Knowledge Bank

We observe a strong diffuse violet sodium fluorescence band (420-457 nm) when irradiating sodium vapor with the UV argon laser lines 333.6 nm, 351-1/4 nm or 363.8 run which excite $Na2C\leftarrow X$ transitions. The band does not correspond to known $Na2$ bound-bound transitions. Its strength is linear in the laser power and its shape is pressure-dependent. The violet band grows relative to the direct $c\to X$ fluorescence band upon increasing the sodium vapor pressure or upon introducing buffer gas. Recently the same band has been observed by Allegrini et al;$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ they ascribe the band to free $\to$ bound $Na2$ transitions during Na(38)-Na(3P) collisions. This interpretation is at variance with our experimental evidence. Since the violet band is analogous to diffuse bands reported for $Rb2\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ and $K2\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ we propose an interpretation similar to that given for those cases: excitation is transferred efficiently by collisions from the $Na2$ C state to one or more near-resonant bound excited states which then decay radiatively to continuum levels of the $Na2$ ground state. The violet emission thus being ascribed to bound $\to$ free $Na2$ transitions is potentially attractive for tunable violet laser action. A rough estimate of the gain coefficient for stimulated emission at the peak of the violet band is $10-1cm-1$ for an upper state concentration of $1014cm-3$. An attractive feature is the possibility of two-photon pumping: we have excited the violet sodium band by two-photon absorption using a CW dye laser. The two-photon excited $Na2$ state branches to the $Na2$ C state, thus producing the violet band.