7. Spectroscopy Flashcards
What is a photon?
Single wave-packet of electromagnetic radiation, which carries an amount of energy defined by its wavelength
Both a wave and particle
What happens to the amount of energy the photon carries when the wavelength is shorter?
The shorter the wavelength, the higher the amount of energy it carries and the more it behaves like a particle under normal conditions
What happens during absorption of a photon of the correct energy (wavelength)?
Leads to excitation of electron to a higher energy level= Photon MUST match the energy level in order to be absorbed
Too high or too low energy which the photon has, absorption does not occur
How are absorption spectra measured for compounds in solution?
Proportional to the concentration of absorbing compound= Simple, general purpose method for determining concentrations of compounds
Colour of compound in solution is determined by absorption of light whose wavelength is the complementary colour
What other types of molecular motion can absorption of photons of suitable wavelengths result in?
High energy level photons= Effects on electron levels
Lower energy level photons= Vibration, Rotation and Molecular motion (heat)
Vibrational molecular motion: Absorption of photons in mid-infrared region (2500-25000nm wavelength) = Shows on infrared spectrum which shows characteristic bands for different bond vibrations
Rotational motions= Result from absorption of photons in far-infrared region (microwave region)
How do microwave ovens work?
Cause warming by emitting photons of suitable wavelength to be absorbed by water molecules in food
Causes rotational motion
How can the amount of radiation absorbed be measured? What is the equation for absorbance using %Transmittance?
A = 2 - log(%T) as at A>2= Very little light is transmitted therefore measured absorbances may be inaccurate
What is the Beer-Lambert law?
A = ε x c x L
Where
A= Absorbance
ε= Molar absorption coefficient (M^-1 cm^-1)
L= path length (length of solution the light passes through) (cm)
What is the Beer-Lambert law used for?
Gives a linear relationship between absorbance and concentration
Allows absorbance to be used as a measure of concentration
The higher the ε, the more strongly the compound absorbs light
What is absorbance widely used for?
As a method for detecting compounds in solution
Changes in chemical structure can change both the absorption maximum and molar extinction coefficient- Changes absorbance
What is fluorescence?
An electron can get ‘stuck’ in a higher energy level= Only way out is to lose the energy by emitting a photon
However, since some energy will have already been dissipated as molecular motion, the emitted photon ALWAYS has lower energy than absorbed photon
Lower energy of emitted photon= Longer wavelength= Different colour= fluorescence
How is fluorescence measured?
Simple fluorimeter
Light emission is measured at right angles to the incident beam= Prevents transmitted light from being detected
