Ramen Spectroscopy

Question 1

What is Ramen spectroscopy?

Answer 1

The study of how EM radiation interacts with a molecule to change its rotational and vibrational energy by inelastic light scattering

Question 2

When does scattering occur?

Answer 2

This occurs when incident radiation interacts with molecules who size is much smaller than the wavelength of the radiation

Question 3

What is Rayleigh scattering?

Answer 3

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

Question 4

What is Ramen scattering?

Answer 4

This is inelastic where energy is exchanged between the photon and the molecule

Question 5

What is stokes Ramen radiation?

Answer 5

This is where the scattered photon has lower energy than the incident photon
The molecules gains energy
The photon now has a different wavenumber

Question 6

What is anti stokes Ramen radiation?

Answer 6

This is where the scattered photon has higher energy than the incident photon
The molecules loses energy
The photon now has a different wavenumber

Question 7

What does the energy difference correspond to?

Answer 7

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

Question 8

How does Ramen spectroscopy work?

Answer 8

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)

Question 9

Differences to IR spectra

Answer 9

Ramen can give peaks with homonuclear as well as heteronuclear

Question 10

Why does an intense laser need to be used?

Answer 10

Rayleigh scattering is weak and Ramen scattering is very weak

Question 11

Which bands are observed?

Answer 11

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

Question 12

Which lasers should be used?

Answer 12

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

Question 13

What is scattering intensity proportional to?

Answer 13

Scattering intensity is proportional to (incident photon)^4 or 1/wavelength^4

Question 14

What are the gross selection rules?

Answer 14

There must be a change in polarisability

Question 15

What is polarisability?

Answer 15

The ease with which electrons within a molecule can be moved by an applied electric field

Question 16

Describe the ease of moving electrons along or away from bond axis

Answer 16

It is easy to move electron density along the bond axis

It is hard to move electron density away from the bond axis

Question 17

What is the polarisability ellipsoid?

Answer 17

This shows the difficult of moving electron density in different directions

Question 18

What is the anisotropic ellipsoid?

Answer 18

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

Question 19

What is the isotropic ellipsoid?

Answer 19

This is the same in all direction

It only applied to spherical rotors

Question 20

What is the benefit of Ramen spectroscopy?

Answer 20

It allows us to study transitions we cannot study by absorption spectroscopy

Question 21

How can a molecule change its energy in rotation Ramen spec?

Answer 21

It can change its rotational energy by inelastic light scattering
Rotation energy levels depend only on the molecule

Question 22

What is the specific selection rule for rotational Ramen scattering?

Answer 22

^J= 0+/-2

This is different from absorption and emission

Question 23

Describe the spectrum of rotational Ramen spectroscopy

Answer 23

There are a range of J levels populated as they are closely spaced
Spectrum will have several lines
The spacing is different from microwave

Question 24

How can a molecule change its energy in vibrational Ramen spectroscopy?

Answer 24

By inelastic light scattering

The vibrational energy levels depend only on the molecule

Question 25

What are the specific selection rules for vibrational Ramen spec??

Answer 25

^V= +/-1

This is the same for absorption and emission

Question 26

Describe the vibrational Ramen spec spectra

Answer 26

Most molecules are in V=0 due to large energy gaps
One stokes band in spectrum
Anti stokes band (1 to 0) not observed

Question 27

What is the mutual exclusion rule?

Answer 27

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

Question 28

What determines band position and strength?

Answer 28

Band positions are determined by the molecule via its energy levels
Band strengths are determined by the spectroscopic technique via its selection rules

Question 29

What region of EM is Ramen spec complementary to?

Answer 29

Microwave and IR spectroscopy