  We investigate the low-energy phase excitations of the quasi-one-dimensional
charge density wave (CDW) in $\mathrm{K}_{0.3}\mathrm{Mo}\mathrm{O}_3$, by
direct probing of infrared-active CDW-lattice modes (phase-phonons) with
ultrafast terahertz spectroscopy. Both the nonequilibrium response and
temperature dependence of the bands are reconciled by generalizing the
time-dependent Ginzburg-Landau theory, beyond that previously applied to the
amplitude-phonons, to include impurity effects, while the photoinduced
blue-shifts are attributed to a reduction of the electron-phonon coupling
induced by a long-lived free-carrier population.
