  Our recent studies of electroluminescence (EL) properties in two-phase argon
detectors for dark matter searches have revealed the presence of unusual
delayed pulses in the EL signal in the form of two slow components with time
constants of about 5 and 50 $\mu$s. These components were shown to be present
in the charge signal itself, which clearly indicates that drifting electrons
are temporarily trapped on two states of metastable negative argon ions which
have never been observed before. In this work, using the pressure dependence of
the ratio of slow component contributions measured in experiment, it is deduced
that these states are those of two types of metastable negative molecular ions,
$\mathrm{Ar}_2^{*-}(b \ ^4\Sigma_u^-)$ and $\mathrm{Ar}_2^{*-}(a \
^4\Sigma_g^+)$ for the higher and lower energy level respectively.
