2,529 research outputs found
Siberian flood basalt magmatism and Mongolia-Okhotsk slab dehydration
Experimental data combined with numerical calculations suggest that fast subducting slabs are cold enough to carry into the deep mantle a significant portion of the water in antigorite, which transforms with increasing depth to phase A and then to phase E and/or wadsleyite by solid-solid phase transition. Clathrate hydrates and ice VII are also stable at PT conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Some cold slabs are expected to deflect while crossing the 410 km and stagnate in transition zone being unable to penetrate through 660 km discontinuity. In this way slabs can move a long way beneath continents after long-lived subduction. With time, the stagnant slabs are heated to the temperature of the ambient transition zone and release free H~2~O-bearing fluid. Combining with transition zone water filter model this may cause voluminous melting of overlying upper mantle rocks. If such process operates in nature, magmas geochemically similar to island-arc magmas are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the south-eastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with fast subduction of the Mongolia-Okhotsk slab and originated by dehydration of the stagnant slab in the transition zone. We show that influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually reduced with increasing distance from the subduction zone
Theoretical Description of Pulsed RYDMR: Refocusing Zero-Quantum and Single Quantum Coherences
A theoretical description of pulsed reaction yield detected magnetic resonance (RYDMR) is proposed. In RYDMR, magnetic resonance spectra of radical pairs (RPs) are indirectly detected by monitoring their recombination yield. Such a detection method is significantly more sensitive than conventional electron paramagnetic resonance (EPR), but design of appropriate pulse sequences for RYDMR requires additional effort because of a different observable. In this work various schemes for generating spin-echo like signals and detecting them by RYDMR are treated. Specifically, we consider refocusing of zero-quantum coherences (ZQCs) and single-quantum coherences (SQCs) by selective as well as by non-selective pulses and formulate a general analytical approach to pulsed RYDMR, which makes an efficient use of the product operator formalism. We anticipate that these results are of importance for RYDMR studies of elusive paramagnetic particles, notably, in organic semiconductors
analytical solutions for spectral patterns and their field dependence
We have obtained analytical solutions for Para-Hydrogen Induced Polarization
(PHIP) for several types of coupled spin systems, namely, for AB-, ABX-,
AA´A´´- and A2B-systems. Scalar spin-spin interactions were considered the
factor, that determines the PHIP spectral pattern; it is the variation of the
spin coupling regime (from strong coupling at low field to weak coupling at
high field), which is responsible for the PHIP magnetic field dependence. The
field dependence of polarization was considered in detail, general
peculiarities of PHIP were found, PHIP patterns were calculated for the
systems mentioned. Special attention was paid to the effects of field
switching on PHIP
Coherent transfer of nuclear spin polarization in field-cycling NMR experiments
Coherent polarization transfer effects in a coupled spin network have been
studied over a wide field range. The transfer mechanism is based on exciting
zero-quantum coherences between the nuclear spin states by means of non-
adiabatic field jump from high to low magnetic field. Subsequent evolution of
these coherences enables conversion of spin order in the system, which is
monitored after field jump back to high field. Such processes are most
efficient when the spin system passes through an avoided level crossing during
the field variation. The polarization transfer effects have been demonstrated
for N-acetyl histidine, which has five scalar coupled protons; the initial
spin order has been prepared by applying RF-pulses at high magnetic field. The
observed oscillatory transfer kinetics is taken as a clear indication of a
coherent mechanism; level crossing effects have also been demonstrated. The
experimental data are in very good agreement with the theoretical model of
coherent polarization transfer. The method suggested is also valid for other
types of initial polarization in the spin system, most notably, for spin
hyperpolarization
Generating and sustaining long-lived spin states in 15N,15N′-azobenzene
Long-Lived spin States (LLSs) hold a great promise for sustaining non-thermal spin order and investigating various slow processes by Nuclear Magnetic Resonance (NMR) spectroscopy. Of special interest for such application are molecules containing nearly equivalent magnetic nuclei, which possess LLSs even at high magnetic fields. In this work, we report an LLS in trans-15N,15N′-azobenzene. The singlet state of the 15N spin pair exhibits a long-lived character. We solve the challenging problem of generating and detecting this LLS and further increase the LLS population by converting the much higher magnetization of protons into the 15N singlet spin order. As far as the longevity of this spin order is concerned, various schemes have been tested for sustaining the LLS. Lifetimes of 17 minutes have been achieved at 16.4 T, a value about 250 times longer than the longitudinal relaxation time of 15N in this magnetic field. We believe that such extended relaxation times, along with the photochromic properties of azobenzene, which changes conformation upon light irradiation and can be hyperpolarized by using parahydrogen, are promising for designing new experiments with photo-switchable long-lived hyperpolarization
Spin mixing at level anti-crossings in the rotating frame makes high-field SABRE feasible
A new technique is proposed to carry out Signal Amplification By Reversible
Exchange (SABRE) experiments at high magnetic fields. SABRE is a method, which
utilizes spin order transfer from para-hydrogen to the spins of a substrate in
transient complexes using suitable catalysts. Such a transfer of spin order is
efficient at low magnetic fields, notably, in the Level Anti-Crossing (LAC)
regions. Here it is demonstrated that LAC conditions can also be fulfilled at
high fields in the rotating reference frame under the action of an RF-field.
Spin mixing at LACs allows one to polarize substrates at high fields as well;
the achievable NMR enhancements are around 360 for the ortho-protons of
partially deuterated pyridine used as a substrate and around 700 for H2 and
substrate in the active complex with the catalyst. High-field SABRE effects
have also been found for several other molecules containing a nitrogen atom in
the aromatic ring
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