Polarization of the CaI* chernilurninescence from the Ca* zyx + CH31 + CaI* + CH3 reaction: Evidence for Hund's case (c)couplingt ~~~ Agustin Laplaza," JoseM. Orea," Carlos Rinaldi,"Gloria Tardajosuvb and Angel Gonzalez Ureiia"* zyxwvutsrq a zyxwvutsrqponmlkjihgf Unidad de Laseres zyxwvutsrqp y Haces Moleculares, Instituto Pluridisciplinar, Universidad Complutense de Madrid, zyxwvutsrqpo Po Juan XXIII-lo, 28040-Madrid Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, 28040-Madrid The polarization emission of the excited CaI* produced in the Ca(3P,, 'D,) + CHJ -, CaI* + CH, reaction has been mea- sured showing evidence for Hund's case (c) in the excited CaI diatom. zyxwvutsrq As is well known, the measurement of the product chemilu- minescence constitutes one of the widely used methods to gain information on the stereodynamics of chemical reactions. ' This technique was pioneered by Zare2 and by Simons4 and their co-workers with studies using alkaline-earth atoms and excited rare gas atoms, respectively. The simplicity of the method is based on the direct assessment of the product rota- tional alignment from an analysis of the degree of polarization of the chemiluminscence. Thus, in many heavy plus heavy- light atom reactions, strong polarization of the product rota- tional angular momentum was found as expected for kinematically constructed reaction^.^.^ Furthermore, devi- ations from such a kinematical limit were also rationalized in the light of several stereodynamical model^.^.^ Most of the available polarization studies have been carried out for parallel transitions i.e. those in which the transition moment lies parallel to the internuclear axis of the product diatom. In these cases, the rotational alignment parameter can be easily related to the (experimental) degree of polariza- ti~n.~.' In the present communication, we report on a product polarization study from the CaI* formed in the Ca* + CHJ -+ CaI* + CH, chemiluminescent reaction. Although our spectral resolution is low, we were able to resolve the X +- B zyxwvutsrqpo '2, X zyxwvutsrqp t A 2r13,2 and X zyxwvutsr e A 2111,2 emission bands. In addition, the polarization degree was measured for each of these transitions showing a positive, negative and positive value, respectively. Whereas the two former values can be expected due to the assumed parallel and perpendicular char- acter of the electronic transition, the latter, e.g. the negative value associated with the X t A 2111,2 transitions is, in prin- ciple, contradictory. These findings have been rationalized assuming that, at least in the present study, Hund's case (c) instead of case (a) is more appropriate for the A excited state of Car. Since in our experiments the relative contributions of the two metastable Ca(,P) and Ca('D,) states can be changed,' the present data correspond to a nearly 50°h contribution (i.e. density x cross-section) from each metastable reaction, leading to CaI in its A or B electronically excited states, respectively. The experiments were carried out in a molecular beam apparatus under beam-gas configuration, which has been described elsewhere.' Briefly, it consists of a high t This work received financial support from the DGICYT of Spain (grant PB91/357) and UE contract no. CI 1-CT94-0128. vacuum chamber containing a high temperature oven where the ground state calcium beam is excited to both ,P, and 'D, metastable states by means of a low current voltage discharge. The beam passes through a 4 mm diameter collimator into a 6 cm length stainless steel scattering cell containing the reac- tant gas. The reactants (MERCK Pro Analyze, purity 98-99%) were used without further purification, and degassed by several freeze and pump cycles. The chemiluminescence is collected perpendicularly to the beam axis by an optical system that focuses the reaction volume onto a 5 mm diameter liquid optical fibre. The other end of the fibre is connected to a monochromator (ISA 320) fitted with a 1200 mm-' grating. The output of the mono- chromator is coupled to an optical multichannel analyser (EG&G OMA-111) with 700 x 0.025 mm pixels, which allows one to take 35 nm spectra with a resolution of 0.4 nm, respec- tively. The OMA averages 25 scans (10 s of acquisition time for each one) and the resulting spectrum is sent to an external computer for further analysis. For polarization measurements, a visible linear polarizer (Ealing, Model 23-5671) is placed between the collecting lens and the optical fibre. This linear polarizer is rotated in order to measure the chemilumine- scence polarized in the planes perpendicular and parallel to the direction of the Ca beam. Typical operating conditions are the same as in ref. 7. Fig. 1 shows a pair of perpendicular (dashed line) and paral- lel (solid line) polarized spectra. The spectral range of the emission bands was restricted to the Av = + 1 sequence because of its higher resolution (for this highly congested system) as compared to that of the Av = 0 one. It can be seen that all the three emission bands are sensitive to the polariza- tion direction as one may expect for this (nearly) kinematically constrained reaction. For a selected molecular transition, the polarization degree, defined by the expression:, can be obtained directly from integration in the spectra recorded with parallel, I I,, and perpendicular, I,, emission intensities. The bottom of Fig. 1 displays the degree of polarization as a function of the wavelength. It is interesting to point out that P is positive (negative) for the X t B (X t A 2113,2) bands as one may expect from the parallel (perpendicular) character of the transition. Surprisingly, P is again positive for the X t A 'Ill,,. electronic transition, con- trary to expectation for a perpendicular transition. This con- tradiction can be eliminated when one considers the possibility that for the present system and conditions, Hund's case (c), instead of case (a), should be more appropriate for describing the components of the A state. Indeed, for this heavy (CaI) diatomic iodide the excited state may well follow this coupling case in which only the projection of J, the elec- tronic angular momentum, on the internuclear axis called Q, is J. Chem. SOC., Faraday Trans., 1996,92(19), 3671-3672 3671 Published on 01 January 1996. Downloaded by University of Warsaw on 30/10/2014 10:18:36. View Article Online / Journal Homepage / Table of Contents for this issue