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This is the continuation of Chapter 1_Part 7 where we describe the resonant transition , fluorescence, phosphorescence and we bring out the distinction between spontaneous and stimulated emission.


Atomic Energy Exchanges are governed by resonance phenomena. If the incident photon energy is approximately equal to the transition energy then photon is said to resonate with transition and the excitation of the electron at the given transition is favored.

For 1 electron atom:

P(transition)= (SinӨ/Ө)^ 2 = Sinc^ 2 (Ө) = Sin^ 2 [π(ν ko -ν)t]/[ π(ν ko -ν)t]^ 2 1.40

Where P(transition) is the probability of transition from ground state to the excited state k.

( Zambuto, Mauro: Handbook of Electrical and Computer Engineering (ed. Sheldon S.L. Chang) , Vol.1, pp. 597- 623, John Wiley&Sons, Inc, New York, 1994;

Leighton, Robert A. Principles of Modern Physics, McGraw Hill, New York, 1954 )

Transition Energy from ground state to excited state = h ν ko = (E n – E 0 );

Incident Photon Energy = h ν;

t = duration of interaction;

For reasonable interaction duration, only incident photon with frequency ν = ν ko will induce excitation. Therefore only resonant photons are absorbed. To the rest of the photons, the atom appears to be transparent.

Probability of Absorption is sharply peaked at resonance as shown in Fig.(1.18)

Fig.(1.18) Probability of Absorption is given by (SincӨ) 2 where Ө= π(ν ko -ν)t

Spontaneous Emission Phenomena .:

When an excited atom returns to ground state in the natural course of events and in the process emits a photon equal to the transition energy , we say spontaneous emission has taken place. This process of spontaneous emission is also known as relaxation.

When relaxation is in nanosecond, we have fluorescence effect. In fluorescence the excited atom relaxes by emitting a lower frequency photon as compared to the higher frequency absorbed. The difference between absorbed and emitted is lost due to collisions with other molecules. The energy absorbed resides in the molecule as rotational energy, vibrational energy and as excited electron energy. The lowering of emitted photon takes place due to the loss of a part of the vibrational energy. We will have a detailed discussion on this topic in the last section, specific heat section, of this Chapter.

When relaxation is in micro and millisecond, we have phosphorescence effect. The photo emission is delayed and prolonged. Here the absorption of photon causes the transition from ground singlet state to excited singlet state. In singlet state total spin quantum number is S =0. Due to collisions with other molecules, the excited singlet state loses a part of vibrational energy and suffers a radiationless transition to excited triplet state (S=1) . The energy difference between the vibrational energy states is lost as the thermal energy of the gas molecules. Now a radiative transition from excited triplet state to ground singlet state is forbidden by SELECTION RULE [ Appendix XXXI A]. Though forbidden nevertheless after some time the excited triplet molecule does relax to the ground singlet state thus giving rise to a delayed and prolonged emission. This is termed as Phosphorescence. The delayed emission may occur after minutes or even after hours depending upon the composition of phosphorescence screen.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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