  The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array
extended the experiment's sensitivity to lower energies, providing new
opportunities for studies of Active Galactic Nuclei (AGNs) with soft intrinsic
spectra and at high redshifts. The high-frequency peaked BL Lac objects PKS
2155-304 (z = 0.116) and PG 1553+113 (0.43 < z < 0.58) are among the brightest
objects in the gamma-ray sky, both showing clear signatures of gamma-ray
absorption at E > 100 GeV interpreted as being due to interactions with the
extragalactic background light (EBL). Multiple observational campaigns of PKS
2155-304 and PG 1553+113 were conducted during 2013 using the full H.E.S.S. II
array (CT1-5). To achieve the lowest energy threshold, a monoscopic analysis of
the data taken with CT5 was developed along with an investigation into the
systematic uncertainties on the spectral parameters which are derived from this
analysis. The energy spectra were reconstructed down to energies of 80 GeV for
PKS 2155-304, which transits near zenith, and 110 GeV for the more northern PG
1553+113. The measured spectra, well fitted in both cases by a log-parabola
spectral model (with a 5 sigma statistical preference for non-zero curvature
for PKS 2155-304 and 4.5 sigma for PG 1553+113), were found consistent with
spectra derived from contemporaneous Fermi-LAT data, indicating a sharp break
in the observed spectra of both sources at E ~ 100 GeV. When corrected for EBL
absorption, the intrinsic spectrum of PKS 2155-304 was found to show
significant curvature. For PG 1553+113, however, no significant detection of
curvature in the intrinsic spectrum could be found within statistical and
systematic uncertainties.
