13 - 14 Flashcards
what are 3 processes of luminescence
(!1) Absorption of external energy
(2) Excitation
(3) Emission of energy in form of light
Describe mechanism of relaxation of luminescence
(1) Excitation of an external electron
(2) Spontaneous relaxation of electron back to ground state
(3) Light emission
What are 2 types of luminescence?
Flourescence and Phosphorescence
What is singlet state?
Paired electrons with net spin of 0
-> , all electron spins are paired.
What is triplet state?
One set of electron spins is unpaired
Describe mechanism of fluorescence
When thermal relaxation is followed by a direct return of electron to the ground state
=> the excess energy is emitted in the form of photons
(Excitation and relaxation (light emission) in singlet states)
State Kasha’s rule
The excited molecule reaches the lowest vibrational level of the 1st excited state through fast transition
=> Photon emission occurs from the relaxation of the lowest vibrational state of 1st excited state to ground state
(radiations released above the thermal radiation of a body - cold light)
What is Stoke’s shift?
The phenomenon that…
- In spectrum of fluorescence, usually a single band which is less structured and is shifted towards the longer wavelength with respect of the absorption band
( the difference between the spectral position of the maximum of the first absorption band and the maximum of the fluorescence emission.)
Describe mechanism of phosphorescence
It occurs during transition from triplet state to singlet ground state by emitting light photon
(= relaxation after spin transition
What are 3 modes of luminescence (bring electrons back to ground state?)
(1) Thermal relaxation
(2) Internal conversion
(3) Intersystem crossing
What is internal conversion?
The transformation of total excitation energy into heat
What is intersystem crossing?
A change in the spin state of an electron (e.g, singlet state to triplet state
What is spectrum?
Distribution of intensities along wavelength
Compare wavelength of excitation band, fluorescence band and phosphorescence band
What is excited state lifetime?
The decrease of number of excites electrons following an exponential decay function
(or the time during which number of electrons decreases to its e th)
Compare excited-state lifetimes pf fluorescence and phosphorescence.
What is the cause?
Excited state lifetime of fluorescence is SMALLER than that of phosphorescence
What is fluorescence quantum yield?
What is Boltzmann-distribution?
The distribution of energy among possible states (Degrees of freedoms)
(number of particles in excited to ground states ni/no)
Describe lifetime of fluorescence
It is inversely proportional to the probability of emission and sum of non-radiative transitions (intersystem crossing, internal conversions,…..)
3 light sources of luminescence
1 application of Low-pressure Hg-vapor lamp
Germicid lamp which kill germs by photon energy released by Hg low pressure
The role of metal vapor lamp
Detect DNA of bacteria
-> bacteria absorb the light
=> DEATH
3 applications of luminescence in dental medicine
(1) Fluorescence image
(2) Amalgam restoration
(3) Caries detection
Applications in dental medicine
-> The role of Red fluorescence?
It indicates the activity of cariogenic bacteria
Applications in dental medicine
What is Auto-fluorescence of teeth?
When teeth are illuminated with high intensity blue light they will start to emit light in green.
=> Diagnoses of real and fake teeth
