  Using quantum Monte Carlo, exact diagonalization and perturbation theory, we
study the spectrum of the $S=1/2$ antiferromagnetic Heisenberg trimer chain by
varying the ratio $g=J_2/J_1$ of the intertrimer and intratrimer coupling
strengths. The doublet ground states of trimers form effective interacting
$S=1/2$ degrees of freedom described by a Heisenberg chain. Therefore, the
conventional two-spinon continuum of width $\propto J_1$ when $g=1$ evolves
into to a similar continuum of width $\propto J_2$ when $g\to 0$. The
intermediate-energy and high-energy modes are termed \emph{doublons} and
\emph{quartons} which fractionalize with increasing $g$ to form the
conventional spinon continuum. In particular, at $g \approx 0.716$, the gap
between the low-energy spinon branch and the high-energy band with mixed
doublons, quartons, and spinons closes. These features should be observable in
inelastic neutron scattering experiments if a quasi-one-dimensional quantum
magnet with the linear trimer structure and $J_2<J_1$ can be identified. Our
results may open a window for exploring the high-energy fractional excitations.
