Translational Energy Effects in the Excited Ca(¹D₂) + HCl → CaCl(A) + H Reaction. Experiments and a Harpooning Mechanism

Abstract

By using the time-of-flight technique under crossed-beam conditions, the collision energy dependence of the Ca(1D) + HCl → CaCl(A) + H reaction cross-section has been measured over the 0.2−0.5 eV collision energy range and found to have a broad peak at 0.3 eV. An empirical description of entrance covalent channelsincluding orbital alignment effects due to the open-shell nature of calcium atomsand a harpooning mechanism, involving ionic charge-transfer states, account for the observed translational energy dependence of the reaction cross section.

Collisional and photoinitiated reaction dynamics in the ground electronic state of Ca–HCl

“…There was an apparent disagreement between the theoretical value of approximately 200 cm −1 for the dissociation energy of Ca-HCl from ab initio calculations 38 and the empirical estimate 13 of approximately 1000 cm −1 . de CastroVítores et al 13 considered two major contributions, one arising from van der Waals interactions, of Ϸ200 cm −1 , and one due to a charge-transfer contribution.…”

“…de CastroVítores et al 13 considered two major contributions, one arising from van der Waals interactions, of Ϸ200 cm −1 , and one due to a charge-transfer contribution. This last contribution was added to match the experimentally estimated dissociation energy of Menéndez et al 62 The latter estimate was based on the energetics of a reactive collision experiment, in which the product recoil energy was neglected.…”

“…The electronically excited MX products emit, and many experimental chemiluminescence studies have been devoted to the collision dynamics from the excited electronic states of the reactants. [8][9][10][11][12][13][14][15][16] An interesting alternative to study such nonadiabatic processes consists of exciting the system from a van der Waals complex formed in the entrance channel of the ground electronic state reaction. [17][18][19][20] The collision reaction dynamics in the ground electronic state has been investigated experimentally for several alkaliearth atoms reacting with different hydrogen halides.…”

“…Most PES's are based on semiempirical approaches [31][32][33] or on models such as the diatomics-in-molecules ͑DIMs͒ method. 34,35 PES's based on accurate ab initio calculations are more scarce.…”

“…Nearly all the investigations involve the electronic excitation of the Ca atom, and detection of the chemiluminescence of CaCl fragments in the excited A or B states. Such experiments were performed either with collisions [8][9][10][11][12][13][14][15][16] or by photoinitiation from the van der Waals complex in the entrance channel. [17][18][19][20] Even though in all these studies the system is electronically excited, the formation of CaCl fragments in the ground electronic state seemed to be a major reaction path.…”