  We calculate the angular resolved photoemission spectra (ARPES) of a spin-1/2
Heisenberg antiferromagnet as a function of magnetic fields using both the
exact diagonalization method and self-consistent Born approximation. Below the
saturation field $B_C$, strong scattering between spin waves and a hole,
created by photoemission of an electron, significantly narrows the
quasiparticle band that is characterized by the lowering of the quasiparticle
energy at $(\pi,\pi)$ with increasing field. Accordingly, in ARPES the
quasiparticle peak gets shaper near $(\pi,\pi)$ and broader elsewhere.
Furthermore, we observe that an anomalous extended van Hove region (EVHR)
around $(\pi,\pi)$ appears in a half saturation field, while EVHRs around
$(\pi,0)$ and $(0,\pi)$ in zero field gradually disappear with increasing
field.
