Patricia R. Levstein

Fourier transform‐electron paramagnetic resonance study of the photochemical reaction of acetone with 2‐propanol

Fourier transform‐electron paramagnetic resonance (FT‐EPR) was used to study the pulsed‐laser induced reduction of acetone with 2‐propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin‐correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2‐propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2‐propanol(D8) at room temperature corresponds to what would be expected ...

Attenuation of polarization echoes in nuclear magnetic resonance: A study of the emergence of dynamical irreversibility in many-body quantum systems

The reversal of the time evolution of the local polarization in an interacting spin system involves a sign change of the effective dipolar Hamiltonian which refocuses the “spin diffusion” process generating a polarization echo. Here, the attenuation of these echo amplitudes as a function of evolution time is presented for cymantrene and ferrocene polycrystalline samples, involving one and two five spin rings per molecule, respectively. We calculate the fraction of polarization which is not refocused because only the secular part of the dipolar Hamiltonian is inverted. The results indicate that, as long as the spin dynamics is restricted to a single ring, the non-inverted part of the Hamiltonian is not able by itself to explain the whole decay of the polarization echoes. A crossover from exponential (cymantrene) to Gaussian (ferrocene) attenuation is experimentally observed. This is attributed to an increase of the relative importance of the spin dynamics, as compared with irreversible interactions, which ...

A nuclear magnetic resonance answer to the Boltzmann–Loschmidt controversy?

A unique experimental tool to deepen into the Boltzmann–Loschmidt controversy is provided by the NMR polarization echoes (PE). These appear when a local spin excitation, evolving with a many-body “diffusive” spin dynamics, is reversed. The attenuation of the PEs represents a progresive failure of the quantum interferences to rebuild the local excitation. Our results indicate that, in the absence of detectable environmental interactions, the characteristic time of this attenuation is determined by the reversible dynamics itself, i.e., spin–spin interaction time. This supports the Boltzmanns hypothesis of molecular “chaos”.

Quantum Dynamical Echoes in the Spin Diffusion in Mesoscopic Systems

The evolution of local spin polarization in finite systems involves interference phenomena that give rise to quantum dynamical echoes and nonergodic behavior. We predict the conditions to observe these echoes by exploiting the NMR sequences devised by Zhang et al. [Phys. Rev. Lett. 69, 2149 (1992)], which uses a rare

Environmentally induced quantum dynamical phase transition in the spin swapping operation.

^{13}\mathrm{C}

Gaussian to exponential crossover in the attenuation of polarization echoes in NMR

as the local probe for a dipolar coupled

Quantum interference phenomena in the local polarization dynamics of mesoscopic systems: an NMR observation

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Photoinduced electron transfer from porphyrins to quinones in micellar systems: an FT-EPR study

spin system. The nonideality of this probe when testing mesoscopic systems is carefully analyzed revealing the origin of various striking experimental features.

FT-EPR study of triplet state C60. Spin dynamics and electron transfer quenching

Quantum information processing relies on coherent quantum dynamics for a precise control of its basic operations. A swapping gate in a two-spin system exchanges the degenerate states |(up arrow, down arrow)> and |(down arrow, up arrow)>. In NMR, this is achieved turning on and off the spin-spin interaction b=DeltaE that splits the energy levels and induces an oscillation with a natural frequency DeltaE/Plancks. Interaction of strength Plancks/tau(SE), with an environment of neighboring spins, degrades this oscillation within a decoherence time scale tau(phi). While the experimental frequency omega and decoherence time tau(phi) were expected to be roughly proportional to b/Plancks and tau(SE), respectively, we present here experiments that show drastic deviations in both omega and tau(phi). By solving the many spin dynamics, we prove that the swapping regime is restricted to DeltaEtau(SE) similar or greater than Plancks. Beyond a critical interaction with the environment the swapping freezes and the decoherence rate drops as 1/tau(phi) proportional to (b/Plancks)2tau(SE). The transition between quantum dynamical phases occurs when omega proportional to sqrt (b/Plancks)2-(k/tau(SE)2 becomes imaginary, resembling an overdamped classical oscillator. Here, 0< or =k2< or =1 depends only on the anisotropy of the system-environment interaction, being 0 for isotropic and 1 for XY interactions. This critical onset of a phase dominated by the quantum Zeno effect opens up new opportunities for controlling quantum dynamics.

Effective one-body dynamics in multiple-quantum NMR experiments

An ingenious pulse sequence devised by Zhang, S., Meier, B. H., and Ernst, R. R., 1992, Phys. Rev. Lett., 69, 2149 reverses the time evolution (‘spin diffusion’) of the local polarization in a dipolar coupled 1H spin system. This refocusing originates a polarization echo, whose amplitude attenuates by increasing the time t R elapsed until the dynamics are reversed. Different functional attenuations are found for a set of dipolar coupled systems: ferrocene, (C5H5)2Fe, cymantrene, (C5H5)Mn(CO)3, and cobaltocene, (C5H5)2Co. To control a relevant variable involved in this attenuation a pulse sequence has been devised to progressively reduce the dipolar dynamics. Since it reduces the evolution of the polarization echo it is referred to as the REPE sequence. Two extreme behaviours were found while characterizing the materials. In systems with a strong source of relaxation and slow dynamics the attenuation follows an exponential law (cymantrene). In systems with strong dipolar dynamics the attenuation is mainly ...