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  • 1
    Electronic Resource
    Electronic Resource
    Laser heating effects in the characterization of carbon fibers by Raman spectroscopy (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 3598-3608 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The first-order Raman spectra of individual 8-μm-diam PAN-derived carbon fibers which had been annealed at five temperatures from 1700 to 2800 °C are measured as a function of incident laser power from 1 to 140 mW. In all fibers studied, the Raman frequencies of the graphite G band at ca. 1580 cm−1 and the disordered-induced D band at ca. 1360 cm−1 shift to lower frequency with increasing laser power. The largest shifts observed before the fiber is physically damaged are about 13 cm−1 at a laser power of 30 mW. The band positions decrease further at higher laser power, up to a maximum of about 20 cm−1 at 40 mW, and at powers above 30 mW the linewidths and the ID/IG intensity ratio change irreversibly as the fiber begins to erode. The irreversible changes extend several hundred μm away from the 2-μm spot illuminated by the laser. The effects are attributed to laser heating. To quantify the degree of laser heating the temperature dependence of the Raman shift (G band) in pure bulk, highly oriented pyrolytic graphite (HOPG) is measured. To study the heating effect in more detail, a Raman imaging experiment is carried out in which a 0.1-mm spot on a fiber is heated by a focused laser while the fiber is illuminated for Raman characterization along a 1.6 mm length by a low power probe laser. The spatially resolved Raman shifts obtained in this way are combined with the HOPG frequency-temperature calibration to obtain the in situ temperature profile of the laser heated fiber. The measured temperature profile along the fiber is in excellent agreement with a simple convective heat-transfer model. Assuming that the temperature dependence of the G-band position is the same in the fibers as in bulk graphite, the present experiments show that a laser power of 30 mW heats an 8 μm fiber to 330 °C and that above 330 °C irreversible changes are produced by erosion of the fiber. Unperturbed room-temperature Raman frequencies for the five groups of fibers are obtained by extrapolating to zero laser power.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.346320
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Raman intensities and interference effects for thin films adsorbed on metals (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2067-2076 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Large oscillations in the vibrational Raman intensity have been observed for thin films adsorbed on a metal substrate as a function of film thickness. The systems studied, N2 and O2 physisorbed on Ag(111) at 14±1 K, have previously been shown to be unenhanced, i.e., the observations are not caused by the surface enhanced Raman (SERS) effect. The observed oscillations are due to multiple beam interference caused by reflections at the film boundaries of the incident laser light, and, to a lesser extent, the Raman scattered light. We extend the two-dimensional theoretical development of Moscovits and co-workers for Raman scattering in thin films to include more general three-dimensional experimental geometries. We derive expressions for the total intensity as a function of film thickness, incident laser polarization and angle, and scattered light polarization and angle. The resultant expressions are applicable to Raman scattering from any dielectric thin film adsorbed on a substrate with known optical parameters. Although complicated, the general expressions have been numerically evaluated for our experimental conditions using a commercial mathematical package on a personal computer. The calculated Raman intensities are in excellent agreement with the experimental results. The observed period of the intensity oscillation relates the film thickness to the exposure allowing the sticking probabilities to be determined. The sticking probabilities of N2 on N2 and O2 on O2 at 14±1 K are 0.84±0.2 and 0.82±0.2, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458040
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  • 3
    Electronic Resource
    Electronic Resource
    Ultrahigh-vacuum apparatus for surface spectroscopy and characterization at cryogenic temperatures (1989)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 1161-1166 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An ultrahigh-vacuum apparatus is described that enables the study of molecular adsorbates on well-defined single crystals at controlled temperatures from 14 to 900 K. Sample cooling is achieved with a closed-cycle helium refrigerator coupled to the sample holder via a flexible copper cable. This configuration maintains a 3-K temperature differential from the 11-K cryocold head to the sample and damps vibrations from the cyrocold head. The cyrogenic sample manipulator allows X, Y, Z translations, tilt, and rotation of an arm holding the sample through an arc of 270° about the vertical central axis of the vacuum chamber. The sample, mounted 12.7 cm from the central axis, may also be rotated 90° about a vertical axis through the sample. Rotation of the sample about a working circle radius of 12.7 cm allows the sample to be accessed for surface cleaning, characterization, and study by instrumentation whose physical size does not allow access by sample rotation about the chamber central axis alone. In our application, unenhanced surface Raman spectroscopy, low-energy electron diffraction, and Auger spectroscopy are carried out while maintaining a controlled sample temperature. Laser entry ports at two incident angles permit the sutdy of substrates with differing optical properties. Unenhanced surface Raman spectra of nitrogen physisorbed on a silver (111) surface are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1140278
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  • 4
    Electronic Resource
    Electronic Resource
    Unenhanced surface Raman spectroscopy of nitrogen physisorbed on silver(111) (1989)
    s.l. : American Chemical Society
    Langmuir 5 (1989), S. 633-639 
    Details
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/la00087a015
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Effect of J dependence of Raman cross sections on CARS product quantum state distributions for the H+D2 → HD+D reaction (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 1605-1608 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: HD product quantum state distributions are recalculated for published CARS measurements using J-dependent Raman cross sections determined from accurate, ab initio calculations. The use of J-dependent cross sections lowers calculated number densities by 5%–10% at high J and therefore systematically shifts the relative product-state distribution somewhat from high J to low J. This shift does not noticeably affect discrepancies between CARS results and multiphoton ionization experiments on the same reaction. The results indicate that J-dependent Raman cross sections should be used in reducing vibrational Q-branch CARS and other Raman data for molecular hydrogen and its isotopes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449397
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    Paper (German National Licenses)
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