Light Absorption + Emission Flashcards
Light scattering (Rayleigh and Mie).
- Lights scattering happens when EM waves encounter particles in air, Light waves cause a dipole moment, so electrons in atom vibrate, Cause them to emit light
- Rayleigh: Depends on wavelength (blue scattered more), When spacing is larger than wavelength there is no interference. Intensity of incident light = intensity of scattered light
- Mie: All wavelengths scatter equally, producing white light, when size of particle is on order of wavelength.
The Beer-Lambert law.
In dilute solution, if solvent doesn’t absorb in applied wavelength, absorption coefficient is proportional to concentration of solute. (Formula)
Properties of the absorption spectrum.
- Absorbance or transmittance (photon goes through something) as a function of wavelength
- Can be used to identify an element
- Absorption requires excitation of electron
Turbidimetry and nephelometry.
- Turbidemitry: Involved with measuring the amount of transmitted light, and calculating absorbed light by particles in suspension to determine conc. Of a substance.
- Nephelometry: At low intensity, measures scattered light. Proportional to conc. Amount of scattered light is much greater than transmitted, this method offers a higher sensitivity than turbidimetry.
- Both Dependent on: Number of particles, Size of particles.
Dynamic light scattering
- Method of analyzing solutions where hydrostatic diameter of particle can be measured.
- Light directed through sample —> Scattering occurs —> light intensity detected on other side —> intensity changes when particles in solution diffuse (brownian motion) —> speed of change depends on size of molecules —> information about intensity used to calculate diffusion coefficient.
Formula: D= k*t/6pinr
Measurement of the absorption spectrum.
- Absorbance vs wavelength of incident light.
- Absorption maxima
- Incident light must be at certain frequency
Energy levels of atoms and molecules: the Jablonski diagram
(Diagram)
Kasha’s rule when electron goes to closest energy level (internal conversion)
And intersystem crossing when going to Triplet or “Forbidden state”
Thermal radiation
- Transfer of heat using EM radiation
- Possible even in Vacuum
- Everything over 0K radiates thermal radiation
Planck’s radiation law
- Studied emission spectrum of black bodies
- Energy emitted resulted from vibration of atoms within the material
- Vibrational energies have discrete values, 1, 2, 3 , never in between.
- E2 - E1 = e = hf
- 6.626 x 10^-34
Light sources based on thermal radiation
- Sun, lightbulbs, heated metal.
Absolute black body
- Ideal, theoretical body which absorbs all radiation incident on it and reemits it.
- Model can be created from closed metal cavity with hole drilled so radiation entering can not easily escape, so absorbed completely.
- Stefan-Boltzmann law describes that emittance of a black body is proportional to the fourth power of the temperature. M black = o T4
Emission spectrum of the absolute black body.
- Emission is in all wavelength spectrum
- Wien’s law: Maximum radiation is in wavelength that is inversely proportional to temperature.
- At low temp, black body is dark, most energy radiated is infra-red
- When temp increases it glows red first, then yellow, then white/blue.
Medical applications of thermal radiation
- Thermography: Test using infrared camera to detect heat patterns and blood flow in body tissues. DITI is the type of thermography used to diagnose breast cancer. (Thermotropic crystals used)
Kirchhoff’s law
- A body which radiates more thermal energy also absorbs thermal energy to a higher extent.
- Ratio between radiant emittance and absorption coefficient is constant with a narrow wavelength range.
The Stefan-Boltzmann law.
Describes that the emittance of a black body is proportional to the 4th power of the temperature.
M black = o T^4
Wien’s displacement law
- The black body radiation curve for different temp peaks at a wavelength inversely proportional to the temperature.
Luminescence: excitation and relaxation.
- Emission of excess energy from excited electrons in form of light.
- Types of excitation: Thermo, bio, photo electro.
- Process: Absorption of external energy causes excitation and emission of energy in the form of light.
- Types of relaxation: Fluorescence, Phosphorescence
Kasha’s rule
- The excited molecule first reaches the lowest vibrational level S1
- Internal conversion
- Photon emission occurs going to ground state.
Fluorescence
- If the luminescence stops as excitation stops
Phosphorescence
- During transition from triplet state to ground state
- Slower luminescence than fluorescence.
Luminescence spectra
- Atoms: Line spectra (Medium pressure Hg lamp)
- Molecules: Band spectrum, (High pressure Hg lamp)
Stokes-shift
Shift difference between peak absorption and peak emission due to loss of energy in the form of heat.
The fluorescence spectrometer
Device which shines light through a sample, measuring excitation spectrum and resulting emission spectrum.
- Parts: Xe lamps, Excitation monochromator, sample cell, emission monochromator, photon detector.
(FRET) Fluorescence Resonance Energy transfer
Energy transfers from donor molecules without emission, to acceptor molecules when they form dipole-dipole interactions.
1) They have to be in the right orientation
2) They have to be really close
3) Spectral overlap between donor and acceptor
Applied to protein-protein interactions
Fluorescence Recovery after photobleaching (FRAP)
- Used to study diffusion of molecules on the lipid membrane.
- Limited cycles of excitation and emission for fluorescent molecules, so they can be bleached.
- When previously bleached area of membrane recovers, this is proof of lateral diffusion.
Notable transitions of luminescence: vibrational relaxation, intersystem crossing.
- Vibrational relaxation: Internal conversion. Kasha’s rule states an excited molecule will first reach lowest vibrational level of S1 by vibration and rotation losing energy as heat.
- Intersystem crossing: Occurs in phosphorescence, a process in which electron transition occurs between the singlet (S1) and the triplet state (T1)
Quantum yield of luminescence.
Measure of the efficiency of photon emission through fluorescence, which is the loss of energy by a substance that has absorbed light via emission of a photon.
Quantum yield = N. Of photons emitted / N. Of photons absorbed
Luminescence lifetime.
Inverse of rates of all transitions
N = No e^t/lifetime
Laser: induced emission.
- Process where emission is stimulated by incoming photon (incoming photon is amplified)
- Laser materials, there are 3 energy levels, and one should be long lifetime.
- Emission from level is achieved only by induced emission.
Laser: the optical resonator.
- Tube with 2 mirrors on both sides to reflect light emitted by laser material to amplify it.
- One mirror allows 1% of light through, this will be utilized.
- Condition for resonator is that wavelength must return in same wavelength.
- 2L = m*wavelength
Laser: population inversion.
- Process where exerting energy on system the electrons are pumped from their ground state, and most molecules are excited (Until induced emission is formed)
Types of lasers.
- CO2 Laser: Surgery
- Krypton Laser: Opthalmology
- Ruby laser: Dermatology
- Solid state, Gas, Dye lasers
Properties of laser light
- Monochromatic
- Coherent (Due to induced emission)
- Small divergence (near to parallel)
- High intensity (beam focusing)
- Often polarized
Applications of lasers.
- Surgery (CO2)
- Photodynamic diagnosis and therapy: fluorophores inserted into body, causing tumor cells to fluoresce. Laser beams directed at that area cause free radicals which kill tumor cells.
