advanced molecular spectroscopy pt 2 (raman & derivative) Flashcards
In raman, incident photons are scattered by molecules with what?
both an increased or decreased energy which is characteristic of sample
Scattering of incident photons are at what angles?
right angles
What is the energy change equal to?
difference in energy between vibrational energy levels
what does the energy change correspond to?
wavelengths in mid-infrared
what 3 ways can light interact with substance?
absorbed, transmitted, scattered
what are the ways that radiation can be scattered?
elastically or inelastically
If it is scattered elastically, how does this affect wavelength? and what is it called?
no change in wavelength, called rayleigh scattering
what is it called when radiation is scattered inelastically?
raman effect
Where does rayleigh scattering occur? how intense is the peak?
at exact wavelength of excitation - most intense peak
Between stokes and antistokes lines, which has greater and smaller energy than rayleigh peak? and which has longer/shorter wavelengths?
stokes = smaller, longer antistokes = greater, shorter
how intense are the stokes and antistokes lines?
stokes less intense than rayleigh, antistokes even weaker
What are virtual states?
assumed state of photon and molecule during short time period
A molecule will only emit monochromatic light and produce raman shifted lines (stokes and antistokes) when what occurs?
when molecule vibrates so that polarizability is changed during vibration
Raman ………… shift = infrared ………… peak frequency
frequency, absorption
Scattering involves a momentary distortion of what?
electrons distributed around a bond
What happens to the molecule in its distorted form? 2
it is temporarily polarised, ie develops a momentary induced dipole
This deforms molcules, leading to what?
vibrations with characteristic frequency
When there is no dipole moment in equilibrium position or when stretched, what does this mean?
no infrared absorption
If the polarizability varies, what does this mean?
it is raman active
In symmetric and anti symmetric modes of stretching, what happens to the dipole moment?
symmetric = no dipole moment, antisymmetric = dipole moment fluctuates
Are symmetricand antisymmetric modes IR active or inactive?
symmetric - inactive, antisymmetric - active
Describe the polarizability of symmetric and anti-symmetric stretching modes?
symmetric = polarizability fluctuates in phase with vibration, antisymmetric = polarizability of one bond increases as other decreases
Are symmetric and antisymmetric stretching modes raman active or inactive?
symmetric = raman active, antisymmetric = raman inactive
does raman require a special sample prep method?
no
Why does water show up on raman and not usually on FTIR?
no background problem in raman
what types of cell might be used in raman?
inexpensive glass or quartz
what types of probes ideally used? 2
non-invasive and immersion probes
Which spectra is less cluttered, raman or FTIR?
raman
What makes this technique well suited for both organic and inorganic species?
spectral range is below 400cm-1
What can be used to do a remote analysis in raman?
fibre optic cables
A major limitation of raman is background signals arising - what causes this?
signal arising from fluorescence of analyte or impurities
Is raman strong or weak effect?
weak
incident flux of … photons will give 1 raman ……
10^8, photon
A ppm impurity with a quantum yield of …. will give ….. fluorescence photons for the same flux
0.1, 10
What range are the detection limits in?
parts per thousand
How can this problem be eliminated?
using FT-Raman
It uses a high power Nd-YAG laser at what wavelength?
1064nm
Is raman more/less expensive than FTIR?
more, due to expensive laser and filters
What are 2 advantages of FT-Raman over dispersive?
1064nm eliminates fluorescence and photodecomposition, superior frequency precision
What is a disadvantage of FT-Raman over dispersive?
Water absorbs in the 1000nm region
What 2 ways can raman be enhanced?
resonance raman effect, surface-enhanced raman spectroscopy (SERS)
In resonance raman effect, how are the raman signals greatly enhanced?
by exciting with wavelength near electronic absorption peak
how does this differ from fluorescence?
no vibrational relaxation
how is this limited?
due to fluorescence of analyte or impurities
Where are samples adsorbed in SERS?
on surface of collodial metal particles
When combined with resonance raman, how much is enhancement increased by?
100x
Name 2 applications of raman?
fingerprinting, paint analysis
What is derivative spectroscopy used for?
removing spectral interferences and increasing selectivity
