Ab initio study of the electronic, spectroscopic, and laser-cooling properties of NaN and NaN − molecular species

Abstract

The laser-cooling feasibility of NaN − anion is investigated using high-level ab initio theoretical methods. Potential energy curves (PECs) for 16 low-lying electronic states of NaN and 28 electronic states of NaN − were determined using the internally contracted multi-reference configuration interaction method. From these high-precision PECs, spectroscopic constants for five strongly bound states of NaN and eight bound states of NaN − were calculated, demonstrating excellent agreement with available literature. The adiabatic electron affinity of NaN is estimated at 0.357 ±0.009 eV. Critical transition parameters, TDMs, including Franck–Condon factors (FCFs), Einstein A coefficients, oscillator strengths, vibrational branching ratios, and radiative lifetimes, were evaluated. The X 2Π–A 2Σ−, X 2Π–B 2Π, and A 2Σ−–B 2Π transitions of NaN − exhibit highly diagonalised FCFs and short radiative lifetimes, facilitating efficient optical cycling. We propose two distinct laser-cooling schemes: a direct cycle and one incorporating an intermediate state, reaching Doppler-limited temperatures between 15 nK and 435 nK. Our results indicate that the X 2Π–B 2Π transition, even when influenced by the A 2Σ− intermediate state, is highly viable for laser-cooling. With the implementation of two to three repumping lasers, this scheme can sustain approximately 2000 optical cycles before cycle exit, with a total initial capacity of 10 4 cycles. This study establishes the NaN − anion as a promising candidate for experimental cold-molecule research.