We recently used selective 2H labeling of
BDPA to investigate
the Overhauser Effect (OE) dynamic nuclear polarization (DNP) mechanism
in insulating solids doped with 1,3-bis(diphenylene)-2-phenylallyl
(BDPA), and established that the α and γ 1H
spins on the fluorene rings are responsible for generating a zero
quantum (ZQ) mediated positive bulk polarization.
Here, we establish that the phenyl 1H spins relax via double-quantum
(DQ) processes and therefore contribute negative enhancements
which attenuate the OE-DNP. With measurements at different magnetic
field strengths, we show that phenyl-d
5-BDPA offers >50% improvement in OE-DNP enhancement compared to h
21-BDPA attaining a maximum of ∼90 at
14.1 T and 5 kHz MAS, the highest observed OE-DNP enhancement to date
under these conditions. The approach may be utilized to optimize other
polarizing agents exhibiting an OE, an important DNP mechanism with
a favorable field and spinning frequency dependence.
We propose a new mechanism for dynamic nuclear polarization that is different from the well-known Overhauser effect, solid effect, cross effect and thermal mixing processes. In particular, we discovered that the evolution of the density matrix with the simple Hamiltonian of a coupled electron-nuclear spin pair with weak microwave irradiation yields a nuclear polarization enhancement when irradiating near the electron Larmor frequency. We denote the mechanism as Resonant Mixing (RM). We believe that this mechanism is responsible for the observed dispersive shaped DNP field profile for trityl samples near the electron paramagnetic resonance center. This new effect is due to mixing of states by the microwave field together with the electron-nuclear coupling, and involves the same interactions as the SE. However, the SE is optimal when the microwave field is off-resonance, whereas RM is optimal when the microwave field is on-resonance.
Laser processing of diamond has become an important technique for fabricating next generation microelectronic and quantum devices. However, the realization of low taper, high aspect ratio structures in diamond remains a challenge. We demonstrate the effects of pulse energy, pulse number and irradiation profile on the achievable aspect ratio with 532 nm nanosecond laser machining. Strong and gentle ablation regimes were observed using percussion hole drilling of type Ib HPHT diamond. Under percussion hole drilling a maximum aspect ratio of 22:1 was achieved with 10,000 pulses. To reach aspect ratios on average 40:1 and up to 66:1, rotary assisted drilling was employed using > 2 M pulse accumulations. We additionally demonstrate methods of obtaining 0.1° taper angles via ramped pulse energy machining in 10:1 aspect ratio tubes. Finally, effects of laser induced damage are studied using confocal Raman spectroscopy with observation of up to 36% increase in tensile strain following strong laser irradiation. However, we report that upon application of 600 °C heat treatment, induced strain is reduced by up to ~ 50% with considerable homogenization of observed strain.
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