Remarkable NLO responses of hyperalkalized species: the size effect and atomic number dependence

Abstract

The term “superalkali” (SA) refers to the species with the ionization energy lower than alkali metals (M). Such species possess an excess electron, which is responsible for their high reducing capacity, enhanced electro-optical properties and tendency to form a variety of charge transfer salts. Utilizing second order Møller–Plesset (MP2) perturbation theory we examine the interaction of SA species (SA = FLi₂, OLi₃, and NLi₄) with M atoms (M = Li, Na, and K), which results in the formation of stable hyperalkalized SA–M species. We have analyzed their structures, bonding and stability. The effect of the size of SA species and the atomic number of M on the electronic properties of SA–M species has also been explored. The ionization energies (IEs) and HOMO–LUMO gaps of SA–M decrease with the decrease in the IE of M (Li > Na >K) and/or increase in the size of SA (FLi₂ < OLi₃ < NLi₄). In contrast, their mean polarizability (α_o) and first order static hyperpolarizability (β_o) increase. For instance, the β_o values of OLi₃–M range from 0 (M = Li) to 2.8 × 10⁴ a.u. (M = K), which reach 3.6 × 10⁴ a.u. for NLi₄–Na. These values are large enough to establish the significant nonlinear optical (NLO) properties of SA–M species. Our CIS calculations show that these larger β_o values result due to lower excitation energies, larger transition dipole moments and oscillator strengths for the crucial excited states of SA–M species. These findings may guide the design of potential NLO materials and a new series of alkalides, the species with negatively charged alkali atoms.