The [formula omitted] and [formula omitted] states of N 2+: hyperfine and nuclear quadrupole coupling constants, electric quadrupole moments, and electron-spin g-factors. A theoretical study

Abstract

The variation with bond distance of hyperfine coupling constants (hfcc) [ A iso, A dip], nuclear quadrupole coupling constants (nqcc) [ χ], molecular electric quadrupole moments [ Θ], and electron-spin g-factors is reported for X 2 Σ g +(1σ u 21π u 42σ g) and B 2 Σ u +(1σ u1π u 42σ g 2) of N 2 +. The hfcc’s, nqcc’s, and Θ’s are studied with ab initio and density functional theory methods. Calculated A iso/ A dip of 91.3/29.7 MHz for the X state have to be compared with experimental values of 102.4/23.3 MHz. For the B state, theoretical values A iso/ A dip=793/6.8 MHz support an experimental A iso=708 MHz (1 σ u is largely s) but disagree with A dip=26 MHz (large p). The nqcc χ( B) is one order of magnitude larger than χ( X), 4.80 versus −0.58 MHz, in line with literature results. Omission of nqcc contributions in the analysis of optical spectra is most probably responsible for the anomalously high “experimental” A dip( B). The quadrupole moment Θ is positive for both X and B states of N 2 +, whereas it is negative for neutral N 2( X). The vibrational absorption intensity for the v=1←0 quadrupole transition of N 2 +( X) is about six times stronger than for N 2( X). The g ⊥-shift (Δ g ⊥= g ⊥− g e ) is evaluated with multireference configuration interaction wavefunctions, via second-order sum-over-states expansions. The Δ g ∥’s are small (≈−100 ppm), while Δ g ⊥( v=0) is −2710 ppm for X 2 Σ g + and −6165 ppm for B 2 Σ u + . Interaction parameters | α el|=31.12 cm −1 and β el=1.01 a.u. calculated for the A 2 Π u∼B 2 Σ u + perturbation are in line with 31.30 cm −1 and 1.07 a.u. averaged over five experimental studies. Our Δ g ⊥( v)’s are compared with Curl’s shifts obtained from experimental spin-rotation ( γ) and rotational constants. Good agreement is found for X but not for the B state, for which it is suggested that experimental γ( B, v)’s recently reported—where γ decreases sharply with vibrational level v—should be revised.