Quantitative Characterization of Materials

Vibrational Spectroscopies

Copyright 2002-2011 Michael J. Kelley

Motivation - Why do This?Molecular information per se is important, a key to working with organic materials Information is needed under conditions native to the materials, e.g., not UHV Need techniques that do not alter materials Need techniques that are easy to use, easy to interpret and affordable

Vibrational Spectroscopy Overview -1Solid structure: ball and stick rigid bonds of fixed length Molecular structure: mass and spring deformable bonds, equilibrium length Bonds have characteristic vibrational frequencies in the range of IR light IR absorption spectroscopy identifies types and amounts of bonds present

IR absorption frequencies Mass and spring

Vibrational Spectroscopy Overview -2IR absorption spectroscopy sees the molecular bonds as such Energy deposition is very low - usually innocuous Less than a tenth the cost of other instruments; use in ambient or nitrogen atmosphere More than 50 years of wide use We have unique resources

IR absorption frequenciesThe bonds connecting atoms in a molecule may be described by a potential: bond energy vs separation Displacements near equilibrium are quite similar to a spring/mass system: simple harmonic oscillator (SHO) Solve the SHO problem for the permitted vibrational energy levels

IR absorption frequencies: SHOLowest energy (ground) state is non-zero Under ambient conditions, only the ground state is populated States are equally spaced in energy Only permitted transitions are single-level - dipole selection rule

Dissimilar vibrational modes couple poorly: characteristic group frequencies

Energy units: cm-1. 8066 cm-1 = 1 eV The number of waves in a cm

Intensity of Absorption -1Atomic bonds may include charge transfer e.g., -C=O appears in many organics The separation d of the centers of positive and negative charge creates at dipole moment: Q = zd, where z = charge The oscillating electric field of the light E=Eoe-i[t alternately stretches and compresses the dipole, depositing energy into that bond Where there is no change of dipole moment, no energy is deposited - not IR active

Mathematical Description Normal CoordinatesDescription in terms of Cartesian coordinates quickly becomes unwieldy Instead use the basic vibrational motions: normal coordinates Any complex molecular motion can be described by a combination of them None of them can be described by any combination of the others

Intensity of Absorption -2Transition rate = constants J1(q) cr J0(q) dq, where q is the normal coordinate and cr is the dipole operator For integral to be non-zero, net symmetry must be even. Since cr is odd, only one of the J s can be odd: change only one vibrational level - the dipole selection rule. Larger Q results in larger rate; carbonyl is a strong absorber 2 X 103 cm-1 is a strong absorber in the IR

Transmission ExperimentsMeasure intensity vs wavenumber

I(R) = IoRe-EctE = extinction coefficient, c =concentration, t = thickness

Transmission or absorbance (ln of T, = optical density) presentation of the data Widely used for fingerprint spectra Require uniform thickness and composition, optimum thickness Also need a reference spectrum (without sample)

Adsorption of CO on Pd:SiO2R.P.Eischens & W.A.Pliskin; Adv.Catal. 10 (1958) 1-56.

IR Spectrum of Kapton Polyimide Film

Reflection ExperimentsSingle reflection at glancing angle RAIRS, IRRAS Repeated reflection between mirrors multiple specular reflectance- MSR Internal reflectance - ATR - attenuated total reflectance Diffuse reflectance - DRIFT

Optics of Reflection - 1The electric field vector of a light wave is perpendicular to the direction of travel. Plane polarized light:

Circularly polarized light has the vector pointed equally in all directions; linearly polarized has it all in one direction.

Optics of Reflection - 2When light encounters an interface between two media, we define a plane of incidence as the plane containing the propagation vector and a normal to the plane. We define the in-plane electric field component as parallel (sub p) and the other as perpendicular (sub s)

Optics of Reflection - 3A light wave propagates through a medium with speed v, characterized as a ratio to the speed in vacuum, the refractive index of the medium: n = c/v For light of wavelength P, absorption (loss) may occur characterized by the extinction coefficient E(P). The effect of loss on the refractive index is described by stating the index as the sum of real and complex parts: = n + iO , where O!EPT. Note also that is equal to the square of the dielectric constant .

