Several factors contribute to this:
1. Collision Broadening. Atoms or molecules in liquid or gaseous phases are in continual motion and collide frequently with each other. These collisions inevitably cause some deformation of the particles and hence perturb, to some extent, the energies of at least the outer electrons in each. This immediately gives a possible explanation for the width of visible and ultra-violet spectral lines, since these deal largely with transitions between outer electronic shells. Equally vibrational and rotational spectra are broadened since collisions interfere with these motions too. In general, molecular interactions are more severe in liquids than in gases, and gas-phase spectra usually exhibit sharper lines than those of the corresponding liquid.
In the case of solids, the motions of the particle are more limited in extent and less random in direction, so that solid phase spectra are often sharp but show evidence of interactions by the splitting of lines into two or more components.
2. Doppler Broadening. Again in liquids and gases the motion of the particles causes their absorption and emission frequencies to show a Doppler shift; since the motion is random in a given sample, shifts to both high and low frequencies occur and hence the spectral line is broadened. In general, for liquids collision broadening is the most important factor, whereas for gases, where collision broadening is less pronounced, the Doppler effect often determines the natural line width.
3. Heisenberg Uncertainty Principle. Even in an isolated staitionary molecule or atom the energy levels are not infinitely sharp, due to the operation of a fundamental and very important principle, the uncertainty principle of Heisenberg.
The Intensity Of Spectral Lines
1. Transition Probability
2. Population Of States
3. Path Length Of Sample
Question 1. Write in detail the factors affecting the intensity of spectral lines?