Resonant SFG Line Shapes on Resonant SFG Line Shapes on Single Crystal SurfacesSingle Crystal Surfaces

Scott K. Shaw,Scott K. Shaw, A. Laguchev, D. Dlott, A. A. Laguchev, D. Dlott, A. Gewirth Gewirth

Department of ChemistryDepartment of Chemistry

University of Illinois at Urbana-ChampaignUniversity of Illinois at Urbana-Champaign

63rd OSU International Symposium on Molecular 63rd OSU International Symposium on Molecular

SpectroscopySpectroscopy

Friday, June 20Friday, June 20thth 2008 - Columbus, Ohio 2008 - Columbus, Ohio

• SFG spectra can reflect simple or

complex line shapes

• Visual interpretation and fitting analysis

can be difficult

• How can we predict/control a desired

amount of derivative phase behavior?

Schultz, Z.D. JACS. 2005, 127,(45). Shaw, S.K. J Electroanal Chem. 2007, 609, (2).

Complex SFG SpectraComplex SFG Spectra

H2O H2O

D2O

Air

A non-linear, second order, optical process

Broad Band IR pulse covers ~ 200 cm-1 window

IR combines with narrowband visible pulse at interface

Sum frequency photons generated at break in symmetry

Sensitive to relative orientation of the vibrational transition

2

1+2

Surface

+

-

+

-

+

-

+

-

+

-

+

-

+ -

+-

+-

+-

+

-

+ -

+-

+

-

+

-1

SFG CharacteristicsSFG Characteristics

SFG CharacteristicsSFG Characteristics

800 nm Vis

femtosecond laser

1 kHz, 120 fs

2.0 mJ, 800 nm

IR

OPA

Fa

bry

-Pe

rot

éta

lon

sp

ec

tro

gra

ph

CCD

short pass filter

ps vis

BB IR

sample

delaySFG

Cartoon of optics layout for

BB-SFG

J. A. Carter. J Phys Chem A. 2008, 112(16).

A non-linear, second order, optical process

Broad Band IR pulse covers ~ 200 cm-1 window

IR combines with narrowband visible pulse at interface

Sum frequency photons generated at break in symmetry

Sensitive to relative orientation of the vibrational transition

Parameters of resonant transitions are extracted by fitting SFG signal to the equation:

Gaussian IR profile

Contribution from resonant transitions

Non-resonant background

202

( ) 22 2(2) (2) (2)

IRni

nSFG IR NR R NR

n IR n n

A eI const e

i

Phase factor

SFG CharacteristicsSFG Characteristics

• Energy of Incident Radiation

• Substrate Material

• Angle of Radiation Incidence

• Media above sample (solvent)

• Applied Potential

• Azimuthal Rotation

• Temporal Overlap

Top: Octadecanethiol on Au in 532 and 1064 nm radiation

Bottom: Octadecanethiol on Ag in 532 and 1064 nm radiation

Potterton. Bain. J. Electroanal Chem. (409) 1996.

SFG CharacteristicsSFG Characteristics

Varying magnitude of XNR is

directly related to changing resonant line shapes

• Yeganeh et al. have reported varying line shape as a function of rotation

• They report both non-resonant and resonant intensity changes• What are possible sources of resonant intensity change? • What is phase term doing?

SFG CharacteristicsSFG Characteristics

Above: single CH3 resonance from alkanethiol on Au(111)

Left: (top) changing resonant intensity and (bottom) changing non-resonant intensity with rotation

Yeganeh. Phys Rev Lett. 74(10) 1995.

Resonant

Non-resonant

1. Use BB-IR SFG to examine a single vibrational transition– Cyanobenzenethiol on Ag Single crystals

2. Examine changes in SFG spectra with sample rotation

3. Explain this dependence

Experimental Set-upExperimental Set-up

SH

N

• Rotation of the sample induced drastic changes in SFG signal line shape

• Non-resonant and phase terms show periodic oscillations

Vibrational Wavenumber (cm-1)

SF

G In

ten

sity

(A

.U.)

000 degrees

050 degrees

040 degrees

030 degrees

020 degrees

010 degrees

SFG Anisotropy Data Ag (111)SFG Anisotropy Data Ag (111)

Vibrational Wavenumber (cm-1)

SF

G In

ten

sity

(A

.U.)

