11

RecapRecap

22

Quantum description of a Quantum description of a particle in an infinite wellparticle in an infinite well

Imagine that we put particle Imagine that we put particle (e.g. an electron) into an (e.g. an electron) into an “infinite well” with width “infinite well” with width L L (e.g. (e.g. a potential trap with sufficiently a potential trap with sufficiently high barrier)high barrier)

In other words, the particle is In other words, the particle is confined within 0 < confined within 0 < xx < < LL

33

Particle forms standing wave Particle forms standing wave within the infinite wellwithin the infinite well

How would the wave How would the wave function of the particle function of the particle behave inside the well?behave inside the well?

They form standing They form standing waves which are waves which are confined within confined within 0 0 ≤≤ xx ≤ ≤ LL

44

Standing wave in generalStanding wave in general

Description of standing waves which ends are Description of standing waves which ends are fixed at fixed at xx = 0 and = 0 and xx = = LL. (for standing wave, the . (for standing wave, the speed is constant), speed is constant), vv = = = constant)= constant)

L = 1/2(n = 1)

L =

(n = 2)

L = 33/2(n = 3)

L

55

mathematical description of mathematical description of standing wavesstanding waves

In general, the equation that describes a In general, the equation that describes a standing wave (with a constant width standing wave (with a constant width LL) is ) is simply: simply:

LL = = nnnn/2/2n n = 1, 2, 3, … (positive, discrete integer)= 1, 2, 3, … (positive, discrete integer)

n n characterises the “mode” of the standing wavecharacterises the “mode” of the standing wave nn = 1 mode is called the ‘fundamental’ or the = 1 mode is called the ‘fundamental’ or the first harminicfirst harminic

nn = 2 is called the second harmonics, etc. = 2 is called the second harmonics, etc. n n are the wavelengths associated with the are the wavelengths associated with the nn-th -th mode standing wavesmode standing waves

The lengths of The lengths of n n is “quantised” as it can take is “quantised” as it can take only discrete values according to only discrete values according to

nn = 2 = 2LL//nn

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Energy of the particle in the boxEnergy of the particle in the box

Recall thatRecall that

For such a free particle that forms standing waves in the box, For such a free particle that forms standing waves in the box, it has no potential energy it has no potential energy

It has all of its mechanical energy in the form of kinetic energy It has all of its mechanical energy in the form of kinetic energy only only

Hence, for the region 0 < Hence, for the region 0 < xx < < L , L , we write the total energy of we write the total energy of the particle asthe particle as

Lx

LxxxV

0,0

,0,)(

EE = = K K + + VV = = pp22/2/2mm + 0 = + 0 = pp22/2/2mm

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Energies of the particle are Energies of the particle are quantisedquantised

Due to the quantisation of the standing wave Due to the quantisation of the standing wave (which comes in the form of(which comes in the form of

nn = 2 = 2LL//nn),),

the momentum of the particle must also be the momentum of the particle must also be quantised due to de Broglie’s postulate:quantised due to de Broglie’s postulate:

L

nhhpp

nn 2

It follows that the total energy of the particle It follows that the total energy of the particle is also quantised:is also quantised:

2

222

2

22 mLn

m

pEE nn

88

2

222

2

22

2

282 mLn

mL

hn

m

pE nn

The The nn = 1 state is a characteristic state called the ground = 1 state is a characteristic state called the ground state = the state with lowest possible energy (also called state = the state with lowest possible energy (also called zero-point energy )zero-point energy )

Ground state is usually used as the reference state when Ground state is usually used as the reference state when we refer to ``excited states’’ (n = 2, 3 or higher)we refer to ``excited states’’ (n = 2, 3 or higher)

The total energy of the The total energy of the n-n-th state can be expressed in term th state can be expressed in term of the ground state energy asof the ground state energy as

(n = 1,2,3,4…)(n = 1,2,3,4…)

The higher The higher nn the larger is the energy level the larger is the energy level

2

22

0 2)1(

mLEnEn

02EnEn

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Some terminologySome terminology nn = 1 corresponds to the ground state = 1 corresponds to the ground state nn = 2 corresponds to the first excited state, etc = 2 corresponds to the first excited state, etc

Note that lowest possible energy for a particle in the box is Note that lowest possible energy for a particle in the box is not zero but not zero but EE00 (= (= EE11 ), the zero-point energy ), the zero-point energy

