  The point-contact spectra of tantalum in the superconducting state, with
$Ta$, $Cu$, and $Au$ counterelectrodes, have been studied. We discovered some
new distinctive features, whose position on the $eV$ axis is determined by the
critical power required for the injection of nonequilibriumquasiparticles. At
this level of power the band gap $\Delta $ decreases abruptly in the vicinity
of the contact. A correction to the point-contact spectrum, with the sign
opposite to that of the usual correction, arises in the region of phonon
energies. The maxima in the $Ta$ spectrum become sharper and their position on
the energy axis becomes stabilized near the values $e{{V}_{ph}}=7.0$, 11.3,
15.5, and 18 $meV$, which correspond to low phonon group velocities $\partial
\omega /\partial q\simeq 0$ in $Ta$. This is confirmed by the existence of
corresponding flattenings on the dispersion relations $\omega (q)$ of lattice
vibrations. Slow phonons are created near the $N-S$ interface in quasiparticle
recombination and relaxation processes and cause a decrease in $\Delta $ and an
increase in the differential resistance in the vicinity of $e{{V}_{ph}}$. An
excess quasiparticle charge is accumulated in the region of the contact,
producing a correction to the resistance, which decreases as $eV$, $T$, and $H$
increase. These mechanisms are particularly effective in dirty contacts, thus
permitting phonon spectroscopy in the superconducting state even when the
current flow occurs in a nearly thermal mode.
