Photoelectron Spectroscopy of High‐Temperature Vapors. I. TlCl, TlBr, and TlI

A cylindrical‐mirror electron energy analyzer has been constructed for use with molecular beams of unstable or nonpermanent gases. The apparatus has been used to study the He(I) photoelectron spectra of TlCl, TlBr, and TlI. For TlCl and TlBr, three peaks are observed. A surprising result of the analysis is that the broad peak, presumably due to removal of an electron from a bonding orbital, corresponds to the ``lone‐pair'' pII orbitals of the halogen. Although removal of such an electron in a system such as HX or CH3X gives rise to sharp peaks, indicating that this orbital is nonbonding in the covalent sense, its removal from an ionic molecule destroys the ionic bond, making the corresponding peak broad. The TlI spectrum is more involved and suggests that the covalent contribution to bonding is competitive with the ionic contribution.

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