Light And Absorbance Flashcards
What is frequency
The amount of waves (occilations) per second
1 oscillation per second is Hz
Relation between frequency and wave length equation
V(wavelength)=c
C is speed of light
V= frequency
What is a photons energy given by
The symbol E
E=hv
Or E=hc/v
H is plancks constant
What is light
Electromagnetic radiation
(Also frequncy)
It can be quantized as packet:units of energy (photons)
What can light do when is interacts with matter
Can be absorbed by the atom/molecule (its energy is taken in and the colour is not shown)
Can scatter off the molecule (the light changes direction in diff way when it interacts) this scattering can be elastic or inelastic
Can be transmitted through the material (light interacts with objects then passes through the object)
Can also be emitted by atoms/molecules (it realesse stored E and we see the light)
What happens to E in elastic scattering?
E does not change during elastic scattering
What happens to the molecules to account for the energy inputs for
X rays
Uv
Infrared
Microwave
X rays: the bonds between molecules break and ionize
Uv: the electrons get excited and jump up in energy
Infrared: molecules vibrates
Microwave: molecules rotate
When light hits atoms, what happens to the electrons
This is atomic absorbance
If absorbed, The electrons are energized and promoted to a higher level energy orbital/level (promoted to excited state)
If the atom emits the photon the energy is lowered
This is atomic absorbance
In molecular absorbance, usually where is the electronic transition going to occur
The electronic transitions are in the molecular orbitals
A HOMO to LUMO transition that involves sigma and pi orbitals
If a larger wavelength, the change in orbitals is also larger
In organic molecules, where are most of the electron trantstions happened
In the UV wavelength,
Most organic molecules absorb the light in the uv range
What is special about longer conjugated pi systems in regard to molecular absorbance
The have a larger area over which the electrons can move
And the electrons require less energy to transition from pi to pi anti binding orbital
This means they have a larger range of wavelength they can absorb (move from uv to visible range)
What are chromophores
Molecules that absorb light a a certain wavelength and emits colour as a result
Most chromophore of molecules are aromatics or conjugated systems
In a molecule, Because each electron transition corresponds to a very specific wavelength that is being introduced to molecule, what do we notice
- If monochromatic light of that specific wavelength the the molecule absorbs is sent through the sample, the lights intensity will dim (because the light got absorbed by the sample
- If polychromatic light (many diff colours/wavelengths) is sent in to that sample, that wavelength that matches the absorption wavelength will be reduced but other wavelengths won’t
Ex. If blue absorbed we see orange, if red and blue absorbed, we see yellow
What does P and P0 stand for
Irradiance
Also called power out and power in
Usually p< or equal to p0
If p is lower that power in, that mess the photons lost were absorbed and more light was lost meaning more of the molecule is absorbing light
1 photon absorbed equals
1 electron transition
What is transmittance
The fraction of the original light that passes through the sample
If the sample is not absorbing, it’s 100% T (blank)
If the sample is absorbing everything, it’s 0% T (dark blank)
What is absorbance
How much the sample is absorbing
If p=P0 the absorbance is 0 since that power out was the same as in
Log scale so if it went from A=1 to 2 that means the sample absorbed 10x more light
A is always less or equal to 2
What is beers law
It describes the relation between absorbance and transmittance
Specific to molecule and wavelength
It’s not specific to the instrument/set up (means we can compare across other labs)
b= path length (length of cuvette)
e=molar absorptivity
c=concentration of the sample
If e is small less light absorbed, if big more light
How do you make an absorbance spectrum
Since absorbance is diff at diff wavelengths, you measure a range of wave lengths and their absorbances
At low concentrations of a compound was happens to the absorbance
We lose some details on the absorbance spectrum graph, it gets blurred
What are the minimun components of a spectrophotometer
A light source (should be reliable and give right wavelength)
A monochromator or a filter to select the wavelength (selects a narrow band of wavelengths to pass through from the light source so all the light is absorbed by the sample)
A sample holder (cuvette)
A detector: which converts the light intensity that passes through the sample to a signal that we can read and use
What should you be careful of with a cuvette during spec
The material is made of needs to allow the wavelength range that we’re interested in to pass through
It should also be compatible with the solvents/sample you’re using
Not made of glass because glass blocks uv light, need quartz for uv
Cuvette that are exposed to IR wavelengths need to be made off Kbr or Nacl after 84000/cm wavelength
Why do we use different shapes of sample holders for measurements
The shape depends of what we want to measure
Longer or shorter path lengths for more or less dilute solutions
Microcells for if the sample is small amount
Flow cuvette for watching the reactions inside it progress without cooling out the samples
Thermal for if you want to keep the temp controlled
Gasses a dilute and have low absorbances, what can we do to increase the signal for gaseous samples
Increase path length (m/km, but not actually a km just by reflection)
Make sure light is collimated so it doesn’t get lost in long distance of the cell (b)
Increase the pressure to increase C
What are the downside to increasing pressure for a gaseous sample to increased its C
It could become more reactive, pressure broadening
What are single beam instruments
They have only one light path
You measure a blank solution to get P0
Then replace the blank with the analyte solution and get P
What are double beam instruments
Splits light evenly between the analyte and blank cuvette so you can get p and p0 without moving anything
It reducers the error of moving the cuvette
What is the first error that affect absorbance
How do we reduce it
Scatter from particles/bubbles in solution which direct light away from the detector and reduces P more that it should be
Absorbances gets increased from true value
Reduced by filtering the samples, using clean solutions, and lids to keep dust out
What is the second error that affect absorbance
Reflection off the cuvette walls
This reduces P by losing light (can be reduced by using same cuvette for P and P0)
Caused by cuvette position/angle (align the cuvette carefully)
A is increased since P decreases
What is the third error that affect absorbance
Stray light can enter if the lid isn’t closed
The extra light from the outside will be included into the P value.
