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  • American Institute of Physics (AIP)  (14)
  • The International Institute of Anticancer Research (IIAR)
  • 1995-1999  (14)
  • 1
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 3254-3256 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have fabricated a two-dimensional photonic band structure based on macroporous silicon with individual gaps for both polarizations in the infrared region between 250 and 500 cm−1 (20–40 μm). A square lattice of circular air rods with a lattice constant of 8 μm was etched 340 μm deep in an n-type silicon substrate by electrochemical pore formation in hydrofluoric acid. The transmission spectra between 50 and 650 cm−1 were in good agreement with the theoretical calculated structure. The pore formation technique should allow the fabrication of photonic lattices with a complete two-dimensional band gap in the middle and near infrared. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The heat capacity of crystalline zinc tris (thiourea) sulphate, has been measured in the range from 220 to 500 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=2.76×10−3 T+0.366 J g−1 K−1. Thermal expansion data have been measured in the range from 150 to 473 K. From these data, the principal thermal expansion coefficients were found to be α1=6.41×10−5 K−1, α2=4.52×10−5 K−1, and α3=−4.32×10−6 K−1. The thermal conductivity tensor of this orthorhombic crystal was calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) crystal planes by the laser flash method. The thermal conductivity coefficients at 295 K are k1=0.27 W m−1 K−1, k2=0.34 W m−1 K−1, and k3=0.54 W m−1 K−1. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first eigenstate resolved, near the infrared spectrum of benzene in the region of the first C–H stretch overtone (6000 cm−1) has been obtained with an IR–IR double-resonance molecular beam optothermal spectrometer. Using a hierarchical tree analysis and level spacing statistics, we show that the intramolecular vibrational relaxation occurs nonergodically over at least seven different time scales ranging from 100 fs to 2 ns.© 1997 American Institute of Physics.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using microwave detected, microwave-optical double resonance, we have measured the homogeneous linewidths of individual rovibrational transitions in the A˜ state of NH3, NH2D, NHD2, and ND3. We have used this excited state spectroscopic data to characterize the height of the dissociation barrier and the mechanisms by which the molecule uses its excess vibrational and rotational energies to help overcome this barrier. To interpret the observed vibronic widths, a one dimensional, local mode potential has been developed along a N–H(D) bond. These calculations suggest the barrier height is roughly 2100 cm−1, approximately 1000 cm−1 below the ab initio prediction. The observed vibronic dependence of levels containing two or more quanta in ν2 is explained by a Fermi resonance between 2ν2 and the N–H(D) stretch. This interaction also explains the observed trends due to isotopic substitution. The rotational enhancement of the predissociation rates in the NH3 21 level is dominated by Coriolis coupling while for the same level in ND3, centrifugal effects dominate. © 1995 American Institute of Physics.
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  • 5
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The intramolecular vibrational relaxation (IVR) of an excited Si–H stretch (second overtone) and C–H stretch (first overtone) in methylsilane has been examined by eigenstate resolved infrared spectroscopy. The experiment probes a molecular beam produced in a supersonic expansion, excited by a laser in a power buildup cavity, and detected by a liquid helium cooled silicon bolometer. The Si–H stretch [local mode (3,0,0), both A and E combinations] is compared with the nearly isoenergetic C–H stretch [predominantly the 2ν70 band]. With the calculated density of states almost unchanged, the two modes exhibit very different IVR behavior, which is quantified in terms of the lifetime of the bright states and the coupling between the bright states and the dark states. © 1997 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 10263-10277 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cavity ring-down is becoming a widely used technique in gas phase spectroscopy. It holds promise for further important extensions, which will lead to even more frequent use. However, we have found widespread confusion in the literature about the nature of coherence effects, especially when the optical cavity constituting the ring-down cell is excited with a short coherence length laser source. In this paper we use the superposition principle of optics to present a general and natural framework for describing the excitation of a ring-down cavity regardless of the relative values of the cavity ring-down time, the input pulse coherence time, or the dephasing time of absorption species inside the cavity. This analysis demonstrates that even in the impulsive limit the radiation inside a high finesse cavity can have frequency components only at the natural resonance frequencies of the cavity modes. As an immediate consequence, a sample absorption line can be detected only if it overlaps at least one of the cavity resonances. Since this point is of particular importance for high resolution applications of the technique, we have derived the same conclusion also in the time domain representation. Finally, we have predicted that it is possible to use this effect to do spectroscopy with a resolution much higher than that of the bandwidth of the excitation laser. In order to aid in the design of such experiments, expressions are derived for the temporal and spatial overlap of a Fourier transform limited input Gaussian beam with the TEMmn modes of the cavity. The expressions we derive for the spatial mode overlap coefficients are of general interest in applications where accurate mode matching to an optical cavity is required. © 1996 American Institute of Physics.
