  The double-slit interference in single-photon ionization of the diatomic
molecular ion $\mathrm{H}_2^+$ is theoretically studied beyond the dipole
approximation. Via simulating and comparing the interactions of the prealigned
$\mathrm{H}_2^+$ and the hydrogen atom with the xuv pulses propagating in
different directions, we illustrate two kinds of effects that are encoded in
the interference patterns of the photoelectrons from $\mathrm{H}_2^+$: (i) the
photon-momentum transfer and (ii) the finite speed of light. While both effects
could modify the maxima of the interference fringes, we show that the former
one hardly affects the interference minima. Our results and analysis show that
the interference minima rule out the influences of the photon-momentum transfer
and, potentially, the multielectron effect, thus performing a better role in
decoding the zeptosecond time delay for the pulse hitting one and the other
atomic centers of the molecule.
