The Collisional Behaviour of Ba[6s5d(3DJ)] with CH3Cl Investigated by Time-Resolved Atomic Emission and Molecular Chemiluminescence BaCl (A2∏1/2,3/2,B2∑+→X2∑+) Following the Pulsed Dye-Laser Excitation of Atomic Barium

Abstract

The collisional behaviour of Ba[6s5d(3DJ)], 1.151 eV above the 6s(S10)2 ground state, is investigated in the presence of CH3Cl at elevated temperature (900 K) following the initial pulsed dye-laser excitation of atomic barium via the allowed transition at λ=553.5nm{Ba[s66p(1P1)]←Ba[6s2(1S0)]} in excess helium buffer gas. The optically mestastable Ba(3DJ) is then subsequently generated in the ‘long-time domain’ by a combination of radiative and collisional processes and may then be monitored b, the spectroscopic marker atomic emission transition at ,λ=791.1nm{Ba[6s6p(3P1)]→Ba[6s2(1S0)]} described previously. Following the collision of {Ba[6s6d(3D1)]} with CH3Cl, the following molecular chemiluminescence transitions of BaCl are also monitored as a function of time: BaCl(A2∏1/2→X2∑+,λ=966nm,△v=0) , BaCl(A2∏3/2→X2∑+,λ=910nm,△v=0)and BaCl(B2∑+→X2∑+,λ=843nm,△v=0), the atomic and molecular profiles all demonstrating exponential decays characterised by decay coefficients which are equal in given reactant mixtures. It may thus be readily seen from the kinetic analysis that the three molecule states, BaCl(A2∏1/2,3/2,B2∑+), are generated directly on collision of Ba(3DJ) with CH3Cl via exothermic reactions. A detailed kinetic analysis employing both the integrated intensities of the atomic emission and these long wavelength molecular emissions, coupled with optical sensitivity calibrations, yields branching ratios in the BaCl(A2∏1/2,3/2,B2∑+) states. These are found to be as follows: BaCl(A2∏1/2)4,68%,BaCl(A2∏3/2)1.29%,BaCl(B2∑+)0.24% The logarithmic variation of these branching ratios with the energies of the states is essentially Boltzmann in character, with an effective temperature close to the ambient temperature of the measurements. This is consistent with the absence of propensity in the yields of these excited molecular states on collision, reflecting the role of late barriers in the potential surfaces involved.