  Since their discovery, the infinite-layer nickelates have been regarded as an
appealing system for gaining deeper insights into high temperature
superconductivity (HTSC). However, the synthesis of superconducting samples has
been proved to be challenging. Here, we develop an ultrahigh vacuum (UHV)
${\mathrm{\textit{in situ}}}$ reduction method using atomic hydrogen as
reducing agent and apply it in lanthanum nickelate system. The reduction
parameters, including the reduction temperature (${\mathrm{\textit{T}_{R}}}$)
and hydrogen pressure (${\mathrm{\textit{P}_{H}}}$), are systematically
explored. We found that the reduction window for achieving superconducting
transition is quite wide, reaching nearly 80$^\circ$C in
${\mathrm{\textit{T}_{R}}}$ and 3 orders of magnitude in
${\mathrm{\textit{P}_{H}}}$ when the reduction time is set to 30 mins. And
there exists an optimal ${\mathrm{\textit{P}_{H}}}$ for achieving the highest
${\mathrm{\textit{T}_{c}}}$ if both ${\mathrm{\textit{T}_{R}}}$ and reduction
time are fixed. More prominently, as confirmed by atomic force microscopy and
scanning transmission electron microscopy, the atomically flat surface can be
preserved during the ${\mathrm{\textit{in situ}}}$ reduction process, providing
advantages over the ${\mathrm{\textit{ex situ}}}$ CaH$_2$ method for
surface-sensitive experiments.
