KINETIC-STUDY OF THE MOLECULAR CHEMILUMINESCENCE SRI(A(2)PI(1/2,3/2),B-2-SIGMA(+)-]X(2)SIGMA(+)) AND THE ATOMIC RESONANCE FLUORESCENCE SR(5P-3(1)-]5S-1(0)) FOLLOWING THE PULSED DYE-LASER GENERATION OF SR(5P-3(J)) IN THE PRESENCE OF CH3I

We present a kinetic study of the reaction of the electronically excited strontium atom, Sr[5s5p(P-3(1))], 1.807 eV above its 5s(2)(S-1(o)) electronic ground state, with the molecule CH3I. The metastable strontium atom was generated by pulsed dye-laser excitation of ground state strontium vapour to the Sr(5(3)P(1)) state at lambda = 689.3 nm [Sr(5(3)P(1)<--5(1)S(o))] at elevated temperature (840 K) in the presence of excess helium buffer gas in a slow flow system, kinetically equivalent to a static system, in which rapid Boltzmann equilibration within the 5(3)P(1) manifold takes place. The atomic emission from Sr(5(3)P(1)-->5(1)S(o)) and the molecular chemiluminescence from SrI(A(2) Pi(1/2,3/2), B-2 Sigma(+)X(2) Sigma(+)) resulting from reaction of the excited atom with CH,I were then monitored in the time-domain. The molecular systems recorded were SrI(A(2) Pi(1/2)-->X(2) Sigma(+),Delta upsilon = 0, lambda = 694 nm), SrI(A(2) Pi(3/2)--> X(2) Sigma(+), Delta upsilon = 0, lambda = 677 nm) and SrI(B-2 Sigma(+)-->X(2) Sigma(+)) (Delta upsilon = 0, lambda = 674 nm). The electronically excited states of SrI(A(2) Pi(1/2,3/2), 172.5, 175.4 kJ mol(-1); B-2 Sigma(+), 177.3 kJ mol(-1)) are energetically accessible on collision by direct I-atomic abstraction between Sr(P-3) and CH3I. Exponential decay profiles for both the atomic and molecular (A, B - X) chemiluminescence emissions are characterised by first-order decay coefficients which are found to be equal under identical conditions and hence SrI(A(2) Pi, B-2 Sigma(+)) are shown to arise from direct reaction. The combination of integrated molecular and atomic intensity measurements yields estimates of the branching ratios into the A(1/2), A(3/2) and B states, arising from Sr(5(3)P(1)) + CH3I, which are found to be as follows: A(1/2), 6.9 X 10(-3); A(3/2), 3.5 x 10(-3); B, 3.0 x 10(-3); yielding Sigma SrI(A(1/2) + A(3/2) + B) = 1.3 X 10(-2). As only the X, A and B states of SrI are accessible on reaction, assuming that removal of Sr(5(3)P(j)) occurs totally by chemical removal, this yields an upper limit for the branching ratio into the ground state of ca. 99%. The present results are compared with previous time-resolved measurements on excited states of strontium halides that we have reported on various halogenated species resulting from reactions of Sr(5(3)P(j)), together with analogous chemiluminescence studies on Sr(3P) from molecular beam measurements, including branching ratio data for Ca(4(3)P(j)).