The frequency of Rotational – Vibrational Spectrum is given by, $$ \overline { \upsilon } = \overline { \omega } + 2BJ\qquad …(1) $$ When \(J = 0\), equation(1) becomes, $$\overline { \upsilon } = \overline { \omega } $$ This indicates that transition for \(J = 0\) is not possible and naturally the corresponding peaks are not observed. This gives flat curve. This frequency is known as band centre. Since the equation (1) does not contain \(v’\) and \(v\), the line corresponding to the vibrational transition is not observed. Instead, a pair of lines on either side of band centre is observed foreach value of \(J\). For each change in vibrational quantum number, one such unit is obtained. The entire vibration rotation spectrum will consist of a number of such units.

The spectral line corresponding to \(\Delta J = -1, -2, -3\) …. etc are called **P-branch**. These lines are at lower frequency.

The spectral line corresponding to \(\Delta J = +1, +2, +3\) … etc. are called **R-branch**.