Sum Frequency Generation in a Co- Propagating Beam Geometry from Thin Films Lei Li PhD Candidate, Department of Physics and Astronomy & The Forest Bioproducts Research Institute, The University of Maine

 Sum Frequency Spectroscopy (SFS) is a surface specific non-linear pulsed laser technique capable of providing detailed molecular level orientation & conformational information of interfacial species.  Application of the technique to thin film structures may result in complex spectra which require deconvolution.  In the co-propagating beam geometry, three periodicities describing the relationship between film thickness & SF intensity are found.

Thickness/Phase Effect In SFS Spectra( 3 SF beam theoretical model) The phase and intensity of the SF signal arising from thin cellulose films on reflective substrates (gold) is a function of the cellulose film thickness. reflective substrates (gold) is a function of the cellulose film thickness. a) water/cellulose/gold interface. interface. IR=2878cm -1 (r+) IR=2878cm -1 (r+) Dominant Periodicity=268nm Minor Periodicities=296nm, 2857nm b) water/cellulose/gold interface. interface. IR=2966cm -1 (r-) IR=2966cm -1 (r-) c) air/cellulose/gold interface. interface. IR=2878cm -1 (r+) IR=2878cm -1 (r+) d) air/cellulose/gold interface. interface. IR=2966cm -1 (r-) IR=2966cm -1 (r-) Dominant Periodicity=267nm Minor Periodicities=294nm, 2857nm Dominant Periodicity=238nm Minor Periodicities=248nm, 6024nm Dominant Periodicity=237nm Minor Periodicities=247nm, 6024nm

 If further sources of SF generation are considered, seven additional periodicities can be simulated.

Air/cellulose/gold interface, IR=2878cm -1, r+ vibrational mode  Applying the 5 SF beam theoretical model, we can describe the cellulose thickness/phase effect more explicitly.  The three periodicities (red) which are also derived from the 3 beam model are predicted by the 5 beam model, although they do not have the highest intensities.  We have not accounted for beam attenuation which will decrease SF intensity.  Substrates of well defined thickness are presently being prepared for experimental verification of the model’s predictions.Periodicity(nm) SF Intensity(arb) d(nm ) Normalized SF Intensity Normalized SF intensity on resonance versus cellulose thickness

Funding  NSF Forest Bioproducts Research Initiative of The University of Maine  The University of Maine Graduate School Acknowledgements : Advisor Dr. David Neivandt Dr. Michael Mason Postdoctoral Dr. Joerg Fick Fellow Graduate Student Sarah Sterling