Raman Spec Flashcards
What is scattering dependent on
Can occur at any wavelength
Depends on polarizability (aplha) of the scattering molecule
What is polarizability
Describes how easy the electron cloud of a molecule can be distorted from its original shape by an external field, ion, or dipole
Slide 2 equation of dipole
In notes
What are the two types of scattering and what do they mean
Elastic (Rayleigh):
wavelength doesn’t change
Amount of scattering depends on the wavelength
- Inelastic scattering:
Molecule changes vibrational or rotational levels
Slide 3 equation of wavelength inverse and energy of vibrational transition
Amount of scattering is inversely proportional to wavelength to the power of 4
In notes
What is Raman spec
A small fraction of the scattered light has a diff wavelength than the incident light
The 1 on 10mill photons will show a shift bc of vibrations and rotations during the interaction with light
The spectrum of this shifted light is Raman spectrum
What is Raman scattering
Scattering of light with the change in frequency/ wavelength
During scattering What do the shifts in wavenumber depend on
The chemical structure of the molecules responsible for the scattering
What components are included when light is scattered from a molecule
The Rayleigh scattering (frequency not changing)
The Raman stokes (red) shift , longer wavelength
The Raman antistokes (blue shift), shorter wavelength
On a excitation plot, the Ir absorption levels are from
Ground to first level
Describe Rayleigh scattering energy level change
Goes up from ground to first virtual energy state then back down to ground
Comes down at same energy diff (not change in frequency or wavelength)
Stokes Raman scattering energy level change
Starts at ground goes up to first virtual energy level
Then down to first energy level (excited energy level) not ground
Anti Stokes Raman scattering energy level change
Starts at first energy level (excited) goes up to first virtual energy level
Then down to ground
What is special about the energy of Raman shifted photons
They can be either higher or lower in energy depending on the vibrational state of the molecule
In what wavelength region does Raman scattering occur
Since it occurs with vibrational and rotational levels
The Raman scatted radiation occurs in the mid infrared region (so in ir spec)
Comparing IR and Raman spec
Resemble each other
But since some groups are more IR active and other groups are more Raman active
The spectra compliment each other (meaning they each give diff useful info of the same molecule, you can better detect something with one method rather than the other)
Which type of scattering causes the most scattering
Least
Rayleigh
Antistokes
Describe what the stocks and antistokes bands look like compared to Rayleigh
Since Rayleigh doesn’t change it’s in the middle
The anti stokes is at a lower wavelength (so higher wave number) than Rayleigh
stokes is higher wavelength (lower wave number)
The distance between the stokes and anti stokes is symmetric (same value but negative or positive compared to reighleigh
In comparison to Rayleigh stokes would have what sign wave number
Antistokes
Stokes is increase wavelength so lower wave number
Negative
Antistokes is lower wave length so increased wave number
Positive
What type of scattering is usually measured and collected
Stokes raman scattering
What is the light source of a Raman spec
Laser
Which is monochromatic
Intense
narrow
Explain how a Raman spectrophotometer works
Laser shines light
A dichromic beam splitter shines light on the sample
Than light scatters off the sample and onto the detector that is above the sample (this is the Raman light)
How can you change the Raman spectrophotometer analysis to get scattering at different parts of the sample
Can move the sample and the lens to get scattering at diff parts of the sample
What is different in the peaks of Raman spec vs or spec
Raman has positive (facing up) peaks
Ir is plotted as transsmittance so the peaks are negative
What is the fingerprint region of a sample
A pattern of bands to identify the molecules in the sample
What is an advantage to Raman spec
Water is not an interferent in Raman spec
It is in IR
Disadvantage of Raman spec
How can they be overcome
Weak signals (can be overcome by strong lasers)
Interference by fluorescence of the sample (or the impurites in the sample)
In a beta solenoid amyloid, what is its amide 11 intensity in comparison to the parallel in register and the alpha syn
The amide II scattering intensity is higher in beta solenoid than the other two
What is the technique called to enhance the signal in Raman spec
How does it work
What is the limitations
Resonance Raman spectroscopy
When the wavelength of the light is close to an absorption peak, the Raman signal gets more intense
This only works for Raman bands that are coming from a chromophore (ex. so they can’t happen with iron atoms in hemoglobin)
What is the technique called to enhance the signal in Raman spec
How does it work
What is the limitations
Resonance Raman spectroscopy
When the wavelength of the light is close to an absorption peak, the Raman signal gets more intense
This only works for Raman bands that are coming from a chromophore (ex. so they can’t happen with iron atoms in hemoglobin)
In resonance Raman spec, how can you change the concentrations of the sample
How does this help
You can use lower concentrations of the sample
This means that all of the peaks that aren’t enchanted by the resonance Raman spec would be weak and won’t interfere with the analysis
What is an example of applying resonance Raman spectroscopy
Checking the different absorbances of bacteriorhodopsin when it has different forms
Every time it goes to a different compound, it has a different maximum absorption
If they wanted to just study one compound they would know to keep it at a specific temperature (less than 120 degrees) and measure it at that point
What is the max absorbance of bacteriophodopsin
570nm
What is the experimental setup of the analysis of bacteriorhodopsin
The sample goes onto a copper tip , and it is cooled to 77K through liquid nitrogen
The copper tip keeps the sample cold
The sample spins and each time different part of it is exposed so that it can then be recooled
The sample gets cold in the dark to not trap intermediates of the sample (and we’re starting with the original bacteriorhodopsin)
What can you explain about the bacteriophodopsin spectra
When just the probe is on, the only get the spectra of bacteriorhodopsin
When both pump and probe, they get both bacteriorhodopsin and it’s intermediate K since it’s a steady state mixture
When they do a difference spectrum of bacteriorhodopsin and it’s intermediate K, the only get the spectra of K
Why is the fluoresce in the bacteriorhodopsin pump and probe less than in just the probe
Because in just the probe BR is fluorescent
In the pump and prove you have BR and K but K isn’t fluorescent so the overall fluorescence goes down
What did they learn about the bacteriophodopsin experiment
The found the the BR was all trans
Then when absorbing light, one of the bonds in the retinal in bacteriorhodopsin shifted to cis
This forms K which is cis
What is surface enhanced Raman spec
The sample is adsorbed on a rough metal surface
Can get 6-12 order of magnitude of intensity in the spec (10^6-10^12)
Makes use of electromagnetic fields
