  We investigate carrier and collective mode dynamics in 2H-NbSe$_2$ using
time-resolved optical pump-probe spectroscopy and compare the results with
first-principle calculations. Broadband ultrafast reflectivity studies of
2H-NbSe$_2$ in a wide temperature interval covering the normal, charge density
wave (CDW) and superconducting phase were performed. Spectral features observed
in the transient reflectivity experiment were associated with specific optical
transitions obtained from band structure calculations. Displacive excitation of
coherent phonons showed CDW-associated coherent oscillations of the soft phonon
mode across the whole spectral range. Temperature evolution of this coherent
phonon mode in the low-excitation linear regime shows softening of the mode
down to the CDW transition temperature T$_{CDW}$ with subsequent hardening
below T$_{CDW}$. The global fit of the broadband probe data reveals four
different relaxation times associated with characteristic electron-electron,
electron-phonon and phonon-phonon relaxation processes. From first principle
calculations of electron-phonon coupling we associate the few picosecond
electron-phonon relaxation time $\tau_2$ with a specific group of phonons with
frequencies around 20 meV. On the other hand, the anomalously long relaxation
time of $\tau_3$~100 ps is associated with anharmonicity-driven phonon-phonon
scattering. All relaxation processes result from anomalies near the second
order CDW phase transition that are reflected in the temperature dependencies
of the characteristic relaxation times and amplitudes of optical densities. At
highest fluences we observe electronic melting of the CDW and disappearance of
the mode hardening below T$_{CDW}$.
