B. Rajasekhar

An ab initio study of dissociative electron attachment to NaHCO3 and NaCO3, and the role of these reactions in the formation of sudden sodium layers

One of the mechanisms that has been proposed to explain the phenomenon of sudden sodium layers (SSLs) in the upper atmosphere is dissociative attachment of electrons to sodium compounds such as NaHCO3 and NaCO3. In this study, abinitio calculations have been used to estimate the reaction enthalpies (ΔH°0(NaX + e− → Na + X−)) for NaHCO3 and NaCO3, which are found to be 72 ± 20 (0.75) and 21 ± 30 (0.22) kJmol−1 (eV), respectively. NaHCO3 is then shown to be relatively stable above the mesopause. This property, together with its rather small threshold for dissociative electron attachment, makes NaHCO3 a possible source of SSLs under certain conditions.

An experimental and theoretical study of the reactions Na+HCl and Na+DCl

An experimental study is presented of the reactions Na+HCl/DCl from 590 to 820 K. Na atoms were produced in an excess of HCl/DCl and He bath gas, by the pulsed photolysis of NaCl vapor. The metal atom concentration was then monitored by time‐resolved laser induced fluorescence of Na atoms at λ=589 nm. A fit of the data to the Arrhenius form yields (2σ uncertainty): k(Na+HCl)=(2.1±0.5)×10−9 exp[(−41.8±1.5 kJ mol−1)/RT] cm3  molecule−1 s−1; k(Na+DCl)=(2.2±1.0)×10−9 exp[(−45.5±3.8 kJ mol−1)/RT] cm3  molecule−1 s−1. The large activation energies are shown to be consistent with vibrational excitation of the hydrogen halides greatly enhancing the reaction cross‐sections. A pair of ab initio potential surfaces for these reactions are then calculated at constant angle cuts through the lowest 2 A’ hypersurface, including a collinear surface and the surface containing the lowest saddle point which is found to occur at a bent configuration with θNaClH=54.7°. Both surfaces exhibit a late reaction barrier. The effect ...

Study of the reaction Li + H2O over the temperature range 850?1000 K by time-resolved laser-induced fluorescence of Li(22 P J ?22 S 1/2)

The first direct study of the reaction between lithium atoms and water vapour is presented. Lithium atoms were produced by the pulsed photolysis of LiOH vapour in the presence of an excess of H2O and N2 both gas. The Li-atom concentration was monitored by laser-induced fluorescence of the metal atoms at λ= 670.7 nm using a pulsed nitrogen-pumped dye laser and boxcar integration of the fluorescence signal. Absolute second-order rate constants were obtained at T= 850, 900, 925, 950, 973 and 1000 K. A fit of these data to the Arrhenius form yields (2σ errors)k(T)=(5.6+ 4.3–2.4)× 10–10 exp [–(7970 ± 544)/T] cm3 molecule–1 s–1.The activation energy of 66.3 ± 4.5 kJ mol–1 may be compared to estimates for the standard enthalpy change (between 850 and 1000 K) of this reaction which rage from 62 to 73 kJ mol–1. To be compatible with our experimental data, and within the range of values derived from studies in flames, D°0(Li—OH) probably lies in the range 437–441 kJ mol–1.The reverse reaction between LiOH and H is important in both the chemistry of Li in flames (T > 1500 K) and in describing the atmospheric chemistry of Li in the mesosphere (T= 200 K). Estimates for this rate constant were obtained in both temperature regimes through detailed balancing.