P. A. Purtov

Evaluation of surface tension and Tolman length as a function of droplet radius from experimental nucleation rate and supersaturation ratio: metal vapor homogeneous nucleation.

Zinc and silver vapor homogeneous nucleations are studied experimentally at the temperature from 600 to 725 and 870K, respectively, in a laminar flow diffusion chamber with Ar as a carrier gas at atmospheric pressure. The size, shape, and concentration of aerosol particles outcoming the diffusion chamber are analyzed by a transmission electron microscope and an automatic diffusion battery. The wall deposit is studied by a scanning electron microscope (SEM). Using SEM data the nucleation rate for both Zn and Ag is estimated as 1010cm−3s−1. The dependence of critical supersaturation on temperature for Zn and Ag measured in this paper as well as Li, Na, Cs, Ag, Mg, and Hg measured elsewhere is analyzed. To this aim the classical nucleation theory is extended by the dependence of surface tension on the nucleus radius. The preexponent in the formula for the vapor nucleation rate is derived using the formula for the work of formation of noncritical embryo [obtained by Nishioka and Kusaka [J. Chem. Phys. 96, 5370 (1992)] and later by Debenedetti and Reiss [J. Chem. Phys. 108, 5498 (1998)]] and Reiss replacement factor. Using this preexponent and the Gibbs formula for the work of formation of critical nucleus the dependence of surface tension on the radius RS of the surface of tension is evaluated from the nucleation data for above-mentioned metals. For the alkali metals and Ag the surface tension was determined to be a strong function of RS. For the bivalent metals (Zn, Hg, and Mg) the surface tension was independent of radius in the experimental range. A new formula for the Tolman length δ as a function of surface tension and radius RS is derived by integration of Gibbs-Tolman-Koenig equation assuming that δ is a monotonic function of radius. The formula derived is more correct than the Tolman formula and convenient for the elaboration of experimental data. Using this formula the values of δ are determined as a function of RS from the experimental nucleation data. It is determined that all the metals considered are characterized by strong dependence of δ on radius; for the bivalent metals δ changes sign.Zinc and silver vapor homogeneous nucleations are studied experimentally at the temperature from 600 to 725 and 870K, respectively, in a laminar flow diffusion chamber with Ar as a carrier gas at atmospheric pressure. The size, shape, and concentration of aerosol particles outcoming the diffusion chamber are analyzed by a transmission electron microscope and an automatic diffusion battery. The wall deposit is studied by a scanning electron microscope (SEM). Using SEM data the nucleation rate for both Zn and Ag is estimated as 1010cm−3s−1. The dependence of critical supersaturation on temperature for Zn and Ag measured in this paper as well as Li, Na, Cs, Ag, Mg, and Hg measured elsewhere is analyzed. To this aim the classical nucleation theory is extended by the dependence of surface tension on the nucleus radius. The preexponent in the formula for the vapor nucleation rate is derived using the formula for the work of formation of noncritical embryo [obtained by Nishioka and Kusaka [J. Chem. Phys. 96, 537...

Anisotropic hyperfine interaction-induced spin relaxation in a low magnetic field

Abstract Spin relaxation caused by modulation of an anisotropic hyperfine interaction (HFI) in a low magnetic field has been correctly considered for a radical with one magnetic nucleus in a framework of Redfield relaxation theory. The analysis of the results obtained revealed that HFI-induced relaxation is very different in a low magnetic field from that in a high field; and that the use of traditional high-field expressions for calculation of T1 and T2 is not correct. It has been shown that taking correct account of the role of HFI-induced relaxation is crucial for the calculations of time-resolved CIDEP and the line width of EPR in a low magnetic field. In calculations of low-field CIDNP and MARY, taking correct account of the relaxation mainly influences the amplitude of the field dependence.

Theoretical and experimental studies of chemically induced dynamic nuclear polarization kinetics in recombination of radical pairs by the method of switched external magnetic field. II. 13C CIDNP of micellized radical pairs

The method of 13C chemically induced dynamic nuclear polarization in a switched external magnetic field (SEMF CIDNP) has been applied for the first time in an experimental investigation of micellized radical pairs (RP). Using the examples of three photochemical reactions it has been shown, that SEMF CIDNP allows the investigation of the kinetics of short-lived micellized RPs with high time-resolution in low and intermediate magnetic fields. The experimental kinetics have been analyzed and simulated on the basis of a previously developed theory [Parnachev et al., J. Chem. Phys. 107, 9942 (1997)]. It has been demonstrated that such an analysis provides information on the rates of radical escape from the micelle, on electron relaxation and on the rate of S–T− transitions. The analysis of the estimated rates of S–T− transitions showed that the exchange interaction is essentially anisotropic in the RPs studied.

