Resonance of Single Molecular Spins

The development of optical single-molecule spectroscopy during the last couple of years has opened a completely new way to obtain information about individual molecules. The natmal question which arises is whether it is possible to perform magnetic resonance experiments on individual, optically selected molecules using 

ODMR-type methods. The answer is affirmative [7, 8] and in this contribution we discuss a number of results on the system pentacene doped into a p-terphenyl host crystal (see Fig. 1) which demonstrate that such experiments are feasible and that it is possible to extract information about a single molecular spin and its interaction with its environment and the resonant radiation field. 

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Photon Antibunching and Magnetic Resonance of Single Molecular Spins... 

Another class of physical phenomena demonstrated for single molecules involves magnetic resonance of a single molecular spin. Historically, the... 

Fig. 2.7. (A) Measured distribution of time delays between successive detected fluorescence photons for a single molecule of pentacene in p-terphenyl showing antibunching at r = 0. For details, see [53]. (B) Magnetic resonance of a single molecular spin. Reductions in fluorescence as a function of microwave frequency for four different single molecules of pentacene in p-terphenyl. For details, see [59]...

Two different experimental arrangements have been successfully utilized for magnetic resonance experiments on single molecular spins [18, 19]. As in the optical single-molecule experiments the essential ingredients are a narrow-band laser, an efficient collection of the emitted fluorescence and a small excitation volume of the sample. To achieve a small sample volume the incoming laser light is either focussed... 

Photons in quantum optical cavities also constitute excellent qubit candidates [52]. Resonant coupling of atoms with a single mode of the radiation field was experimentally achieved 25 years ago [53], and eventually the coherent coupling of quantum optical cavities with atoms or (simple) molecules was suggested as a means to achieve stable quantum memories in a hybrid quantum processor [54]. There might be a role to play for molecular spin qubits in this kind of hybrid quantum devices that combine solid-state with flying qubits. 

We report an electron spin resonance (ESR) study on a C60 anion and a metal (M) encapsulated in fullerene (C ) (a metallofullerene M C ). The anisotropy components of the g-factor of Cg0 were determined accurately from the analysis of angular-dependent ESR spectra of single crystal Cg0 salt. The evaluation of the g-factor was performed according to the classification of symmetry of the C60 geometry. It was found out from the evaluation that the molecular structure of Cg0 should he distorted to lower symmetry, C2h or C,. The variety of ESR spectra of metallofullerenes of La C s was obtained in terms of a g-factor, a hyperfine coupling constant, and a line width. In the case of the isomer I of La C80 and the isomer II of La C84, an abnormally large line width was measured. The molecular structure with high symmetry would reflect on the specific spin dynamics. 

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