000 degrees

100 degrees

080 degrees

060 degrees

040 degrees

020 degrees

SFG Anisotropy Data Ag(110)SFG Anisotropy Data Ag(110)

• Rotation of the sample induced drastic changes in SFG signal line shape

• Non-resonant and phase terms show periodic oscillations

Azimuthal Rotation (degrees)

Ph

ase

()

)2( NR

• Phase and non-resonant parameters for thiolated faces of Ag

• Three-fold and two-fold symmetry patterns

• Similarities to SHG – red lines are fits to equation:

2))(3cos())(2cos()cos( dDcCbBAI NR

Bilger, C. Pettinger B. Chem. Phys. Lett. 1998, 294, (4,5).

SFG Anisotropy DataSFG Anisotropy Data

• Overlay of non-resonant SFG with SHG for bare surfaces

• Clear resemblance of (111) face

• More complicated in (110) face… (reconstruction and (100) oxides)2

))(3cos())(2cos()cos( dDcCbBAI NR

Comparison to SHG ResponseComparison to SHG Response

Azimuthal Rotation (degrees)

Rel

ati

ve

SF

G I

nte

nsi

ty

SFG and SHG response for (111) surface

SFG and SHG response for (110) surface

Bilger, C. Pettinger B. Chem. Phys. Lett. 1998, 294, (4,5). Georgiadis, R. Richmond G.L. J. Phys. Chem. 1991, 95, (7).

SHG of Ag(111) (+)

SFG of Ag(111) ( )

Dashed line is fit to equation

SHG of Au(110) (O)

SFG of Ag(110) ( )

As visible beam is delayed, less sampling of the NR response is up-converted to SFG

Delay of Visible Pulse (ps)

SF

G In

ten

sity

-0.5 0.0 0.5 1.0 1.5 2.0

-1.0

-0.5

0.0

0.5

1.0

amp

litu

de

(arb

)

PIRNR(t)

PIRR(t)

ps vis at different temporal positions

Time Delay Scheme for BB-SFG

Temporal Delay: Scheme and EffectsTemporal Delay: Scheme and Effects

SFG response as function of vis beam delay

J. A. Carter. J Phys Chem A. 2008, 112(16).

Lower non-resonant contribution eliminates phase term

-0.5 0.0 0.5 1.0 1.5 2.0

-1.0

-0.5

0.0

0.5

1.0

amp

litu

de

(arb

)

PIRNR(t)

PIRR(t)

ps vis at different temporal positions

Time Delay Scheme for BB-SFG

Temporal Delay: Scheme and EffectsTemporal Delay: Scheme and Effects

J. A. Carter. J Phys Chem A. 2008, 112(16).

2)2()2()2()2(2)2()2( iRNRRNRRNRSFG eI

• CBT decorated Ag surfaces as a function of temporal overlap• Constant azimuthal angle maintained• Drastic changes in resonant line shape• Decreasing intensity of non-resonant term

Vibrational Wavenumber (cm-1)

Re

lati

ve

SF

G In

ten

sit

y No delay

~ 1.5 ps delay

~ 3.0 ps delay

No delay

~ 1.9 ps delay

~ 3.7 ps delay

Temporal Delay DataTemporal Delay Data

Ag (111) Surface

Ag (110) Surface

• Rotational data with a temporal delay to suppress non-resonant term • Line shape changes with respect to rotation are absent• Changing line shape is definitely associated with the non resonant term

Vibrational Wavenumber (cm-1)

Re

lati

ve

SF

G In

ten

sit

yTemporal Delay DataTemporal Delay Data

Ag (111) Surface

Ag (110) Surface

• SFG from single crystal surfaces is azimuthally dependent

• Can minimize non-resonant response to simply resonant line shapes

• Allows more consistent approach to future vibrational SFG studies

• Will simplify analysis of SFG spectra

• Explains discrepancies in previous data

Conclusions: Conclusions:

• Jonathan Arambula (synthesis of CBT)

• Alexi Lagutchev, Dana Dlott, Andrew Gewirth*

• Mauro Sardela (X-ray work)

•

– DMR 050438

– CHE-06-03675

• Air Force Office of Scientific Research

– FA9550-06-1-0235

Acknowledgements: Acknowledgements: