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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

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
Do somebody tell me a best nano engineering book for beginners?
s. Reply
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?
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
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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