Spectroscopy Basics

Question 1

What is a spectrum?

Answer 1

A plot of energy vs absorption, transmission or emission

Question 2

Visible spectrum?

Answer 2

400-700nm

Question 3

What is the EM spectrum?

Answer 3

Simple harmonic waves of interconnected, perpendicular electric and magnetic fields

Question 4

Energy of radiation?

Answer 4

The energy of radiation is quantised – takes discrete values – and carried by photons

Question 5

Energy levels in molecules?

Answer 5

Energy levels in molecules are also quantised electrons in atoms and molecules can be described by wave functions

Question 6

Hydrogen spectrum?

Answer 6

Discrete lines

Question 7

Basic principle of spectroscopy?

Answer 7

Transitions between energy levels occur when photons of energy with exactly the correct value are absorbed or emitted

Question 8

What do different forms of spectroscopy involve?

Answer 8

Different forms of spectroscopy involve

energy levels arising from different processes in atoms and/or molecules

Question 9

Three types of spectroscopy?

Answer 9

Absorption (can also be displayed in Transmission)
Emission
Scattering

Question 10

Transitions within electronic energy levels in atoms/molecules?

Answer 10

Some electrons are
promoted to upper levels by high temperatures or electronic discharge. photons are emitted at discrete energies as they fall back to the ground state

Question 11

Energy levels and spectroscopy?

Answer 11

In fact, all types of energy are quantised and

give rise to energy levels and spectroscopy

Question 12

Molecules motion?

Answer 12

Translation, rotation, vibration

Question 13

Size of energy levels in molecules?

Answer 13

Translation - motion of the whole molecules tiny change in energy
Nuclear/spin - change in spin of electron and positron very small change in energy
Rotational - rotation of the whole molecules small change one energy
Vibrational - motion of the bonds between atoms medium change in energy
Electronic - movement of electrons between levels

Question 14

Selection rules?

Answer 14

Not all transitions between energy levels are possible, calculating the wave-functions of the two energy levels sometimes gives zero probability of a transition – a forbidden transition, selection Rules tell us which transitions can and can’t occur

Question 15

Gross selection rule?

Answer 15

In terms of molecular properties for particular types of absorption

Question 16

Specific selection rule?

Answer 16

In terms of between which levels transition can take place

Question 17

What does spectroscopy involve?

Answer 17

Spectroscopy involves the interaction and exchange of electromagnetic radiation with matter

Question 18

What does absorption of energy involve?

Answer 18

Absorption of energy can only take place when the energy of the radiation exactly matches the difference between molecular energy levels

Question 19

Transition between energy levels?

Answer 19

Transitions between energy levels involve discrete quantities of energy and give rise to spectroscopy

Question 20

How can spectra be presented?

Answer 20

Spectra can be presented as absorption or transmission or emission spectra

Question 21

Positions of peak in spectra?

Answer 21

The position of a peak in a spectrum is related to the difference in energy between energy levels

Question 22

Electromagnetic spectrum?

Answer 22

The electromagnetic spectrum contains radiation with a large range of energies that can interact with various processes within molecules

Question 23

Molecular spectroscopy?

Answer 23

Molecular spectroscopy can be used to identify compounds, to measure how much of a compound is present and to determine molecular properties

Question 24

Molecules absorption of a photon?

Answer 24

A molecule can absorb a photon and be raised from one molecular energy level to another the energy of the photon must correspond exactly to the difference in energy between the initial and final level, the spacing between the energy levels depends on the type of molecular transitions, different types of molecular transition involve absorption of energy from different parts of the electromagnetic spectrum

Question 25

Movement of a molecule?

Answer 25

The movement of a molecule as a whole through space is associated with translational energy, the molecule can rotate giving rise to rotational energy and the atom within the molecule can move relative to one another as if they were on springs giving vibrational energy, all of these forms are quantised and there are energy levels associated with each of them the molecule cannot take up energy continuously but can only do so in discrete amounts leading to transitions between the energy levels

Question 26

Absorption spectroscopy?

Answer 26

Whenever the energy exactly corresponds to a gap between energy levels within the sample the radiation may be absorbed this is known as absorption spectroscopy the lowest energy state of the molecule is known as the ground state and absorption of radiation increases the energy of the molecule and promotes it to an excited state

Question 27

Absorption spectrum?

Answer 27

The amount of radiation absorbed is plotted against the energy of the radiation the peaks rise up from a base line in the spectrum

Question 28

Transmission spectrum?

Answer 28

Measures the proportion of energy transmitted through the sample if no absorption takes place 100% of the radiation is transmitted through the sample, when absorption takes place a downward peak is produced since the amount of radiation transmitted goes down

Question 29

Emission spectroscopy?

Answer 29

Radiation is emitted when a species in an excited state falls back to the ground state, the peaks in the spectrum risk upwards from the baseline

Question 30

Raman spectroscopy?

Answer 30

Involves scattering rather than absorption or emission of light, radiation is directed onto a sample and some of the radiation that is scattered has a different frequency from the incident radiation, the difference in frequency provides information about the energy levels of the sample

Question 31

What is the position of a peak in the spectrum determined by?

Answer 31

The energy of the transitions, by the difference between two energy levels in the molecule

Question 32

What determines the size/intensity of the peaks?

Answer 32

The concentration of the sample The distance the radiation travels in the sample (path length) How many molecules within the sample are in the correct energy state to absorb or emit at this frequency How likely it is that the transition will occur

Question 33

Best way to see how the amount of material affect the spectrum?

Answer 33

By considering a compound in solution, can use the Beer lambert law

Question 34

How does population of energy levels affect intensity of the spectral peaks?

Answer 34

Absorption of a photon results in the promotion of a molecule from a lower level to a higher level, the more molecules there are in the lower energy level the more likely it is that a photon will be absorbed and a molecule promoted

Question 35

How are populations described by the Boltzmann distribution if change in energy is small?

Answer 35

the populations in the energy levels are described by the Boltzmann distribution, if energy levels are closely spaced so that change in energy is small such less than KbT the value of the exponential term is close to one this means that the ratio of the populations is approximately equal to the ratio of the degeneracies, if the degeneracies of both states are one the population of n upper and n lower are similar

Question 36

How are populations described by the Boltzmann distribution if change in energy is large?

Answer 36

If the energy levels are widely spaces so that change in energy is large much greater than KbT the ratio of the populations is now close to zero and very few molecules are in the upper state

Question 37

Absorption peak and populations of energy levels?

Answer 37

For an absorption peak the population of the lower energy levels in the transitions affects the intensity of the peak

Question 38

Emission peak and populations of energy levels?

Answer 38

For an emission peak the intensity depends on the population of the upper level