Optics of Reflection - 4We require that the fields be continuous at the interface for both polarizations and get the Fresnel equations. The reflected fractions are:

Calculated Reflectance Ratios

From Harrick

RAIRS/IRRASGlancing Angle, Single Bounce

On metal surfaces, Es = 0

RAIRS Example - Coating TransferA polyester film coated with Bisphenol A containing 5% silicone was used as a release film. Did it transfer silicone ? RAIRS of stainless steel mirror pressed against the film compared to transmission Look for silicone - broad band at 11001200 cm-1

Multiple Specular ReflectanceRepeated reflections between opposed mirrors accumulate total absorption Angular alignment is less critical Metal surface selection rule, saturation

Comparison of MSR and RAIRS for a metal film having reflectance Ro with coating having reflectance R. A.Sondag and M.C.Raas; Appl.Spec 43 (1989) 107

Attenuated Total Reflectance (ATR)-1Light coming from a more dense medium deflects away from the surface normal. At the critical angle, the light propagates along the interface. Above the critical angle, it reflects back into the optically denser medium

Attenuated Total Reflectance (ATR)-2Because the electric field must be continuous at the interface, a small amount of intensity is present in the rarer phase: the evanescent wave. The penetration distance is given by: dp = [Pr/2T][sin2U -(n1/n2)2]-1/2n1

n2

where the wavelength is in the rarer medium

Materials for ATR crystals. R is loss to reflection when the IR beam enters the crystal.From: N.J.Harrick, Internal Reflection Spectroscopy, Wiley 1967

Attenuated Total Reflectance (ATR)-3absorption in the rarer phase causes light to leak in

From Harrick

ATR - Multiple Bounce

Material studied is pressed against the top surface

ATR Example PET Film AmorphizationPET film is about half crystalline, half amorphous. The free volume of the amorphous phase facilitates uptake of dyes, etc. Heating the surface to above the glass temperature and cooling within microseconds quenches the crystallization, leaving an amorphous layer Excimer laser and FEL accomplish the RTP

Characterization by ATR/FTIR Band AssignmentsCrystalline: 1341 cm-1 Amorphous: 1371 cm-1 Reference: 1410 cm-1

Penetration DepthKRS-5: 2.08 Qm Ge: 0.49 Qm

Polyethylene Terephthalate (PET) film

Further ATR ConfigurationsCylindrical element, conical ends. Easy to seal for gas, liquid-tight cell. High temperature, high pressure studies Micro-element on optical fiber. Insert as IR probe. Long fiber. Embed in (e.g.) composites to monitor curing. Small single bounce - ATR microscopy fibers, particles. Interface imaging

ATR Experimental IssuesGood contact of sample to crystal Absorption not too strong Correction for wavelength dependence Residues on crystal surface Alignment can be sensitive Micro-ATR is most SEM-like IR

ATR - semiconductor applicationY.J.Chabal; Surf.Sci.Repts.8(1988) 211.

ATR - Single Bounce

ATR Single Bounce Equipment

Monitor microbial fouling of water purification membrane

Effect of biocide on foulingP.R.Campbell et al. Biotechnol.Bioeng.64 (1999) 527

Diffuse Reflectance - DRIFTThe surface viewed consists of many randomly-oriented facets, e.g., mineral grains, roughened metal Surface is highly reflective - not organics. May add reflective material: KBr No specular component in reflected light Sensitive to sample packing

DRIFT ModelAbsorbance is small compared to scattering Sample thickness is infinite Ratio of scattered intensity from sample to that from perfect scatterer: Rg Remission function: F(Rg) = (1-Rg)2/2Rg F(Rg) = absorbance

DRIFT Set-ups

Interpretation of coal IR spectraP.C.Painter et al. Coal and Coal Products E.L.Fuller ed. ACS Symp 205 (1982) 47.

