislocations are able to attract both Li and O vacancies, potentially affecting the kinetics of the material as well as acting as a precursor for further degradation mechanisms.[3]Lastly, we mention our recent analysis again focusing on NiLi defects. Our simulations show that such Ni ions remain in a +2 oxidation state independent on the degree of lithiation. This contradicts the broad opinion of an oxidation state change to +3 at low Li content, which is believed to lead to a local constriction of the layer that can hardly be relithiated again. Nevertheless, we show that NiLi indeed leads to an attraction of Li vacancies within two lattice sites and the potential to split fast divacancies into two considerably slower single vacancies. These observations together with the fact that any NiLi depicts an obstacle for diffusing Li ions or Li vacancies shed light on the kinetics of non-stoichiometric LiNiO2 and support the development of further optimization schemes.[4][1] Chem. Mater. 2020, 32, 23, 10096–10103[2] J. Mater. Chem. A , 2021,9, 14928-14940[3] Chem. Mater. 2023, 35, 2, 584–594[4] https://doi.org/10.26434/chemrxiv-2023-hkmsn-v3

Spin-Orbit-Lattice Entangled State in A_{2}MgReO_{6} (A=Ca, Sr, Ba) Revealed by Resonant Inelastic X-Ray Scattering.

The 5d^{1} ordered double perovskites present an exotic playground for studying novel multipolar physics due to large spin-orbit coupling. We present Re L_{3} edge resonant inelastic x-ray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A_{2}MgReO_{6} (A=Ca, Sr, Ba) family of 5d^{1} double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric line shape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbit-lattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibronic coupling for a complete description of the ground state physics of 5d^{1} double perovskites.

Unraveling dynamic Jahn-Teller effect and magnetism in FeTiF6×6H2O single crystal

Hydrated iron fluoridotitanate (FeTiF6 × 6 H2O) single crystals are fascinating magnetic materials with unique properties. To understand the underlying mechanisms, this study combines X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) techniques, complemented by density functional theory (DFT) calculations. Polarization-dependent X-ray absorption spectroscopy, encompassing XANES and XMCD, is a powerful technique for probing the local structures and magnetic properties of materials. It is element-selective, bulk-sensitive, and compatible with a wide range of experimental conditions. In this study, we used XANES and XMCD spectroscopies to investigate the local structures and magnetic properties of Fe and Ti in FeTiF6 × 6 H2O single crystals. XANES analysis revealed distinct local environments around Fe and Ti, providing insights into their coordination environments. Element-selective magnetization measurement at the Fe K-edge demonstrated that iron sites in the oxidation state Fe2+ have an unambiguous paramagnetic contribution to the magnetization along the b-axis. Notably, the absence of an XMCD signal at the Ti K-edge confirmed the absence of a magnetic moment in Ti atoms within the crystal. DFT calculations corroborate the experimental findings and provide insights into the electronic structure and magnetic interactions. The combined results provide a comprehensive understanding of the dynamic Jahn-Teller effect in FeTiF6 × 6 H2O single crystals, highlighting the significance of polarization-dependent X-ray absorption spectroscopy in unraveling the intricate magnetic behavior of such materials. This study contributes to the fundamental understanding of magnetism in these materials and paves the way for the development of novel magnetic materials with tailored properties.

Multi-state vibronic dynamics of the benzene cation: Combined multi-mode Jahn-Teller and Pseudo Jahn-Teller Effects

The multi-state vibronic interactions in the benzene radical cation are investigated theoretically, based on an ab initio quantum dynamical approach. The three lowest doubly degenerate and two lowest non-degenerate electronic states are included, amounting to eight electronic component states and 28 vibrational degrees of freedom. The multi-mode dynamical Jahn-Teller as well as pseudo Jahn-Teller effects are included on an equal footing. This becomes possible by employing the Multiconfigurational Time-Dependent Hartree Method and its multi-layer extension for the wavepacket propagation underlying the dynamical treatment. The results indicate a step wise population transfer from higher to lower-energy electronic states. The transfer between the highest ( E∼ and D∼ states) is extremely fast which is made plausible by their energetic proximity. On the other hand the transfer is not complete and does not comprise the X∼ ground state. Rather, a substantial part of the population gets trapped in the lowest excited ( B∼ ) state. The phenomenon is briefly discussed and calls for future work on this intricate dynamical system.

