Nondegenerate two-photon down conversion: Coherent inputs and nonclassical effects

We study the nondegenerate two-photon down conversion described by a quantum trilinear Hamiltonian. The idler mode is initially prepared in the vacuum while the pump (laser) and the signal mode are prepared in coherent states which at high intensities resemble classical inputs. Such setup with a coherent signal mode allows us to scan the dynamics from the regime of the down conversion (empty signal) up to the frequency conversion (highly excited signal). The analysis concentrates on the entanglement properties of the modes which are compared with their other statistical properties such as squeezing and antibunching to give a more complete characterization of the modes. We show that the single mode nonclassical effects (squeezing and antibunching) disappear when an initial signal intensity highly exceeds that of the pump. In this regime the numerical results are confirmed by approximate analytical calculations. We point out that initially comparable intensities of the signal and pump mode lead to the effect of the spontaneous disentanglement of the signal mode from others and to the production of its squeezed and sub-Poissonian state which is pure to a good approximation.On leave from: Institute of Physics, Slovak Acad. Sci., Dúbravská cesta 9, 842 28 Bratislava, Slovakia.This work was supported in part by the Grant Agency of the Slovak Acad. Sci. Grant No. GA SAV 2/999143/93. We thank Dr. V. Buek for stimulating discussion. The financial support from the Max-Planck Society for one of us (I.J.) is acknowledged.     

Elimination kinetics of tertiary alcohols catalyzed by HBr in the gas phase

The homogeneous, molecular, gas phase elimination kinetics of several tertiary alcohols in seasoned, static reaction vessels, catalyzed by hydrogen bromide at temperatures of 320–392 °C and pressures of 40–174 torr are described. The steric factor appears to be responsible for rate enhancement in the dehydration process. A six-membered cyclic transition state appears to be a reasonable explanation for the mechanism of these reactions.

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, 320–392°C 40–174 . . .

     

Novel podands and macrocycles with diacetal (tetraoxadecalin) cores: synthesis, structure, stereochemistry and cation inclusion

A series of functionalized 2,6-dialkyl-cis-1,3,5,7-tetraoxadecalin podands (3-10, alkyl = hydroxy-, mesyloxy-, halo-, azido- and aminomethyl and -ethyl) were prepared, characterized, and used as precursors for a new and interesting class of macrocycles and cryptands (12-21), with the aim to use these as host-guest inclusion systems. Extensive spectroscopic work was performed, structural endorsement was obtained from X-ray diffraction analyses and further insight into the structures was obtained from theoretical/computational studies. A number of macrocycles in the series exhibited good complexation with alkaline-earth metal ions.     

Syntheses of bis(imino)thietans

The reactions of sulfonyliminothiirans ($\text{1}$) with isonitriles and those of ketenimines with arylsulfonyl isothiocyanates yield the title compounds $\text{2}$. In contrast, benzoyl isothiocyanate undergoes a Diels-Alder reaction with ketenimines to give 1,3,5-oxadiazine-4-thiones ($\text{3}$).     

Evaluation of viability of retinal photoreceptor cells by using their endogenous electrical field

The rod photoreceptor cells are electrical dipoles sustained by cell metabolic energy. Polarity of photoreceptor cells is directly connected to the so-called "dark current" which circulate along the living photoreceptors. Since only the living cells in a good functional state display electrical polarity, the orientation of photoreceptors in static electric field reflects their viability as long as it depends on the functionality of molecular mechanisms that maintain the dark current.Studying the rod cells' orientation in static electric field at different times after their isolation is thus an accurate way to evaluate the cell viability/degeneration. Retinal transplant experiments in animals and humans, which are presently in progress, require a quick and reliable viability test of cells/tissue to be transplanted. Checking the orientation pattern of rod photoreceptors in static electric field prior to transplantation is a candidate method for an accurate cell viability test.     

On the nature of dendrimer cross-linking by ring-closing metathesis

The ring-closing metathesis (RCM) mediated cross-linking of dendrimer 1 and it subunits was examined. Through the combined use of MALDI-MS-TOF, SEC, NMR, and X-ray crystallographic analysis it was shown that the cross-links are formed with the following order of preference: type d >/= type c > type b > type a.     

