Doppler Broadening

M.Zeeshan Khalid

09050610-039

Introduction

• It is the broadening of the spectral line due to Doppler

Effect of the random thermal motion of the molecules.

• Lack of sharpness in the spectrum line causes the

broadening and is related to the distribution of velocities

of the atoms or molecules.

• Causes red shifted and blue shifted wavelengths

• Emitting atoms are in motion with a range of speeds Δν along the line of sight to the observer then Doppler Broadening is given as

• For a gas of atoms of mass m at a temperature T, the velocity dispersion is given by

where k is the Boltzmann constant (1.38 × 10−23 J K−1)

Thermal Doppler broadening

• Atoms and molecules move in a haphazard direction with average velocity is given as (1/2mv2 )= 3/2kT

• The spreading of the spectral line due to the temperature of the emitting medium is called as thermal Doppler broadening.

• Spreading of the spectral line depends on the frequency, mass of the emitting particles and the temperature.

Doppler broadening on nuclear reactors

• The Doppler effect arises when the temperature of the fuel changes.

• When the uranium is heated, its nuclei move more rapidly in random directions, and therefore generate a wider range of relative neutron speeds.

• Pebble bed nuclear reactors

Doppler Broadening Applications and other causes

• Thermal Doppler broadening

• Causes of velocity distribution

Turbulent motion Accretion disc

• Stark broadening