Investigation of the molecular chemiluminescence SrCl(A2Π1/2,3/2, B2Σ+ → X2Σ+) and the atomic resonance fluorescence Sr(53P1 → 51S0) in the time‐domain following the pulsed dye laser generation of Sr(53 PJ) in the presence of CH2Cl2

Abstract

AbstractTime‐resolved investigations of the atomic resonance fluorescence Sr(53P1 → 51S0) and the molecular chemiluminescence from SrCl(A2Π1/2,3/2, B2Σ+ → X2Σ+) are reported following the reaction of the electronically excited strontium atom, Sr(5s5p(3PJ)), 1.807 eV above its 5s2(1S0) electronic ground state, with CH2Cl2. The optically metastable strontium atom was generated by pulsed dye‐laser excitation of ground state strontium vapor to the Sr(53P1) state at λ = 689.3 nm (Sr(53P1 ← 51S0)) at elevated temperature (850 K) in the presence of excess helium buffer gas in which rapid Boltzmann equilibration within the 53PJ manifold takes place. Sr(53PJ) was then monitored by time‐resolved atomic fluorescence from Sr(53P1) at the resonance wavelength together with chemiluminescence from electronically excited SrCl resulting from reaction of the excited atom with CH2Cl2. The molecular systems recorded in the time‐domain were SrCl(A2Π1/2 → X2Σ+) (Δν = 0, λ = 674 nm), SrCl(A2Π3/2 → X2Σ+) (Δν = 0, λ = 660 nm), and SrCl(B2Σ+ → X2Σ+) (Δν = 0, λ = 636 nm). Both the A2Π (179.0 kJ mol−1) and (B2Σ+(188.0) kJ mol−1) states of SrCl are energetically accessible on collision between Sr(3P) and CH2Cl2. Exponential decay profiles for both the atomic and molecular (A,B – X) chemiluminescence emission are observed and the first‐order decay coefficients characterized in each case. These are found to be equal under identical conditions and hence SrCl(A2Π, B2Σ+) are shown to arise from direct Cl‐atom abstractions on reaction with this halogenated species. The combination of integrated molecular and atomic intensity measurements, coupled with optical sensitivity calibration, yields estimations of the branching ratios into the A1/2,3/2, B, and X states arising from Sr(53 PJ) + CH2Cl2 which are found to be as follows: A1/2, 3.0 × 10−3; A3/2, 1.7 × 10−3; B, 4.4 × 10−4 yielding ΣSrCl(A1/2 + A3/2 + B) = 5.1 × 10−3. As only the X, A and B states of SrCl are accessible on reaction, this indicates an upper limit for the branching ratio into the ground state of 0.995. The present results are compared with previous time‐resolved measurements on SrF, Cl, Br(A2Π,B2Σ+ − X2Σ+) that we have reported on various halogenated species and with analogous chemiluminescence studies on Sr(3P) with other halides obtained from molecular beam measurements. The results are further compared with those from a series of previous analogous investigations in the time‐domain we have presented of molecular emissions from CaF, Cl, Br, I (A,B – X) arising from the collisions of Ca(43PJ) with appropriate halides and with branching ratio data for Ca(43PJ) obtained in beam measurements. © 1995 John Wiley & Sons, Inc.