No 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, Qq, 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 Qq on relative collision speed, g. Employing this method, Qq 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 Qq 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 Qq 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, Qq[K* (5 2P)] > Qq[Na* (3 2P)] ≳ Qq[K* (4 2P)]. The difference between K* (52P) and K* (4 2P) [or Na* (3 2P)] is sometimes quite large; for CH4, for example, Qq[K* (5 2P)]/Qq[K* (4 2P)] > 60. For I2, however, the trend is dramatically reversed with Qq[K* (5 2P)]/Qq[K* (4 2P)]=0.2. Effects of isotopic substitution were also examined; no difference in Qq was measured for H2 vs D2, H2O vs D2O, C2H4 vs C2D4, or HCl vs DCl. However, for K* (5 2P), Qq(CH4)/Qq(CD4) ≈ Qq(C2H6)/Qq(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.

Previous reports on the thermal or CO2-laser induced decomposition of trichloroethylene have identified only one condensible product, hexachlorobenzene (in addition to HCl and mono- and dichloroacetylene). We have found that trichloroethylene vapor exposed to cw irradiation on the P(24) line of the (001 - 100) band of the CO2 laser at incident power levels from 8 - 17 W produces numerous products, of which the 13 major ones have been identified using IR, GC/MS, GC/FTIR, and NMR methods. All of these products have 4, 6, or 8 carbons, are highly unsaturated, and are completely chlorinated or contain a single hydrogen. C4HCl5 and C6Cl6 isomers (three of each) account for S 55% to 85% of total products (based on peak areas in the total ion chromatograms in GC/MS runs), depending on reaction conditions. In addition to characterizing the products, we discuss the dependence of the product distribution on laser power, irradiation time, and cell geometry, and we outline a possible mechanism.

Two concept maps have been developed to represent the organization of the material in a first-semester general chemistry course into two overall themes: a structure and properties theme and a quantitative chemical relationships theme. By providing these maps to students and referring to them in class, it is hoped that the instructor can assist students in maintaining a grasp of the "big picture" during the progress of the course.