Bound–free B²Σ⁺ –X²Π, A²Σ⁺ emission in the BAr van der Waals complex

Abstract

This pnper is dedicated to Professor John C. Polclnyi on the occasion ofhis 65th birthday E m s o o~ HWANG, PAUL J. DAGDIGIAN, and MILLARD H. ALEXANDER. Can. J. Chem. 72, 821 (1994).Spectrally resolved bound-free fluorescence emission spectra for the excitation of several vibrational levels in the excited B 2x+ electronic state of the van der Waals molecule " B A~ are presented. This excited state emits to the ground X 2n and low-lying A 'z+ states, both of which correlate with the ground state atomic asymptote B(2p 2~) + Ar. Because of the large differences in equilibrium internuclear separations, the emission occurs mainly to the repulsive walls. In order to gain more information on this portion of the potential energy curves, the experimental emission spectra were compared with simulated spectra derived from ob initio calculated B-Ar interaction potentials. The simulated spectra reproduce the experimental spectra well if the lower-state potential energy curves are shifted slightly inward. This discrepancy is consistent with our previous observation that the a b irlitio calculations slightly overestimate the vibrationally averaged internuclear separation, which we determined experimentally. This reflects the difficulty of accurately calculating weak van der Waals interaction energies. Introduction Supersonic beam and laser spectroscopic methods have been extensively employed to observe transitions between bound vibrational levels of weakly bound diatomic metal -rare gas van der Waals complexes, as illustrated by recent representative studies involving metals in a number of chemical groups (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). With the resulting derived spectroscopic data, considerable information has been obtained on the attractive portions of the potential energy curves of the ground and excited electronic states. It would also be interesting to extract similar information on the repulsive parts of these curves from analysis of continuum spectra.In a recent paper (17), we reported a collaborative experimental and theoretical study of the BAr van der Waals complex. This metal -rare gas complex contains the lightest element of the group 13 family and has a relatively small number of electrons so that accurate electronic structure calculations could be carried out for the ground and excited electronic states of this system. Similar all-electron calculations on other weakly bound metal -rare gas complexes have performed (1 8-2 1). A number of the complexes of heavier members of this series (AlRg, InRg) have been spectroscopically studied by other groups (1 1-16). In our study of BAr, we observed (17), using laser fluorescence excitation of a pulsed supersonic beam, a progression of

Fluorescence excitation and depletion spectroscopy of the BAr complex: Electronic states correlating with the excited valence B(2s2p2 2D) asymptote

“…In the absence of nonradiative decay processes, we expect that the lifetimes of the BAr(D 2 ⌸) vibrational levels should be the same as for atomic level, B(2s2 p 2 2 D), to which the D state correlates since the electronic transition moment should not be affected by the approach of the Ar atom ͑except at very short internuclear separations͒, as was found for the B 2 ⌺ ϩ -X 2 ⌸ transition moment. 3 Hence, we conclude that the BAr(D 2 ⌸) state is subject to a weak nonradiative decay process. This has the effect of reducing the fluorescence quantum yield of these excited levels.…”

“…1 In our laboratory, we have observed laser fluorescence excitation ͑FE͒ spectra of the B 2 ⌺ ϩ -X 2 ⌸ electronic transition of the BNe and BAr complexes and characterized the three lowest electronic states ͓X 2 ⌸,A 2 ⌺ ϩ ,B 2 ⌺ ϩ ͔ of these species in conjunction with ab initio calculations by Alexander and co-workers. [2][3][4][5] The X 2 ⌸ and A 2 ⌺ ϩ electronic states correlate with the lowest atomic asymptote B(2s 2 2p 2 P)ϩRg ͑RgϭNe,Ar͒, while the B 2 ⌺ ϩ electronic state correlates with B(2s 2 3s 2 S)ϩRg. With our experimental observations and the collaborative ab initio calculations, it was possible to derive potential energy curves for the B 2 ⌺ ϩ state of both BNe and BAr.…”

New and Unusual Bonding in Open Shell van der Waals Molecules Revealed by the Heavy Atom Effect: The Case of BAr

“…While methods exist for evaluating the magnitude of the BSSE, such as the counterpoise correction of Boys and Bernardi, 34 they are unwieldy to apply in MRCI calculations. 32 It is unlikely given the size of D e and the extended nature of the basis set used here that BSSE would be a serious problem. This is supported by the work by Hwang et al 21 who report calculations for the X 2 Π, A 2 Σ + , and B 2 Σ + states of BAr using comparable basis sets.…”

Experimental investigation of weakly bound B(2p,3s)–H2/D2 complexes through laser fluorescence excitation spectroscopy

“…The BAr B 2 ⌺ ϩ -X 2 ⌸, A 2 ⌺ ϩ transition moments were found to be essentially unaltered from their asymptotic atomic values for all internuclear separations larger than the inner classical turning point in the B 2 ⌺ ϩ state. 19 We thus write the space-fixed Z component of the transition moment operator for the atom-diatom complex as…”