Physics 203/204Physics 203/204

10 Interaction of Light with Matter

•Fluorescence and Phosphorescence•Lasers•Diatomic Molecules•Rotational and Vibrational Motion of a Molecule•Molecular Spectra

Fluorescence and Phosphorescence

•Fluorescence

•Light emission ends instantly incident radiation does

• Phosphorescence

•Light emission continues after incident radiation is shut off

•LIFETIME of excited state shortLIFETIME of excited state short

•FLUORESCENCEFLUORESCENCE

•LIFETIME of excited state longLIFETIME of excited state long

•PHOSPHORESCENCEPHOSPHORESCENCE

h

Spontaneous EmissionSpontaneous Emission

•Emitted frequency does not have to equal incident frequency

•Must be less than or equal to incident frequency

•Incident blue light can produce red light

•Incident red light CANNOT produce blue light

h

STIMULATED STIMULATED EMISSIONEMISSION

h

h

h

•Emitted light is

•In phase

•Going in same direction

•More intense

•same frequency

•with incident light

•Thus we get intense MONOCHROMATIC

•light emitted

•Need long lived excited state•in order to have high probability•of Stimulated emission•META STABLE STATE

•If all light produced by stimulated emission

•is made to travel in same direction

•and not diverge (Attenuate)

•We would have a strong beam of parallel intense monochromatic light

•L ight•A mplified by•S timulated•E mission of•R adiation

Helium States Neon States

Pumping

Electric discharge

Metastable State

He-Ne collisions

E2

E1

E0

h E2-E1

Laser light

632.8 nm

rapid decay

ground state

Diatomic Molecules

n1l1ml1ms1nNlNmlNms N

r1,r2, rN Antisymmetric combination of the products

n1l1ml1ms1

r1 n2l2ml2ms2

r2 nNlNmlNms Nr N

Motion of an object can be analyzed into three

types

•translation of the center of mass•rotation (rigid) about the center of mass •vibration (elastic) about center of mass

The energy of a molecule can be broken down

into four parts

Etot Eelec Etrans Erot EvibThe motion of the molecule is coupled to the

electronic standing wave patterns and effects

their energyfrequency by changing their

environment, i.e. different translational or

rotational or vibrational motions of the nuclei

produce electronic standing wave patterns

electronic stationary states of slightly different

energies.

The rotational energy of a molecule is given by

Erot 1

2Icm2

1

2Icm

Icm 2 1

2Icm

Lcm2

Icm

m1m2m1 m2

r2 r 2

reduced mass, r inter atomic distance

The angular momentum of the molecule is quantized

Lcm

= J(J 1) ; J 0,1,2,3,

Erot

2

2IcmJ(J 1); J 0,1,2,3,

The frequency differences between these levels lie in

the microwave frequency range

Near the equilibrium bond length,

the bonding force between the nuclei

can be approximated by a spring force

and a diatomic molecule simulates

a simple harmonic oscillator of frequency

f 1

2 k

; k spring constant

motion of reduced mass about spring whose

equilibrium length is the equilibrium bond

length

E vib v 1

2

h ; v 0 ,1, 2 ,

The frequency differences between these levels

lie in the infrared range

Molecular spectra will have both vibrational and rotational levels. The spacing between rotational levels is much smaller.

Hence for each electronic level there will be a set of vibrational levels superimposed and for vibrational levels there will be a

set of rotational levels superimposed . The total energy for a fixed electronic standing wave pattern

labeled by "n"is the sum of the energies due to vibrational and rotational modes

EnvJEn 2J J1

2

2I v 1

2

h; J 0,1,2,;v=0,1,2,

E000

E001

E010

E020

E100Molecules will absorb or emit photons that

have frequencies equal to the

DIFFERENCE of natural

frequencies

E101

E110

E120