spectroscopy

Question 1

What is spectroscopy?

Answer 1

The interaction of light with matter. The bsorption, emission and scattering of electromagnetic radiation by atoms and molecules.

Question 2

What are the types of energy in a molecule?

Answer 2

Electronic energy, vibrational energy, rotational energy, translational energy.

Question 3

What happens in absorption spectroscopy?

Answer 3

Incident photon is absorbed,
resulting in a higher energy
state

Question 4

What happens in emission spectroscopy?

Answer 4

Photon is emitted, resulting
in a lower energy state.

Question 5

What happens in stimulated emission spectroscopy?

Answer 5

Photon (1) induces transition to lower energy level (2). Two photons are emitted (3) that are “in phase” (same wavelength and phase)

Question 6

What is the Beer-Lambert law and where does it fail?

Answer 6

The Beer-Lambert law states that there is a linear relationship between the concentration and the absorbance of the solution. 𝑨 = 𝜺𝒄𝒍
It fails:
At high concentrations, there are more interactions between molecules. At low concentrations the signal intensity is too low, compared to noise

Question 7

Electronic spectrum of H, series

Answer 7

L1 - Lyman
L2 - Balmer
L3 - Baschen
∝ 𝑡𝑟𝑎𝑛𝑠𝑖𝑡𝑖𝑜𝑛 = 𝑙𝑜𝑤𝑒𝑠𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑛𝑠𝑖𝑡𝑖𝑜𝑛 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑒𝑟𝑖𝑒𝑠

Question 8

What is the Rydberg formula and what dooes it indicate?

Answer 8

The Rydberg formula calculates the wavelengths of a spectral line chemical elements. Units have to be cm-1 and n1 and n2 are pure numbers.

Question 9

Describe UV-vis spectroscopy and what is the isobestic point?

Answer 9

Photons have enough energy (𝜆= 200-800nm) to move molecular electrons from bonding orbitals or non-bonding orbitals to antibonding.
Isobestic point indicates a direct change from one molecule to another, where all the graphs cross over

Question 10

Decribe vibrational (IR) spectroscopy.

Answer 10

All molecules vibrate and can absorb incident photons to increase the vibrations
Vibrational spectroscopy which wavelengths of light are absorbed by a molecule and can be used to identify an unknown molecule by comparing to other, known molecules.

Question 11

Describe the harmonic oscillator approximation.

Answer 11

𝐸𝑣=ℎ𝜔 (𝑣 + 1/2) 𝑤ℎ𝑒𝑟𝑒 𝑣 𝑛𝑜𝑡 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦! = 0, 1, 2 … …
ℎ = 𝑝𝑙𝑎𝑛𝑐𝑘 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝐽𝑠 ,
𝜔 = 𝑓𝑢𝑛𝑑𝑎𝑚𝑒𝑛𝑡𝑎𝑙 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑠 − 1,(𝜔 is related to the spring constant (ie how stiff the spring is)
and the mass of the objects attached to the spring)
𝑤ℎ𝑒𝑛 𝑣 = 0, 𝐸0 = ℎ𝜔(0 + 1/2) = 1/2 ℎ𝜔
𝑤ℎ𝑒𝑛 𝑣 = 1, 𝐸1 = ℎ𝜔(1 + 1/2) = 3/2 ℎ𝜔
𝑤ℎ𝑒𝑛 𝑣 = 2, 𝐸2 = ℎ𝜔(2 + 1/2) = 5/2 ℎ𝜔

Question 12

Selection rules for IR spectroscopy

Answer 12

Gross selection rule – (property of molecule needed to absorb radiation) to interact with infrared radiation a vibration must involve a changing dipole.
Specific selection rule - ∆𝒗 = + 𝒐𝒓 − 𝟏

Question 13

What is the degrees of freedom?

Answer 13

Degrees of freedom is the total number of variables used to define the motion of a molecule completely. A molecule consisting of (N) number of atoms has a total of 3N degrees of freedom, corresponding to the Cartesian coordinates of each atom in
the molecule. In a non-linear molecule, 3 of these degrees of freedom are rotational (can rotate around the 3 axis-all atoms
move), 3 are translational and the remainder are fundamental vibrations

Question 14

How many vibrational moodes are there?

linear and non-linear

Answer 14

For non-linear molecules there are 3N-6 Vibrational modes.
For linear molecules there are 3N-5 vibrational modes.
where N is the number of atoms

Question 15

Describe the anharmonic oscillator.

Answer 15

The Morse function/oscillator describes this
curve (empirical, measured not derived).
Putting into Schrodinger’s equations allows
solution to derive allowed energy levels in
anharmonic oscillator. The levels are not evenly spaced.

Question 16

What are the specific selection rules for the harmonic aqnd anharmonic oscillators?

Answer 16

Harmonic oscillator - ∆𝑣 = ± 1
Anharmonic oscillator - ∆𝑣 = ± 1, ± 2, ± 3

Question 17

Selection rules for rotational spectroscopy.

Answer 17

Gross selection rule:
A molecule must have a permanent dipole to show a “pure” rotational spectrum
Pure-excited by microwave radiation
Specific selection rule - Linear rotors ∆𝐽 = ±1
Symmetric rotors ∆𝐽 = 0, ±1

Question 18

Formula for roational energy levels.

Answer 18

𝐸𝐽 = ℎ𝐵𝐽 (𝐽 + 1)

Question 19

Explain the population of energy levels of rotational spectroscopy.

Answer 19

The position of a peak in the spectrum is determined by the energy of the transition, that is, by the difference between two energy levels in the molecule.
The peak size (intensity) depends on the number of absorption or emission events.

Question 20

Selection rules for Raman spectroscopy.

Answer 20

Gross selection Rules
Vibrational Raman : Polarizability must change in the vibration (Change in dipole moment during the vibrtation)
Rotational Raman : Anisotropic (uneven) polarizability in the molecule (has to have a permanent dipole)
Specific selection Rules : Vibrational Raman ∆𝑣 = ±1
Rotational ∆𝐽 = +2 𝑠𝑡𝑜𝑘𝑒𝑠 − 2 (𝑎𝑛𝑡𝑖𝑠𝑡𝑜𝑘𝑒𝑠)

Question 21

What is the Zeeman effect?

Answer 21

In the presence of a strong magnetic field, a spectral line normally at one wavelength splits into two or three components
The amount of splitting corresponds to the strength of the magnetic field.
In a magnetic field split the single electron state into 2