Molecular Beam Kinetics: Reactions of Deuterium Atoms with Halogen Molecules

Abstract

Angular distributions and velocity distributions have been measured for deuterium halides reactively scattered from crossed beams of D atoms and five halogen molecules, Cl2, Br2, I2, ICl, and IBr. The D beam was obtained either from a tungsten furnace at ∼2800 ° K or a microwave discharge at ∼350 ° K, the halogen beam from a supersonic nozzle source at ∼300–350 ° K. A small computer was employed as a multiscalar to record the time of flight spectrum of the product at each observation angle. Data of satisfactory quality (signal-to-noise ratio > lim ∼ 8) were obtained with rather short counting times (> lim ∼ 1000 sec) for reactive scattering signals of a few counts per second. Contour maps of the differential reaction cross sections are derived from an approximate transformation to the center-of-mass coordinate system. The main results found with D atoms at ∼2800°K are as follows: (1) In the Cl2 reaction, most of the DCl recoils into the backward hemisphere with respect to the incoming D atoms; the angular distribution peaks beyond ∼150°, nominally near 180°. For the Br2 reaction, the DBr is much less strongly pitched backwards, peaking near ∼120°-140 °. For the I2 reaction, most of the DI scatters sideways, peaking near ∼100°-110°. (2) The angle and velocity dependence of the DX distributions is approximately separable, but the forward-scattered part shows a distinct upward trend in velocity (∼15% at 60°) as the scattering angle decreases. (3) The mean energy released into relative translation of the products is 25, 16, and 12 kcal/mole, respectively, for the Cl2, Br2, and I2 reactions, as compared with total available energy (∼10 kcal/mole mean collision energy plus reaction exoergicity) of 55, 51, and 44 kcal/mole. (4) For the IBr reaction, the yields of DBr and DI are comparable. The distributions in angle and translational energy are different, for DBr resembling closely the Br2reaction, for DI the I2 reaction. (5) For ICl, the yield of DI exceeds DCl by a factor of at least 3 or 4 despite the much lower exoergicity for forming DI (20 kcal/mole) rather than DCl (52 kcal/mole). The DI angular distribution peaks sideways, as for I2; the DCl peaks backwards, as for Cl2. However, the translational energy of DI is much lower (mean 7 kcal/mole) than for I2 and that of DCl much lower (mean ∼10 kcal/mole<) than for Cl2. With the D atoms at ∼350°K: (6) The reaction yield with Cl2was not detectable, very small with Br2, and comparable to that for hot D atoms with I2. (7) In the I2 case, the product angular distribution shifts backwards by ∼15° as compared to the hot D atom reaction. The mean energy released into product translation decreases substantially (from 12 to 9 kcal/mole) but about in proportion to the decrease in the total available energy due to the temperature change. Qualitative implications of these findings are discussed, with emphasis on the role of the rapid motion of D relative to X and Y and electronic structure aspects. The energy distributions indicate an impulsive interaction governed by repulsion between the halogen atoms. The angular distributions indicate that migration of D between the halogens does not occur and that the preferred reaction geometry changes from essentially collinear D-X-Y to strongly bent configurations as the central atom changes from Cl to Br to I.