11.12 Luminescence and laser Flashcards
What is luminescence?
Spontaneous emission of a photon due to relaxation of an excited electron
Fluorescence lifetime
10^(-10) - 10^(-7) seconds
Phosphorescence lifetime
10^(-5) - 10s
What is molecular vibration?
change in shape, stretching, bending of a molecule
What does the Jablonski diagram illustrate?
transitions between electronic states of molecules
What is spin multiplicity?
number of possible orientations of the spin angular momentum
What is emitted / released in vibrational relaxation?
heat, NO PHOTON RELEASED
What is intersystem crossing?
transition between two electronic states : from singlet to triplet state. Radiation-less process
Which has more energy? Fluorescence or phosphorescence?
Fluorescence
What is Kasha’s rule?
The excited molecule first reaches the lowest vibrational level of S1 and photon emission always occurs from this state to any vibrational level of the ground S0 state
What is Stoke’s rule?
Wavelength of emitted light is almost always longer than that of the excitation photon due to loss of E via heat
What is the stoke-shift?
Difference in the luminescence spectrum between aborbed and emitted wavelengths due to loss of energy as heat
What is the quantum yield?
measure of efficiency of emission : number of photons emitted / number of photons absorbed
What is the lifetime of luminescence?
decay of emitted luminescence
How do you calculate lifetime of fluorescence?
τ = 1/(Kf+Knr)
τ : lifetime
Kf : rate of photon producing transitions
Knr : rate of non-radiating transitions
What is the function of a fluorescence spectrometer?
analyses fluorescence properties of a sample to determine the concentration of an analyte in solution
What does FRET stand for?
Fluorescence resonance energy transfer
What is FRET used for?
To measure distance and detect molecular interactions between organisms
What is the process used in FRET?
energy transfer from donor to acceptor without emission, thanks to dipole-dipole interactions
What does FRAP stand for?
fluorescence recovery after photobleaching
What is photobleaching?
photochemical alteration of a dye or fluorescent molecule - permanently unable to emit fluorescence
What is a dichroic mirror?
mirror that lets only a small range of light pass through and reflects the rest
What is induced emission?
emission is induced by an incoming photon : incoming radiation is amplified (since they have same phase, constructive interference)
What is a coherent process?
a process is coherent if the difference of their phase is constant or changes in a regular way
What does LASER stand for?
light amplification by stimulated emission of radiation
What does the optical resonator ensure?
ensures positive feedback and choice of frequency corresponding to the resonance
What is the condition for laser resonance?
2L = m * lamda
m : integer
What does the frequency of the laser depend on?
On the spectral properties of the mirror and the length between mirrors
What are the 6 properties of laser light
- small divergence : parallel beam
- Large power
- Small spectral bandwidth (monochromatic)
- Often polarized
- Possibility of short pulses (ps, fs)
- Coherence
What is coherent length?
largest distance at which interference can still be observed (large for lasers) bc close emission points
What are 4 types of lasers?
- Solid state laser
- Gas laser (Co2)
- Dye laser (coumarine)
- semiconductor (diode) laser
What is a solid state laser?
Laser that uses solid medium instead of a liquid or gas one. For example neodymium-YAG laser
2 examples of gas lasers
CO2, HeNe
2 examples of dye laser
coumarine, rhoatamine
