Fourier-transform spectroscopy of ${\mathrm{Sr}}_2$ and revised ground-state potential

Precise potentials for the ground-state $X{}^1\Sigma _g^{+}$ and the minimum region of the excited state $2{}^1\Sigma _u^{+}$ of ${\mathrm{Sr}}_2$ are derived by high-resolution Fourier-transform spectroscopy of fluorescence progressions from single-frequency laser excitation of ${\mathrm{Sr}}_2$ produced in a heat pipe at $950°\mathrm{C}$. A change of the rotational assignment by four units compared to an earlier work [G. Gerber et al., J. Chem. Phys. 81, 1538 (1984)] is needed for a consistent description leading to a significant shift of the potentials toward longer interatomic distances. The huge amount of ground-state data derived for the three different isotopomers ${}^{88}\mathrm{Sr}_2$, ${}^{86}\mathrm{Sr}{}^{88}\mathrm{Sr}$, and ${}^{87}\mathrm{Sr}{}^{88}\mathrm{Sr}$ (almost 60% of all excisting bound rovibrational ground-state levels for the isotopomer ${}^{88}\mathrm{Sr}_2$) fixes this assignment beyond a doubt. The presented ground-state potential is derived from the observed transitions for the radial region from $4\text{to}11\mathrm{\AA }$ ($9{\mathrm{cm}}^{-1}$ below the asymptote) and is extended to the long-range region by the use of theoretical dispersion coefficients together with already available photoassociation data. New estimations of the scattering lengths for the complete set of isotopic combinations are derived by mass scaling with the derived potential. The data set for the excited state $2{}^1\Sigma _u^{+}$ was sufficient to derive a potential energy curve around the minimum.

Figure 1

An example spectrum averaged over 50 scans excited by a frequency of ${\nu }_L=17397.62{\mathrm{cm}}^{-1}$. In total, 14 progressions could be assigned which belong to the $2{}^1\Sigma _u^{+}–X{}^1\Sigma _g^{+}$ transition; the quantum numbers of only six of them are given on top of the figure, including the isotope assignment.

Figure 2

(Color online) Overview of the observed energy levels of the ground state $X{}^1\Sigma _g^{+}$.

Figure 3

(Color online) Observed energy levels of the excited state $2{}^1\Sigma _u^{+}$.

Figure 4

(Color online) Residuals of the $X{}^1\Sigma _g^{+}$ potential fit as a function of the transition frequencies. The range $\delta$ marks the uncertainty interval estimated for most lines.

Figure 5

(Color online) $X{}^1\Sigma _g^{+}$ ground-state potential (solid line) compared to the RKR potential from [4] (dotted) and the ab initio potential from [6] (dash-dotted).

Figure 6

(Color online) Residuals of the $2{}^1\Sigma _u^{+}$ potential fit as a function of the term energies.

Figure 7

(Color online) Derived potential (solid line) compared to the RKR potential from Gerber et al. [4] (dotted) for the state $2{}^1\Sigma _u^{+}$ and the ab initio potentials from Boutassetta et al. [6, 33] (dash-dotted) and Czuchaj et al. [10, 34] (dashed).