We present the c -axis optical conductivity σ 1 c ( ω , T ) of underdoped ( x = 0.12 ) and optimally doped ( x = 0.15 ) L a 2 − x S r x C u O 4 from 4 meV to 1.8 eV obtained by a combination of reflectivity and transmission spectra. In addition to the opening of the superconducting gap, we observe an increase of conductivity above the gap up to 270 meV with a maximal effect at about 120 meV. This may indicate a new collective mode at a surprisingly large energy scale. The Ferrell-Glover-Tinkham sum rule is violated for both doping levels. Although the relative value of the violation is much larger for the underdoped sample, the absolute increase of the low-frequency spectral weight, including that of the condensate, is higher in the optimally doped regime. Our results resemble in many respects the observations in Y B a 2 C u 3 O 7 − δ .