  We have investigated whether the nu_4 feature of NH4+ is a viable candidate
for the 6.85 micron absorption band seen towards embedded young stellar
objects. To produce NH4+ astrophysical ice analogs consisting of H2O, CO2, NH3
and O2 were UV photolysed. The IR spectra reveal peaks that are identified with
the NH4+, NO2-, NO3- and HCO3- ions. It is shown that the NH4+ matches two
absorption features that are observed towards embedded young stellar objects,
i.e., the strong 6.85 micron feature and the 3.26 micron feature. The
characteristic redshift with temperature of the interstellar 6.85 micron
feature is well reproduced. The abundance of NH4+ in interstellar ices would be
typically 10 % relative to H2O. The experiments show that the counterions
produce little distinct spectral signature but rather a pseudo-continuum if a
variety of them is present in a H2O dominated environment. The anions could
therefore go undetected in IR spectra of interstellar ice. In the ISM, where
additional mechanisms such as surface chemistry and additional elements such as
sulfur are available many acids and an even wider variety of anions could be
produced. These components may be detectable once the ices sublime, e.g, in hot
cores.