EPR parameters and optical spectrum of α-Al2O3:Ti3+ with dynamic Jahn-Teller effect

The quenching phenomenon of trigonal crystal field (CF), spin–orbit (SO) coupling operators, and orbital operators reflected in electron paramagnetic resonance (EPR) parameters and energy splitting intervals observed in experiments of α-Al2O3:Ti3+ is theoretically investigated by the calculation of EPR g factors and the energy differences of the lowest three levers E3/2, 1E1/2 and 2E1/2 with the formulas derived with high-order perturbation method. In the calculation, the coupling of 2T2g and 2Eg with e vibrational model of dynamic Jahn-Teller effect are considered. The calculated results are in agreement with the experimental values. The local defect structure around Ti3+ ions (bond lengths and angles of [TiO6]9-) and trigonal CF parameters ν and v′ are determinedand the covalence effects on SO coupling parameters ζ, ζ’ and orbital reduction parameters k、k’ are determined also.

Lattice source for charge and spin inhomogeneity in 2D perovskite cuprates

In the work we highlight the structural features of 2D perovskite cuprates (tilted CuO6 octahedra with different orientation with respect to spacer rocksalt layers), where sources of charge and spin inhomogeneity can be hidden. We used the impurity Anderson model with the Jahn–Teller(JT) local cells to show the charge inhomogeneity arises at any low doping concentration x, but disappears when the doping level exceeds threshold concentration xc, and the smaller the magnitudes xc, the larger the area of regions with JT pseudo effect. It is expected that spontaneous chiral symmetry breaking in the dynamic JT state of the stripe CuO2 layer as a whole can lead to the appearance of the goldstone phonon mode. As consequence, the giant thermal Hall effect could be observed in the 2D perovskite cuprates with CuO6 octahedra, rather than with CuO4 squares, e.g. in the Tl-based n layer cuprates or cuprates based on infinite-layer CaCuO2 structure.

Vibronic order and emergent magnetism in cubic d1 double perovskites

The synergistic interplay of different interactions in materials leads to the emergence of novel quantum phenomena. Spin-orbit and vibronic couplings usually counteract each other; however, in cubic ${d}^{1}$ double perovskites they coexist and give rise to spin-orbit-lattice entanglement with unquenched dynamic Jahn-Teller effect on the metal sites. The correlation of these entangled states induced by intersite interactions has not been assessed so far. Here, we investigate the joint cooperative effect of spin-orbit and vibronic interactions on the formation of the ordered phases in ${d}^{1}$ double perovskites. We found that the magnetically ordered states in these systems coexist with a dynamic vibronic order characterized by the ordering of vibronic quadrupole moments on sites. This treatment allows for the rationalization of a number of unexplained features of experimentally investigated phases.

Vibronic excitations in resonant inelastic x-ray scattering spectra of K2RuCl6

We present the fingerprints of dynamic Jahn-Teller effect in resonant inelastic x-ray scattering (RIXS) spectra of K$_2$RuCl$_6$. We determined the dynamic Jahn-Teller model Hamiltonian of an embedded Ru$^{4+}$ ion using post Hartree-Fock methods, and derived the vibronic states by numerically diagonalizing the Hamiltonian. With the obtained vibronic states, we reproduced the RIXS spectra. The shape and the temperature dependence of the RIXS spectrum agree well with the experimental data. We found that some peaks emerge due to the dynamic Jahn-Teller effect rather than the crystal field splitting. Our study indicates the significance of the Jahn-Teller coupling to adequately interpret RIXS spectra.

