  We investigate phonon induced electronic dynamics in the ground and excited
states of the negatively charged silicon-vacancy ($\mathrm{SiV}^-$) centre in
diamond. Optical transition line widths, transition wavelength and excited
state lifetimes are measured for the temperature range 4-350 K. The ground
state orbital relaxation rates are measured using time-resolved fluorescence
techniques. A microscopic model of the thermal broadening in the excited and
ground states of the $\mathrm{SiV}^-$ centre is developed. A vibronic process
involving single-phonon transitions is found to determine orbital relaxation
rates for both the ground and the excited states at cryogenic temperatures. We
discuss the implications of our findings for coherence of qubit states in the
ground states and propose methods to extend coherence times of $\mathrm{SiV}^-$
qubits.
