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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5313-5315 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν3 (O–Si stretch) fundamental bands of the HOSi+ and DOSi+ molecular ions in the 9 μm region have been detected for the first time, using a tunable infrared diode laser spectrometer and a hollow cathode discharge cell. Analysis of the results yielded accurate values for the molecular rotational and centrifugal distortion parameters, as well as for the band origins, which are 1127.009 cm−1 for HOSi+ and 1103.112 cm−1 for DOSi+ . The ground vibrational state parameters are in excellent agreement with those determined from the ν1 bands of the two isotopes.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7273-7286 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Precise measurements of the frequencies of a number of new rotational transitions of a 3Π CO (v=0–4) in the range 250–500 GHz are reported. In addition earlier measurements of the J=0–1 frequencies near 90 GHz have been repeated at higher precision and extended to include the v=6 lines. The nonlinear least-squares analysis of this microwave data combined with the lambda doublet transition frequencies of Klemperer and co-workers was carried out for v=0–4 by direct diagonalization of a Hamiltonian matrix that included the relevant v subblock of the a 3Π state along with the subblocks for v'=0–10 of the a' 3Σ+ state. The new frequencies make possible the determination from the rf-microwave data set of several parameters that previously had to be assumed, and the method of treating most of the a–a' perturbations directly in the large matrix yields parameters, especially lambda doubling constants, which vary less, but more smoothly, with J than before. The later smoothness is found to be enhanced by inclusion of an off-diagonal, spin–spin interaction in the a–a' perturbation Hamiltonian, as was done in published fits of FTIR spectra of the a' 3Σ+–a 3Π system. We also discuss the theory of the perturbed Zeeman effect of a 3Π CO, along with some preliminary measurements, and report a value of the pressure broadening parameter for a typical rotational transition in this state.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5310-5312 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν1 (H–O) stretches of HOSi+ and DOSi+ have been observed with the difference frequency laser system in the hollow cathode discharge. The ions were observed either by discharge of (CH3)3SiOH or (CH3)3SiOD in a buffer containing H2 (or D2) and He, or by discharging SiH4 and N2O in a buffer of H2 or H2 and He. Discharging SiH4 and H2O in buffers of H2 and H2 and He was not successful in producing HOSi+. The constants obtained were: ν0=3662.364 67(15) cm−1, B000=18 260.75(22) MHz, D000=20.44(31) kHz, B100=18 195.05(23) MHz, and D100=19.91(34) kHz for HOSi+. For DOSi+ we obtained: ν0=2716.558 13 (12) cm−1, B000=16 231.05(24) MHz, D000=15.69(38) kHz, B100=16 154.41(22) MHz, and D100=15.60(33) kHz. These values agree well with the theory. The combination of the two B0 values results in r0(H–O)=0.940 A(ring) and r0(O–Si) =1.537 A(ring). The differences between these values and the calculated equilibrium values are similar to the corresponding results for HOC+.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 4008-4013 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using classical elasticity theory, the rise in free energy upon crosslinking nematogenic polymers into a network is calculated for the isotropic and nematic phases. Spontaneous strains are allowed for in the network. The consequence of network formation upon nematic–isotropic equilibria is calculated by adding these elastic contributions to a conventional Landau theory. Memory of the crosslinking conditions yields quartic and quadratic additions to the standard Landau theory. We find that crosslinking in the isotropic state lowers the nematic–isotropic phase transition temperature compared with the unlinked case and the application of suitable stress raises it again. Crosslinking in the nematic state raises the transition temperature. We recover the mechanical critical point proposed long ago by de Gennes. Our Gaussian theory encompasses both main- and side-chain polymers. The hairpin limit for main chain networks yields a modulus varying exponentially with temperature. The Landau–de Gennes free energy for comb polymers is presented for the first time.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 1308-1313 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new microprocessor-controlled multichannel spectrometer for the measurement of fluorescence detected circular dichroism (FDCD) is presented. Spectra of model steroid compounds are provided to demonstrate the analytical utility of multichannel FDCD over the classical circular dichroism measurement. A detection limit of 3.05×10−8 M was determined for a solution of estrone based on a signal/noise ratio of 2 as a lower limit. The optics and electronics for the rapid acquisition of multichannel FDCD data are described in detail.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 3640-3642 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The performance of a high-pressure xenon arc lamp is enhanced by application of a combination of two techniques which (1) reduces noise due to arc wander and (2) increases the lamp intensity. Fluctuations in lamp intensity due to arc wander are minimized by the superimposition of an alternating current (ac) on the direct-current (dc) source voltage in conjunction with detector averaging. Lamp intensity is increased by the application of a static magnetic field to the arc plasma. Arc stability and intensity are monitored indirectly by measuring arc-excited Rhodamine B fluorescence. Data illustrating the effects of each technique individually and in combination are presented. Lamp output is typically maintained to less than 1% relative standard deviation while intensity is increased by as much as 127%.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 90-95 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A simple, versatile power supply designed to control xenon flashlamps is described. The circuit is configured to utilize an N-channel enhanced metal-oxide-semiconductor field effect transistor (MOSFET), taking the place of commonly used triggering devices such as a silicon controlled rectifier (SCR), or triac. Incorporation of the MOSFET permits flexible control of flash duty cycle and frequency by applying one signal to the transistor's gate. The circuit's versatility is demonstrated using an 8.3-W xenon flashlamp as an excitation source for a photodiode array based HPLC fluorescence detector. A detailed description of the power supply circuit is given, along with information on interfacing the circuit and the photodiode array.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 5514-5517 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Low-resistivity contacts to bulk high Tc superconductors have been prepared by resistive evaporation with values of ρs(Ω cm2) as low as 10−10 (77 K) for Ag/YBaCuO, 10−9 (77 K) for Ag/TlPbBaCaCuO, and 10−7 (60 K) for Ag/BiCaSrCuO. ρs has been found to be improved by increasing the length of a preevaporation step. This effect has been further investigated by secondary ion-mass spectrometry analysis on several Ag/Si samples. Both three-terminal and four-terminal methods have been used to determine ρs; the three-terminal method shows less dependency on the sample/contact geometry for measurements taken above Tc.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 1545-1547 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An automated device for determination of the kinetics of drug delivery from colloidal latex particles is described. The system utilizes ultrafiltration and spectrophotometric techniques simultaneously, allowing rapid, in situ, drug release profiles to be obtained without need for removal of the dispersed phase. The device has been used successfully in our laboratory for numerous drug delivery experiments.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 945-949 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An optical multichannel analyzer system for obtaining high-quality spectra of intermediates formed by nanosecond laser photolysis is described. The system is useful for samples which undergo irreversible photochemistry and which are available in limited amounts. Sample volumes as small as 25 μl are possible and a syringe pump driven by a linear stepper motor is used to deliver fresh sample after each photolysis pulse. The microchannel-plate intensified detector used here in gated mode can collect an entire spectrum in 5 ns, but proper synchronization of the detector scans is required to avoid severe distortion which can appear as random noise in signal averaging. Noise from various sources such as phosphor lag in the detector, time jitter, source intensity fluctuations, and the photon character of light are discussed. Experimental design for noise reduction is described including diagnostic procedures and light source filtering to produce more uniform signal-to-noise ratio throughout the spectrum. Finally, a comparison is made between the noise observed to occur between detector channels and the noise predicted to occur based on the photoelectron gain of the intensifier. The disagreement between these is discussed in terms of the spatial resolution of the detector.
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