  Liquid Xe TPCs are among the most popular choices for double beta decay and
WIMP dark matter searches. Gaseous Xe has intrinsic advantages when compared to
Liquid Xe, specifically, tracking capability and better energy resolution for
double beta decay searches. The performance of gaseous Xe can be further
improved by molecular additives such as trimethylamine(TMA), which are expected
to (1) cool down the ionization electrons, (2) convert Xe excitation energy to
TMA ionizations through Penning transfer, and (3) produce scintillation and
electroluminescence light in a more easily detectable wavelength (300 nm).
These features may provide better tracking and energy resolution for
double-beta decay searches. They are also expected to enhance columnar
recombination for nuclear recoils, which can be used for searches for WIMP dark
matter with directional sensitivity. We constructed a test ionization chamber
and successfully measured scintillation and ionization yields at high precision
with various Xe and TMA mixtures and pressures. We observed the Penning effect
and an increase in recombination with the addition of TMA. However, many
undesired features for dark matter searches, such as strong suppression of the
scintillation light and no sign of recombination light, were also found. This
work has been carried out within the context of the NEXT collaboration.
