PN: Raman spectroscopy Flashcards
what effect does raman use
Raman spectroscopy relies upon inelastic scattering of photons, known as Raman scattering.
-use intense UV lasers to irradiate, Rayleigh adn raman exit
egneral pretend B field non existent since raman not invovle it
- use filter to remove Rayleight light (due to elastic scatterring) collect light with lens and send to spectromter
uses of raman
technique typically used to determine
1) vibrational modes of molecules, although
2) rotational and other
3) low-frequency modes of systems may also be observed.
Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.
Decription of photon energies vs orignin
Gamma, X ray nucleas, inner atoms and molecule trnasitions
UVVIS- Electornic trans
Raman adn IR - trans betwn Vibrational levels (change in config)
Microwave - trans betwen roatational levels (chang eof orient)
Diff between e levels of harminic and anharmonic/morse
The energy levles are no longer = spaced and sep dit with increasing freq get closer till eventually not bound states and in a continuum
What assumption on state pop do we make in raman
Most molecules in gs at room temp
After illum with uV laser what comes out of raman spectrum
Rayleigh and raman scatter,
Ray-strong I peak and the same as the incident beam freq
Raman scattering is wek and has freq v0 -+ vm called Stokes and Antisokes
stokes -> ——
antistokes -> ++++
raman thus measrues vibrational freq shift vm as a shift from incident beam frequency v0
what region of specturm do raman exitation adn emitted Raman lines fall
UV VIS for both
what are the releative stize of energy gaps for stokes anit sotkes and floro
Floro usulaly dominates in dintensity light
this is purley between elec transitions
Condition to be raman active
q is nuclear displacement
alpha is a contnat called the polarizability
dalpha/dq must not be zero
the rate of change of polarizability with the vibration
antistokes vs stoeks energy output
always get a central rayleigh lines and then symmetric on either side you get the stokes (LHS lower freq so lower raman shift) and antistokes on RHS (larger energy given off so larger raman shift)
regular start and ened at sample vibr level
stokes: end at a highver vibr level than you start at
antistoke: start at a v>0 vibr level and end up at a lower level tha you started
so antistokes energy is larger in spectrum(bigger jump down)
Can you use raman with any Y
Yes since you always measure either stokes or antistokes RELATIVE to the rayleigh line and Y sets the rayleigh line (just dont choose an absorbing wavelngth, then the inelastic and elastic scattering is weak and low signla)
why is the anti stokes signal typically smaller in intensity than the stokes
same wavenumber relative to rayleigh on the x axis
but on y intensity axis the population is lower of the startign state (stokes starts in v=0 gs vibr level, while antistokes requires starting at v>0 evs(elec gs) which has fewer electrons)
why would you use a short wavelength in Raman
scattering proportional to 1/Y^4
so short wavelength Y gives very large scattering probability so a very large raman signal
downside: this is going to be high freq w and then you oftne gen fluo which washes out the resonance
normal vibr modes
Molecule will vibrate at all of these normal modes simultanously and measuring a moe with a particu,ar wavelngth is a projection onto the basis set
Motion_tot =a1M1 + a2M2 +……
where m1,m2 are the basis normal vibrational modes
how do we det if raman is appropriate for a chem (Raman sensitivity)
Check its polarizability (alpha) since da/dq determines the strength of q the nuclear displace ment which acts a source term for the raman radiation in maxwell eqn
if the polarizability ellipsoid changes (shape size orientation) during normal vibration) then the vibration is Raman active
Polarisation and alpha
P = alphaE
matrix alpha is polarizability tensor
types of vibrations
Symmetric, antisymmetric stretch
bending
mutual exclusion principle
how IR and Raman complementary:
The vibrations that are not raman active are IR active
holds for any molecules havinga centre fo symmetry
8 Advatges of raman over IR
1) some samples not IR active only raman , totally symmetric vibrations are always raman active
2) some vibrations weak in IR and strong in Raman (covalent bonds also)
3) symmetry of molecule can be deduced using polarizability (Raman activity)
4) using resonance raman you can select particular gorups in molecules for vibrational study
5) raman reuqires lser 1-2mm so only need small sample , IR requires large smaple
6) water is a weak Raman scatterer si you can spectrocopy with bio compountds in water ( IR water abs lots)
7) can put things in glasss (nto raman active), IR glass is active and abs
8) raman coveres full wavenumebr region ina single recording, in IR you need to move rgatings and fitlers etc over a single region
when is a raman virbation strong in raman
strong in raman if covalent bond
strongin IR is bond is ionic
4 disav of raman spectros
1) Laser required to obsevre weak raman scatterring -> heating or decomposition of sample, esp resonance raman where the freq is tuned to be absorbed -> bleaching
2) some compounds fluoresce when laser light incident
3) Raman in VIS and not IR,so more difficult to get high resolution roational adn rotation vibrational info (there rotatoion IR is okay)
4) state of the art raman much mroe expensive than FTIR
