ATOMS: ALMA Three-millimeter Observations of Massive Star-forming
  regions -- I. Survey description and a first look at G9.62+0.19
  The "ATOMS," standing for {\it ALMA Three-millimeter Observations of Massive
Star-forming regions}, survey has observed 146 active star forming regions with
ALMA Band 3, aiming to systematically investigate the spatial distribution of
various dense gas tracers in a large sample of Galactic massive clumps, to
study the roles of stellar feedback in star formation, and to characterize
filamentary structures inside massive clumps. In this work, the observations,
data analysis, and example science of the "ATOMS" survey are presented, using a
case study for the G9.62+0.19 complex. Toward this source, some transitions,
commonly assumed to trace dense gas, including CS $J = 2-1$, HCO$^+$ $J = 1-0$
and HCN $J = 1-0$, are found to show extended gas emission in low density
regions within the clump; less than 25\% of their emission is from dense cores.
SO, CH$_3$OH, H$^{13}$CN and HC$_3$N show similar morphologies in their spatial
distributions and reveal well the dense cores. Widespread narrow SiO emission
is present (over $\sim$1 pc), which may be caused by slow shocks from
large--scale colliding flows or H{\sc ii} regions. Stellar feedback from an
expanding H{\sc ii} region has greatly reshaped the natal clump, significantly
changed the spatial distribution of gas, and may also account for the
sequential high-mass star formation in the G9.62+0.19 complex. The ATOMS survey
data can be jointly analyzed with other survey data, e.g., "MALT90", "Orion B",
"EMPIRE", "ALMA\_IMF", and "ALMAGAL", to deepen our understandings of "dense
gas" star formation scaling relations and massive proto-cluster formation.
