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Spectroscopic Lineshapes

Introduction

The width and shape of spectroscopic transitions will affect the ability to extract qualitative and quantitative information from a spectrum. The lineshapes of spectroscopic transitions depend on the broadening mechanisms of the initial and final states, and include natural broadening, collisional broadening, power broadening, and Doppler broadening. Natural, collisional, and power broadening are homogeneous mechanisms and produce Lorentzian lineshapes, and Doppler broadening is a form of inhomogeneous broadening and has a Gaussian lineshape. The general form of Lorentzian and Gaussian lineshapes are shown below. Combinations of Lorentzian and Gaussian lineshapes can be approximated by a Voigt profile.

Gaussian and Lorentzian spectral lineshapes


Collisional or Pressure Broadening

Collisions broaden spectroscopic linewidths by shortening the lifetime of the excited states.


Power Broadening

Power broadening occurs by shortening the lifetime of the excited state due to stimulated emission.


Doppler Broadening

Doppler broadening is due to the distribution of atomic velocities (speed and direction), which each have a Doppler shift with respect to an observer.


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