NCAFM2023 Programme Booklet

ILLUSTRATING SELECTION RULES OF ELECTRON SPIN RESONANCE IN REAL SPACE

Lisanne Sellies*, Raffael Spachtholz, Sonja Bleher, Philipp Scheuerer, Jascha Repp*

Institute for Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany *Email: lisanne.sellies@ur.de; jascha.repp@ur.de

Recently, we combined the high energy resolution of electron spin resonance (ESR) with the spatial resolution offered by atomic force microscopy (AFM). This ESR-AFM technique relies on driving electron spin transitions between the non-equilibrium triplet state levels of a single molecule. Since these triplet states typically have different lifetimes, driving such transitions modifies the overall triplet lifetime [1,2], which can be detected by an electronic pump-probe scheme [3]. ESR-AFM allows to measure ESR signals with a sub-nanoelectronvolt energy resolution, as we demonstrated for pentacene on thick NaCl films [4]. Thereby, molecules only differing in their isotopic configuration can be distinguished. Moreover, due to the minimally invasive nature of the ESR-AFM technique, the electron spins of pentacene can be coherently manipulated over tens of microseconds, as shown in Fig. 1. After introducing the basic principles of ESR-AFM, we will show the systematic relationship between the orientation of the pentacene molecules and the frequency of their respective Rabi oscillations. This experiment illustrates the selection rules at play and provides information on the direction of the RF magnetic field. It thereby demonstrates the power of relating local single-molecule information with the high spectral resolution provided by ESR-AFM.

Fig. 1 : Rabi oscillations from driving the transition between two of the zero-field split triplet states of pentacene (T X and T Z ). Upon increasing the duration of the RF pulse, the triplet population after a fixed delay time is coherently transferred from the T Z to the T X state and back. The measured normalized AFM signal is proportional to the triplet population. Figure adapted from [4]. References [1] J. Köhler, J. A. Disselhorst, M.C. J. M. Donckers, E. J. Groenen, J. Schmidth, W. E. Moerner, Nature, 1993, 363 , 242. [2] J. Wrachtrup, C. Von Borczyskowski, J. Bernard, M. Orrit, R. Brown, Nature, 1993, 363 , 244. [3] J. Peng, S. Sokolov, D. Hernangómez-Pérez, F. Evers, L. Gross, J. M. Lupton, J. Repp, Science, 2021, 373 , 452. [4] L. Sellies, R. Spachtholz, P. Scheuerer, J. Repp, arXiv, 2022, arXiv:2212.12244.

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