  We report the first detection of the phosphorus monoxide ion (PO$^{+}$) in
the interstellar medium. Our unbiased and very sensitive spectral survey
towards the G+0.693$-$0.027 molecular cloud covers four different rotational
transitions of this molecule, two of which ($J$=1$-$0 and $J$=2$-$1) appear
free of contamination from other species. The fit performed, assuming Local
Thermodynamic Equilibrium conditions, yields a column density of
$N$=(6.0$\pm$0.7)$\times$10$^{11}$ cm$^{-2}$. The resulting molecular abundance
with respect to molecular hydrogen is 4.5$\times$10$^{-12}$. The column density
of PO$^{+}$ normalised by the cosmic abundance of P is larger than those of
NO$^{+}$ and SO$^{+}$, normalised by N and S, by factors of 3.6 and 2.3,
respectively. The $N$(PO$^{+}$)/$N$(PO) ratio is 0.12$\pm$0.03, more than one
order of magnitude higher than those of $N$(SO$^{+}$)/$N$(SO) and
$N$(NO$^{+}$)/$N$(NO). These results indicate that P is more efficiently
ionised in the ISM than N and S. We have performed new chemical models that
confirm that the PO$^+$ abundance is strongly enhanced in shocked regions with
high values of cosmic-ray ionisation rates (10$^{-15}-$10$^{-14}$ s$^{-1}$), as
occurs in the G+0.693$-$0.027 molecular cloud. The shocks sputter the
interstellar icy grain mantles, releasing into the gas phase most of their P
content, mainly in the form of PH$_3$, which is converted into atomic P, and
then ionised efficiently by cosmic rays, forming P$^+$. Further reactions with
O$_2$ and OH produce PO$^{+}$. The cosmic-ray ionisation of PO might also
contribute significantly, which would explain the high $N$(PO$^{+}$)/$N$(PO)
observed. The relatively high gas-phase abundance of PO$^{+}$ with respect to
other P-bearing species stresses the relevance of this species in the
interstellar chemistry of P.
