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1.
The low density behaviour of the spectral lineshape function associated with the relaxation of the transverse components of the nuclear magnetization of a homonuclear diatomic gas of spin nuclei is studied from the point of view of a recent kinetic theory approach to NMR in molecular gases. It is shown that as a critical density is approached (from above), the relaxation (in time-domain) passes over from exponential to non-exponential behaviour. For the special case in which the spin—rotation relaxation mechanism predominates, an analytic solution to the problem has been given, while for molecular hydrogen which has, in addition to the aforementioned mechanism, dipolar relaxation, the behaviour of the lineshape function has been obtained numerically. The critical density at which the relaxation passes from exponential to nonexponential in H 2 is found to be of the order of 3 × 10 ?3 amagats which lies, at present, still outside experimental accessibility. One important consequence of this result is that the traditional Abragam formula for the transverse relaxation time T2 is clearly seen to be invalid below ≈ 5 × 10 ?3 amagats. 相似文献
2.
The 44 atom core Ni38Pt6 in the metal cluster compound [HNi 38Pt 6(CO) 48] 5- is the largest metal atom core, that has been realized in a single crystal. By 195Pt NMR we have studied the electron density at the Pt-core atoms in powders and single crystals of two varieties of this compound. The large line widths can be explained by charge fluctuations between the metal cores, which are about 17 Å apart. The relaxation rates, which resemble those in randomly-packed Pt309 cluster compounds, confirm such an interpretation. 相似文献
3.
The electron spin distribution in aniline and alkylaniline-Ni(AA) 2 complexes is deduced from 1H, 13C and 14N contact shifts. The nitrogen hybridization state is given by the experimental values of aNHH/ aN compared to the results of INDO calculations. The 13C relaxation times in complexed 4-ethyl aniline indicates an N-Ni distance of 2 A and an electron relaxation time T1 of the order of 10 ?10 s. 相似文献
4.
The electronic relaxation of O 2 is investigated by an absorption excitation and fluorescence detection technique. The relaxation rate constant of O 2( 1Δ g) is measured in the density range from 10 21 to 3 × 10 22 cm −3 at temperatures between 90 and 295 K. The experimental results are compared with theoretical models based on the pair distribution functions of the fluid. The effects of intermolecular potentials with hard or soft cores are discussed. 相似文献
5.
We develop a simple semiempirical model that correlates the Auger parameter to the ground state valence charge of the core-ionized atom with closed shell electron configuration. Until now, the Auger parameter was employed to separate initial and final state effects that influence the core electron binding energy. The model is applied to Cu(I) and Cu (II) compounds with the Auger parameter defined as α' = EbFL (2p 3/2) + EkFL (L 3M 45M 45; 1G). The Auger parameter shift for Cu(I) ion in CuI, CuBr, CuFeS 2, Cu 2S, and Cu 2O compounds—with respect to the copper free atom—increases with the electronic polarizability of the nearest-neighbour ligands suggesting a nonlocal screening mechanism. This relaxation process is interpreted as due to an electron transfer from the nearest-neighbour ligands toward the spatially extended 4sp valence orbitals of the core-ionized Cu(I) ion. In agreement with our model, a linear relationship is found between the Auger parameter shift and the ground state Bader valence charge obtained by density functional theory calculations. The Auger parameter shift for the Cu (II) ion in CuF 2, CuCl 2, CuBr 2, CuSO 4, Cu (NO 3) 2•3H 2O, Cu 3(PO 4) 2, Cu (OH) 2, and CuO compounds is very close to the Auger parameter of metallic copper, and therefore, it is not related to the calculated ground state Bader valence charge. The relaxation process in the final state is dominated by the local screening mechanism, which involves an electron transfer from the nearest-neighbour ligands toward the spatially contracted 3d orbitals of the core-ionized Cu (II) ion. 相似文献
6.
