Some Propensity Rules in Collision-Induced Rotational Quantum Jumps

When a mixture of lithium vapor and argon is irradiated with the light from an argon ion laser and the resulting fluorescence of the ${\mathrm{Li}}_2{B}^1{\Pi }_u{\mathrm{\hspace{0.17em}-\hspace{0.17em}X}}^1{\Sigma }_g^{+}$ band system is examined under high resolution, a pattern of collision‐induced satellite lines is observed to accompany the parent resonance fluorescence series. Under conditions of low pressure these satellite lines originate from single inelastic events which alter the rotational state of the excited Li₂ molecule. The relative intensities of the satellite lines are found to be markedly different depending on whether the collision‐induced transition originates from the upper or lower component of the $\mathrm{Lgr;}$ doublet of the $\Pi$ state, referred to as $c$ or $d$ , respectively. An increase in $\mathrm{J(+\hspace{0.17em}\Delta J}\mathrm{jump})$ is favored over a decrease $\mathrm{(-\hspace{0.17em}\Delta J)}$ for $\mathrm{d\hspace{0.17em}\to \hspace{0.17em}c}$ jumps whereas $\mathrm{-\hspace{0.17em}\Delta J}$ is favored over $\mathrm{+\hspace{0.17em}\Delta J}$ for $\mathrm{c\hspace{0.17em}\to \hspace{0.17em}d}$ jumps. On the other hand $\mathrm{\pm \hspace{0.17em}\Delta J}$ changes that preserve the character of the $\Lambda$ component, i.e., $\mathrm{c\hspace{0.17em}\to \hspace{0.17em}c}$ and $\mathrm{d\hspace{0.17em}\to \hspace{0.17em}d}$ , occur with nearly equal probability for the same value of $\mathrm{\Delta J}$ . This behavior has been observed for the satellite lines corresponding to $\text{ΔJ\hspace{0.17em}=\hspace{0.17em}±\hspace{0.17em}1, ±\hspace{0.17em}2}$ , and in some cases ± 3 for $\mathrm{(\upsilon \prime ,\; J\prime ,\; c,}\mathrm{or}\mathrm{d)}$ levels $\text{(2, 31, c), (3, 30, c), (9, 38, c), (4, 24, d)}$ , and $\text{(7, 61, d)}$ of ⁷Li₂ and $\text{(0, 45, d)}$ of ⁶Li ⁷Li. In ⁷Li₂ only collisional transitions between symmetric or between antisymmetric levels are allowed. For ⁶Li ⁷Li, which does not have this symmetry, more satellite lines are observed. However, these additional lines are significantly weaker since the ⁶Li ⁷Li molecule is nearly homonuclear. A simple classical model is suggested which may help to explain the different rotational quantum jump propensities for the two $\Lambda$ components.