foundations of spectroscopy Flashcards
(31 cards)
spectroscopy definition
the study of how EM radiation interacts with matter
give 5 uses of spectroscopy
- identification of molecules
- checking success of reactions
- checking purity
- determining molecular properties/structure
- quantitative analysis
spectrum definition
a plot of energy of excitation vs absorption, transmission or emission
absorption definition
measures amount of radiation absorbed/taken in depending on wavelength
transmission definition
measures amount of radiation transmitted/passed through depending on wavelength
emission definition
measures amount and wavelength of radiaiton emitted/released by a molecule following irradiation
wavelength, λ definition
distance between successive waves
units = m
frequency, v definition
number of waves per second
units = 2^-1 or Hz
wave number definition
= 1/λ , number of waves per metre
units = m^-1
what is the velocity of a wave?
= frequency x wavelength = speed of light
2.997x10^8 ms^-1
what are the 5 energy stores that make up total energy in a molecule?
translational (motion of whole molecule)
nuclear spin (changes in spin of electrons/protons)
rotational (rotation of molecule)
vibrational (motion of bonds between atoms)
electronic (movement of electrons between levels)
ΔE increases from translational (tiny ΔE) to electronic (large ΔE)
what is the born-oppenheimer approximation?
the assumption that the energy stores in a molecule are independent of each other
explain why the idea of quanitisation is fundamental to spectroscopy
energy of EM radiation is quantised, meaning it takes discrete values carried by photons
energy can only be released/absorbed in small packets (photons)
electrons can only occupy certain energy levels
snapshot definition
when measurement time is faster than process being measured e.g. electronic spectroscopy gives snapshots
- usually many molecules are being observed
so spectra produced in a combination of snapshots of each molecules electronic / vibrational / etc properties
average definition
when measurement time is slower than process being measured - as usually molecules are undergoing dynamic processes e.g. NMR gives an average
- lowering temp may allow a snapshot to be taken in process becomes slower than measurement timescale
how can electrons move between energy levels?
the molecules must absorb or emit a photon of equal energy to the gap between the initial energy level and the new energy level
absorption spectra definition
molecules absorb photons with energy corresponding to energy level differences, the electron moves to a higher energy level
emission spectra definition
molecules emit photons with energy corresponding to energy level differences, the electron moves to a lower energy level
e.g. atomic spectra
what does absorption depend on?
absorption is related to the number of molecules in a particular energy level which can be promoted to the next state by absorbing a photon
what does intensity of peaks depend on?
occupancy/population and degeneracy
this is because if more molecules are at a particular energy level then more molecules will be able to absorb a specific photon and be promoted, if levels are degenerate more molecules can occupy the same state, or undergo transitions equal to the same photon energy
occupancy/population definition
number of molecules in each state
degeneracy definition
levels with the exact same energy within a molecule
what condition affects occupancy + why?
temperature dependent
at room temperature it is unlikely than all molecules are in the lowest state, but also few molecules will be in high energy states, molecules will occupy a range of energy levels depending on available thermal energy, or size of ΔE
how does the boltzmann distribution link to spectroscopy?
it tells us that the intensity of absorption peaks depends on the number of molecules that absorb the radiation (aka the population)
