Photodissociation of NaBr, Nal, and KI vapors and collisional quenching of Na* (3 2P), K* (4 2P), and K* (5 2P) by foreign gases

Abstract

Fluorescence from excited alkali atoms (A*) may be produced by photodissociation of alkali halide (AX) vapor. Fluorescence efficiencies have been determined as a function of the photodissociation wavelength, λ₀, for Na* (3 ²P) from NaBr and for K* (4 ²P) and K* (5 ²P) from KI. Employing the Stern‐Volmer relation, cross sections, Q_q, for the collisional quenching of the A* electronic excitation may be determined from the attenuation of the A* fluorescence that is observed upon introduction of a foreign gas. Because A* may be produced with different average speeds by varying λ₀, this method permits the determination of the dependence of Q_q on relative collision speed, g. Employing this method, Q_q was determined to decrease monotonically with increasing g for Na* (3 ²P)+Br₂ (AX = NaBr) and K* (4 ²P)+C₂H₄, CF₃Cl, and SO₂ (AX = KI). Moreover, values of Q_q were determined at a particular g value for K* (4 ²P)+I₂ and K* (5 ²P)+I₂, HCl, and DCl. Alternatively, premixing the quenching gas in a large (∼ 100‐fold) excess of Ar thermal moderator (Ar failed to quench any of these A* levels) makes possible the measurement of Q_q for a thermal distribution in g. Thermal results (800–900°K) obtained by this method are reported for the quenching of Na* (3 ²P) (AX=NaI), K* (4 ²P), and/or K* (5 ²P) by N₂, CF₃Cl, H₂, H₂O, CH₃OH, CF₄, CH₄, C₂H₆, C₂H₄, and SO₂. In general, for any particular quenching gas, $Q_q[\mathrm{K}{\text{*\hspace{0.17em}(5\hspace{0.17em}}}^2{\mathrm{P)]\; >\hspace{0.17em}Q}}_q[\mathrm{Na}{\text{*\hspace{0.17em}(3\hspace{0.17em}}}^2{\mathrm{P)]\hspace{0.17em}\gtrsim \hspace{0.17em}Q}}_q[\mathrm{K}{\text{*\hspace{0.17em}(4\hspace{0.17em}}}^2\mathrm{P)]}$ . The difference between K* (5²P) and K* (4 ²P) [or Na* (3 ²P)] is sometimes quite large; for CH₄, for example, Q_q[K* (5 ²P)]/Q_q[K* (4 ²P)] > 60. For I₂, however, the trend is dramatically reversed with Q_q[K* (5 ²P)]/Q_q[K* (4 ²P)]=0.2. Effects of isotopic substitution were also examined; no difference in Q_q was measured for H₂ vs D₂, H₂O vs D₂O, C₂H₄ vs C₂D₄, or HCl vs DCl. However, for K* (5 ²P), Q_q(CH₄)/Q_q(CD₄) ≈ Q_q(C₂H₆)/Q_q(C₂D₆) ≈ √2. All of these results are discussed in terms of the likely forces between the reactants and the possibility of a change‐transfer intermediate.