The chemical zeno effect in exchange-coupled radical Pairs: 1. Triplet radical pairs

It was shown that spin-selective intracage recombination could influence the limiting triplet-singlet conversion stage and, therefore, the whole chemical dynamics. An increase in the rate constant for intracage recombination w decreases the S-T evolution frequency and changes its character by transforming the oscillating spin conversion mode into “kinetic.” As a result, an increase in w can decrease the yield of intracage recombination products and increase the yields of products formed in competing extracellular radical reaction channels. The chemical Zeno effect and its consequences are an analogue of the quantum Zeno effect (quantum evolution “deceleration” caused by successive measurements), and spin-selective recombination is similar to quantum state measurements.     

Continuous and pulsed quantum zeno effect

Continuous and pulsed quantum Zeno effects were observed using a 87Rb Bose-Einstein condensate. Oscillations between two ground hyperfine states of a magnetically trapped condensate, externally driven at a transition rate omega(R), were suppressed by destructively measuring the population in one of the states with resonant light. The suppression of the transition rate in the two-level system was quantified for pulsed measurements with a time interval deltat between pulses and continuous measurements with a scattering rate gamma. We observe that the continuous measurements exhibit the same suppression in the transition rate as the pulsed measurements when gammadeltat=3.60(0.43), in agreement with the predicted value of 4. Increasing the measurement rate suppressed the transition rate down to 0.005 omega(R).     

Atomic quantum zeno effect for ensembles and single systems

The so-called quantum Zeno effect is essentially a consequence of the projection postulate for ideal measurements. To test the effect, Itanoet al. have performed an experiment on an ensemble of atoms where rapidly repeated level measurements were realized by means of short laser pulses. Using dynamical considerations, we give an explanation why the projection postulate can be applied in good approximation to such measurements. Corrections to ideal measurements are determined explicitly. This is used to discuss how far the experiment of Itanoet al. can be considered as a test of the quantum Zeno effect. We also analyze a new possible experiment on a single atom where stochastic light and dark periods can be interpreted as manifestation of the quantum Zeno effect. We show that the measurement point of view gives a quick and intuitive understanding of experiments of the above type, although a finer analysis has to take the corrections into account.     

Watching it boil: Continuous observation for the quantum zeno effect

The quantum Zeno effect (QZE) is often associated with the ironic maxim, “a watched pot never boils”, although the notion of “watching” suggests a continuous activity at odds with the usual (pulsed measurement) presentation of the QZE. We show how continuous watching can provide the same halting of decay as the usual QZE, and, for incomplete hindrance, we provide a precise connection between the interval between projections and the response time of the continuous observer. Thus, watching closely, but not so closely as to halt the “boiling”, is equivalent to—gives the same degree of partial hindrance as—pulsed measurements with a particular pulsing rate. Our demonstration is accomplished by treating the apparatus for the continuous watching as a fully quantum object. This in turn allows us a second perspective on the QZE, in which it is the modified level structure of the combined system/apparatus Hamiltonian that slows the decay. This and other considerations favor the characterization “dominated time evolution” for the QZE.     

Hindered decay of an unstable system: A quantum zeno effect

A direct test of the so-called “quantum Zeno effect” is proposed for a truly decaying system. It is suggested that the lifetime of an unstable atom can be extended by illuminating it with an intense laser beam at the frequency of another of its transitions. The “Zeno” time is also compared to the lifetime     

Nonexponential decay via tunneling in tight-binding lattices and the optical zeno effect

An exactly-solvable model for the decay of a metastable state coupled to a semi-infinite, tight-binding lattice, showing large deviations from exponential decay in the strong coupling regime, is presented. An optical realization of the lattice model, based on discrete diffraction in a semi-infinite array of tunneling-coupled optical waveguides, is proposed to test nonexponential decay and for the observation of an optical analog of the quantum Zeno effect.     

Quantum zeno effect in Cooper-pair transport through a double-island Josephson system

Motivated by recent experiments, we analyze transport of Cooper pairs through a double-island Josephson qubit. At low bias in a certain range of gate voltages, coherent superpositions of charge states play a crucial role. Analysis of the evolution of the density matrix allows us to cover a wide range of parameters, including situations with degenerate levels, when dissipation strongly affects the coherent eigenstates. At high noise levels, the so-called Zeno effect can be observed, which slows down the transport. Our analysis explains certain features of the I-V curves, in particular, the visibility and shape of resonant peaks and lines.     

