Rotational Coupling in Methyl-Tunneling Electron Spin Echo Envelope Modulation

Coherence between tunnel-split states of a methyl quantum rotor can be generated and observed in stimulated and spin-locked echo experiments, if hyperfine coupling of a nearby electron spin to the methyl protons breaks C$$_3$$ 3 symmetry and is of the same order of magnitude as the tunnel splitting. Here, we consider the case of two methyl groups bound to the same sp$$^{3}$$ 3 -hybridized atom, which is important in the context of common nitroxide spin labels. For a simple form of the rotor-rotor coupling Hamiltonian, we provide an approach that allows for density operator computations of this system with 1152 quantum states with moderate computational effort. We find that, in the regime where the ratio between rotor-rotor coupling and rotational barrier is much smaller than unity, three-pulse ESEEM and hyperfine-decoupled ESEEM depend only on the tunnel splitting, but not on this ratio. This finding may simplify the treatment of tunnel-induced electron decoherence in systems where the methyl groups are bound to sp$$^{3}$$ 3 -hybridized atoms.

Природа тонкой структуры вращательных уровней основного X-=SUP=-2-=/SUP=-Sigma-=SUP=-+-=/SUP=--состояния радикала CN

By means of ab initio high-level quantum-chemical calculations of non-diagonal matrix elements of spin-orbital and electron-rotational coupling between the ground X2Σ+ and excited (1--4)2Π states the observed regular effect of γ-doubling of the rotational levels of the X2Σ+ state was shown to be mainly determined by intramolecular interactions of the mentioned state with remote states (2--4)2Π. In terms of the nonadiabatic model of the effective radial Hamiltonian of the isolated electronic state, it was possible to create the analytical potential of the X2Σ+ state and the corresponding function γ (R) reproducing the frequencies of rotational and vibrational-rotational transitions (for the lowest vibrational levels ν≤3) of the CN molecule at the experimental (spectroscopic) level of accuracy.

Competing effect of wind braking and interior coupling in the rotational evolution of solar-like stars

Solar-like stars (M ≲ 1.3 M⊙) lose angular momentum through their magnetized winds. The resulting evolution of the surface rotation period, which can be directly measured photometrically, has the potential to be an accurate indicator of stellar age, and is constrained by observations of rotation periods of coeval stars, such as members of Galactic open clusters. A prominent observational feature of the mass–rotation period diagrams of open clusters is a sequence of relatively slower rotators. The formation and persistence of this slow-rotator sequence across several billion years imply an approximately coherent spin-down of the stars that belong to it. In particular, the sequence is observed to evolve coherently toward longer periods in progressively older clusters. Recent observations of the ≈700 Myr Praesepe and the 1 Gyr NGC 6811 clusters, however, are not fully consistent with this general pattern. While the stars of 1 M⊙ on the slow-rotator sequence of the older NGC 6811 have longer periods than their counterparts in the younger Praesepe, as expected, the two sequences essentially merge at lower masses (≲0.8 M⊙). In other words, it seems that low-mass stars have not been spinning down in the intervening 300 Myr. Here we show that this behavior is a manifestation of the variable rotational coupling in solar-like stars. The resurfacing of angular momentum from the interior can temporarily compensate for that lost at the surface due to wind braking. In our model the internal redistribution of angular momentum has a steep mass dependence; as a result, the re-coupling occurs at different ages for stars of different masses. The semi-empirical mass dependence of the rotational coupling timescale included in our model produces an evolution of the slow-rotator sequence in very good agreement with the observations. Our model, in particular, explains the stalled surface spin-down of low-mass stars between Praesepe and NGC 6811, and predicts that the same behavior should be observable at other ages in other mass ranges.

The impact of Jean-Paul Zahn discoveries on rotation and evolution

We first review the main effects of stellar rotation on evolution along the fundamental discoveries by Jean-Paul. Then, we examine some of the consequences of rotation in the evolution of single and binary stars. The proper account of meridional circulation in close binaries tends to increase the synchronization time because meridional currents always counteract the tidal interaction. We consider the case of the very low metallicity Z stars, in particular the CEMP-no stars, where rotational mixing may have played a dominant role in their strange chemical composition. Then, turning to “What are the mysteries?”, we emphasize that all over the evolution and for various masses the present models seem to still have a lack of rotational coupling between cores and envelopes. We suggest that magnetic fields may produce this missing internal coupling.

Molecular dynamics and the translational–rotational coupling of an ionically conducting glass-former: amlodipine besylate

We studied the conductivity relaxation originating from a glass-former composed of cations and anions, and the relation to the structural α-relaxation at temperatures above and below the glass transition temperature.