OBSERVATION OF AN UNGERADE RYDBERG STATE OF $^{7}Li_{2}$

Abstract

Some weak, collisionally induced transitions have been observed following excitation of the two lowest vibrational levels of the $5d {^{1}}\Pi_{g}$ state of $^{7}Li_{2}$ (produced in a heatpipe source). The transitions were recorded in Fourier transform resolved fluorescence spectra in the near infrared $(7900 - 8800 cm^{-1})$, at an instrumental resolution of $0.08 cm^{-1}$, and they have been assigned as transitions from a Rydberg $^{1}\Pi_{u}$ state to $v = 0, 1$ and 2 of the $1 ^{1}\Delta_{g}$ electronic state. It seems highly likely that the two upper state levels populated through the collisions are $v = 0$ and $v = 1$ of $a ^{1}\Pi_{u}$ state which lies very close in energy to the $5d {^{1}}\Pi_{g}$ state. The energy transfer occurs to $v = 0$ of the $^{1}\Pi_{u}$ state from $v = 0$ of the $5d ^{1}\Pi_{g}$ state, and (at low J only) to $v = 1$ of the $^{1}\Pi_{u}$ state from $v = 1$ of $5d ^{1}\Pi_{g}$. Six bands were analysed: $0-0, 0-1, 0-2, 1-0, 1-1$ and 1-2. The rotational constants of this $^{1}\Pi_{u}$ state are too large to be high levels of the other ungerade Rydberg state of $Li_{2}$ documented in the literature, namely the $D ^{1}\Pi_{u}$ state studied by Theiss and $co-workers^{b}$. They are however sufficiently close to those of the $5d ^{1}\Pi_{g}$ state $(T_{e} = 37257.7 cm^{-1}, B_{e} = 0.48 cm^{-1}, \omega_{e} = 237.4 cm^{-1})$ given in earlier $work^{c}$, that we suggest the Rydberg state is most likely to correlate at long range with $Li(2s) + Li(5p)$ atoms. Principal spectroscopic constants are $T_{o} = 37390.198(5) cm^{-1}, B_{o} = 0.50627(12) cm^{-1}, \Delta G_{v} = 262.4 cm^{-1}$.