Sum Frequency Generation in a Co-Sum Frequency Generation in a Co-Propagating Beam Geometry from Propagating Beam Geometry from

Thin FilmsThin Films

Lei LiLei LiPhD Candidate, Department of Physics and AstronomyPhD Candidate, Department of Physics and Astronomy

& The Forest Bioproducts Research Institute, The University of Maine& 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 Thickness/Phase Effect In SFS Spectra( 3 SF beam theoretical model)theoretical model)

The phase and intensity of the SF signal arising from thin cellulose films on 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 a) water/cellulose/gold interface. interface. IR=2878cmIR=2878cm-1-1(r+)(r+)

Dominant Periodicity=268nmDominant Periodicity=268nmMinor Periodicities=296nm, 2857nmMinor Periodicities=296nm, 2857nm

b) water/cellulose/goldb) water/cellulose/gold interface.interface. IR=2966cmIR=2966cm-1-1(r-)(r-)

c) air/cellulose/gold c) air/cellulose/gold interface. interface. IR=2878cmIR=2878cm-1-1(r+)(r+)

d) air/cellulose/gold d) air/cellulose/gold interface. interface. IR=2966cmIR=2966cm-1-1(r-)(r-)

Dominant Periodicity=267nmDominant Periodicity=267nmMinor Periodicities=294nm, 2857nmMinor Periodicities=294nm, 2857nm

Dominant Periodicity=238nmDominant Periodicity=238nmMinor Periodicities=248nm, 6024nmMinor Periodicities=248nm, 6024nm

Dominant Periodicity=237nmDominant Periodicity=237nmMinor Periodicities=247nm, 6024nmMinor Periodicities=247nm, 6024nm

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

210-6 410-6 610-6 810-6 0.00001

1

2

3

4

210-7 410-7 610-7 810-7 110-6

0.5

1

1.5

2

2.5

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)Periodicity(nm) SF Intensity(arb)SF Intensity(arb)

6024.16024.1 0.8600620.860062

2551.022551.02 0.4383710.438371

1798.561798.56 16.439416.4394

1392.761392.76 7.268527.26852

342.231342.231 7.86517.8651

302.115302.115 0.3444390.344439

287.853287.853 21.765821.7658

274.876274.876 8.64498.6449

248.26248.26 0.4841840.484184

238.55238.55 18.920218.9202

d(nm)

d(nm)

Nor

mal

ized

SF

Inte

nsity

Nor

mal

ized

SF

Inte

nsity

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