The CIDNP kinetics in recombination of successive radical pairs

A theory of chemically induced dynamic nuclear polarization (CIDNP) formed in recombination of successive radical pairs (PRs) is developed. The theory is based on that of RP recombination with the spin Hamiltonian instantaneously changing in time. With kinematics approximation it is shown that general relations for CIDNP are fully expressed via the quadratures of Green functions, which characterize the molecular motion of reagents. Analytical formulae for the time dependence of CIDNP both of primary and secondary RPs have been derived in the strong magnetic field approximation (S-T₀ approximation); field dependences of stationary CIDNP effect for some model cases have been analyzed. For long-lived systems the sensitivity of secondary RP CIDNP to the singlet-triplet evolution of primary RP has been demonstrated. It is shown that sometimes the correct analysis of the effect calls for taking into account the reactivity anisotropy.     

Freezing coherent field growth in a cavity by the quantum zeno effect

We have frozen the coherent evolution of a field in a cavity by repeated measurements of its photon number. We use circular Rydberg atoms dispersively coupled to the cavity mode for an absorption-free photon counting. These measurements inhibit the growth of a field injected in the cavity by a classical source. This manifestation of the quantum Zeno effect illustrates the backaction of the photon number determination onto the field phase. The residual growth of the field can be seen as a random walk of its amplitude in the two-dimensional phase space. This experiment sheds light onto the measurement process and opens perspectives for active quantum feedback.     

Weakly coupled radical pairs in solids: ELDOR in ESE structure studies

Possibilities of the structure determination of radical pairs having fixed geometry with the help of ELDOR in ESE technique are considered. It is demonstrated that one can obtain information on relative orientation of paramagnetic centers in weakly coupled pairs in addition to the energy parameters of the spin Hamiltonian. Appropriate requirements for such experiments are formulated.     

Studies of radical pairs in potassium hydrogen malonate I. E.S.R.

Irradiation of single crystals of potassium hydrogen malonate with X-rays at 77 K yields two types of radical pairs, which differ in the orientation of the vector r linking the radicals of a pair but are composed of the same monoradicals. This monoradical is the main radical species found at room temperature and is characterized by hydrogen abstraction from the methylene group. Each type of pair exists in three modifications with respect to the fine structure parameters D and E, depending on the temperature treatment of the crystals after irradiation. The principal values of the dipole splitting tensor and the direction cosines of r for all pairs were determined. By application of the appropriate theory the signs of D and E could be deduced from second order effects observed at 9·5 GHz and 35 GHz. The two types of pairs exhibit marked differences in their fine structure parameters, their microwave saturation and their temperature and light stability, all of which can be attributed to different orientations of the linking vectors r relative to the molecular planes. Finally temperature coefficients of the dipole interactions were measured yielding opposite signs for the two types of pairs.     

Spin-vibrational interactions in geminate radical pairs: Effective spin Hamiltonian including anharmonicity

An equation for the effective spin Hamiltonian including the interaction of radical pair spins with vibrations in both linear and higher approximations was obtained. Even in the linear approximation, the Hamiltonian was found to be non-Heisenberg. The influence of a sound field on the probability of geminate radical pair recombination is considered in the Appendix. The results obtained can be of use for solving certain nanochemistry problems.     

Spin dynamics and EPR spectra of consecutive spin-correlated radical pairs. Model calculations

Time-resolved continuous wave EPR signals of two consecutive radical pairs are found in the linear response limit. Numerical simulations of the EPR observables visualize two characteristic features. First, there is a shift of a phase of quantum beats of the EPR line intensities of the secondary pairs. This phase shift originates from a certain time delay in a formation of the secondary pairs (due to time spent by electron spins in the primary radical pair state) and from the difference of the spin dynamics in the secondary and the primary pairs. This phase shift might be detected even in the cases when the primary radical pair has the very short lifetime and, as a result, the EPR spectrum of the primary pair cannot be detected directly. Second, for two consecutive radical pairs, there might be a pronounced non-equality of intensities of EPR lines at the EPR resonance frequencies of the secondary pairs. Indeed, in a case of two consecutive pairs there is the additional mechanism which induces the non-equality of the EPR line intensities: a polarization transfer from the primary to secondary pair and the change of a electron spin quantization axis when a primary radical pair transforms to a secondary radical pair. A possibility to detect experimentally these features of the EPR signals when studying consecutive charge separated states in photosynthetic reaction centers is discussed briefly.     

