REACTION DYNAMICS OF THE CA(D-1(2), P-3(J))-]CAI-ASTERISK+CH3 SYSTEM - CHEMILUMINESCENCE, ENERGY DISPOSAL AND PRODUCT POLARIZATION(CH3I)

The Ca(D-1(2), P-3(J)) + CH3 --> CaI(A,B) + CH3 reactions system has b een studied by measuring its chemiluminescence under beam-gas conditio ns. Absolute values of the state-to-state reaction cross-sections were determined at low collision energy (E) over bar(T) less than or equal to 0.20 eV. In addition, the electronic branching ratio and product e nergy disposal have been determined for each metastable reaction. The major change observed in the chemiluminescence when comparing the Ca(D -1(2)) reaction versus that of Ca(P-3(J)) is the total yield associate d with the former reaction. To the best of our spectral resolution nei ther the electronic branching ratio e.g. CaI(A)/CaI(B) nor the interna l CaI energy disposal change significantly as the metastable Ca(D-1(2) )/Ca(P-3(J)) ratio is varied. In spite of the fact that the Ca(P-3(J)) reaction is less exoergic, the CaI product appears with a higher frac tion of internal energy than that of Ca(D-1(2)) reaction. Thus, the fr action of the total energy appearing in CaI internal energy amounts to 57.5% in the Ca(3(J)(P)) reaction while it is 19.3% only for the Ca(D -1(2)) reaction. This difference is discussed in the light of a distin ct mechanism associated with the attack of the excited Ca atom into th e C-I bond. No significant chemiluminescence yield was found for the e nergetically open CaCH3 channels. The product chemiluminescence polar ization was also measured as a function of the metastable concentratio n. A significant degree of polarization was found depending upon the s pecific electronic excitation. The analysis of the polarization emissi on associated to the parallel CaI(X-2 Sigma(+) <-- B-2 Sigma(+)) emiss ion led into a strong polarization of the product rotational angular m omentum. The comparison of the product rotational alignment for the ki nematically identical Ca(D-1(2), P-3(J), P-1(1)) + CH3 --> CaI(B-2 Si gma(+)) + CH3 reaction system showed that the CaI rotational polarizat ion diminishes in the P-3(J) --> D-1(2) --> P-1(1) sequence, e.g. as t he reaction exothermicity increases. In addition the degree of polariz ation associated with other emission bands as for example CaI(X-2 Sigm a(+) <-- A(2)II(1/2)) indicates the presence of a parallel transition which has been interpreted as mixing of Hund's case (a) and (c) approp riate for this heavy CaI diatom produced with a high rotational excita tion. (C) 1997 Elsevier Science B.V.