3 Observations

The $B^2\Sigma^+{\rightarrow}X^2\Sigma^+$ system of LaO, lying in the green-yellow spectral range between 500 and 650 nm, is composed of red-degraded, double-headed bands. Each head appears finely split (Fig. 1), this being a feature of the systems involving $X^2\Sigma^+$. It arises from the special $b_{\rm \beta S}$ coupling in the ground state, which splits each rotational level into two equidistant components separated by a constant value of about 0.5 cm^-1 (see Sect. 4).

  

Figure 1: The $B^2\Sigma^+{\rightarrow}X^2\Sigma^+(3-2)$ and (4-3) relaxed bands following excitation by Kr+ 520.8-nm and Ar+ 501.7-nm laser lines. Some direct fluorescence lines are shown. R23(113) and R24(113) lines in the (2-1) band are also noted

The fluorescence spectrum induced by the Kr⁺ 520.8-nm and Ar⁺ 501.7-nm laser lines consists of the pairs of strong lines (P(J+1), R(J-1) patterns) of the direct fluorescence, accompanied by many lines corresponding to transitions from upper state rotational levels populated by collisional transfer of energy. The Kr⁺ 520.8-nm line excites the v=2 and v=3 levels in $B^2\Sigma^+$ from the ground state v=0 and v=1, respectively, giving rise to fluorescence transitions to v=1, 2 and v=2, 3 in $X^2\Sigma^+$. The Ar⁺ 501.7-nm line, which populates v=4 in $B^2\Sigma^+$ from $X^2\Sigma^+(v=1)$, induces fluorescence transitions towards $X^2\Sigma^+(v=3,\ 4,\ 5)$. The relaxed spectrum is generally well developed, except for the 2$\leftarrow$0 excitation by Kr⁺. The summary of excitation and direct fluorescence transitions is given in Table 1. Parts of the observed fluorescence spectra are shown in Fig. 1.

Let us mention that under our experimental conditions the very first bands of the $\Delta v=0$ sequence are also observed, with rapidly decreasing intensity and development. The strong 0-0, moderately intense 1-1, and weak 2-2 bands have been rotationally analysed.