chapter 18 - raman spectroscopy

reflection

a change in direction of light at a fixed angle; it bounces off

transmission

light goes through a material with no energy loss

absorbance

the transfer of light energy into material

scattering

a change in light direction without energy loss (goes through)

what is the principle of Rayleigh scattering?

light scatters with an intensity

elastic scattering

there is no change in light velocity; also known as Rayleigh scattering

inelastic scattering

there is a change in light velocity; aka Raman scattering

stokes scattering

when the scattered radiation is of a lower frequency than the excitation radiation (towards red)

antistokes scattering

when the scattered radiation is of a higher frequency than the source of radiation (towards blue)

what is a requirement of Raman spectroscopy?

there needs to be a change in polarizability

are the vibrations in Raman spec symmetric or asymmetric?

symmetric

virtual state

a short-lived distortion of electron cloud due to molecular vibrations

is stokes or anti-stokes emission favored?

stokes because it is toward lower frequency/energy

how would an increase in temperature change the ratio of anti-stokes to stokes intensities?

there would be more molecules in the first vibrational excited state, so anti-stokes scattering would be more probable

is Rayleigh or Raman scattering more probable?

Rayleigh

what is the typical source for Raman spec?

lasers because of their high intensity which is needed to produce Raman scattering of sufficient S/N

who do you determine the stokes and anti-stokes shifts in wavenumbers?

convert the excitation wavelength to wavenumbers, add and then subtract what the Raman band is, then convert it back to nm

what are some things that vibrational modes are sensitive to?

atomic mass, molecular geometry, bond strength, bond order, local chemical environment (think H-bonding etc)

mutual exclusion rule

• center of symmetry, Raman active vibrations are IR inactive and vice versa

• no center of symmetry, some or all of the vibrations may be both Raman and IR active

what happens when there is a molecule with no symmetry?

usually all the vibrational modes are Raman actiev

what happens when the molecule has symmetry?

• symmetric vibrations give stronger Raman lines

• non-symmetric vibrations are weaker or unobservable

  • bending modes are usually very weak

what kind of sampling instruments can be used for Raman?

glass can be used, as well as others because the the laser source can be easily focused onto a small sample area and the emitted radiation efficiently focused on a slit

what liquid solvent is a weak Raman scatterer?

water

what substances can Raman scan?

liquids and solids

spatial resolution

knowing how near two objects are; the level of detail captured

what happens when the source of radiation intersects an electronic excitation band for the analyte?

markedly increased Raman line intensity

surface-enhanced Raman spec

obtain raman spectra in the usual way on samples that are adsorbed on the surface of nano colloidal metal particles