2 sion starting from J ′ = 25 a resolution of 0.05 cm − 1 was used, but because of the small spacings between the lines with v $^{′′}$> 50 it was expected that for the smaller J ′ the P, R doublets of the highest vibrational levels would not be completely resolved with this resolution. Therefore, the resolution was increased for the remaining measure- ments to 0.02 cm − $^{1}$. For increasing the sensitivity of the measurements without saturating the amplifiers in the detection unit of the spectrometer selected optical filters were used. For the progressions of the four larger J ′ (25 to 13) a holographic notch filter and a OG 590 color glass filter were inserted to suppress the internal HeNe laser of the spectrometer and the stray light of the dye laser, respectively. For the three progressions (J ′ = 25, 21, and 17) up to 600 scans of the spectrometer were averaged and for J ′ = 13 about 800 scans were necessary to identify safely lines to the highest existing levels with v ′′ = 59 and 60. To be able to observe v ′′ = 60 also for J ′ = 9, where the intensity is already significantly lower because of the smaller degeneracy of that level, the experiment had to be further optimized. With a single bandpass filter with 10 nm FWHM (full width at half maximum) around 600 nm, where the expected fluores- cence for v ′′ > 30 should appear, and a laser power of more than 200 mW in front of the heatpipe it was pos- sible to identify two more rotational levels for v ′′ = 60 (J ′′ = 8 and J ′′ = 10) by averaging 600 scans. The method, mentioned in [10] and successfully used for Ca$_{2}$ in [18], of excitation spectroscopy by tuning the laser over the transition frequencies to the desired levels and filtering the detected fluorescence with a monochro- mator, was not used for observing even higher v $^{′′}$, i.e. 61 and 62, because for these levels, which exist only for J ′′ ≤ 8, the line separations of the P, R doublets become already smaller than the Doppler widths of the individ- ual lines. Beam experiments would be needed to obtain sufficient resolution for this purpose. available of the excited states of the molecules are input to the program. Additional lines which cannot be auto- matically identified to belong to the progression, because they extend significantly beyond the precisely known re- gion of the ground state potential, are individually se- lected by the user before the program automatically as- signs the best fitting quantum numbers. The data set published in [10] consisted of 4673 dif- ferent rovibrational levels for the ground state, contain- ing 3163 of the ca. 5400 existing ground state levels of the main isotopologue $^{88}$Sr$_{2}$ . The current ground state data set is extended to 6077 different rovibrational lev- els, of which 4159 belong to the isotopologue $^{88}$Sr$_{2}$ and 1301 and 617 belong to the isotopologues $^{86}$Sr$^{88}$Sr and $^{87}$Sr$^{88}$Sr, respectively. The complete data field is shown in figure 1. The assignment of the 15700 lines (5400 new since pub- lication of [10]) of the current data set was done using a partially automated software as already described in [10] (in a prior version in [19]). This software uses the differ- ences between individual lines of a selected progression to automatically assign them to a molecule and an elec- tronic transition and to quantum numbers v $^{′}$, J$^{′}$, v′′ and J $^{′′}$, provided that the data of the ground and as much as In [10] the uncertainties for the differences of the line frequencies were estimated to be 0.005 cm − 1 in most cases. This is one tenth of the applied resolution of 0.05 cm − $^{1}$. We keep this uncertainty for the lines which are taken from the earlier data set and also the lines newly measured using the same resolution. For the new lines measured with the higher resolution of 0.02 cm − 1 we esti- mate a smaller uncertainty of 0.002 cm − 1 for the stronger lines (S/N ratio larger than 5). For these lines a signifi- cant contribution of any Doppler shift can be excluded, since the excitation frequency was optimized on the cen- ter of the Doppler profile and thus the velocity compo- nents of the excited molecules parallel to the direction of observation will be very small. The specified uncer- tainty of the Fourier spectrometer for frequency differ- ences within a spectrum is 0.001 cm − $^{1}$, still below the assumed experimental uncertainty. For weak lines and lines belonging to accidentally overlapping excitations an uncertainty of 0.004 cm − 1 is assumed. The complete list of assigned lines can be found in the additional online material. For the ground state a potential energy curve is cal- culated using the same type of representation as in [10] IV. RESULTS III. DATA SET FIG. 1: (Color online) Overview of the observed energy levels of the ground state X $^{1}$Σ+ g , including all measurements of ref. [10]. 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