INTRODUCTION

• Characteristics of Thermal Radiation• Thermal Radiation Spectrum

• Two Points of View

• Two Distinctive Modes of Radiation• Physical Mechanism of Absorption and Emission• Transition of Energy States

• Physical Mechanism of Scattering

Characteristics of Thermal Radiation

1.Independence of existence and temperature of medium

Ex) ice lens

black carbon paper

ice lens

2. Acting at a distance

• electromagnetic wave or photon

conduction

• photon mean free path

• volume or integral phenomena

• ballistic transport

diffusion or differential phenomena as long as continuum holds

free electronsolid: lattice vibration

(phonon)

fluid: molecular random motion

Ex) sky radiation

d Phonon

Phonon

Phonon-Boundary Scattering

Si@300K = 300 nm

d < 60 nm (channel length of SOI transistor)

100

120

140

160

102 103

Silicon layer thickness (nm)

40

60

80

Th

erm

al con

du

cti

vit

y (

W/m

K)

40

Phonon boundary scattering predictions

Asheghi et al. (1998)

Savvides et al (1973)Yu et al. (1998) Ju & Goodsen (1999)

Escoba & Amon (2004)

T = 300 KBulk silicon

104

Thermal Conductivity of Silicon

ad

ibati

c

wal l

cold wall at Tc

L

a) m << L : normal pressure

b) m ~ L : rarefied pressure

c) m >> L

gas

ad

ibatic

w

all

hot wall at Th

3. Spectral and Directional Dependence• quanta• history of path

surface emission

CO2 at 830 K, 10 atm

Thermal Radiation Spectrum

10-2 10-1 1 10 102 103

ultra violet

infraredvisibl

e

0.4 0.7

thermal radiation

Two Points of View

1.Electromagnetic wave • Maxwell’s electromagnetic

theory • Useful for interaction between radiation and matter

2. Photons • Planck’s quantum theory • Useful for the prediction of

spectral properties of absorbing, emitting medium

EM theory

Two Distinctive Modes of Radiation1.Thermal radiation through

transparent media: surface radiation Theoretical frame work

Micro-physical properties r, ,

Optical constants n,

Solid state theory

q

T

Surface radiative properties , ,

Transport theoryGeometric integral eq.

Transport theory

Radiative Transfer Eq. (RTE)

2. Thermal radiation in participating media: gas radiation

Theoretical frame work

Molecular or particle parameters

Radiation properties a,

q

T

Quantum theory

Mie theory

Physical Mechanism of Absorption and Emission

• composition of radiating gas: molecules, atoms, ions, free electrons• photon: basic unit of radiation energy• emission: release of photons of energy• absorption: capture of photons of energy• 3 types of transition

bound-boundbound-freefree-free

bound state

free state

ionized energy

Energy transition for atom or ion

E1 = 0

E2

E3

E4

EI

bound-bound absorption

bound-bound emission

bound-free absorption

free-bound emission

free-free transition

Bound-Bound Transition• When a photon is absorbed or emitted

by an atom or a molecule and there is no ionization or recombination of ions or electrons

• Magnitude of energy transition:

related to frequency of emitted or absorbed radiation

E3 E2 emission, E3 - E2 = h a photon emitted with h 3 2E E

h

or

fixed frequency associated with the transition of energy level

E1 E2, E3, E4 absorption

2 1 3 1 4 1, ,E E E E E E

h h h

in the form of spectral lines

Broadening Effect• natural broadening

(Heisenberg uncertainty principle)• Doppler broadening• collision broadening• Stark broadening (strong electric field)

Carbon dioxide gas at 830 K, 10 atm

Transition of Energy States

1.bound-bound transition

• molecules: rotational states vibrational states electronic states

• atoms: electronic state

electronic state 1

Internuclear separation distance(diatomic molecule)

En

erg

y

dissociation energy for state 1

dissociation energy for state 2

electronic state 2

vibrational state

rotational state

Transition between rotational levels of same vibrational state in same electronic state

Transition between rotational levels in different electronic state

Transition between rotational levels in different vibrational states of same electronic state

1)Rotational transition within a given vibrational state:

associated energies at long wavelength 8 ~ 1000 m

2) Vibration-rotation transition: at infrared 1.5 ~ 20 m

3) Electronic transition: at short wavelength in the

visible region 0.4 ~ 0.7 mEngineering industrial

temperature:vibration-rotation transition

2. bound-free transition

• sufficient energy of ionization or recombination

• bound-free absorption (photoionization)

• free-bound emission (photorecombination)

• continuous absorption coefficient

3. free-free transition

• in ionized gas (bremsstrahlung)

reflection, refraction, diffraction

Scattering

• Redirection of photons

• Elastic scattering (coherent) Inelastic scattering

• Isotropic scattering Anisotropic scattering

• Dependent scattering Independent scattering

Scattering Regime

size parameter: D/

• Rayleigh scattering: molecular scattering D/• Mie scattering: Mie theory D/

• Geometric scattering: principle of geometric optics D/