Proton spin lattice relaxation times have been measured for a variety of single crystal orientations of the (φ 3AsCH 3) +(TCNQ) 2? ion radical salt over the temperature range of approximately 90 to 370°K. It is shown that the relaxation rate is directly proportional to the triplet exciton density where the exciton production energy is significantly dependent on temperature. The data suggests that exciton—exciton exchange is an important aspect in the relaxation mechanism. 相似文献
7.
Magnetic anisotropy is the key element in the construction of single‐ion magnets, a kind of nanomagnets for high‐density information storage. This works describes an unusual large easy‐plane magnetic anisotropy (with a zero‐field splitting parameter D of +40.2 cm ?1), mainly arising from the second‐order spin‐orbit coupling effect in a trigonal‐planar Co II complex [Li(THF) 4][Co(NPh 2) 3], revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field‐induced slow magnetic relaxation in this complex was mainly attributed to the Raman process. 相似文献
8.
Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins. Herein, we demonstrate that electron decoupling with chirped microwave pulses enables improved observation of DNP‐enhanced 13C spins in direct dipolar contact with electron spins, thereby leading to an optimal delay between transients largely governed by relatively fast electron relaxation. We report the first measurement of electron longitudinal relaxation time (T 1e) during magic angle spinning (MAS) NMR by observation of DNP‐enhanced NMR signals (T 1e=40±6 ms, 40 mM trityl, 4.0 kHz MAS, 4.3 K). With a 5 ms DNP period, electron decoupling results in a 195 % increase in signal intensity. MAS at 4.3 K, DNP, electron decoupling, and short recycle delays improve the sensitivity of 13C in the vicinity of the polarizing agent. This is the first demonstration of recovery times between MAS‐NMR transients being governed by short electron T 1 and fast DNP transfer. 相似文献
9.
The effect of orbital relaxation on the properties of atomic and molecular hydrogen systems H, H
2
+
, H 2, H
2
−
, and H
3
+
calculated using the minimal basis set, the split valence shell basis set including the polarization function, and an extended basis set of grouped natural orbitals is considered. Inclusion of orbital relaxation in calculations results in a decreased total energy and more accurate energies of electron affinity. The strongest effect is produced on the calculated characteristics of the anions. The calculated activation energy of the radical reaction of hydrogen elimination. H 2 + H = H + H 2 depends strongly on the degree to which electron correlation is taken into account. Due to inclusion of orbital relaxation, the activation energy also approximates the experimental value, although to a lesser extent. The semiempirical PM3 method fails to adequately describe the transition state of this reaction, but this disadvantage is eliminated by using the exponent of the relaxed orbital of hydrogen.Original Russian Text Copyright © 2004 by A. I. Ermakov, A. E. Merkulov, A. A. Svechnikova, and V. V. Belousov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 979–985, November–December, 2004. 相似文献
10.
For the evaluation of transport in high density plasmas numerical models have been developed in which simultaneously the conservation laws for mass, momentum and energy are solved. For high density plasmas, which are not too far from equilibrium the commonly used thermodynamic quantities are, electron temperature Te, electron density ne, heavy particle temperature and neutral density (or pressure). In this contribution an alternative formulation is described in which the plasma state is described by electron density ne and total pressure p and two non-equilibrium parameters: the deviation from Saha equilibrium of the neutral ground state (δb1 = n1/n1
saha−1) and the deviation from thermal equilibrium between electrons and heavy particles δΘ = 1−Th/Te. The latter two parameters are zero in local thermodynamic equilibrium. The advantage of this formulation is, that the transport coefficients and radiative properties can be reformulated as function of mainly ne (at constant pressure), as the influences of non zero δb1 and δΘ are small or can be explicitly given. As a result a simpler approximate formulation of the transport problem can be obtained. As an example the procedure is illustrated for atmospheric argon plasmas and for one aspect a comparison is made with work from e.g. E. Pfender. 相似文献
11.
