Ramen Spectroscopy Flashcards
What is Ramen spectroscopy?
The study of how EM radiation interacts with a molecule to change its rotational and vibrational energy by inelastic light scattering
When does scattering occur?
This occurs when incident radiation interacts with molecules who size is much smaller than the wavelength of the radiation
What is Rayleigh scattering?
This is elastic as the scattered photon has the same energy as the incident photon
There is no change to energy of photon or molecule
The wavenumber of the photon does not change
What is Ramen scattering?
This is inelastic where energy is exchanged between the photon and the molecule
What is stokes Ramen radiation?
This is where the scattered photon has lower energy than the incident photon
The molecules gains energy
The photon now has a different wavenumber
What is anti stokes Ramen radiation?
This is where the scattered photon has higher energy than the incident photon
The molecules loses energy
The photon now has a different wavenumber
What does the energy difference correspond to?
The energy different results in displacement of the scattered radiation to either a higher or lower energy than the incident radiation
The different in E corresponds to the different between two energy levels in the molecule
How does Ramen spectroscopy work?
A laser light is directed at the sample and the Ramen shift is measured
The frequency difference proves info about energy levels in the molecules (include r and v levels)
Differences to IR spectra
Ramen can give peaks with homonuclear as well as heteronuclear
Why does an intense laser need to be used?
Rayleigh scattering is weak and Ramen scattering is very weak
Which bands are observed?
Ramen bands are observed due to transitions when the molecule: - accepts energy v~=v~exc + ^V~ - releases energy v~=v~exc - ^V~ v~exc is the incident photon ^V~ is release or gain of energy
Which lasers should be used?
A visible light laser- observe changes in v or r energy
Light of high incident photons (short wavelength) give stronger scattering
(Shorter the wavelength, the stronger the energy)
Blue light scatters the most
What is scattering intensity proportional to?
Scattering intensity is proportional to (incident photon)^4 or 1/wavelength^4
What are the gross selection rules?
There must be a change in polarisability
What is polarisability?
The ease with which electrons within a molecule can be moved by an applied electric field
Describe the ease of moving electrons along or away from bond axis
It is easy to move electron density along the bond axis
It is hard to move electron density away from the bond axis
What is the polarisability ellipsoid?
This shows the difficult of moving electron density in different directions
What is the anisotropic ellipsoid?
This is different in different directions
It changes as the molecule rotates
Rotation of Ramen transition is allowed
True for all molecules except isotropic ellipsoid
What is the isotropic ellipsoid?
This is the same in all direction
It only applied to spherical rotors
What is the benefit of Ramen spectroscopy?
It allows us to study transitions we cannot study by absorption spectroscopy
How can a molecule change its energy in rotation Ramen spec?
It can change its rotational energy by inelastic light scattering
Rotation energy levels depend only on the molecule
What is the specific selection rule for rotational Ramen scattering?
^J= 0+/-2
This is different from absorption and emission
Describe the spectrum of rotational Ramen spectroscopy
There are a range of J levels populated as they are closely spaced
Spectrum will have several lines
The spacing is different from microwave
How can a molecule change its energy in vibrational Ramen spectroscopy?
By inelastic light scattering
The vibrational energy levels depend only on the molecule
What are the specific selection rules for vibrational Ramen spec??
^V= +/-1
This is the same for absorption and emission
Describe the vibrational Ramen spec spectra
Most molecules are in V=0 due to large energy gaps
One stokes band in spectrum
Anti stokes band (1 to 0) not observed
What is the mutual exclusion rule?
If a molecule has a centre of inversion, it’s Ramen active modes are IR inactive and vice versa (won’t be both)
If a molecule does not have a centre of inversion, the some of all the modes may be Ramen and IR active
What determines band position and strength?
Band positions are determined by the molecule via its energy levels
Band strengths are determined by the spectroscopic technique via its selection rules
What region of EM is Ramen spec complementary to?
Microwave and IR spectroscopy
