  We analyze the observations of a quiescent prominence acquired by the
Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites
Solaires (THEMIS) in the He I 5876 A (He I D3) multiplet aiming to measure the
spectral characteristics of the He I D3 profiles and to find for them an
adequate fitting model. The component characteristics of the He I D3 Stokes I
profiles are measured by the fitting system approximating them with a double
Gaussian. This model yields an He I D3 component peak intensity ratio of
$5.5\pm0.4$, which differs from the value of 8 expected in the optically thin
limit. Most of the measured Doppler velocities lie in the interval $\pm5$ km/s,
with a standard deviation of $\pm1.7$ km/s around the peak value of 0.4 km/s.
The wide distribution of the full-width at half maximum has two maxima at 0.25
A and 0.30 A for the He I D3 blue component and two maxima at 0.22 A and 0.31 A
for the red component. The width ratio of the components is $1.04\pm0.18$. We
show that the double-Gaussian model systematically underestimates the blue wing
intensities. To solve this problem, we invoke a two-temperature multi-Gaussian
model, consisting of two double-Gaussians, which provides a better
representation of He I D3 that is free of the wing intensity deficit. This
model suggests temperatures of 11.5 kK and 91 kK, respectively, for the cool
and the hot component of the target prominence. The cool and hot components of
a typical He I D3 profile have component peak intensity ratios of 6.6 and 8,
implying a prominence geometrical width of 17 Mm and an optical thickness of
0.3 for the cool component, while the optical thickness of the hot component is
negligible. These prominence parameters seem to be realistic, suggesting the
physical adequacy of the multi-Gaussian model with important implications for
interpreting He I D3 spectropolarimetry by current inversion codes.