The spatial distributions of electron temperature and density in a dc glow discharge that is created by a pair of planar electrodes were obtained by using double Langmuir probes. The contribution of double Langmuir probes measurement is to provide a relatively quantitative tool to identify the electron distribution behavior. Electrons gain energy from the imposed electric field, and electron temperature (T e) rises very sharply from the cathode to the leading edge of the negative glow where T e reaches the maximum. In this region, the number of electrons (N e) is relatively small and does not increase much. The accelerated electrons lose energy by ionizing gas atoms, and T e decreases rapidly from the trailing edge of the negative glow and extends to the anode. N e was observed to increase from the cathode to the anode, which is due to the electron impact ionization and electron movement. The electron density was observed to increase with increasing discharge voltage while the electron temperature remained approximately. At 800 V and 50 mTorr argon glow discharge, Te ranged from 15 to 52 eV and Ne ranged from 6.3×10 6/cm 3 to 3.1×10 8/cm 3 in the DC glow discharge, and T e and N e were dependent on the axial position. 相似文献
12.
The electron density in a laser produced plasma experiment was measured utilizing the Stark broadening of the H α-line at 656.27 nm. This line results from the interaction of the Nd:YAG laser at the fundamental wavelength of 1.06 μm with a plane solid aluminum target in a humid air. The measurements were repeated at several delay times (0–10 μs) and at a fixed gate time of 1 μs. The electron density from the optically thin Al II-line at 281.62 nm was measured in parallel from the same spectra. The electron density was found in the range from 10 18 cm − 3 down to 6 × 10 16 cm − 3 at longer delay time. The electron density from the H α-line using the Griem's standard theory was compared with the predictions of other model due to Gigosos et al. The agreement between the measured electron density from both the H α-line and the Al II-line would confirm the reliability of utilizing the H α-line as an electron density standard reference line in LIBS experiments. Several important features characterize the H α-line: it is a well isolated line, it gives large signal to background ratio, it lasts a long time after the termination of the laser (up to 10 μs), its Stark width is relatively large and does not exhibit self-absorption. 相似文献
13.
The overall rate constants for collisional relaxation of metastable excited states of Fe + by He, Ar, Kr, H 2, 2H 2, CO, N 2, NO, CH 4, and CH 3OH have been studied by using charge-exchange ion-molecule reaction chemistry. The rate constants vary according to the nature of the quenching reagent as well as the energy level and electron configuration of the Fe + ions. In general, NO, CH 4, and CH 3OH are the most efficient quenching reagents with rate constants that approach the Langevin collision rate, whereas the reaction rates for the rare gas atoms are slow and vary depending upon the specific electron configuration of the Fe + ion. The mechanism of collisional relaxation is discussed with emphasis on a curve-crossing. mechanism for the rare gas atoms. An electron-transfer mechanism is described for the relaxation of high lying (Fe +)*. 相似文献
14.
Calculations of the absorption spectra of e
t
–
were performed using the model of electron localization in amorphous media. The model describes electron localization as the radiationless transition of an electron from the conduction state to the binding level of one of a statistical variety of the pre-existing traps in the matrix. Electron capture by very deep traps is considered as a two-stage process-electron capture by the excited level of the trap followed by relaxation to the ground state. The time evolution and the temperature dependence of the optical absorption spectra of e
t
–
were computed for polar and nonpolar glasses. The spectral changes predicted by the model follow the experimental results provided by pulse radiolysis. 相似文献
15.
Zerfo field splitting plays an important role in determining the electron spin relaxation of Gd(III) in solution. We understand the ZFS as an effect depending on the f electron structure and treat it in the framework of ligand field-density functional theory (LF-DFT). We apply this theory to calculate the ZFS of [Gd(DOTA)(H 2O)] ? from first principles, having an insight concerning the contributions determining the ZFS. 相似文献
16.
The non-equilibrium vibrational kinetics of H 2 in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H 2( v) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H −) shows a satisfactory agreement, thus indicating the basic correctness of our model. 相似文献
17.
