State selective reactions prepared through the excitation of orbital states in van der Waals complexes of Ca–HX*

Abstract

We have observed the chemiluminescent reaction Ca*+HX→CaX*+H, where the reactants are prepared in a van der Waals complex formed in a supersonic expansion. This preparation, combined with tunable laser excitation, allows access to well-defined electronic states of the reaction complex corresponding to different orientations of the calcium excited orbital. In the case of the Ca–HBr complex, a remarkable effect of this preparation is observed on the branching ratio to the final excited states A 2Π and B 2Σ of CaBr: Depending upon the selected state of the complex, the A/B ratio varies by a factor of 2. This is interpreted by the conservation of the orbital orientation during the reaction involving the departure of the hydrogen.

Quantum stereodynamics of the Li+HF(v,j) reactive collision for different initial states of the reagent

“…A method for the direct spectroscopic investigation of the transition state is the study of the photoinitiated reaction from the van der Waals complex of the reactants. [33][34][35][36][37][38][39][40] In these kinds of experiments on M ϩHX systems, the complex is promoted to an excited electronic state of the metal atom, [34][35][36][37][38][39][40] and the generated product fragments detected show a certain rotational polarization due to the relative orientation of the reactants in the van der Waals complex. Moreover, in a recent work 41 we have shown the possibility of accessing the vicinity of the transition state on the ground electronic state by infrared excitation of the complex of Li-HF, obtaining a high efficiency in forming LiF products at total energies where the collisional reaction cross section is rather low.…”

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

“…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. On the basis of the present results it may be considered that the Ca-HCl van der Waals complex formed in the experiments of Soep and co-workers [17][18][19][20] is not bound, but corresponds to one or several of these long-lived resonances, which have dissociation energies up to Ϸ650 cm −1 with respect to the corresponding HCl͑͒ asymptote. The latter value of the Ca-HCl binding energy would explain the formation of Ca-HCl complexes in competition with Ca 2 dimer formation.…”

“…Experimentally this is studied both by collisions [8][9][10][11][12][13][14][15][16] and through electronic excitation of the Ca atom in the van der Waals complex in the reactant channel. [17][18][19][20] Here, we expect nonadiabatic effects to be more important than in the ground state and we include multiple-coupled potential-energy surfaces.…”

“…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. 12,20 The Ca+ HCl reaction in the ground electronic state has not been studied before, possibly because it is highly endoergic.…”

“…[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. [21][22][23][24][25][26][27][28][29] These studies concerned the effects of the collision energy and initial vibrational excitation of HX, as well as the final rovibrational state distribution of the fragments.…”