  We study the energy and momentum of the surface plasmon-polariton (SPP)
excited in a symmetric 3-layer "insulator-metal-insulator" structure, which is
known to support the symmetric (S) mode with the negative group velocity as
well as the antisymmetric (AS) mode with only positive energy flow. The
electric and magnetic field vectors are calculated via both the
phenomenological and the microscopic approach; the latter involves the
hydrodynamic model accounting for the quantum statistical effects for the
electron gas in metal. Explicit representation for the energy and momentum
constituents in the dielectric and in the metal film are obtained, and the
wavenumber dependences of the energy and momentum contributions for the whole
SPP are analyzed numerically. The various energy and momentum constituents are
classified with respect to their origin: "field" or "material", and the
physical nature: orbital (canonical) and spin (Belinfante) momentum
contributions. The pictures characteristic for the S and AS modes are
systematically compared. The results can be useful for the studies and
applications of the SPP-induced thin-film effects, in particular, for the
charge and spin dynamics in thin-film plasmonic systems.