1% coal in KBr A.F.Gaines in Y.Yurum ed. New Trends in Coal Science (1988) 218

Use of SiO2 overtone absorbance to indicate temperature. S.Sharma et al. J.Catal.110 (1988) 103

DRIFT: Silanation StudiesWant to put oxides into polymers reinforcing fibers, modulus raisers, lowcost diluents, fire retardants Mineral/glass to polymer bonds aren t very stable against water Pre-treat minerals with coupling agent, usually a silane How to do it right/do it best ?

Clays - common fillers

IR MicroscopyMany important objects are on the microns to tens of microns size scale Examples include synthetic fibers, microbes, contaminant particulates and layered structures IR microscopy chiefly uses reflective - not refractive optics Optical efficiency is critical

IR Microscopy of Aramid FibersPatrick H. Young; Spectros.3(9)

Effect of orientation on fiber modulus

Apertureless Scanning Near-field Optical Microscope (SNOM)(source: Ed Gillman) Optical Antenna At sufficiently high frequencies any inhomogeneties exposed to a field becomes a source of radiation; an antenna. This radiation can be detected many wavelengths from its origin in the far-field. In the farfield the scattering observed depends on the on near-field zone that surrounds the source, its dielectric and magnetic properties and the mode of excitation.

Fourier Transform MethodsAn IR spectrum consists of the intensity measured at each wavelength in a range. Early IR instruments measured the intensity at each wavelength in succession - serial data collection These dispersive instruments traded sensitivity for resolution and wasted most of the signal Fourier transfrom instruments have replaced them: FTIR

Michaelson Interferometer FTIR

Mixing of three wavelengths gives intensity maximum when the inteferometers arms are the same length

Mixing of many wavelengths gives the interferogram centerburst . A Fourier transform gives the spectrum

Raman Spectroscopy MotivationIR spectroscopy requires that we generate and manipulate a white IR beam. Intense lab sources are a problem IR beam is not simple to focus and manipulate Some environments of interest are not IR transparent - water Some absorptions are too intense - metaloxygen fundamentals Some vibrations of interest are not IR-active

Raman Absorption MechanismLight polarizes a molecule vibrating in mode q: P = E Eosin [Et ; q = qosin [vt Unfortunately, E is used for polarizability also. The amplitude of the vibration affects the polarizability: E = Eo + (xE/xq) q (Taylor series expansion) Substituting above and using a trig identity for the product of sines and collecting terms gives: P = ESEosin [Et +[(1/2) (xE/xq) qoEo] [sin([E+[v)t + sin([E-[v)t] The first term is elastic (Rayleigh) scattering at the original wavelength; the others are light higher or lower by the vibrational frequency: anti-Stokes or Stokes lines

Raman MechanismQuantum Picture

The polarizability of carbon dioxide may be represented by the ellipsoid. Modes that cause a change in polarizability are Raman-active. Usually they are not IR active

Raman ExperimentsDetect at 90o to drive laser beam Problem: Raman effect is weak (10-6); Scattered main beam not excluded Sample degrades from laser beam Many organics fluoresce Solutions: IR laser - FT Raman (goes as P-4); doesn t excite fluorescence UV laser - Raman lines above fluorescence Tunable laser - resonance Raman Picosecond laser- gate faster than fluorescence

Benefit of FT RamanD.B.Chase in Analytical Raman Spectroscopy Wiley (1991) 21.

Raman Applications to CarbonM.S.Dresselhaus et al. Analytical Applications of Raman Spectroscopy M.Pelletier ed. Blackwell (1999) 367

Raman Applications to CarbonM.S.Dresselhaus et al. Analytical Applications of Raman Spectroscopy M.Pelletier ed. Blackwell (1999) 367

Raman Applications to CarbonM.S.Dresselhaus et al. Analytical Applications of Raman Spectroscopy M.Pelletier ed. Blackwell (1999) 367

Raman Application: MineralsP.C.Stair, C.Li; J.Vac.Sci.Technol. A15(1997) 1679

R.Manoharan et al. Photochem. Photobiol. 67 (1998) 15.

