PubMed Health⌕ Search

Biomedical subjects

N A Sergeev

Publications and source records attributed to N A Sergeev.

15 recordsLinked to original sources
PubMed2004

Simple two-pulse time-reversal sequence for dipolar and quadrupolar-coupled spin systems.

We demonstrate, both theoretically and experimentally, that two-pulse sequence (2n+1) X 90 degrees(Y) - 90 degrees(X) -Acq(t) without delay between pulses, yields the reverse of time evolution of spin system with dipolar and quadrupole interactions. This process results in refocusing of the spin magnetization into magic echo at te = t1/2 after the second pulse, where t1 is the length of the first pulse.

Journal Article↗
PubMed2004

Calculation of dipolar correlation function in solids with internal mobility.

A general equation for the dipolar correlation function, to be used to analyze various kinds of independent internal motions, described by some correlation times tau(cm) (m = 1,2 em...k), has been obtained. The obtained expression has been used to analyze the temperature dependencies of different NMR measured values: second moment: spin-lattice relaxation times; amplitude of solid echoes signals.

Journal Article↗
PubMed2002

Solid echo in the slow-motion region. Effects of the finite pulse widths.

The effects of nonzero radio-frequency pulse widths on the echo signals in solids with molecular motions have been investigated. It has been shown that in the slow-motion region a time position and an amplitude of the echo signal depend not only on the width of the pulse, but also on the shape of potential wells and the correlation time, describing the molecular motion. A comparison of the developed theory with experimental results obtained for polycrystalline NH4Cl demonstrates a good agreement between them.

Ammonium Chloride↗
PubMed1997

Effects of finite pulse width on free induction decay.

The shape of the free induction decay for the case in which the amplitude B1 (B1 = omega 1/gamma) of the RF pulse is comparable with local magnetic fields at sites of the nuclei is considered. It is shown that the shape of the FID G(t + tau) (where tau is the width of a RF pulse, t is the time after a RF pulse) depends on the shape of the FID G0(t) following a hard, delta function RF pulse and on the shape of the function F0(tau), which describes the evolution of the longitudinal nuclear magnetization Mz under the effect of the hamiltonian (H0-omega 1 I gamma), where H0 is the interaction hamiltonian of the nuclear spin system. Calculations of F0(tau) and G(t + tau) for different interaction hamiltonians H0 are presented.

Magnetic Resonance Spectroscopy↗
PubMed1997

Effects of the finite pulse widths on solid echo signals.

The quadrupolar and dipolar interactions of spins during the radio frequency (RF) pulses are considered. It is shown that due to these interactions the two-pulse echo signal is observed at the shifted time t(e) = tau + t1/2 (t1 = width of the first RF pulse, tau = time interval between the pulses).

Magnetic Resonance Spectroscopy↗