Raman Spec Flashcards by Zunaira Ali

What is scattering dependent on

Can occur at any wavelength

Depends on polarizability (aplha) of the scattering molecule

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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

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Slide 2 equation of dipole

In notes

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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

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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

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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

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What is Raman scattering

Scattering of light with the change in frequency/ wavelength

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During scattering What do the shifts in wavenumber depend on

The chemical structure of the molecules responsible for the scattering

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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

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On a excitation plot, the Ir absorption levels are from

Ground to first level

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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)

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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

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Anti Stokes Raman scattering energy level change

Starts at first energy level (excited) goes up to first virtual energy level

Then down to ground

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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

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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)

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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

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