The free radical scavenging activity of lespedezacoumestan toward ˙OH radical: A quantum chemical and computational kinetics study

The free radical activity of lespedezacoumestan was investigated toward hydroxyl (˙OH) radical in polar and nonpolar media using density functional theory. Four reaction mechanisms including radical adduct formation, hydrogen atom transfer, sequential single electron‐proton transfer (SET‐PT), and sequential proton loss electron transfer were considered. The rate constants and branching ratio for all possible sites of reaction were calculated and reported for the first time. According to the obtained results, lespedezacoumestan reacts faster with ˙OH radical in aqueous solution than in nonpolar media. Also, lespedezacoumestan is an excellent ˙OH radical scavenger regardless of the environment polarity.     

Fractal chemical kinetics: Reacting random walkers

Computer simulations on binary reactions of random walkers (A + A A) on fractal spaces bear out a recent conjecture: ( ^–1 – ₀ ^–1) t ^f , where is the instantaneous walker density and ₀ the initial one, andf=d _s /2, whered _s is the spectral dimension. For the Sierpinski gaskets:d=2, 2f=1.38 (d _s =1.365);d=3, 2f=1.56 (d _s =1.547); biased initial random distributions are compared to unbiased ones. For site percolation:d= 2,p=0.60, 2f= 1.35 (d _s =1.35); d=3,p=0.32, 2f=1.37 (d _s =1.4); fractal-to-Euclidean crossovers are also observed. For energetically disordered lattices, the effective 2f (from reacting walkers) andd _s (from single walkers) are in good agreement, in both two and three dimensions.Supported by NSF Grant No. DMR 8303919.     

Prebreakdown generation of nonequilibrium electron-hole pairs: The multiphoton avalanche effect

A new mechanism of prebreakdown generation of electron-hole pairs is considered. It includes a cascade of interband multiphoton transitions as well as Auger-type processes involving two or three photons. A combination of these processes leads to the multiphoton avalanche effect. The threshold pumping radiation intensities required for triggering the avalanche mechanism lie in the range of 10¹¹–10¹²W/cm². The band population balance equations describing the kinetics of pair production are obtained and solved numerically. It is shown that the proposed mechanism of production of nonequilibrium electron-hole pairs is more effective than “conventional” multiphoton absorption for intensities exceeding the threshold values.     

Studies on coherent and incoherent spin dynamics that control the magnetic field effect on photogenerated radical pairs

Since 1970s, magnetic field effects (MFEs) on photogenerated radical pairs have been the centre of focus in the field of spin chemistry. The MFE attributes to quantum mechanical interconversion between the singlet and triplet radical pair states and subsequent spin-selective recombination reactions. In this New View article, the author picks up two hot topics studied during the last two decades, which are (i) so-called low field effect (LFE) and (ii) 2J-resonance MFE on fixed distance donor–acceptor linked molecules. In both of the topics, quantum mechanical explanations are given referring to recent reports, and some novel calculations have been carried out for bridging theoretical and experimental data for long-lived radical pairs. For the first topic, time domain calculations of coherent state mixing have been carried out for elucidation of hyperfine (HF) structure dependence of the LFE. For the second topic, Monte Carlo simulations of the torsional motion of polyaromatic linker unit have been carried out for the demonstration of fast decoherence in such rigid molecules. From these considerations, future possibilities of MFE studies on photo-functional materials and biomolecules have been indicated.     

The effect of ion pairs on water structure

A Monte Carlo study of lithium chloride and potassium fluoride in water clusters is reported. The ion-water and water-water interactions are modelled using Hartree-Fock potentials. Results are given for clusters of 50 molecules with a fixed ion-ion distance of 4 Å at a temperature of 298 K. It is shown that all four ions have strongly bound first-neighbour shells and that some water structure is discernible beyond the second-neighbour shell. When ion pairs are close together, as in this study, their coordination numbers are not independent.