  The NEXT experiment aims at searching for the hypothetical neutrinoless
double-beta decay from the ${}^{136}$Xe isotope using a high-purity xenon TPC.
Efficient discrimination of the events through pattern recognition of the
topology of primary ionisation tracks is a major requirement for the
experiment. However, it is limited by the diffusion of electrons. It is known
that the addition of a small fraction of a molecular gas to xenon reduces
electron diffusion. On the other hand, the electroluminescence (EL) yield drops
and the achievable energy resolution may be compromised. We have studied the
effect of adding several molecular gases to xenon (CO${}_{2}$, CH${}_{4}$ and
CF${}_{4}$) on the EL yield and energy resolution obtained in a small prototype
of driftless gas proportional scintillation counter. We have compared our
results on the scintillation characteristics (EL yield and energy resolution)
with a microscopic simulation, obtaining the diffusion coefficients in those
conditions as well. Accordingly, electron diffusion may be reduced from about
10 mm/$\sqrt{\mathrm{m}}$ for pure xenon down to 2.5 mm/$\sqrt{\mathrm{m}}$
using additive concentrations of about 0.05%, 0.2% and 0.02% for CO${}_{2}$,
CH${}_{4}$ and CF${}_{4}$, respectively. Our results show that CF${}_{4}$
admixtures present the highest EL yield in those conditions, but very poor
energy resolution as a result of huge fluctuations observed in the EL
formation. CH${}_{4}$ presents the best energy resolution despite the EL yield
being the lowest. The results obtained with xenon admixtures are extrapolated
to the operational conditions of the NEXT-100 TPC. CO${}_{2}$ and CH${}_{4}$
show potential as molecular additives in a large xenon TPC, CH${}_{4}$ showing
the best performance and stability to be used in the NEXT-100 TPC, with an
extrapolated energy resolution of 0.4% at 2.45 MeV for concentrations below
0.4%.
