Optical absorption probes of LixNa4−x (x=1,2): Geometry, substitution isomers, and composition-dependent electron delocalization

Photodepletion spectra (415–763 nm, 10 Å effective resolution) were obtained for LiNa3 and Li2Na2 and compared to previous Na4 and Li4 measurements as well as to model calculations ranging from classical electrostatic to ab initio molecular orbital. Findings include (i) the assignment of planar rhombic singlet ground states for both molecules; (ii) in Li2Na2, the possibleobservation of predominantly one of three energetically close-lying planar rhombic or near rhombic substitution isomers; and (iii) in the sodium/lithium tetramer series, evidence for increasing localization of itinerant valence electrons with increasing cluster lithium content.