Photodissociation of NaI Vapor and the Energy Dependence of the Quenching of Na* (3p 2P) by Foreign Gases

Abstract

Fluorescence of the Na^* (3p ²P) D lines is observed upon photodissociation of NaI vapor by 1900–2500 Å radiation obtained through a monochromator from a high current, low pressure H₂ arc continuum source; the properties of this source are briefly described. The D lines fluorescence efficiency exhibits a threshold at $\text{∼ 2500 Å}$ , a relative maximum at $\text{∼ 2225 Å}$ , and a relative minimum at $\text{∼ 2050 Å}$ . An expression is derived for the distribution in laboratory speeds of an atom produced by photodissociation of a diatomic molecule at thermal equilibrium. This expression is then employed to calculate the distributions in speeds of the photodissociatively generated Na^* and these computed Na^* speed distributions are employed to analyze the observed attenuations of the D‐lines fluorescence upon addition of foreign gases. In this manner, the dependences on relative collision velocity g of the cross sections Q_q for collisional quenching of Na^* by CO₂, C₂H₄, CH₃CN, CF₃Cl, C₆H₆, SO₂, and I₂ have been determined. Over the range in g studied in this work $\text{(∼ 1.0 ‐ 2.5\hspace{0.17em}}\mathrm{km}/\sec )$ , Q_q varies approximately as g^−4/8, with s in the range of 4–6, for all seven quenching gases. At a fixed value of g, Q_q varies as: $I_2>{\mathrm{SO}}_2>{\mathrm{C}}_6{\mathrm{H}}_6>{\mathrm{CH}}_3\mathrm{CN}>{\mathrm{CF}}_3\mathrm{Cl}>{\mathrm{C}}_2{\mathrm{H}}_4>{\mathrm{CO}}_2$ . These results on the quenching cross sections are discussed in terms of the long‐range forces between the reactants, including a possible long‐range electron transfer curve crossing for the collision of Na^* with I₂ or SO₂. These measured quenching cross sections are also compared with previous literature values; good agreement is found in most cases, although some discrepancies are noted.