Selection Rules for Electronic Transitions
in Diatomic Molecules

The selection rules for optical transitions between different electronic states of a diatomic molecule are shown in Table 4.

Table 4: Selection Rules for Electronic Transitions in Diatomic Molecules

Allowed Transitions	Examples
$\Delta\Lambda = 0, \pm 1$	$\Sigma \leftrightarrow \Sigma$, $\Pi \leftrightarrow \Pi$, $\Sigma \leftrightarrow \Pi$, $\Delta \leftrightarrow \Pi$
$\Delta S =0$	$^1\Sigma \leftrightarrow\: ^1\Sigma$, $^3\Pi \leftrightarrow\: ^3\Pi$, $^1\Sigma \leftrightarrow\: ^1\Pi$, $^3\Sigma \leftrightarrow\: ^3\Pi$
$+ \leftrightarrow +$	$\Sigma^+ \leftrightarrow \Sigma^+$
$- \leftrightarrow -$	$\Sigma^- \leftrightarrow \Sigma^-$
$g \leftrightarrow u$	$\Sigma_g^+ \leftrightarrow \Sigma_u^+$, $\Sigma_g \leftrightarrow \Pi_u$

If we consider the rotational states as well, it is required that the total angular momentum of photon and molecule remains constant. In general, the selection rules for the total angular momentum $J$ are as follows: $\Delta J = 0, \pm 1$ however, for $\Sigma \leftrightarrow \Sigma$ transitions the $\Delta J =0$ transition is forbidden. These rules are summarized in table 5 below.

Table 5: Selection Rules for rotational quantum number $J$ in Electronic Transitions

Electron Transition	Allowed transitions	Name
$\Sigma \leftrightarrow \Sigma$	$\Delta J = -1$	P branch
	$\Delta J = 1$	R branch
	$\Delta J = -1$	P branch
all others	$\Delta J = 1$	R branch
	$\Delta J =0$	Q branch

Auf diesem Webangebot gilt die Datenschutzerklärung der TU Braunschweig mit Ausnahme der Abschnitte VI, VII und VIII.