The formation and detection of some low-lying excited electronic states of BrCl and other halogens

Abstract

Forty-nine bands arising from 8 $\geqslant$ v' $\geqslant$ 2 and 14 $\geqslant$ v'' $\geqslant$ 1 have been detected in the emission spectrum of BrCl($^3\Pi_0+ \rightarrow ^1\Sigma^+$) from two sources: (i) the reaction of Br or Br$_2$ with ClO$_2$, in which BrCl($^3\Pi_0$+) is populated in the transfer reaction, \begin{equation*}\tag{3}Br+ClO_2 \rightarrow BrCl + O_2,\end{equation*} and (ii) the radiative combination of bromine and chlorine atoms, \begin{equation*}\tag{5}Br + Cl + (M) \rightarrow BrCl(^3\Pi_0+) (+M).\end{equation*} The populations of the higher levels of BrCl ($^3\Pi_0$+) formed in reaction (3) are less than in reaction (5). In reaction (3), BrCl($^3\Pi_0$+) is evidently formed via a triangular transition state similar to that postulated previously; the rate constant, k$_3$, was found to be $(3.1 \pm 0.3) x 10^{10} cm^3 mole^{-1} s^{-1} at 300 ^\circ K.$ The combination of ground state iodine (I$^2$P$_\frac{3}{2}$), and chlorine (Cl$^2$P$_\frac{3}{2}$) atoms was also found to be chemiluminescent in the red and near infrared region, and fifty-seven bands of the system ICl($^3\Pi_1 \rightarrow ^1\Sigma^+$), with 0 $\leqslant$ v'' $\leqslant$ 7 and 1 $\leqslant$ v' $\leqslant$ 18, have been recorded. The radiative combination of ground-state chlorine atoms has been reinvestigated, and transitions of Cl$_2$($^3\Pi_{0u+} \rightarrow ^1\Sigma^+_g$) from v' $\leqslant$ 14 have been detected. Combining band head analyses of these emission spectra with previous data, the following constants (in cm$^{-1}$) are reported: \begin{align*}\omega_e \quad \omega_ex_e \quad -10^2\omega_ey_e \quad T_e \quad \nu_{00} \\ \mathrm {BrCl}(X^1\Sigma^+) \quad 440 \pm 2 \quad 1\cdot6 \pm 0\cdot1 \quad - \quad 0 - \\ (A^3II_0+) \quad 243 \pm 3 \quad 6\cdot7 \pm 0\cdot2 \quad - 16795 \pm 2 \quad 16695 \pm 2 \\ \mathrm {ICl}(X^1\Sigma^+) \quad 384\cdot15 \pm 0\cdot05 \quad 1\cdot44 \pm 0\cdot05 \quad 1\cdot3 \pm 0\cdot2 \quad 0 - \\ (B^3II_1) \quad 212\cdot3 \pm 0\cdot3 \quad 2\cdot39 \pm 0\cdot05 \quad 1\cdot2 \pm 0\cdot2 \quad 13742 \pm 1 \quad 13656 \pm 1 \\ \mathrm {Cl}_2(X^1\Sigma^+_g) \quad 559\cdot8 \pm 0\cdot5 \quad 2\cdot7 \pm 0\cdot0.5 \quad 0\cdot6 \pm 0\cdot1 \quad 0 \quad - \\ (A^3II_{ou}+) \quad 259\cdot5 \pm 0\cdot2 \quad 5\cdot3 \pm 0\cdot1 \quad - \quad 17809 \pm 1 \quad 17658 \pm 1\end{align*} The reasons for the predominant radiation, during atom combination processes, from $^3\Pi_{0 u)}$ + type states for Cl$_2$ and BrCl, and from the $^3\Pi_1$ state for ICl, are discussed in relation to Brown & Gibson's (1932) finding of a maximum in the potential energy curve of ICl($^3\Pi_0$+) leading to I($^2$P$_\frac{3}{2}$) and Cl($^2$P$_\frac{3}{2}$) as dissociation products.