Mechanisms and Relative Rates of MX₂* Chemiluminescence in the Reactions of Ca, Sr, and Ba(¹S) Atoms with Dihalogen Molecules

Abstract

Chemiluminescent reactions of ground state (Ca, Sr, and Ba) atoms with (Cl 2 , Br 2 , I 2 , ICl, and IBr) were studied in a beam-gas arrangement. The MX 2 * pseudo-continua were measured as a function of target gas pressure in the 0.0001-0.25 Pa range. To identify the main reaction channels that contribute to MX 2 * formation and to obtain their relative contributions, kinetic models were fitted to the data. The following channels were considered: (1) radiative two-body recombination (R2BR), (2) radiative three-body recombination (R3BR), (3) two consecutive harpooning steps involving MX † intermediate (two-step chemiluminescent reaction TSCR), and (4) combinations of the above. On their own, none of the mechanisms (1-3) provide satisfactory data fits. The best agreement is obtained by a model involving both R2BR and TSCR. The branching ratios of these channels were determined for p ) 0-0.25 Pa. At 0.16 Pa these R2BR fractions vary from 7% (for Ca, Sr + ICl) to 79% (for Ba + IBr). Absolute CL cross sections and lower limits of photon yields were estimated by cross-calibrations. Photon yields for R2BR varied from 0.0004% to 0.037%, depending on the collision partners.

Chemical and Electronic Branching Ratios in the Chemiluminescent Reactions of Hyperthermal Ca(3P) Atoms with CF2Cl2 and CF2CCl2

“…The study of chemical reactions with electronically excited atoms constitutes one of the most active fields of research in reaction dynamics. In these reactions, measurement of the product chemiluminescence has provided to be one of the best methods to obtain information on the internal energy distribution and electronic populations in the reaction products. − This technique has been extensively used to study the reactions of alkaline-earth-metal atoms (M) with halogenated molecules, which are of interest because of the large number of reactive channels that can be open. − …”

“…Although many efforts were made in the past to understand the reactions of M with halogen molecules (X 2 ) and a very good model to describe these reactions was developed, , it seems interesting to study the reactions of M with diatomic interhalogens (XY). These systems double the number of exit channels and bring in the question of chemical branching, where two different product molecules are formed in several electronic states.…”

“…In these reactions, measurement of the product chemiluminescence has provided to be one of the best methods to obtain information on the internal energy distribution and electronic populations in the reaction products. [1][2][3][4][5][6][7][8][9][10][11][12][13] This technique has been extensively used to study the reactions of alkaline-earth-metal atoms (M) with halogenated molecules, which are of interest because of the large number of reactive channels that can be open. [1][2][3][4][5][6][7][8][9][10][11][12][13] Although many efforts were made in the past to understand the reactions of M with halogen molecules (X 2 ) and a very good model to describe these reactions was developed, 9,13 it seems interesting to study the reactions of M with diatomic interhalogens (XY).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] This technique has been extensively used to study the reactions of alkaline-earth-metal atoms (M) with halogenated molecules, which are of interest because of the large number of reactive channels that can be open. [1][2][3][4][5][6][7][8][9][10][11][12][13] Although many efforts were made in the past to understand the reactions of M with halogen molecules (X 2 ) and a very good model to describe these reactions was developed, 9,13 it seems interesting to study the reactions of M with diatomic interhalogens (XY). These systems double the number of exit channels and bring in the question of chemical branching, where two different product molecules are formed in several electronic states.…”