Band strengths for electric dipole transitions from ab initio computation: Lio (X 2Π −X 2Π), (A 2Σ+−A 2Σ+), (X 2Π − A 2Σ+); AlO (X 2Σ+−X 2Σ+), (A 2Π −A 2Π), (X 2Σ+−A 2Π), (B 2Σ+−B 2Σ+), (X 2Σ+−B 2Σ+)

Intensity relations in diatomic molecule spectra are reviewed and computationally useful expressions given for band strengths and band oscillator strengths in terms of electronic expectations values, which are the normal output of ab initio calculation. Calculations of these properties are reported for ⁷Li¹⁶O and ²⁷Al¹⁶O. With very accurate ${\mathrm{X\hspace{0.17em}}}^2\Pi$ and A ²Σ⁺ LiO wavefunctions, band strengths and band oscillator strengths for all transitions involving ν,′ ν″ up to four are given, both within and between the electronic states. Results, given as band strengths (band oscillator strengths), are ${\mathrm{X\hspace{0.17em}}}^2{\mathrm{\Pi \; -X\hspace{0.17em}}}^2\text{Π\hspace{0.17em}0–1}\mathrm{band}$ , $\text{0.038\hspace{0.17em}}\mathrm{a.u.}{\text{\hspace{0.17em}(2.4× 10}}^{\text{−6}})$ . A ²Σ⁺ −A ²Σ⁺ 0–1 band, $\text{0.029\hspace{0.17em}}\mathrm{a.u.}{\text{\hspace{0.17em}(3.7× 10}}^{\text{−5}}\mathrm{);}$ and ${\mathrm{X\hspace{0.17em}}}^2{\mathrm{\Pi \; -A\hspace{0.17em}}}^2{\Sigma }^{+}\text{\hspace{0.17em}0–0}\mathrm{band}$ , ${\text{6.4× 10}}^{\text{−4}}\mathrm{a.u.}{\text{\hspace{0.17em}(1.1× 10}}^{\text{−6}})$ . Similar data are presented for the X ²Σ⁺ and ${\mathrm{A\hspace{0.17em}}}^2\Pi$ states of ²⁷Al¹⁶ 0 using wavefunctions of comparable quality. The dipole moment in the X ²Σ⁺ state is shown to be essentially constant over the range of nuclear vibration in the lowest vibrational levels. Results, given as band strengths (band oscillator strengths), are X ²Σ⁺ − X ²Σ⁺ 0–1 band, ${\text{2.1× 10}}^{\text{−5}}\hspace{0.17em}\mathrm{a.u.}{\text{\hspace{0.17em}(3.1× 10}}^{\text{−8}}{\mathrm{);\; A\hspace{0.17em}}}^2{\mathrm{\Pi \; -A\hspace{0.17em}}}^2\text{Π\hspace{0.17em}0–1}\mathrm{band}$ , $\text{0.035\hspace{0.17em}}\mathrm{a.u.}{\text{\hspace{0.17em}(2.0× 10}}^{\text{−5}}{\mathrm{);\; X\hspace{0.17em}}}^2{\Sigma }^{+}{\mathrm{-A\hspace{0.17em}}}^2\text{Π\hspace{0.17em}0–0}\mathrm{band}$ , $\text{0.0158\hspace{0.17em}}\mathrm{a.u.}{\text{\hspace{0.17em}(6.8× 10}}^{\text{−5}})$ . Adjusting this last band oscillator strength by use of the observed rather than the computed energy separation of the X ²Σ⁺ and ${\mathrm{A\hspace{0.17em}}}^2\Pi$ states increases it from ${\text{6.8× 10}}^{\text{−5}}\hspace{0.17em}\mathrm{to}{\text{\hspace{0.17em}1.2× 10}}^{\text{−4}}$ . Finally more limited and less accurate calculations on the B ²Σ⁺ state of ²⁷Al¹⁶0, give the estimates B ²Σ⁺ − B ²Σ⁺ 0–1 band, $\text{0.08\hspace{0.17em}}\mathrm{a.u.}{\text{\hspace{0.17em}(1× 10}}^{\text{−4}})$ and X² Σ⁺ − B ²Σ⁺ 0–0 band, 1.8 a.u. (0.07).