Search

Hyperpolarization methods, which can enhance nuclear spin signals by orders of magnitude, open up important new opportunities in magnetic resonance. However, many of these applications are limited by spin lattice relaxation, which typically destroys the hyperpolarization in seconds. Significant lifetime enhancements have been found with "disconnected eigenstates" such as the singlet state between a pair of nearly equivalent spins, or the "singlet-singlet" state involving two pairs of chemically equivalent spins; the challenge is to populate these states (for example, from thermal equilibrium magnetization or hyperpolarization) and to later recall the population into observable signal. Existing methods for populating these states are limited by either excess energy dissipation or high sensitivity to inhomogeneities. Here we overcome the limitations by extending recent work using continuous-wave irradiation to include composite and adiabatic pulse excitations. Traditional composite and adiabatic pulses fail completely in this problem because the interactions driving the transitions are fundamentally different, but the new shapes we introduce can move population between accessible and disconnected eigenstates over a wide range of radio-frequency (RF) amplitudes and offsets while depositing insignificant amounts of power. [ABSTRACT FROM AUTHOR]

For more than three decades, the classical or mean-field picture describing the distant dipolar field has been almost always simplified to an effective field proportional to the local longitudinal magnetization, differing only by a scale factor of 1.5 for homomolecular (identical resonance frequency) and heteromolecular interactions. We re-examine the underlying assumptions, and show both theoretically and experimentally that the mathematical framework needs to be modified for modern applications such as imaging. We demonstrate new pulse sequences which produce unexpected effects; for example, modulating an arbitrarily small fraction of the magnetization can substantially alter the frequency evolution. Thus, matched gradient pulse pairs (a seemingly innocuous module in thousands of existing pulse sequences) can alter the time evolution in highly unexpected ways, particularly with small flip angle pulses such as those used in hyperpolarized experiments. We also show that specific gradient pulse combinations can retain only dipolar interactions between unlike spins, and the dipolar field can generate a secular Hamiltonian proportional to Ix. [ABSTRACT FROM AUTHOR]

This report introduces a new approach that enhances nonlinear solution magnetic resonance signals from intermolecular dipolar interactions. The resulting signals can theoretically be as large as the full equilibrium magnetization. Simple, readily implemented pulse sequences using square-wave magnetization modulation simultaneously refocus all even order intermolecular multiple quantum coherences, leading to a substantial net signal enhancement, complex nonlinear dynamics, and improved structural sensitivity under realistic conditions. [ABSTRACT FROM AUTHOR]

We examine the propagation of shaped (amplitude- and frequency-modulated) ultrafast laser pulses through optically dense rubidium vapor. Pulse reshaping, stimulated emission dynamics, and residual electronic excitation all strongly depend on the laser pulse shape. For example, frequency swept pulses, which produce adiabatic passage in the optically thin limit (independent of the sign of the frequency sweep), behave unexpectedly in optically dense samples. Paraxial Maxwell optical Bloch equations can model our ultrafast pulse propagation results well and provide insight. [ABSTRACT FROM AUTHOR]

Dipolar effects in solution nuclear magnetic resonance lead to additional peaks in two-dimensional experiments. These peaks, which have the experimental properties of intermolecular multiple-quantum coherences, have been used in a variety of applications. Most efforts have focused on intermolecular zero-quantum or double-quantum coherences, which originate in two-spin terms from the equilibrium density matrix. In this paper, we characterize the “third-order experiments” (Hahn echo decay and triple-quantum CRAZED, which both originate in the three-spin terms in the equilibrium density matrix) both theoretically and experimentally. For example, in the coupled-spin picture, Hahn echo decays in concentrated solutions arise initially from intermolecular, 3-spin, -1-quantum coherences, which are partially converted to 3-spin, +1-quantum coherences by the second pulse, and hence survive the 1:1 coherence transfer echo. Such terms require two dipolar couplings to become observable. We discuss the general properties of both of these sequences, and show that they only give information that is already present in the “second-order” double-quantum and zero-quantum experiments. Finally, we also show that relaxation and diffusion can be introduced into the coupled-spin picture in a straightforward manner. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The time evolution of spins-1/2 subject to radiation damping, which is commonly encountered for solvent peaks at high field, is examined in detail. The well-known analytic results for rectangular pulses on undamped spin-1/2 systems are extended to the radiation damped case, and reveal surprisingly complex dynamics. Explanations in terms of Bloch vectors are also presented, and composite pulse sequences which would also be insensitive to radiation damping are proposed. In addition, gradient optimization programs were developed to find shaped π and π/2 pulses insensitive to radiation damping. The optimized pulses compensate for radiation damping effects even when the characteristic damping time is shorter than the pulse length. [ABSTRACT FROM AUTHOR]

