LASER-EXCITED BROADBAND VIOLET EMISSION FROM $Na_{2}$ MOLECULES
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Date
1978
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Ohio State University
Abstract
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 $Na_{2} C \leftarrow X$ transitions. The band does not correspond to known $Na_{2}$ 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 $c \rightarrow X$ fluorescence band upon increasing the sodium vapor pressure or upon introducing buffer gas. Recently the same band has been observed by Allegrini et al;$^{1}$ they ascribe the band to free $\rightarrow$ bound $Na_{2}$ 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 $Rb_{2}{^{2}}$ and $K_{2}{^{3}}$ we propose an interpretation similar to that given for those cases: excitation is transferred efficiently by collisions from the $Na_{2}$ C state to one or more near-resonant bound excited states which then decay radiatively to continuum levels of the $Na_{2}$ ground state. The violet emission thus being ascribed to bound $\rightarrow$ free $Na_{2}$ 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 $10^{-1}\,cm^{-1}$ for an upper state concentration of $10^{14}\,cm^{-3}$. 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 $Na_{2}$ state branches to the $Na_{2}$ C state, thus producing the violet band.
Description
$^{1}$ M. Allegrini, G. Alzetta, A. Kopystynska, L. Moi and G. Orriols, Opt. Commun. 22 (1977) 329. $^{2}$ J. M. Brom and H. P. Broida, J. Chem. Phys. 61 (1974) 982; D. L. Feldman and R. N. Zare, Chem. Phys. 15 (1976) 415. $^{3}$ M. M. Rebbeck and J. M. Vaughan, J. Phys. B4 (1971) 258.""
Author Institution: Philips Research Laboratories
Author Institution: Philips Research Laboratories