Electric-dipole forbidden transitions in C60: oscillator strengths induced by the spin–orbit coupling (original) (raw)
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Electronic Excitations and Stability of the Ground State of C60 Molecules
1998
A model study of the singlet excitons in the C60 molecule with emphasis on the Coulomb interaction between excited electron and hole leads to a physical understanding of the interaction effects on the absorption spectra and to a new identification of the forbidden excitons in the third-harmonic generation spectra. These conclusions may be tested experimentally on the model predictions related to the optical Kerr effect. The model shows that, with sufficiently strong interatomic than onsite interaction, a T_{2G} exciton could have very low energy or become unstable against the closed-shell ground state. Properties of these interesting cases beyond the C60 are briefly examined.
First principles study of the vibronic coupling in positively charged C60+
Authorea
Vibronic coupling parameters for C + 60 were derived via DFT calculations with hybrid B3LYP and CAM-B3LYP functional, based on which the static Jahn-Teller effect were analyzed. The global minima of adiabatic potential energy surface (APES) shows a D 5d Jahn-Teller deformation, with stabilization energies of 110 and 129 meV (with B3LYP and CAM-B3LYP respectively), which are two times larger than that in C − 60 , suggesting the crucial role of the dynamical Jahn-Teller effect. Present results enable us to assess the actual situation of dynamical Jahn-Teller effect in C + 60 and excited C 60 in combination with the established parameters for C − 60 .
Normal vibrational analysis of the triplet excited state ofC60
Physical Review B, 2001
The vibrational frequencies and infrared intensities for the lowest triplet excited state (T 1) of the C 60 molecule have been calculated using a density-functional theory. Despite the fact that the I h to D 5d structural distortion on the ground state (S 0) to T 1 electronic transition was not large, the calculated infrared stick spectrum of T 1 is very different from that of S 0. The change in the geometry and vibrational structures from S 0 to T 1 reflects the difference between their potentials. The difference was discussed in consideration of both the translation and rotation of the normal coordinates for T 1 relative to S 0 .
Excited states of the C60 dimer
Synthetic Metals, 1996
Recent investigations are reported on two different sorts of fullerene dimer. The first type is a simple cluster of two CeO molecules, i.e., a van der Waals complex. The second one is a covalently bound system. The nature of the electronic excitations has been studied for these model systems. It is found that for the van der Waals complex the low-lying excitations remain localized on one monomer and, thus, they are precursors of the Frenkel-type excitons in the crystal. At significantly higher energies, one obtains so-called charge transfer type excitations, which are precursors of the Wannier excitons. The picture of different excitations resembles that of a very weakly bound system, e.g., two benzene molecules at 5 A from each other. For the covalent dimer, we considered the binding situation where there are two single bonds between the monomers. The extent of the conjugation between the two CbO molecules is monitored by the change in the excitation energies. The lowest-lying transitions were observed at higher energies than those in the monomer. This feature is quite unusual for a system exhibiting at least partial conjugation, and it is in agreement with recent band structure calculations and hybridization studies in the fullerene polymer.
Optics and Spectroscopy, 2005
The singlet-singlet and triplet-triplet absorption spectra of C 60 fullerene are calculated using the density functional method and taking into account the theory of linear and quadratic responses. The B3LYP density functional and the 6-31G and 3-21G atomic basis sets are used. The calculations are performed using the D 2 h and D 5 d symmetry groups, although the real symmetry of the ground state is described by the I h symmetry group. The matrix elements of the operator of the spin-orbit coupling are calculated and the probabilities of some singlet-triplet transitions are estimated. Taking into account the data in the literature on vibronic interactions of vibrations of the t 1 u , t 2 u , g u , and h u symmetry species, the radiative lifetime of the 1 3 T 2 g 1 1 A g phosphorescence was estimated to be 45 s. The fact that this time proved to be considerably greater than the experimentally observed total lifetime of the triplet testifies to a fast nonradiative deactivation of the lowest triplet state of C 60 fullerene and agrees with a low phosphorescence intensity. The zero-field splitting of some triplets and the intensities of magnetic dipole transitions are discussed.
