# What is P and R Branch in Rotational – Vibrational Spectra.

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.

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