Question

When an optical spectroscopic transition is measured, either by emission or absorption, a so-called lineshape of given intensity and frequency dependence is obtained. For any spectroscopy there exist quantum mechanical or other basic connections to all of the experimental parameters that determine the lineshape. List these experimental parameters and explain how they are related to the previously mentioned connections.

Answer 1

The so called Lineshape or Spcetral line is the characteristic feature of the given molecule/atom/ion, which basically resembles to change in the energy of the given atom/molecule etc. The shape of ideal spectral line is given by three functions such as Lorentzian, Gaussian and Viogt functions. Basically they differ on the use of parameters such as Line position for Lorentzian function, maximum height for Gaussian function and half width for Viogt function respectively.

So for much simplicty, I will categorize the Spectroscopic technique according to the line shape, where i am gonna mention about the parameters affecting them.

1. AS (Atomic absorption or emission spectroscopy) -

In this technique the atoms are in the gas phase and the line shape is mostly influenced by two parameters such as Doppler effect and Pressure effect. Gaussian line is observed here.

Effect of Pressure : - The emission process in disrupted by the collision of particles with emitting particle thus shortening the time scale of the process. The duration of the collision is way much lesser than the overall emsission process.

Sometimes, the spectral line is also influnecd by density and temperature of the gas.

2. MS (Molecular spectroscopy : Rotational spectroscopy, Vibrational Spectroscopy)

Molecules can be either in gaseous or liquid state. The molecules in gaseous phase are influenced by Doppler effect and Pressure effect. But, the molecules in the liquid state are influenced by Collisions and proximity. Gaussian line is observed here too. (Note : Collisions occur between molecules during their motion in liquid state whereas Proximity indicates the effect of neighbouring atom)

So what happens in Gaussian line ? Yhe distribution of velocities in the gas depends on the temperature. Higher the temperatur of the gas, more is the distribution of velocities in the gas. Since the spectral line is a combination of all of the emitted radiation, the higher the temperature of the gas, the broader the spectral line emitted from that gas.

3. NMR (Nuclear Magnetic Resonance Spectroscopy) -

Here, the line observed is Lorentzian and is influenced by process of Free inductive decay.