First-principles study on electronic responses of a C 60 molecule to external electric fields
Chemical Physics, 2007
Using a density-functional electronic structure calculation method, the electronic structure of an isolated C60 molecule are calculated under consideration of finite external electric fields, and changes in electron density induced by the external electric fields are investigated in detail. Main finding of this study is that field-induced change in the electron density of an isolated C60 molecule significantly differs depending on molecular orientation relative to electric field direction, even though the field-induced total dipole moment of a whole molecule hardly shows such orientational dependence. Moreover, such orientational dependence also arises both in total energy and in dielectric constant. These orientational dependence would affect the chemical reactivity and orientational preference of rotating C60 molecules in a gas phase under finite external electric fields.
A theoretical investigation of four electronic states of C6− and the ground state of linear C6
Chemical Physics Letters, 1995
Large-scale open-shell coupled cluster calculations have been carried out for the four lowest doublet states of C~ and the ground state of linear C 6. They strongly support the assignment of the peaks at 607.7 and 586.5 nm observed in neon matrix isolation experiments to the 0 ° and 31 transitions within the electronic band (~ 2IIg ~-X 21-Iu of C~. The vibrational structure of the photoelectron spectrum of C~ has been calculated; the 15 transition is calculated to have 42% intensity with respect to the adiabatic peak. Some misassignments of the experimental threshold photodetachment and autodetachment spectra are corrected. Excellent agreement with experiment is obtained for the electron affinity of linear C 6.
Absorption at the dipole-forbidden optical gap of crystalline C60
Chemical Physics Letters, 1995
We report detailed optical studies of crystalline C6o applying several optical spectroscopic techniques such as absorption, photoluminescence, and one-photon and two-photon photoluminescence excitation spectroscopy. The energetically lowest purely electronic 0'-0 transition is found at 1.846 eV. At low temperatures, the absorption spectrum systematically exhibits doublet features with the same energetic splitting as observed for the low-temperature fluorescence spectrum. These transition pairs either represent two distinct vibration-induced transitions or only one vibration-induced transition and the splitting is then due to the two distinct alignments of neighbouring C6o molecules (merohedral disorder). Temperature-dependent absorption measurements show that the two molecular alignments might indeed be responsible for the double resonances.
Theoretical calculations for the fullerene cation are important for several aspects. Jahn-Teller vibronic interactions are symmetry-allowed in this ion and the expectation is that such interactions lead to static geometrical effects. In addition it is interesting to study the change of the vibrational spectrum of C 60 upon ionization. We have used semi-empirical AM1/UHF and PM3/UHF and density functional theoretical methods (B3LYP and LDA) to calculate the electronic ground state equilibrium geometry and vibrational frequencies for C 60 + . A small but significant reduction from icosahedral symmetry to D 5d obtained from DFT calculations without symmetry constraints supports the presence of static Jahn-Teller distortions. Structure optimizations constrained to D 5d and D 3d symmetry have also been carried out. The JT stabilization energy in the D 5d and D 3d structures is estimated according to two different methods. Vibrational frequencies calculated at B3LYP/6-31G level for the fullerene cation of D 5d geometry are reported and compared to literature values of neutral C 60 .
FT-EPR study of triplet state C60. Spin dynamics and electron transfer quenching
Chemical Physics Letters, 1993
A Ff-EPR study was made of paramagnetic species formed by pulsed-laser excitation of Cm in fluid solution. Earlier findings that photoexcitation of Cso in fluid solution gives rise to an EPR signal with narrow linewidth were confirmed. The lifetime of the signal corresponds to that of the CGO triplet as measured by flash photolysis. In the presence of donors, the rate of signal decay is increased and matches the growing in of EPR signals from oxidized donors. The time dependence of the FT-EPR spectra gives values for electron transfer rate constants which agree with those derived from flash photolysis measurements on the same systems. Based on these findings, we assign the narrow line signal to the triplet of &,. The time evolution of the FT-EPR signal establishes that the triplets are born with less spin polarization than the thermal equilibrium value. As a result, signal growth is controlled by spin-lattice relaxation.