  Many parameters constraining the spectral appearance of exoplanets are still
poorly understood. We therefore study the properties of irradiated exoplanet
atmospheres over a wide parameter range including metallicity, C/O ratio and
host spectral type. We calculate a grid of 1-d radiative-convective atmospheres
and emission spectra. We perform the calculations with our new
Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary
Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric
structures and spectra are made available online. We find that atmospheres of
planets with C/O ratios $\sim$ 1 and $T_{\rm eff}$ $\gtrsim$ 1500 K can exhibit
inversions due to heating by the alkalis because the main coolants CH$_4$,
H$_2$O and HCN are depleted. Therefore, temperature inversions possibly occur
without the presence of additional absorbers like TiO and VO. At low
temperatures we find that the pressure level of the photosphere strongly
influences whether the atmospheric opacity is dominated by either water (for
low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot,
carbon-rich objects this pressure level governs whether the atmosphere is
dominated by methane or HCN. Further we find that host stars of late spectral
type lead to planetary atmospheres which have shallower, more isothermal
temperature profiles. In agreement with prior work we find that for planets
with $T_{\rm eff}$ $<$ 1750 K the transition between water or methane dominated
spectra occurs at C/O $\sim$ 0.7, instead of $\sim$ 1, because condensation
preferentially removes oxygen.
