As explained earlier, a molecule has different types of quantized energy levels i.e. translational, rotational, vibration and electronic. The molecular spectra arise due to the transitions taking place between these energy levels. The energy absorbed for any transition is equal to the difference in the energies of the two levels involved. It is found that these energies for transition are in the order
                                                   E₁ << E_r <<E_v << E_e *

In fact the difference between the successive translational energy levels is so small ( 10^-60 J mol^-1)that it cannot be observed experimentally. For this reason, for practical purposes, translational energy is considered as types of molecular spectra generally observed, depending upon the energies absorbed and the regions of the electromagnetic spectrum in which they are observed, are briefly explained below:

(1)    Pure rotational (Microwave) spectra.

If the energy absorbed by the molecules is so low the tit can cause transitionally form one rotational level to another within the same vibration level, the result obtained is called the rotational spectrum. These spectra are, therefore, observed in the far infra-red region or in the microwave reign whose energies are exceed-ingly small . The spectra is, therefore also called microwave spectra.

(2)    Vibration rotational spectra.

If the exciting energy is sufficiently large so that it can cause a transition form one vibration level to another witching the same electronic level then as the energies required for the transitions between the rotational levels are still less, bother types of transition will take place. The result is, therefore, a vibration-oration spectrum. Since such energies are available in the near infra-red region, these pectoral are observed in this region and are also called infra-red spectra.

(3)    Electronic Band Spectra.

If the exciting energy is still higher such that it can result in a transition form one electronic level to another, then these will also be accompanied by vibration level changes and each of these is further accompanied by rotational level changes. For each vibration change, a set of closely spaced lines is observed due to rotational level changes. Such a group of closely spaced rotational lines is called a band. Thus for a given electronic transition, a set of bands is observed. This say of bands is called band group or a band system. such electronic transition gives a band system. The complete set of band systems obtained due to different electronic transition gives the electronic band spectrum of the gaseous molecule. Thus whereas atoms give line spectra, molecules give band spectra. As such high excitation energies are available in the visible and ultraviolet regions, these spectra are observed in the visible region (12,500-25,000 cm^-1) and ultraviolet region ( 25,000-70,000 cm-1 ) .

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