  Monolayer tinene presents rich absorption spectra in electric fields. There
are three kinds of special structures, namely shoulders, logarithmically
symmetric peaks and asymmetric peaks in the square-root form, corresponding to
the optical excitations of the extreme points, saddle points and
constant-energy loops. With the increasing field strength, two splitting
shoulder structures, which are dominated by the parabolic bands of ${5p_z}$
orbitals, come to exist because of the spin-split energy bands. The frequency
of threshold shoulder declines to zero and then linearly grows. The third
shoulder at ${0.75 \sim 0.85}$ eV mainly comes from (${5p_x,5p_y}$) orbitals.
The former and the latter orbitals, respectively, create the saddle-point
symmetric peaks near the M point, while they hybridize with one another to
generate the loop-related asymmetric peaks. Tinene quite differs from graphene,
silicene, and germanene. The special relationship among the multi-orbital
chemical bondings, spin-orbital couplings and Coulomb potentials accounts for
the feature-rich optical properties.