Sum Frequency GenerationAn emerging vibrational spectroscopy Michael J. KelleyJefferson Lab and The College of William & Mary

Vibrational SpectroscopiesView molecular architecture directly IR Absorption - deliver and detect wavelength that drives the vibrational excitation; 50+ years experience; affordable and accessible; not inherently surface sensitive. E-> QIR Raman Scattering - deliver a single wavelength and detect emission at wavelengths longer by the energy required to drive the vibrational excitation; inherently weak effect - need tricks; not inherently surface sensitive. Polarizability: Elm. Sum Frequency Generation - deliver intense fixed wavelength visible and variable wavelength IR beams and detect sum signal; basically understood; inherently surface sensitive; many tricks possible; we can get it, but should we ?

SFG - Doing It !

The optical beams in the experimental cell below are delivered through the prism at the bottom. TIR arrangements are also possible. From: Vidal et al.

The SFG beam exits close to the reflected visible beam so that usually a monochromator is used in addition to the slits. The two input beams usually source from a single laser. Polarization of each input beam and SFG beam is separately selected. From: Miranda & Shen

SFG Signal Intensity - Basic Model: product of termsInput beams: IIR x Ivis x X - how much light Fresnel factor: |F| = C Fil Fjm Fkn el em en - matches fields G2total = |Gnr + Gr|2 Non-linear susceptibility: Gijk = N Hyperpolarizability: Flmn = Eij Qk /{2(R - Rr - i+r)} Applies to 2-D charge sheet between two bulk phases Notation about polarization: ssp etc.

SFG Results - Water3700 cm-1 is free hydroxyl - about 1/4 of OH - surface layer only 3150 cm-1 is seen in ice 3400 cm-1 is seen in liquidM.J.Schultz et al.; J.Phys.Chem.B 106 (2002) 5313

Solutes that partition to the surface extinguish the free OH feature. Other polarizations are dark.

SFG Results: E-Al2O3 in waterThe combined intensities of the 3150 cm-1 and 3400 cm-1 bands were monitored vs pH. They reflect the amount of structured water . The minimum is associated with the isoelectric point. This consistent with the notion that surface charge promotes water structure and suggest using SFG to track surface charge.Yaganeh, Dougal & Pink 1999

SFG Results: Organics in waterPlacing PEG or PPG on a hydrophilic surface eliminates all spectral features associated with CH2 and CH3, compared with a hydrophobic surface. This is interpreted as evidence for ordered orientation toward the hydrophobic surface vs disorder otherwise. Kim et al. (2003)

SFG: Broadband IRScanning the IR laser step-by-step through the frequency range is slow and risks noise from system instability. Making the IR pulse short (~100 femto) broadens its spectral range to cover the methyl region. Leave vis pulse long. Not broad enoungh for hydroxyl ?? Experiment: open the receiving slit and replace the PMT with a linear detector array- count all channels Beginning to be how it s done . Nd:YAG -> Ti:sapph(s!)

SFG: Resonant enhancementChoose a vis wavelength to drive an electronic transition whose excited state distortion is similar to the vibrationally excited state:Rascke et al. 2002

Electronic effects are possible on metal surfaces. Situation on oxides is complex.

EllipsometryAn optical technique closely related to the vibrational spectroscopies The Fresnel equations describe reflection at an interface in terms of the optical constants: = n + iO (functions of P) The reflected intensities Rp and Rs may be stated as: Rp/Rs = tan = ei(

Ellipsometry MeasurementReflection from successive interfaces sums at the detector: interference Measure Rs and Rp as a function of angle and wavelength with a laser source

Ellipsometry SchematicSee website: jawoollam.com

Ellipsometry Data Fitting

1.25 Qm GaN on sapphire, modeled with and without considering 0.53 Qm buffer layer. C.Pickering; Surf.Int.Analysis 31 (2001) 927