Molecular spectra

When atom or molecule makes transition from a state of energy E1 to another state with energy E2, it absorbs or emits energy, where:

E2-E1=hv

For atoms, energy states are determined by arrangements of electron clouds (electronic states). Each line of spectrum observed corresponds to an electronic transition.

For molecules, the spectra are more complicated. Their states are not only depending on the electron cloud but also by the geometrical arrangement of the nuclei and their movements. This is because molecules have additional internal degree of freedom.

Molecules:

• possess more electronic states than atoms
• nuclei can vibrate around their equilibrium position 
• may rotate around axes through its center of mass.

For each electronic state, various vibrational and rotational energy levels are exist.

Molecular spectra can be categorized as below:
1) Transitions between different rotational levels for the same vibrational (and electronic) state lead to pure rotational spectra with wavelengths in the microwave region
2) Transitions between rotational levels in different vibrational levels of the same electronic state lead to vibration-rotation spectra in the mid-infrared
3) Transitions between two different electronic states have wavelengths from the UV to the near infrared. Each electronic transitions comprises many vibrational bands corresponding to transitions between the different vibrational levels of the two electronic states involved. Each of these bands contains many rotational lines with wavelengths or frequencies.

Source: Molecular Physics: Theoretical Principles and Experimental Methods by Wolfgang Demtroder