Spin States of C603-and C120On-(n= 2, 3, 4) Anions Using Electron Spin Transient Nutation Spectroscopy

Drew, Simon C.; Boas, John F.; Pilbrow, John R.; Boyd, Peter D. W.; Paul, Parimal; Reed, Christopher A.

doi:10.1021/jp035632x

“…However, there was a controversy 27 on whether the low-spin/high-spin gap is very small (below one wavenumber 28 ) or rather of the order of 600 wavenumbers 29 . Recently, this problem was carefully reconsidered and it was shown that activated behaviors of C N − 60 which were observed so far and used to determine the gap are in fact due to C 120 O impurities 30,31 . However, the work clearly reconfirms that iso-lated C N − 60 ions (N = 2, 3) are in the low-spin state.…”

Section: Results For C60mentioning

confidence: 99%

Jahn-Teller effect versus Hund’s rule coupling inC60N−

Wehrli

¹

,

Sigrist

²

2007

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We propose variational states for the ground state and the low-energy collective rotator excitations in negatively charged C N− 60 ions (N = 1 . . . 5). The approach includes the linear electron-phonon coupling and the Coulomb interaction on the same level. The electron-phonon coupling is treated within the effective mode approximation (EMA) which yields the linear t1u ⊗Hg Jahn-Teller problem whereas the Coulomb interaction gives rise to Hund's rule coupling for N = 2, 3, 4. The Hamiltonian has accidental SO(3) symmetry which allows an elegant formulation in terms of angular momenta. Trial states are constructed from coherent states and using projection operators onto angular momentum subspaces which results in good variational states for the complete parameter range. The evaluation of the corresponding energies is to a large extent analytical. We use the approach for a detailed analysis of the competition between Jahn-Teller effect and Hund's rule coupling, which determines the spin state for N = 2, 3, 4. We calculate the low-spin/high-spin gap for N = 2, 3, 4 as a function of the Hund's rule coupling constant J. We find that the experimentally measured gaps suggest a coupling constant in the range J = 60 − 80 meV. Using a finite value for J, we recalculate the ground state energies of the C N− 60 ions and find that the Jahn-Teller energy gain is partly counterbalanced by the Hund's rule coupling. In particular, the ground state energies for N = 2, 3, 4 are almost equal. I. INTRODUCTIONThe t 1u ⊗ H g Jahn-Teller problem, where electrons in a threefold degenerate orbital interact with a fivefold degenerate phonon multiplet, is known since more than 30 years. It first arised for the particular case of p-electrons in a cubic systems which are equally coupled to E g and T 2g vibrational modes 1 . On the level of linear coupling, an equivalent problem arises in negatively charged C N − 60 ions (N = 1 . . . 5). These materials experienced particular interest when superconductivity was observed in alkali-doped A 3 C 60 (A=K,Cs,Rb, for a review see Ref. 2). The neutral C 60 molecule is a closed shell system and highly symmetric. It has icosahedral symmetry which is the largest threedimensional point-group with 1-, 3-, 4-and 5-dimensional irreducible representations (IR) 3 . The lowest unoccupied molecular orbital (LUMO) of C 60 is threefold degenerate and has t 1u symmetry. It couples to two non-degenerate A g phonon modes and eight 5-fold degenerate H g phonon multiplets 4,5 . In the present work we focus on the nontrivial coupling to the H g multiplets. We restrict our attention to linear coupling and approximate the eight H g multiplets by one effective multiplet which gives rise to the linear, single-mode t 1u ⊗ H g Jahn-Teller problem. Furthermore, we will use the fact that the icosahedral IR's t 1u and H g correspond to the L = 1, 2 IR of SO(3) which don't split under the icosahedral symmetry 3 . As a consequence, the linear t 1u ⊗ H g Jahn-Teller problem is equivalent to the problem of p-electrons interac...

