Raman spectroscopy of organic molecules physisorbed onto graphitic carbon surfaces

• Main Author: Kagan, Mark Richard
• Format: Dissertation
• Language: English
• Subjects: chemistry, analytical; chemistry, physical
• ISBN: 9780591050684; 0591050684

The utility of Raman spectroscopy for studying organic molecules physisorbed onto various graphitic carbon surfaces was explored. A solution phase procedure was used to deposit the adsorbate molecules. The majority of the adsorbate spectra were acquired under ambient atmospheric conditions. The adsorbate molecules studied were $\beta$-carotene, 2,6-anthraquinonedisulfonate (AQDS), Rhodamine 6G (R6G), 1,4-bis(2-methylstyryl)benzene (BMB), and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazol (BPBD). The graphitic carbon surfaces studied were highly order pyrolytic graphite (HOPG), pyrolytic graphite, a spectroscopic graphite rod, glassy carbon, and an activated carbon pellet. Adsorbate surface coverages on glassy carbon were determined from adsorption isotherms, constructed by plotting adsorbate Raman signal intensities as a function of solution phase concentration. The detection limits for strongly resonance enhanced Raman features was less than 1% of a monolayer. For Raman features with solution phase scattering cross sections comparable to that of the 992 cm$\sp{-1}$ band of benzene, the detection limits were about one monolayer.
A procedure was developed for the determination of adsorbate Raman scattering cross sections. These values were compared with solution phase values. For the nonresonant (or preresonant) scatterers BMB, BPBD, and AQDS; the adsorbate Raman scattering cross sections were enhanced by a factor of $\sim$3 relative to the corresponding solution phase values. For the resonant Raman scatterer $\beta$-carotene, the adsorbate cross sections were significantly reduced relative to solution phase values. Partial Raman excitation profiles for solution phase and adsorbed $\beta$-carotene were also determined. The adsorbate profile was significantly flattened, and possibly red-shifted, relative to the solution phase profile.
The ability of graphitic carbon surfaces to quench the fluorescence of adsorbed molecules was explored with the goal of estending the utility of Raman spectroscopy to a wider range of chemical systems. The effect of graphitic carbon surface roughness and microporosity on adsorbate Raman signal intensities was also explored. Finally, the potential use of adsorbate Raman spectra as probes of various graphitic carbon properties was investigated.