Time-Resolved Investigation of the Molecular Chemiluminescence SrI(A²∏_1/2,3,2,B²∑⁺→X²∑⁺) and the Atomic Resonance Fluorescence Sr(5³P₁→5¹S₀) Following The Pulsed Dye Laser Generation Of Sr(5³P_J) in the Presence of CF₃I

Abstract

A time-resolved investigation is presented of the electronic energy distribution in SrI following the collision of the optically metastable strontium atom, Sr [5s5p(³P_J)], with the molecule CF₃I. Sr[5s5p(³P_J)], 1.807 eV above its 5s²(¹S₀) electronic ground state, was generated by pulsed dye-laser excitation of ground state strontium vapour to the Sr(5³P₁) state at , λ =689.3 nm {Sr(5³P₁←5¹S₀)} at elevated temperature (840 K) in the presence of excess helium buffer gas in which rapid Boltzmann equilibration within the 5³P_J spin-orbit manifold takes place. Time resolved atomic emission from Sr(5³P₁→5¹S₀) at the resonance transition and the molecular chemiluminescence from SrI(A²∏_1,2,3/2,B²∑⁺→X²∑⁺) resulting from reaction of the excited atom with CF₃I were recorded and shown to be exponential in character. SrI in the A²∏_1/2,3/2 (172.5, 175.4 kJ mol^-1) and B²∑⁺ (177.3 kJ mol^-1) states are energetically accessible on collision by direct-I-atomic abstraction between Sr(³P) and CF₃I. The first-order decay coefficients for the atomic and molecular emissions are found to be equal under identical conditions and hence SrI(A²∏_1/2,3/2, B²∑⁺) are shown to arise from direct I- atom abstraction reactions. The molecular systems recorded were SrI (A²∏_1/2→X²∑⁺, Δv=0, λ=694 nm), SrI(A²∏_3/2→X²∑⁺, Δv=0, λ=677 nm) and SrI(B²∑⁺→X²∑⁺) (Δv=0, λ=674 nm), dominated by the Δv=0 sequences on account of Franck-Condon considerations. The combination of integrated m61ecular and atomic intensity measurements yields estimates of the branching ratios into the specific electronic states, A_1/2, A_3/2 and B, arising from Sr(5³P_J)+CF₃I which are found to be as follows: A_1/2,1.2 × 10^-2; A_3/2, 6.7 × 10^-3; B, 5.1 × 10^-3 yielding ∑SrI(A_1/2+A_3/2+B)=2.4 × 10^-2. As only the X, A and B states SrI are accessible on reaction, assuming that the removal of Sr(5³P_J) occurs totally by chemical removal, this yields an upper limit for the branching ratio into the ground state of ca. 98%. 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(5³P_J), together with analogous chemiluminescence studies on Sr(³P_J) and Ca(4³P_J) from molecular beam measurements.