Highly efficient Sb3+ emitters in 0D cesium indium chloride nanocrystals with switchable photoluminescence through water-triggered structural transformation

All-inorganic lead-free luminescent metal halide nanocrystals (NCs) have shown great promise in optoelectronics but their applications are limited by the low photoluminescence (PL) efficiency. Herein, we report a strategy via Sb3+ alloying to achieve highly emissive 0D In-based halide NCs and investigate the effect of NC size on the optical properties and excited-state dynamics of Sb3+. Owing to the strong electron-phonon coupling of Sb3+ in the spatially confined 0D structure of Cs3InCl6, Sb3+ ions experience a dynamic Jahn-Teller distortion in the 3P1 excited state and an off-center position in the 1S0 ground state, which results in intense broadband emission of Sb3+ from the inter-configurational 3P1 → 1S0 transition with a large Stokes shift and a high PL quantum yield (QY) of 52.3%. Furthermore, through an interfacial reaction with water, the green-emitting Cs3InCl6: Sb3+ NCs can be transformed into the orange-emitting Cs2InCl5·H2O: Sb3+ NCs with a PLQY up to 75.3%. These findings reveal the unique advantage of the 5s2-metal Sb3+ luminescence in 0D metal halide NCs, thereby opening up a new avenue for exploring novel and versatile lead-free luminescent metal halide NCs through ns2-metal ion doping or alloying.

Enhancement of the Magnetoelectric Effect Using the Dynamic Jahn-Teller Effect in a Transition-Metal Complex.

In this Letter, a novel mechanism to enhance the magnetoelectric (ME) coupling between electric polarization and magnetism using the dynamic Jahn-Teller (JT) effect is demonstrated. Electric polarization of over 100 μC/m^{2} is induced by the magnetic field owing to the second-order ME effect in the noncentrosymmetric transition metal complex [Mn^{III}(taa)]. This appearance of electric polarization does not require magnetic order in contrast to the linear ME effect in ME multiferroic materials. The value of the electric polarization is 1 order larger than that induced by the second-order ME effect, which originates from the p-d hybridization. Our calculation, taking into account the single-ion-type magnetic anisotropy originating from the spin-orbit interaction and ferrodistortive intermolecular interaction, verifies that the alignment of the JT distortion by the magnetic field results in the large electric polarization observed. Thus, our results provide a new method to gain strong ME coupling by tuning the atomic displacement using a magnetic field.

Electronic structure of the solvated benzene radical anion

The benzene radical anion is a molecular ion pertinent to several organic reactions, including the Birch reduction of benzene in liquid ammonia. The species exhibits a dynamic Jahn-Teller effect due to its open-shell nature and undergoes pseudorotation of its geometry. Here, we characterize the complex electronic structure of this condensed-phase system based on ab initio molecular dynamics simulations and GW calculations of the benzene radical anion solvated in liquid ammonia. Using detailed analysis of the molecular and electronic structure, we find that the spatial character of the excess electron of the solvated radical anion follows the underlying Jahn-Teller distortions of the molecular geometry. We decompose the electronic density of states to isolate the contribution of the solute and to examine the response of the solvent to its presence. Our findings show the correspondence between instantaneous molecular structure and spin density; provide important insights into the electronic stability of the species, revealing that it is, indeed, a bound state in the condensed phase; and offer electronic densities of states that aid in the interpretation of experimental photoelectron spectra.

Resolving Jahn-Teller induced vibronic fine structure of silicon vacancy quantum emission in silicon carbide

Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from $n$-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with $1.45\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ and $1.59\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ energy spacing after H and He irradiation and increase linearly in intensity with fluence---reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect.

Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity

We report the observation of an antipolar phase in cubic GaNb_{4}S_{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb_{4}S_{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T_{JT}=31 K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T_{JT}. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb_{4}S_{8} may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.

