Raman Spectroscopy and Thin Films

1. Raman Spectroscopy and Thin Films Alexander Couzis ChE5535

2. Principle of Raman Spectroscopy Incident Light Scattered Light Sample Raleigh Scatter (same wavelength as incident light) Raman Scatter (new wavelength)

3. Raman Scattering: Classical Description Consider a diatomic molecule: If light with an electrical field E=E0coswt is incident on this molecule, the molecule will oscillate. The induced dipole moment m, is then given by: a, is the polarizability of the molecule, and it is a function of the separation between the atoms. If x is the displacement from the equilibrium separation between the atoms we can expand the polarizability about x0, the equilibrium separation:

4. Raman Scattering: Classical Description The molecule also vibrates at its resonant (natural) vibration frequency w0, so that x = acoswt and substituting we get: Since the dipole radiates light at its oscillating frequency, the molecule will emit light at three frequencies, w, w-w0, and w+w0. w-w0 Stokes frequency w+w0 Anti-Stokes frequency

5. Principle of Raman The polarizability depends on how tightly the electrons are bound to the nuclei. In the symmetric stretch the strength of electron binding is different between the minimum and maximum internuclear distances. Therefore the polarizability changes during the vibration and this vibrational mode scatters Raman light (the vibration is Raman active). In the asymmetric stretch the electrons are more easily polarized in the bond that expands but are less easily polarized in the bond that compresses. There is no overall change in polarizability and the asymmetric stretch is Raman inactive.

6. Experimental Set-Up Filters

9. Integrated Optics Raman Spectroscopy

10. IO-Raman on Thin Films • Rabe, J. P., J. D. Swalen, et al. (1978). “Order-Disorder transitions in Langmuir-Blodgett Films. III. Polarized Raman Studies of Cadmium Arachidate Using Integrated Optical Techniques.” J. Chem. Phys.86(3): 1601-1607. • Rabolt, J. F., R. Santo, et al. (1979). “Raman Spectroscopy of Thin Polymer Films Using Integrated Optical Techniques.” Applied Spectroscopy33(6): 549-551. • Rabolt, J. F., R. Santo, et al. (1980). “Raman Measurements on Thin Polymer Films and Organic Monolayers.” Applied Spectroscopy34(5): 517-521. • Rabolt, J. F., N. E. Schlotter, et al. (1981). “Spectroscopic Studies of Thin Film Polymer Laminates Using Raman Spectroscopy and Intergated Optics.” J. Phys. Chem85: 4141-4144. • Rabolt, J. F., N. E. Schlotter, et al. (1983). “Comparative Raman Studies of Molecular Interactions at a Dye/Polymer and a Dye/Glass Interface.” Journal of Polymer Science: Polymer Physics Edition21: 1-9. • Schlotter, N. E. and J. F. Rabolt (1984). “Measurements of the Optical Anisotropy of Trapped Molecules in Oriented Polymer Films by Waveguide Raman Spectroscopy (WRS).” Applied Spectroscopy38(2): 208-211. • Schlotter, N. E. and J. F. Rabolt (1984). “Raman Spectroscopy in Polymeric Thin Film Optical Waveguides. 1. Polarized Measurements and Orientational Effects in Two-Dimensional Films.” J. Phys. Chem.88: 2062-2067.

11. Internal Reflection Raman Iwamoto, R., M. Miya, et al. (1981). J. Chem. Phys.74(9): 4780-4790.

12. Surface-enhanced Raman scattering Light incident on a molecule can lose energy to a vibrational mode and be scattered at a lower frequency. The Raman signal normally has a small cross section but systems comprising rough silver surfaces with adsorbed molecules show huge enhancements to the cross section. The effect only works for rough surfaces, and only for metals whose conductivity is very high. The effect is believed to be due to surface resonances enhancing the local intensity of the light, but detailed quantitative calculations have only recently become meaningful with the availability of a theory for photonic materials, and the experimental possibility of making well characterised ordered arrays of metallic nanospheres.

13. Raman at the Air-Liquid Interface

14. DDPA Stearic Acid 52 mN/m 35mN/m 25 mN/m

15. Head Group Structure