Flashcards in Spectroscopy Basics Deck (38)
What is a spectrum?
A plot of energy vs absorption, transmission or emission
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
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)
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
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
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
The electromagnetic spectrum contains radiation with a large range of energies that can interact with various processes within molecules
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
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
The amount of radiation absorbed is plotted against the energy of the radiation the peaks rise up from a base line in the 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
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