An investigation of local structures for 3d3, 3d5 and 3d7 ions doing in the layered LiCoO2 cathode materials

Doping transition metal ions in LiCoO2 batteries has been proved an efficient method to improve the electrochemical properties in experiment, which is deeply related with the distorted local structure of impurities ions. According to the observed EPR spectra, the distorted local structures for 3d3 (Mn4+), 3d5 (Fe3+) and 3d7 (Ni3+) ions in LiCoO2 batteries are theoretically studied based on the perturbation calculations of Spin Hamilton Parameters (g factors, hyperfine structure constants A and Zero-field splittings parameters D and E). By considering the Jahn-Teller effect and mismatch radius of above transition metal ions (TMs), the formed [MnO6]8-, [FeO6]9- and [NiO6]9- clusters in LiCoO2 exhibit various distorted symmetries compared with experimental single one, characterized by trigonal distortion parameter Δθ as well as rhombic distortion parameters ΔZ and Δϕ in the 3d3 center and the 3d5 center and tetragonal distortion parameter ΔZ in the 3d7 center. In addition, compared with asymmetrical signal of EPR spectra at 30K, the role of relatively high temperature for 100K is investigated by dynamic Jahn-Teller, making the isotropic EPR signal and regular octahedron for Ni3+ center. Comparing with the obtained structure properties of isolate-doped transition metal ion, the co-doped Mg in LiCoO2 makes the further elongated distortion of local structure. Based on the well fitted EPR parameters and obtained local environment of doped transition metal ions, the inactive of electrochemical properties of Mn4+ can be attribute to the largest crystal field (Dq) and the smallest difference with host Co3+ site. Therefore, a comprehensively study of the local structure may be helpful to understand and predict the electrochemical behaviors of batteries.

Unconventional magnetic excitations and spin dynamics of exotic quantum spin systems BaCo$$_2$$V$$_2$$O$$_8$$ and Ba$$_3$$CuSb$$_2$$O$$_9$$

We review terahertz (THz) electron spin resonance studies of two types of exotic quantum spin systems, namely, the spin(S)-1/2 one-dimensional (1D) Ising-like antiferromagnet BaCo $$_2$$ V $$_2$$ O $$_8$$ and the S=1/2 two-dimensional (2D) honeycomb-like antiferromagnet Ba $$_3$$ CuSb $$_2$$ O $$_9$$ in magnetic fields of up to 50 T. For the former subject, unconventional magnetic excitations were identified below a critical magnetic field $$H_c$$ ( $$\sim $$ 4 T), where the exotic field-induced order-to-disorder transition occurs, and magnetic excitations in a Tomonaga-Luttinger liquid state were observed above $$H_c$$ . The novel magnetic excitations were analyzed with an S=1/2 1D XXZ model by considering the peculiar structure of this compound. For the latter subject, the orbital quantum dynamics of the spin liquid candidate Ba $$_3$$ CuSb $$_2$$ O $$_9$$ was revealed using multifrequency electron spin resonance ranging from 9.3 GHz to 0.73 THz. The g-factor of the hexagonal Ba $$_3$$ CuSb $$_2$$ O $$_9$$ single crystal possesses a weak six-fold symmetry at low frequencies, while two-fold symmetry is manifested at high frequencies. From the critical point between the two frequency regions, the frequency of the dynamic Jahn-Teller distortion is determined to be approximately 10 GHz. This dynamic distortion, accompanied by orbital quantum tunneling, proves the spin-orbital liquid state in Ba $$_3$$ CuSb $$_2$$ O $$_9$$ .

Electronic Spectrum of the Tropylium Cation in the Gas Phase

The structure and properties of the tropylium cation (C7H7+) have enthralled chemists since the prediction by Hückel in 1931 of the remarkable stability for cyclic, aromatic molecules containing six π-electrons. However, probing and understanding the excited electronic states of the isolated tropylium cation have proved challenging, as the accessible electronic transitions are weak, and there are difficulties in creating appreciable populations of the tropylium cation in the gas phase. Here, we present the first gas-phase S1 ←S0 electronic spectrum of the tropylium cation, recorded by resonance-enhanced photodissociation of weakly bound tropylium-Ar complexes. We demonstrate that the intensity of the symmetry-forbidden S1 ←S0 transition arises from Herzberg-Teller vibronic coupling between the S1 and S2 electronic states mediated by vibrational modes of e2' and e3' symmetry. The main geometry change upon excitation involves elongation of the C-C bonds. Multiconfigurational ab initio calculations predict that the S1 excited state is affected by the dynamical Jahn-Teller effect, which should lead to the appearance of additional weak bands that may be apparent in higher-resolution electronic spectra.