  Spectroscopically, TDEs are characterized by broad ( 10$^{4}$ km/s) emission
lines and show large diversity as well as different line profiles. After
carefully and consistently performing a series of data reduction tasks
including host galaxy light subtraction, we present here the first detailed,
spectroscopic population study of 16 optical/UV TDEs. We report a time lag
between the peaks of the optical light-curves and the peak luminosity of
H$\alpha$ spanning between 7 - 45 days. If interpreted as light-echoes, these
lags correspond to distances of 2 - 12 x 10$^{16}$ cm, one to two orders of
magnitudes larger than the estimated blackbody radii (R$_{\rm BB}$) of the same
TDEs and we discuss the possible origin of this surprisingly large discrepancy.
We also report time lags for the peak luminosity of He I $\lambda$5876 line;
smaller than the ones of H$\alpha$ for H TDEs and similar or larger for N III
Bowen TDEs. We report that N III Bowen TDEs have lower H$\alpha$ velocity
widths compared to the rest of the TDEs in our sample and we also find that a
strong X-ray to optical ratio might imply weakening of the line widths.
Furthermore, we study the evolution of line luminosities and ratios with
respect to their radii (R$_{\rm BB}$) and temperatures (T$_{\rm BB}$). We find
a linear relationship between H$\alpha$ luminosity and the R$_{\rm BB}$ and
potentially an inverse power-law relation with T$_{\rm BB}$ leading to weaker
H$\alpha$ emission for T$_{\rm BB}$ $\geq$ 25000 K. The He II/He I ratio
becomes large at the same temperatures possibly pointing to an ionization
effect. The He II/H$\alpha$ ratio becomes larger as the photospheric radius
recedes, implying a stratified photosphere where Helium lies deeper than
Hydrogen. We suggest that the large diversity of the spectroscopic features
seen in TDEs along with their X-ray properties, can potentially be attributed
to viewing angle effects.
