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771.
772.
We investigate the second-order nonlinear interaction as a means to generate entanglement between fields of differing wavelengths and show that perfect entanglement can, in principle, be produced between the fundamental and second-harmonic fields in these processes. Neither pure second-harmonic generation nor parametric oscillation optimally produce entanglement; such optimal entanglement is rather produced by an intermediate process. 相似文献
773.
Nonlinear optical effects for frequency conversion require a phase-matching condition to efficiently generate a coherent field at the new wavelength. We find that the phase-matching condition can be replaced by a resonance condition when the nonlinear effect takes place in a waveguide directional coupler. We apply this theory to second-harmonic generation and find a theoretical conversion efficiency of 100%, equivalent to the perfect phase-matching condition. An example of the design of such waveguide directional coupler is presented. 相似文献
774.
Egdal RK Hazell A Larsen FB McKenzie CJ Scarrow RC 《Journal of the American Chemical Society》2003,125(1):32-33
The first structurally characterized Fe(II)-Fe(III) complex containing a M2(mu-OH)2 diamond core is a Robin and Day class II mixed-valence complex. 相似文献
775.
Dr. Tom Coleman Joel Z. H. Lee Alicia M. Kirk Daniel Z. Doherty Matthew N. Podgorski Dilshi K. Pinidiya Dr. John B. Bruning Prof. James J. De Voss Dr. Elizabeth H. Krenske Dr. Stephen G. Bell 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(67):e202201895
The cytochrome P450 (CYP) family of heme monooxygenases catalyse the selective oxidation of C−H bonds under ambient conditions. The CYP199A4 enzyme from Rhodopseudomonas palustris catalyses aliphatic oxidation of 4-cyclohexylbenzoic acid but not the aromatic oxidation of 4-phenylbenzoic acid, due to the distinct mechanisms of aliphatic and aromatic oxidation. The aromatic substrates 4-benzyl-, 4-phenoxy- and 4-benzoyl-benzoic acid and methoxy-substituted phenylbenzoic acids were assessed to see if they could achieve an orientation more amenable to aromatic oxidation. CYP199A4 could catalyse the efficient benzylic oxidation of 4-benzylbenzoic acid. The methoxy-substituted phenylbenzoic acids were oxidatively demethylated with low activity. However, no aromatic oxidation was observed with any of these substrates. Crystal structures of CYP199A4 with 4-(3′-methoxyphenyl)benzoic acid demonstrated that the substrate binding mode was like that of 4-phenylbenzoic acid. 4-Phenoxy- and 4-benzoyl-benzoic acid bound with the ether or ketone oxygen atom hydrogen-bonded to the heme aqua ligand. We also investigated whether the substitution of phenylalanine residues in the active site could permit aromatic hydroxylation. Mutagenesis of the F298 residue to a valine did not significantly alter the substrate binding position or enable the aromatic oxidation of 4-phenylbenzoic acid; however the F182L mutant was able to catalyse 4-phenylbenzoic acid oxidation generating 2′-hydroxy-, 3′-hydroxy- and 4′-hydroxy metabolites in a 83 : 9 : 8 ratio, respectively. Molecular dynamics simulations, in which the distance and angle of attack were considered, demonstrated that in the F182L variant, in contrast to the wild-type enzyme, the phenyl ring of 4-phenylbenzoic acid attained a productive geometry for aromatic oxidation to occur. 相似文献
776.
Kirk A. Peterson Angela K. Wilson David E. Woon Thom H. Dunning Jr. 《Theoretical chemistry accounts》1997,97(1-4):251-259
Using systematic sequences of the newly developed correlation consistent core-valence basis sets from cc-pCVDZ through cc-pCV6Z,
the spectroscopic constants of the homonuclear diatomic molecules containing first row atoms, B–F, are calculated both with
and without inclusion of 1s correlation. Internally contracted multireference configuration interaction (IC-MRCI) and singles and doubles coupled cluster
(CCSD) theory with a perturbational estimate of connected triple excitations, CCSD(T), have been investigated. By exploiting
the convergence of the correlation consistent basis sets, complete basis set (CBS) limits have been estimated for total energies,
dissociation energies, equilibrium geometries, and harmonic frequencies. Based on the estimated CBS limits the effects of
1s correlation on D
e
(kcal/mol), r
e
(?), and ω
e
(cm−1) are: +1.1, −0.0070, +10 for B2; +1.5, −0.0040, +13 for C2; +0.9, −0.0020, +9 for N2; +0.3, −0.0020, +6 for O2; and −0.1, −0.0015, +1 for F2.
Received: 20 January 1997 / Accepted: 6 May 1997 相似文献
777.
Photoacoustic microscopy (PAM) offers label-free, optical absorption contrast. A high-speed, high-resolution PAM system in an inverted microscope configuration with a laser pulse repetition rate of 100,000 Hz and a stationary ultrasonic transducer was built. Four-dimensional in vivo imaging of microcirculation in mouse skin was achieved at 18 three-dimensional volumes per second with repeated two-dimensional (2D) raster scans of 100 by 50 points. The corresponding 2D B-scan (50 A-lines) frame rate was 1800 Hz, and the one-dimensional A-scan rate was 90,000 Hz. The lateral resolution is 0.23 ± 0.03 μm for Au nanowire imaging, which is 2.0 times below the diffraction limit. 相似文献
778.
This paper proposes a frequency compounding method to reduce speckle interferences, where a concentric annular type high-frequency dual element transducer is used to broaden the bandwidth of an imaging system. In frequency compounding methods, frequency division is carried out to obtain sub-band images containing uncorrelated speckles, which sacrifices axial resolution. Therefore, frequency compounding often deteriorates the target-detecting capability, quantified by the total signal-to-noise ratio (SNR), when the speckle’s SNR (SSNR) is not improved as much as the degraded axial resolution. However, this could be avoided if the effective bandwidth required for frequency compounding is increased. The primary goal of the proposed approach, hence, is to improve SSNR by a factor of two under the condition where axial resolution is degraded by a factor of less than two, which indicates the total SNR improvement to higher than 40% compared to that of an original image. Since the method here employs a dual element transducer operating at 20 and 40 MHz, the effective bandwidth necessary for frequency compounding becomes broadened. By dividing each spectrum of RF samples from both elements into two sub-bands, this method eventually enables four sets of the sub-band samples to contain uncorrelated speckles. This causes the axial resolution to be reduced by a factor of as low as 1.85, which means that this method would improve total SNR by at least 47%. An in vitro experiment on an excised pig eye was performed to validate the proposed approach, and the results showed that the SSNR was improved from 2.081 ± 0.365 in the original image to 4.206 ± 0.635 in the final compounding image. 相似文献
779.
Timothy W. Butler Kirk R. Karwan James R. Sweigart 《The Journal of the Operational Research Society》1992,43(7):665-675
Strategic decision making in hospitals involves the assessment of linkages between decisions that are typically made in a hierarchical fashion. In hospitals, as in most large organizations, overall system performance is a function of how well the critical decisions are integrated. This paper focuses on the multi-level nature of the decisions and policies that typically need to be evaluated in hospital planning, highlighting that both optimization and simulation approaches may be required. An application involving a large general purpose urban hospital is used to illustrate the interdependency between the levels in the planning hierarchy. An optimization model is formulated to deal with facility layout and capacity allocation while a simulation model is proposed to capture the complexities of hospital operations. The linkages and information feedback between the models are shown to be critical in the design of a system that performs well and facilitates strategic hospital planning. 相似文献
780.