  We present a new method to probe atmospheric pressure on Earthlike planets
using (O2-O2) dimers in the near-infrared. We also show that dimer features
could be the most readily detectable biosignatures for Earthlike atmospheres,
and may even be detectable in transit transmission with the James Webb Space
Telescope (JWST). The absorption by dimers changes more rapidly with pressure
and density than that of monomers, and can therefore provide additional
information about atmospheric pressures. By comparing the absorption strengths
of rotational and vibrational features to the absorption strengths of dimer
features, we show that in some cases it may be possible to estimate the
pressure at the reflecting surface of a planet. This method is demonstrated by
using the O2 A band and the 1.06 $\mu$m dimer feature, either in transmission
or reflected spectra. It works best for planets around M dwarfs with
atmospheric pressures between 0.1 and 10 bars, and for O2 volume mixing ratios
above 50% of Earth's present day level. Furthermore, unlike observations of
Rayleigh scattering, this method can be used at wavelengths longer than 0.6
$\mu$m, and is therefore potentially applicable, although challenging, to
near-term planet characterization missions such as JWST. We have also performed
detectability studies for JWST transit transmission spectroscopy and find that
the 1.06 $\mu$m and 1.27 $\mu$m dimer features could be detectable (SNR$>$3)
for an Earth-analog orbiting an M5V star at a distance of 5 pc. The detection
of these features could provide a constraint on the atmospheric pressure of an
exoplanet, and serve as biosignatures for oxygenic photosynthesis. We have
calculated the required signal-to-noise ratios (SNRs) to detect and
characterize O2 monomer and dimer features in reflected spectra and find that
SNRs greater than 10 at a spectral resolving power of R=100 would be required.
