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(3PJ)], with the molecule CF3I. Sr[5s5p(3PJ)], 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 53PJ 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, A1/2, A3/2 and B, arising from Sr(53PJ)+CF3I which are found to be as follows: A1/2,1.2 × 10-2; A3/2, 6.7 × 10-3; B, 5.1 × 10-3 yielding ∑SrI(A1/2+A3/2+B)=2.4 × 10-2. As only the X, A and B states SrI are accessible on reaction, assuming that the removal of Sr(53PJ) 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(53PJ), together with analogous chemiluminescence studies on Sr(3PJ) and Ca(43PJ) from molecular beam measurements.

Determination of Branching Ratios in a Kinetic Investigation of the Atomic Resonance Fluorescence and Molecular Chemiluminescence, CaCl(A2Π1/2,3/2<

“…Measurements of the time-resolved atomic fluorescence and molecular chemiluminescence following the reaction of Ca(43pj)/ CH3X(X F, C1, Br, I) [15][16][17][18] indicated that CaX(,,4:II1/E,a/2,B2 +) arise from direct reaction rather than secondary processes involving electronic energy transfer and can thus be related to analogous studies on energy distribution in molecular beams. These studies, in some cases, are in contrast to analogous later measurements in the time-domain on the reactions Sr(53pj) [20][21][22][23][24], 1.807 eV above the 5s2(1S0) ground state, following pulsed dye-laser excitation of atomic strontium in the presence of various halides which did yield molecular electronic product branching ratios. This arose from improvements in data capture and optical calibrations [20][21][22][23][24].…”

Determination of the Branching Ratios of CaBr(A2Π1/2,3/2, B2Σ+) Resulting from the Reaction of Ca[4s4p (3PJ)] with CH3Br by Time-Resolved Atomic Emission and Molecular Chemiluminescence Following Pulsed Dye-Laser Excitation of Atomic Calcium

“…In contrast to previous measurements on Ca(4 3 P J ) + CH 3 X in the timedomain [16][17][18][19], the present system requires a combination of the comparisons of time-dependences for atomic and molecular emissions together with determinations of integrated intensities using an optically calibrated system [20,21,[23][24][25][26][27]. Ca[4s4 p( 3 P 1 )] was generated by the pulsed dye-laser excitation (10 Hz) of calcium vapour at elevated temperature (T = 940 K) at λ = 657.3 nm {Ca[4s4 p( 3 P 1 )] ← Ca[4s 2 ( 1 S 0 )]} in the presence of methyl bromide and excess helium buffer gas in a slow flow system, kinetically equivalent to a static system.…”

“…As in related investigations on branching ratios for reactions of Ca( 3 P J ) with CH 3 F [20] and CH 3 Cl [21], and also Sr[5s5 p( 3 P J )]with halides in the timedomain [23][24][25][26][27], the integration of the appropriate rate equations leading to the intensities of atomic and molecular states are presented here without immediate correction for the response of the optical system at different wavelengths and the electronic gain of the photomultiplier at different operating voltages but these corrections are included in obtaining the final values of the integrated intensities. As in related investigations on branching ratios for reactions of Ca( 3 P J ) with CH 3 F [20] and CH 3 Cl [21], and also Sr[5s5 p( 3 P J )]with halides in the timedomain [23][24][25][26][27], the integration of the appropriate rate equations leading to the intensities of atomic and molecular states are presented here without immediate correction for the response of the optical system at different wavelengths and the electronic gain of the photomultiplier at different operating voltages but these corrections are included in obtaining the final values of the integrated intensities.…”

“…Those measurements may be contrasted to analogous later investigations in the time-domain on the reactions Sr(5 3 P J ) [23][24][25][26][27], 1.807 eV above the 5s 2 ( 1 S 0 ) ground state, following pulsed dye-laser excitation of atomic strontium in the presence of various halides which did yield molecular electronic product branching ratios. This development for collisions of Sr(5 3 P J ) arose from improvements in data capture and optical calibrations [23][24][25][26][27]. In this paper, this general approach [23][24][25][26][27] is used to characterise molecular electronic branching ratios yielding CaBr(A 2 Π 1/2,3/2 , B 2 Σ + ) following the reactions of Ca(4 3 P J ) with CH 3 Br.…”

“…This development for collisions of Sr(5 3 P J ) arose from improvements in data capture and optical calibrations [23][24][25][26][27]. In this paper, this general approach [23][24][25][26][27] is used to characterise molecular electronic branching ratios yielding CaBr(A 2 Π 1/2,3/2 , B 2 Σ + ) following the reactions of Ca(4 3 P J ) with CH 3 Br. The results are compared with related results for reactions of Ca(4 3 P J ) in molecular beams, with some very recent branching ratio measurements on Ca(4 3 P J ) with CH 3 F and CH 3 Cl in the time-domain [19,20] and with branching ratio measurements for Sr(5 3 P J ) with halides in the timedomain [22][23][24][25][26], particularly the production of SrBr(A 2 Π 1/2,3/2 , B 2 Σ + ) from the reactions of Sr(5 3 P J ) with CH 2 Br 2 [24].…”