A 0D numerical approach including a Collisional-Radiative model is elaborated in the purpose of describing the behavior of the nascent plasma resulting from the interaction between a 4 ns/65 mJ/532 nm Q-switched Nd:YAG laser pulse and an aluminum sample in vacuum. The heavy species considered are Al, Al +, Al 2+ and Al 3+ on their different excited states and free electrons. The translation temperatures of free electrons and heavy species are assumed different ( Te and TA respectively). Numerous elementary processes are accounted for as electron impact induced excitation and ionization, elastic collisions, multiphoton ionization and inverse Bremsstrahlung. Atoms passing from the sample to gas phase are described by using classical vaporization theory so that the surface temperature is arbitrarily limited to values less than the critical point one at 6700 K. The laser flux density considered in the study is therefore moderate with a fluence lower than 7 J cm ? 2.This model puts forward the major influence of multiphoton ionization in the plasma formation, whereas inverse Bremsstrahlung turns out to be quasi negligible. The increase of electron temperature is mainly due to multiphoton ionization and Te does not exceed 10,000 K. The electron induced collisions play an important role during the subsequent phase which corresponds to the relaxation of the excited states toward Boltzmann equilibrium. The electron density reaches its maximum during the laser pulse with a value ≈ 10 22, 10 23 m ? 3 depending highly on the sample temperature. The ionization degree is of some percents in our conditions. 相似文献
18.
Vertically aligned few layered graphene (FLGs) nanoflakes were synthesized by microwave plasma deposition for various time durations ranging from 30 to 600 s to yield graphene films of varying morphology, microstructure and areal/edge density. Their intrinsic electrochemical properties were explored using Fe(CN) 6 3?/4? and Ru(NH 3) 6 3+/2+ redox species. All the FLG electrodes demonstrate fast electron transfer kinetics with near ideal Δ Ep values of 60–65 mV. Using a relationship between electron transfer rate and edge plane density, an estimation of the edge plane density was carried out which revealed a moderation of edge plane density with increase in growth time. The pristine FLGs also possess excellent electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solutions. This ORR activity can be further enhanced by exposing the pristine FLGs to nitrogen electron cyclotron resonance plasma. The metal free N-doped FLGs exhibit much higher electrocatalytic activity towards ORR than pristine FLGs with higher durability and selectivity than Pt-based catalysts. The excellent electrochemical performance of N-doped FLGs is explained in terms of enhanced edge plane exposure, high content of pyridinic nitrogen and an increase in the electronic density of states. 相似文献
19.
The measured linewidths of NMR spectra for 15N 2 and 14N 2 have been used to calculate rotational (τ J) and orientational (τθ 2) relaxation times. Within the density range studied (0–100 amagat) τθ 2 is proportional to τ J in line with the impact theory of orientational and rotational relaxation. 相似文献
20.
Abstract A MNDO and 300-MHz 1H NMR study of some trigonal-bipyramidal (TBP) five-coördinated phosphorus (P v) compounds is reported. It is shown by the MNDO calculations that, in the oxaphosphole P v TBP compounds 5a-c, the electron distribution in the axial bonds of the TBP is affected by the electronegativity of the substituent at C 4 of the oxaphosphole ring. With increasing electronegativity of the substituent at C 4, the electron density on the axial exocyclic oxygen atom O 1 decreases whereas the electron density on the axial endocyclic atom O 1 increases. This is supported by a 1H NMR conformational analysis of the C 1[sbnd]C 2 bond of the oxaphosphole P v TBP compounds 6–11. The gauche(-) rotamer fraction (O 1 and O 1 trans situated) of these compounds, which is correlated to the electron density on O 1, is reduced to 30% as compared to the absolute axial g ?rotamer fraction (59%) of the dioxaphosphole P v TBP compound 13, most likely because of the presence of the carbonyl group at C 4 of the oxaphosphole ring. So, both the 1H NMR and MNDO study show that electron withdrawing substituents on the oxaphosphole ring of P v TBP compounds reverse the electron transfer in the axial P[sbnd]O bonds of the TBP (as compared to dioxaphosphole compounds), from exocyclic O 1 towards endocyclic O 1. 相似文献
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