  The complex Fermi surfaces of transition-metal dichalcogenides (TMDCs)
challenge the standard Peierls-instability-driven charge-density-wave (CDW)
formation. Recently, evidence has been accumulating of a prominent role of
ionic thermal fluctuations, which frozen out below $T_{CDW}$ inducing a
periodic lattice distortion (PLD). We focus on $2H$-NbSe$_2$, displaying a
quasi-commensurate CDW below $T_{CDW}$ $\approx$33 K, and use time-resolved
optical spectroscopy (TR-OS) to detect and disentangle the electronic and
lattice degrees of freedom. We reveal a fingerprint of the ordered phase at
h$\nu$ $\sim$1 eV: at $T_{CDW}$, a divergent relaxation timescale and a
sign-change of the differential reflectivity indicate that CDW gap opening and
PLD formation occur at the same temperature. However, we show that these
effects can be decoupled under moderate photoexcitation, forming a long-lived
state in which the electronic order is destroyed, but the lattice distortion is
not. Our results and computations suggest an unconventional CDW mechanims in
$2H$-NbSe$_2$, highlighting the dominant role of the lattice in driving the
ordered-phase formation.
