Excitation spectra for the photoassociation of Krndash;F and Xendash;I collision pairs in the ultraviolet (208ndash;258 nm)

摘要

Photoassociation, the boundlarr;free absorption of a photon by a colliding pair of atoms, has been studied experimentally in the Krndash;F and Xendash;I systems and is demonstrated to be an effective tool for deducing molecular interaction potentials and for determining the dependence of the relative electronic transition moment on internuclear separation (R) in small molecules having dissociative ground states. Specifically, the excitation (lsquo;lsquo;actionrsquo;rsquo;) spectrum for the photoassociation of thermalized Kr(4p6thinsp;1S0)ndash;F(2p5thinsp;2P) collision pairs, measured at 300 K in the 208ndash;250 nm wavelength region, reveals deeply modulated Franckndash;Condon structure associated with transitions of Krndash;F pairs from the ground statersquo;s thermal and vibrational continua to the lowest 27 bound vibrational states (vrsquo;=0ndash;26) of the KrF(Bthinsp;2Sgr;) excited state. Under these conditions, photoassociation occurs in a narrow (Dgr;R=0.7ndash;0.8 Aring;) Franckndash;Condon region in which the difference potential is a single valued function ofR. Similar spectra have been observed for Xe(1S0)ndash;I(2P) pairs in the 208ndash;258 nm spectral region that arise from boundlarr;free transitions to the lowest sim;40 vibrational levels of the XeI(B) state. Since the Krndash;F and Xendash;I collision pairs are photoexcited from the vibrational continuum of the ground state, the atomic pair energy distribution is thermalized and the envelope of the excitation spectrum can be interpreted directly in terms of the relativeBndash;Xtransition moment, mgr;(R). Consequently, the behavior of mgr;(R) has been determined over a limited range inRand, for XeI, mgr; is found to fall by a factor of 2 in the sim;0.4 Aring; region extending fromRminus;ReBape;0.25ndash;0.62 Aring;. The XeI ground state (Xthinsp;2Sgr;1/2+) potential in the 3.0le;Rle;5.0 Aring; interval, derived from photoassociation spectra, is similar to that deduced from scattering experiments. Excited and ground state structural constants derived from numerical quantum simulations of the experimental spectra are presented.