10-1

Application of IRRaman Spectroscopy

• 3 IR regions• Structure and Functional Group

• Absorption IR• Reflection IR• Photoacoustic IR• IR Emission• Micro

10-2

Mid-IR

• Mid-IR absorption Samples

Placed in cell (salt) Combined with oil

Need cell that does not absorb IR KBr, NaCl* Tends to absorb water

Gases Solutions

Solvent issues* Dissolution of cell

10-3

Analysis• Can calculate group

frequencies C-H, C=O, C=C, O-H

Variations of frequencies for group

• Fingerprint region Compare to standards Absorption of

inorganics Sulphate,

phosphate, nitrate, carbonate

• Search spectra against library

10-4

Mid-IR

10-5

10-6

10-7

10-8

Interpretation• Alcohols and amines display strong broad O-H and N-H stretching bands in the

region 3400-3100 cm-1 bands are broadened due to hydrogen bonding and a sharp 'non-bonded'

peak can around 3400 cm-1 . • Alkene and alkyne C-H bonds display sharp stretching absorptions in the region

3100-3000 cm-1 bands are of medium intensity often obscured (i.e., OH).

• Triple bond stretching absorptions occur in the region 2400-2200 cm-1 Nitriles are generally of medium intensity and are clearly defined Alkynes absorb weakly unless they are highly asymmetric

symmetrical alkynes do not show absorption bands• Carbonyl stretching bands occur in the region 1800-1700 cm-1

bands are generally very strong and broad Carbonyl compounds (acyl halides, esters) are generally at higher wave

number than simple ketones and aldehydes amides are the lowest, absorbing in the region 1700-1650 cm-1

• Carbon-carbon double bond stretching occurs in the region around 1650-1600 cm-1

bands are generally sharp and of medium intensity Aromatic compounds will display a series of sharp bands

• Carbon-oxygen single bonds display stretching bands in the region 1200-1100 cm -1 bands are generally strong and broad

10-9

Quantitative IR

• Difficult to obtain reliable quantitative data based on IR Deviations from Beer’s law

Narrow Bands and wide slit widths required* Require calibration sources

Complex spectra Weak beam Lack of reference cell

Need to normalize refraction * Take reference and sample with same cell

10-10

Other methods

• Reflectance IR Measurement of absorbance from reflected IR

Surface measurement• Photoacoustic IR

can use tunable laser• Near IR

700 nm to 2500 nm Quantitative analysis of samples * CH, NH, and OH

Low absorption • Emission IR

10-11

Raman Spectroscopy

• Scattering of light Fraction of scattered light in the visible differs

from incident beam Difference based on molecular structure

* Based on quantized vibrational changes

* Difference between incident and scattered light is in mid-IR region

No water interference Can examine aqueous samples

Quartz or glass cells can be used Competition with fluorescence

10-12

Raman Spectroscopy

• Theory• Instrumentation• Application

• Method Excitation with UV or NIR Measurement of scatter at 90 °

Measurement 1E-5 of incident beam

10-13

Theory• 3 types of scattered radiation

Stokes Lower energy than Anti-

Stokes* Named from

fluorescence behavior More intense Used for Raman

measurements Anti-Stokes

No fluorescence interference

Rayleigh Most intense Same as incident radiation

• Shift patterns independent of incident radiation wavelength

10-14

Theory

• Excitation From ground or 1st vibrationally excited state

Population of excited state from Boltzmann’s equation*Molecule populates virtual states with

energy from photon* Can be effected by temperature

Elastic scattering is Rayleigh Energy scattered=energy incident

Energy difference due to ∆ ground and 1st excited state hE is Stokes scattering HE is anti-Stokes scattering

10-15

10-16

Theory• Variation in polarizability of bond with length• Electric field (E) due to excitation frequency

with E0

• Dipole moment (m) based on polarizability of bond ()

• For Raman activity must vary with distance along bond

is polarizability at req

)2cos(0 tEE ex

)2cos(0 tEEm ex

))((0 rrr eq

)2cos(max trrr veq

10-17

Theory

• Equation has Rayleigh, Stokes, and Anti-Stokes component

• Complementary to IR absorbance Overlap not complete

))(2cos()(2

))(2cos()(2

)2cos(

0

000

tr

rE

tr

rE

tEm

exm

exmex

10-18

10-19

Instrumentation

• Laser source Ar (488 nm, 514.5 nm) Kr (530.9 nm, 647.1 nm) He/Ne (623 nm) Diode (782 nm or 830 nm) Nd/YAG (1064 nm) Tunable lasers

Intensity proportional to 4

*Consider energy and chemical effect of absorbing energy

10-20

Instrumentation

• Sample holder Glass Laser focusing allows small sample size Liquid and solid samples can be examined Use of fiber optics

10-21

Applications• Laser microprobes

Use of laser permits small sampling area

• Resonance Raman Use electronic

absorption peak Low concentrations can

be examined Lifetimes on 10 fs

• Surface enhanced Raman Increase of sensitivity by

1000 to 1E6

10-22

10-23