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.