Substrate temperature effects on film chemistry in plasma deposition of organics. III. Analysis by static secondary ion mass spectrometry

Static secondary ion mass spectrometry (SIMS) was used to examine the effect of reducing the substrate temperature during the radio frequency plasma deposition of organic films. Studies of two polymerizable plasma precursors (2-hydroxyethyl methacrylate and acrylic acid) and one nonpolymerizable precursor (acetone) deposited without substrate cooling and with liquid nitrogen cooling are presented. Acetone deposited with methanol/dry ice cooling was also investigated. Spectra of polymerizable precursors were analyzed by comparison to spectra for the corresponding conventionally-polymerized polymer films [i.e., poly(hydroxyethyl methacrylate) and poly(acrylic acid)]. Acetone spectra were interpreted by reference to SIMS analysis of plasma-deposited films prepared from isotopically-labelled acetone and to reference homopolymers. Comparison of the SIMS spectra of films deposited at different substrate temperatures indicates that a reduction in substrate temperature generally results in higher intensity of peaks characteristic of oxygenated ion structures. SIMS also suggests that the reduction of substrate temperature results in less polymer unsaturation and fewer structures which form by hydrogen redistribution during the deposition process. These results support the hypothesis that deposition at low substrate temperatures leads to an increase in the proportion of precursor incorporated into the film without substantial fragmentation. Corroborative results from high resolution x-ray photoelectron spectroscopy (XPS) and assays for precursor functional groups by chemical derivatization reactions in conjunction with XPS are also presented. © 1992 John Wiley & Sons, Inc.     

Time-dependent quantum dynamics of reactive scattering and the calculation of product quantum state distributions — A study of the collinear F+H2(v=0) → HF(v′)+H reaction

Summary A new method for the calculation of partial cross sections in the time-dependent quantum theory of molecular reactive scattering processes is discussed. Preliminary calculations are presented which clearly illustrate the power of the method. They show how all the partial cross sections associated with a single initial quantum state may be computed over a very wide energy range from a single propagation of a prepared wavepacket. The resonance behaviour is obtained without difficulty and the energies of the reactive scattering resonances are exactly reproduced.     

Calorimetric and structural studies of 1,2,3-trisubstituted cyclopropanes as conformationally constrained peptide inhibitors of Src SH2 domain binding.

Isothermal titration calorimetry and X-ray crystallography have been used to determine the structural and thermodynamic consequences associated with constraining the pTyr residue of the pYEEI ligand for the Src Homology 2 domain of the Src kinase (Src SH2 domain). The conformationally constrained peptide mimics that were used are cyclopropane-derived isosteres whereby a cyclopropane ring substitutes to the N-Calpha-Cbeta atoms of the phosphotyrosine. Comparison of the thermodynamic data for the binding of the conformationally constrained peptide mimics relative to their equivalent flexible analogues as well as a native tetrapeptide revealed an entropic advantage of 5-9 cal mol(-1) K(-1) for the binding of the conformationally constrained ligands. However, an unexpected drop in enthalpy for the binding of the conformationally constrained ligands relative to their flexible analogues was also observed. To evaluate whether these differences reflected conformational variations in peptide binding modes, we have determined the crystal structure of a complex of the Src SH2 domain bound to one of the conformationally constrained peptide mimics. Comparison of this new structure with that of the Src SH2 domain bound to a natural 11-mer peptide (Waksman et al. Cell 1993, 72, 779-790) revealed only very small differences. Hence, cyclopropane-derived peptides are excellent mimics of the bound state of their flexible analogues. However, a rigorous analysis of the structures and of the surface areas at the binding interface, and subsequent computational derivation of the energetic binding parameters, failed to predict the observed differences between the binding thermodynamics of the rigidified and flexible ligands, suggesting that the drop in enthalpy observed with the conformationally constrained peptide mimic arises from sources other than changes in buried surface areas, though the exact origin of the differences remains unclear.