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The high sensitivity of cavity ring-down spectroscopy has allowed us to observe a few perpendicular vibrational overtone transitions of HCN in the visible. These transitions display a sizable Herman–Wallis effect, that is an asymmetry in the relative intensities of the R and P branch lines. We have developed a theory for the first-order Herman–Wallis effect based upon using variational vibrational wave functions but treating the vibration–rotation interaction by first-order perturbation theory. In the specific case of perpendicular transitions, the first-order effect is dominated by Coriolis mixing of Σ and Π overtone states. We used the empirical energy surface by Carter, Mills, and Handy [J. Chem. Phys. 99, 4379 (1993)] restricted to the stretching degrees of freedom. Bending was included by multiplication of these stretching wave functions by harmonic wave functions of the bend. Vibrational transition moments were calculated using a polynomial surface fit to ab initio CCSD(T) dipole moment points by Botschwina et al. [Chem. Phys. 190, 345 (1995) and private communication]. We expected that this treatment would be accurate but the calculated Herman–Wallis effect is about one order of magnitude too large. To gain further insight into the poor agreement between theory and experiment, we have calculated the sensitivity of the Herman–Wallis coefficient and of the transition moment to the dipole and energy surface parameters. From this, it appears that the dipole surface, while producing accurate band intensities, could at the same time be inadequate to account for the Herman–Wallis effect. A similar possibility stands for the energy surface, which however is highly constrained by the requirement to fit the observed band origins. © 1996 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 633-642 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper reports the results of our use of Cavity Ring Down Spectroscopy to extend the study of highly excited vibrational states in HCN. We extend our previous study of the H12C14N isotopomer, reporting on some weaker bands between 17 500 and 19 500 cm−1. We also report spectra of overtone and combination bands with six, seven, and eight quanta of stretching vibration in the isotopomers H12C15N and H13C14N in the interval from 17 500 to 23 000 cm−1. The observed spectroscopic constants and band intensities are compared with calculated values. All but one of the observed bands can be fit to within experimental accuracy (∼0.02 cm−1) to the standard distortable–rotor Hamiltonian. The one perturbed band has been successfully analyzed in terms of three anharmonically coupled levels. An anomalous line intensity distribution has been observed in the 1115 and 0116 Π←Σ bands, which we believe is produced by Coriolis coupling. © 1995 American Institute of Physics.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microwave detected, microwave-optical double resonance was used to record the A˜ state electronic spectrum of NH3, NH2D, and NHD2 with both vibrational and rotational resolution. To investigate ND3 with the same resolution as we had with our hydrogen containing isotopomers, a strip-line cell was constructed allowing the simultaneous passage of radio-frequency and ultraviolet radiation. Rotational constants were obtained as a function of ν2 excitation and an A˜ state equilibrium bond length was estimated at 1.055(8) A(ring). In addition, the harmonic force field for the A˜ state has been experimentally determined. fhh, fαα−fαα', and frr were found to be 1.06(4) aJ/A(ring)2, 0.25(2) aJ, and 4.9 aJ/A(ring)2, respectively. This calculated harmonic force field predicts that the asymmetry observed in the NH3 24 band is due to a strong anharmonic interaction with the 43 level and the broad feature observed in the dispersed fluorescence spectrum previously assigned to the 11 band is more likely attributable to the 42 level. © 1995 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 747-749 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have fabricated a two-dimensional photonic band structure based on macroporous silicon with a gap common to both polarizations and centered at 5 μm. A triangular lattice of circular air rods with a lattice constant of 2.3 μm was etched 75 μm deep in an n-type silicon substrate by electrochemical pore formation in hydrofluoric acid. The porous layer was then micromechanically structured in such a way that 200 μm thick free-standing bars of porous material were left over on the silicon substrate. These bars were then used for measuring the transmission of the photonic lattice. The results showed an excellent agreement with the theoretically calculated structure. © 1996 American Institute of Physics.
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