This a result consistent with the Heisenberg uncertainty This a result consistent with the Heisenberg uncertainty principleprinciple

n =1

n =2

n =3

n = 1 is the ground state (fundamental mode): 2 nodes, 1 antinode

n = 2 is the first excited state, 3 nodes, 2 antinodes

n = 3 is the second excited state, 4 nodes, 3 antinodes

1010

Simple analogy

• Cars moving in the right lane on the highway are in ‘excited states’ as they must travel faster (at least according to the traffic rules). Cars travelling in the left lane are in the ``ground state’’ as they can move with a relaxingly lower speed. Cars in the excited states must finally resume to the ground state (i.e. back to the left lane) when they slow down

Ground state excited states

1111

Example Example on energy levelson energy levels

Consider an electron confined by electrical Consider an electron confined by electrical force to an infinitely deep potential well force to an infinitely deep potential well whose length whose length LL is 100 pm, which is is 100 pm, which is roughly one atomic diameter. What are the roughly one atomic diameter. What are the energies of its three lowest allowed states energies of its three lowest allowed states and of the state with and of the state with nn = 15? = 15?

SOLUTIONSOLUTION For For nn = 1, the ground state, we have = 1, the ground state, we have

eV7.37J103.6

m10100kg101.9

Js1063.6

8)1( 18

21231

234

2

22

1

Lm

hE

e

1212

The energy of the remaining states The energy of the remaining states ((nn=2,3,15) are =2,3,15) are

eV8481eV7.37225)15(

eV339eV7.379)3(

eV150eV7.374)2(

12

15

12

3

12

2

EE

EE

EE

When electron make a transition from the n = 3 When electron make a transition from the n = 3 excited state back to the ground state, does the excited state back to the ground state, does the energy of the system increase or decrease? energy of the system increase or decrease?

1313

Solution: the energy of the system Solution: the energy of the system decreases as energy drops from 339 eV to decreases as energy drops from 339 eV to 150 eV150 eV

The lost amount |The lost amount |E| = EE| = E3 3 - E- E11 = 339 eV – = 339 eV –

150 eV is radiated away in the form of 150 eV is radiated away in the form of electromagnetic wave with wavelength electromagnetic wave with wavelength obeying obeying E = E = hchc//

1414

ExampleExampleradiation emitted during de-excitationradiation emitted during de-excitation

Calculate the wavelength of the Calculate the wavelength of the electromagnetic radiation emitted when electromagnetic radiation emitted when the excited system at the excited system at n n = 3 in the = 3 in the previous example de-excites to its previous example de-excites to its ground stateground state

SolutionSolution ==hchc/|/|E|E|

= 1240 nm. eV / (|E= 1240 nm. eV / (|E3 3 - E- E11|)|) = 1240 nm. eV / (299 eV – 150 eV) = = 1240 nm. eV / (299 eV – 150 eV) =

= 8.3 nm= 8.3 nm

n = 1

n = 3

Photon with = 8.3 nm

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ExampleExamplemacroscopic particle’s quantum macroscopic particle’s quantum

statestate

Consider a 1 microgram speck of dust Consider a 1 microgram speck of dust moving back and forth between two rigid moving back and forth between two rigid walls separated by 0.1 mm. It moves so walls separated by 0.1 mm. It moves so slowly that it takes 100 s for the particle to slowly that it takes 100 s for the particle to cross this gap. What quantum number cross this gap. What quantum number describes this motion?describes this motion?

1616

SolutionSolution

The energy of the The energy of the particle is particle is

J105s/m101kg1012

1

2

1)( 222692 mvKE

Solving for Solving for nn in in 2

222

2mLnEn

Yields Yields 141038 mEh

Ln

This is a very large number. It is experimentally impossible to distinguish between the n This is a very large number. It is experimentally impossible to distinguish between the n = 3 x 10= 3 x 101414 and n = 3 x 10 and n = 3 x 101414 + 1states, so that the quantized nature of this motion would + 1states, so that the quantized nature of this motion would never reveal itself never reveal itself

1717

The quantum states of a macroscopic The quantum states of a macroscopic particle cannot be experimentally discerned particle cannot be experimentally discerned (as seen in previous example)(as seen in previous example)

Effectively its quantum states appear as a Effectively its quantum states appear as a continuumcontinuum

E(n=1014) = 5x10-22J

allowed energies in classical system – appear continuous (such as energy carried by a wave; total mechanical energy of an orbiting planet, etc.)

descret energies in quantised system – discrete (such as energy levels in an atom, energies carried by a photon)

E ≈ 5x10-22/1014 =1.67x10-36 = 10-17 eV

is too tiny to the discerned