Spin relaxation of radicals in low and zero magnetic field

Spin relaxation of radicals in solution in low and zero magnetic field has been studied theoretically. The main relaxation mechanisms in low magnetic field [modulation of anisotropic and isotropic hyperfine interaction, and modulation of spin–rotational interaction] are considered within a Redfield theory. The analytical results for a radical with one magnetic nucleus (I=12) and for a radical with two equivalent magnetic nuclei (I=12) are obtained and analyzed. It is shown that the probabilities of relaxational transitions in low and zero magnetic fields differ significantly from the probabilities in high magnetic fields. The use of high-field expressions in low and zero magnetic fields is not correct. Taking exact account of spin relaxation is important in calculations of much low-field magnetic resonance data.

Magnetic field effects on stilbene sensitized trans-cis photoisomerization

Abstract The magnetic field dependence ratio of cis and trans forms in sensitized photoisomerization of some stilbenes was studied. It is shown that the observed magnetic field effect arises through the “radical pair” mechanism. The experimental magnetic field dependence is compared with the calculated one. The good agreement found confirms the radical mechanism of sensitized trans-cis photoisomerization of stilbene is the basic one.

Theoretical and experimental studies of CIDNP kinetics in recombination of radical pairs by the method of switched external magnetic field. I. Theory

The theory of chemically induced dynamic nuclear polarization (CIDNP) of radical pairs in restricted volume formed in experiments with instantaneously switched external magnetic field is developed. For this purpose the theory of recombination of radical pairs with instantaneously changed spin Hamiltonian is extended to the case of rather weak magnetic fields. Using kinematic approximation it is shown that the recombination probability of radical pairs is fully expressed via the matrix formulas in terms of averages of Green’s functions of Liouville equation over both the reaction zone and that of energy level crossing. The effect of field switching on the CIDNP value is calculated for the typical parameters of the radical pairs. Simple biexponential expressions are derived for the CIDNP time dependencies for the case of strongly averaged S−T0 transitions and full mixing in configuration space.

Theoretical and experimental studies of chemically induced electron-nuclear polarization in low magnetic fields

Chemically induced electron-nuclear polarization at low magnetic fields is considered theoretically for a radical with one magnetic nucleus. It is shown that large non-equilibrium populations of the radical spin levels are expected to exist under low magnetic fields. This large electron-nuclear polarization has been observed experimentally during the photolysis of two phosphine oxides using a modified L-band TREPR setup. The TREPR spectra and kinetics of dimethoxyphosphonyl and diphenylphosphonyl radicals have been measured and analysed in low magnetic fields, and excellent agreement between experiment and theory has been achieved.

FMR study of superparamagnetic Ni particles with weak and strong magnetic anisotropy

The method of ferromagnetic resonance (FMR) was used to study the process of thermal decomposition of the layered double hydroxides of lithium-aluminum and nickel-aluminum with intercalated EDTA complexes of nickel. The magnetic resonance spectra of nickel superparamagnetic nanoparticles were recorded at two temperatures (300 and 77 K). A computer simulation of FMR spectra was based on a modified statistic model which assumes the resonance of single-domain particles randomly oriented in an amorphous matrix. It is suggested that the line of the magnetic resonance of superparamagnetic particles narrows due to effects similar to those of dynamic narrowing in electron spin resonance and nuclear magnetic resonance spectra. In the framework of the model used, a fairly good agreement was achieved between calculated and experimental data. The formation of the two types of particles with strong (about 2·106 erg/cm3) and weak (about 2·105 erg/cm3) effective magnetic anisotropy was established.

Theoretical and experimental studies of CIDNP kinetics in recombination of radical pairs by the method of switched external magnetic field. III. Free radicals in homogeneous solution

The method of chemically induced dynamic nuclear polarization in a switched external magnetic field (SEMF CIDNP) is applied here for the first time to an experimental study of short-lived neutral radicals in homogeneous solutions. With three photochemical reactions it is exemplified, that SEMF CIDNP allows investigations of the kinetics of the transient species with high time-resolution as well as a determination of their spin relaxation times in low magnetic fields. A theoretical approach is developed, which permits simulation and analysis of the experimental data. In weak magnetic fields (0.5–2.0 mT) the effective spin-lattice relaxation times for the decay of the chemically induced spin polarizations in benzyl, tert-butyl, and 2-hydroxy-2-propyl radicals are found to be T1=(3.8±0.5) μs, T1=(7.8±0.5) μs, and T1=(2.5±0.5) μs, respectively, in benzene solution at room temperature. They are in fair agreement with relaxation times determined by time-resolved X-band electron paramagnetic resonance spectrosco...

CIDNP in photolysis of acetone solutions in plastic crystals of cyclohexane-d12

The emissive CIDNP was observed during photolysis of acetone solutions in solid cyclohexane-d12. The polarization is assumed to arise in contact triplet radical pairs as a result of their T_-S, conversion and their recombination.