Spectroscopy Basics Flashcards
What is a spectrum?
A plot of energy vs absorption, transmission or emission
Visible spectrum?
400-700nm
What is the EM spectrum?
Simple harmonic waves of interconnected, perpendicular electric and magnetic fields
Energy of radiation?
The energy of radiation is quantised – takes discrete values – and carried by photons
Energy levels in molecules?
Energy levels in molecules are also quantised electrons in atoms and molecules can be described by wave functions
Hydrogen spectrum?
Discrete lines
Basic principle of spectroscopy?
Transitions between energy levels occur when photons of energy with exactly the correct value are absorbed or emitted
What do different forms of spectroscopy involve?
Different forms of spectroscopy involve
energy levels arising from different processes in atoms and/or molecules
Three types of spectroscopy?
Absorption (can also be displayed in Transmission)
Emission
Scattering
Transitions within electronic energy levels in atoms/molecules?
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
Energy levels and spectroscopy?
In fact, all types of energy are quantised and
give rise to energy levels and spectroscopy
Molecules motion?
Translation, rotation, vibration
Size of energy levels in molecules?
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
Selection rules?
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
Gross selection rule?
In terms of molecular properties for particular types of absorption
Specific selection rule?
In terms of between which levels transition can take place
What does spectroscopy involve?
Spectroscopy involves the interaction and exchange of electromagnetic radiation with matter
What does absorption of energy involve?
Absorption of energy can only take place when the energy of the radiation exactly matches the difference between molecular energy levels
Transition between energy levels?
Transitions between energy levels involve discrete quantities of energy and give rise to spectroscopy
How can spectra be presented?
Spectra can be presented as absorption or transmission or emission spectra
Positions of peak in spectra?
The position of a peak in a spectrum is related to the difference in energy between energy levels
Electromagnetic spectrum?
The electromagnetic spectrum contains radiation with a large range of energies that can interact with various processes within molecules
Molecular spectroscopy?
Molecular spectroscopy can be used to identify compounds, to measure how much of a compound is present and to determine molecular properties
Molecules absorption of a photon?
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
Movement of a molecule?
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
Absorption spectroscopy?
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
Absorption spectrum?
The amount of radiation absorbed is plotted against the energy of the radiation the peaks rise up from a base line in the spectrum
Transmission spectrum?
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
Emission spectroscopy?
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
Raman spectroscopy?
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
What is the position of a peak in the spectrum determined by?
The energy of the transitions, by the difference between two energy levels in the molecule
What determines the size/intensity of the peaks?
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
Best way to see how the amount of material affect the spectrum?
By considering a compound in solution, can use the Beer lambert law
How does population of energy levels affect intensity of the spectral peaks?
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
How are populations described by the Boltzmann distribution if change in energy is small?
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
How are populations described by the Boltzmann distribution if change in energy is large?
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
Absorption peak and populations of energy levels?
For an absorption peak the population of the lower energy levels in the transitions affects the intensity of the peak
Emission peak and populations of energy levels?
For an emission peak the intensity depends on the population of the upper level