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“…EPR spectra indicated a triplet state, while a study using electron spin transient nutation spectroscopy suggested an extremely low separation of only 5 cm −1 between the singlet and triplet states for C 120 O 2− in solution. 16,17 Our calculation predicted a small ͑2 kJ/ mol͒ spin excitation for the isolated dianion. However, it should be noted that for molecular systems of such large size even much more demanding computational methods are unlikely to provide a level of accuracy comparable to the true gap.…”

Section: A Spin Coupling: Is C 120 O 2− Singlet or Triplet?mentioning

confidence: 73%

“…Anions of C 120 O ͑C 120 O n− ͒ are common impurities in solutions of C 60 n− and have been studied by electron paramagnetic resonance ͑EPR͒ spectroscopy. 16,17 The isolated C 120 O 2− dianion is an interesting model system to study spin couplings and electron correlations. There are several important questions: How are the two excess electrons distributed in the molecule?…”

Section: Introductionmentioning

confidence: 99%

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C120O2−

Wang

¹

,

Matheis

²

,

Ioffe

³

et al. 2008

The Journal of Chemical Physics

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C(120)O comprises two C(60) cages linked by a furan ring and is formed by reactions of C(60)O and C(60). We have produced doubly charged anions of this fullerene dimer (C(120)O(2-)) and studied its electronic structure and stability using photoelectron spectroscopy and theoretical calculations. High resolution and vibrationally resolved photoelectron spectra were obtained at 70 K and at several photon energies. The second electron affinity of C(120)O was measured to be 1.02+/-0.03 eV and the intramolecular Coulomb repulsion was estimated to be about 0.8 eV in C(120)O(2-) on the basis of the observed repulsive Coulomb barrier. A low-lying excited state ((2)B(1)) was also observed for C(120)O(-) at 0.09 eV above the ground state ((2)A(1)). The C(120)O(2-) dianion can be viewed as a single electron on each C(60) ball very weakly coupled. Theoretical calculations showed that the singlet and triplet states of C(120)O(2-) are nearly degenerate and can both be present in the experiment. The computed electron binding energies and excitation energies, as well as Franck-Condon factors, are used to help interpret the photoelectron spectra. A C-C bond-cleaved isomer, C(60)-O-C(60) (2-), was also observed with a higher electron binding energy of 1.54 eV.

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“…However, there was a controversy 27 on whether the low-spin/high-spin gap is very small (below one wavenumber 28 ) or rather of the order of 600 wavenumbers 29 . Recently, this problem was carefully reconsidered and it was shown that activated behaviors of C N − 60 which were observed so far and used to determine the gap are in fact due to C 120 O impurities 30,31 . However, the work clearly reconfirms that iso-lated C N − 60 ions (N = 2, 3) are in the low-spin state.…”

Section: Results For C60mentioning

confidence: 99%

Jahn-Teller effect versus Hund's rule coupling in C60N-

Wehrli,

Sigrist

2007

Preprint

0

View full text Add to dashboard Cite

We propose variational states for the ground state and the low-energy collective rotator excitations in negatively charged C N− 60 ions (N = 1 . . . 5). The approach includes the linear electron-phonon coupling and the Coulomb interaction on the same level. The electron-phonon coupling is treated within the effective mode approximation (EMA) which yields the linear t1u ⊗Hg Jahn-Teller problem whereas the Coulomb interaction gives rise to Hund's rule coupling for N = 2, 3, 4. The Hamiltonian has accidental SO(3) symmetry which allows an elegant formulation in terms of angular momenta. Trial states are constructed from coherent states and using projection operators onto angular momentum subspaces which results in good variational states for the complete parameter range. The evaluation of the corresponding energies is to a large extent analytical. We use the approach for a detailed analysis of the competition between Jahn-Teller effect and Hund's rule coupling, which determines the spin state for N = 2, 3, 4. We calculate the low-spin/high-spin gap for N = 2, 3, 4 as a function of the Hund's rule coupling constant J. We find that the experimentally measured gaps suggest a coupling constant in the range J = 60 − 80 meV. Using a finite value for J, we recalculate the ground state energies of the C N− 60 ions and find that the Jahn-Teller energy gain is partly counterbalanced by the Hund's rule coupling. In particular, the ground state energies for N = 2, 3, 4 are almost equal.

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Spin States of C₆₀^3-and C₁₂₀Oⁿ^-(n= 2, 3, 4) Anions Using Electron Spin Transient Nutation Spectroscopy

Cited by 14 publications

References 16 publications

Jahn-Teller effect versus Hund’s rule coupling inC60N−

Jahn-Teller effect versus Hund’s rule coupling inC60N−

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C120O2−

Jahn-Teller effect versus Hund's rule coupling in C60N-

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