X-ray absorption of cold gas: Simulating interstellar molecular clouds
  in the laboratory
  Galactic and extra-galactic sources produce X-rays that are often absorbed by
molecules and atoms in giant molecular clouds (GMCs), which provides valuable
information about their composition and physical state. We mimic this
phenomenon with a laboratory Z-pinch X-ray source, which is impinged on neutral
molecular gas. The novel technique produces a soft X-ray pseudo continuum using
a pulsed-current generator. The absorbing gas is injected from a 1 cm long
planar gas-puff without any window or vessel along the line of sight. An X-ray
spectrometer with a resolving power of $\lambda/\Delta\lambda\sim$420,
comparable to that of astrophysical space instruments, records the absorbed
spectra. This resolution clearly resolves the molecular lines from the atomic
lines; therefore, motivating the search of molecular signature in astrophysical
X-ray spectra. The experimental setup enables different gas compositions and
column densities. K-shell spectra of CO$_2$, N$_2$ and O$_2$ reveal a plethora
of absorption lines and photo-electric edges measured at molecular column
densities between $\sim$10$^{16}$ cm$^{-2}$ -- 10$^{18}$ cm$^{-2}$ typical of
GMCs. We find that the population of excited-states, contributing to the edge,
increases with gas density.