We numerically investigate optical dephasing from as many as 500 interacting two-level absorbers by making the semiclassical approximation on the radiation–matter interaction which includes retarded dipole–dipole coupling and superradiant coupling. We calculate the superradiant decay rate following single pulse optical excitation, the photon echo decay and the fluorescence. By carrying out an expansion to the second order in time we demonstrate a fundamental difference between the homogeneous free-induction decay and photon echo decay which is due to radiation damping. We show that an echo sequence is capable of reversing superradiant damping in some cases. Specific computations for pentacene doped mixed molecular crystals, where ‘‘domains’’ of resonant impurities are likely to exist, are presented and compared with experimental results. [ABSTRACT FROM AUTHOR]

We show that a fundamental assumption used in analyzing photon echo decays from interacting molecules—the assumption that the ‘‘pure’’ dephasing rate can be obtained by subtracting one half of the fluorescence decay rate (2T1)-1 from the photon echo decay rate—is generally inconsistent with a fully quantum mechanical treatment. The density matrix evolution for a system of noninteracting dimers is calculated using the retarded electromagnetic coupling between two oscillating dipoles. We are able to properly include the effects of spontaneous emission and Dicke superradiance on the echo decay. [ABSTRACT FROM AUTHOR]

The feasability of selective and complete population transfer between atomic or molecular levels by stimulated Raman scattering with delayed pulses involving spectrally broad light with characteristics typical for pulsed lasers is investigated. In extension of previous work, the effect on the transfer efficiency of phase fluctuations and of the detuning ΔR of the laser frequencies from the two-photon resonance is analyzed. The minimum pulse energy Pmin required to achieve a transfer efficiency of nearly unity is derived analytically, with some restrictions imposed on the type of phase fluctuations. Pmin increases approximately proportional to the square of the bandwidth of the laser radiation and to the square of ΔR. The conclusions are confirmed by the results of extensive numerical calculations. These studies also reveal a high sensitivity of the transfer efficiency to the autocorrelation of the fluctuating light. [ABSTRACT FROM AUTHOR]

Frequency modulated picosecond laser pulses are applied to the X→B electronic transition in I2 vapor. In the adiabatic limit, these pulses generate more excitation in I2 vapor than do ‘‘transform-limited’’ laser pulses. Multilevel effects make it important to match the pulse bandwidth to the material absorption spectrum, as demonstrated experimentally. [ABSTRACT FROM AUTHOR]

Experimental observation of anomalous intermolecular cross-peaks in two-dimensional solution NMR spectra have attracted significant recent attention. Extremely simple pulse sequences on extremely simple samples with large equilibrium magnetization give resonances in the indirectly detected dimension which are simply impossible in the conventional density matrix framework of NMR. Here we extend a recently proposed density matrix treatment [Science 262, 2005 (1993)] to calculate the exact time evolution for a variety of pulse sequences. This density matrix treatment explicitly removes two fundamental assumptions of the standard theory—it includes the dipolar interaction between spins in solution (which is only partially averaged away by diffusion) and completely removes the high temperature approximation to the equilibrium density matrix [exp(-βH)≊1-βH]. We compare this quantum mechanical treatment to a corrected classical model, which modifies the dipolar demagnetizing field formulation to account for the effects of residual magnetization, and show that the quantum picture can be reduced to this corrected classical model when certain assumptions about the retained dipolar couplings are valid. The combination of quantum and classical pictures provides enormously better predictive power and computational convenience than either technique alone. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present detailed experimental and theoretical results on population transfer with frequency-swept picosecond laser pulses. Here, we demonstrate that intense frequency-swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than do unmodulated laser pulses. The experimental work is performed on quasi-two-level systems (pentacene/p-terphenyl crystal and Na vapor), quasi-three-level systems (Na vapor), and on more complex multilevel systems (I2 vapor). We discuss the different characteristics of adiabatic population transfer in both few-level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer. [ABSTRACT FROM AUTHOR]

Answers comments involving multiple quantum nuclear magnetic resonance of water. Information on the classical radiation damping theory; Discussion of the possibility of multiple-quantum coherences in water; Multiple-quantum diffusion measurements in water.

We show experimentally and theoretically that coupling between a spin system and the NMR probe, which is responsible for radiation damping, can also act to generate multiple-quantum coherences, even in molecules such as water which ordinarily have no multiple-quantum spectrum. We discuss the implications of this observation for solvent suppression sequences and for preparing nonequilibrium spin states. [ABSTRACT FROM AUTHOR]