A will be decreasing since P is increased and we can tell where light came from or went
What determine what analyte concentration we use to measure absorbances
We need an analyte concentration that gives an absorbance between 0.1-1
Outside the range is more error prone
What does is mean if absorbance is too low
P is very close to P0
Any variation in intensity can cause fluctuations in A
What does is mean if absorbance is too high
The P is very low which may be due to electronic noise in the detector
This may influence the reading you do (cause uncertainty)
Affects like shadowing can happen (another molecule in solution doesn’t get to absorb any light)
What is the slope of the calibration curve for absorbance
The y int
eb from beers law
Ideally zero
What does e in beta law depend on
Analyte and wavelengths
How do we correct for path length changes throughout labs when measuring absorbance
Subtract A value from blank from all sample A values and itself
This brings the initial absorbace to zero and cancels out the cuvette scattering light and any small A from the solvent
Calculation of mass and concentration of unknown using eb and A and linear equation
Slide 25
What is non radiative decay
When the light is absorbed by something the energy from it is dissipated through collisions with other molecules as kinetic (heat) energy
What is luminescence
After light that has been absorbed, the energy from it is released as a photon from the excited state of the molecule
In this category is phosphorescence and fluorescence
What are the characteristics of fluorescence compared to absorbance
absorption is 10^-15 seconds (Fast) but fluorescence is slower than this, the exited state lasts for 10^-8 to 10-4 seconds
The emmision of a photon is at a lower energy meaning it comes in at a longer wavelength
Singlet electron transition (electron keeps its original spin)
What is stokes shift
The emmision of a photon is at a lower energy meaning it comes in at a longer wavelength
Losing energy
Absorption is at low wavelength and fluorescent is at a higher
Explain the first step in how a photon travels during fluorescence
- The photon is absorbed and excites an electron from and electron pair in the molecule from ground state (S0) to excited state (s1)
One electron from the pair stays in the ground state
Explain the second step in how a photon travels during fluorescence
- The electron in the excited state (s1) undergoes vibrational relaxation
it becomes less excited loses energy and moves down a few energy levels (non radiative decay, no light emitted )
Explain the third step in how a photon travels during fluorescence
- Emmision: the electron undergoes radiative decay (emits light) to any vibration level in S0
The energy from this decay is released/emitted as a photon
The electron then relaxes and goes back to ground state and keeps the spin pairing
Explain what internal conversion And inter system crossing is
Internal: the nonradiative transition of the electron that keeps is original spin
Intersystem: the nonradiative transition of the electron that changes its orginal spin
Explain phosphorescence
It involves an intersystem crossing to a triplet (T1) excited state (the spin changes)
Long time in the excited state (10^-4 to 10^-2 seconds
Has a stokes shift larger than fluoresces because of the 2 relaxations. Losing more energy so the wavelength is longer
Explain the first step in how a photon travels via phosphorescence
The electron gets excited, goes to s1
then does nonradiative decay to T1 while changing its spin
This is how it does intersystem crossing
Explain the second step in how a photon travels via phosphorescence
After the intersystem crossing, the flipped electron does another non radiative relaxation to a lower level in T1
Is stays there for longer because the electron now has the same spin as the other one so it won’t go to the ground state as readily
Explain the third step in how a photon travels via phosphorescence
The electron that has just relaxed in T1 (triplet excited state) relaxes radiatively
This releases the photon after a longer waiting time
The electron converts its spin again (to singlet) and goes back to ground state and relaxes
What is chemiluminescenes
What things use this
When we get emmision of photons even through no light has hit the sample
A energy of a chemical reaction produces the excited state electron and the light that is emitting
Luminol and glow sticks
The amount of light out give the concentration
What is internal conversion
External
Conversion to a lower energy electron within the same molecule
Conversion to a lower energy electron by losing energy to other molecules
Slide 31 comparison of absorption and emmision
Ok look at it actually
What is included in a flourecense spectra
Both excitation and and emission spectra
What is an excitation spectrum
It’s made by measuring a varied range of excitation wavelengths while keeping a the emmited light at a consant wavelength
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