X-ray absorption of cold gas: Simulating interstellar molecular clouds in the laboratory

Galactic and extragalactic 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. This technique produces a soft x-ray pseudocontinuum 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 similar to 420, comparable to that of astrophysical space instruments, records the absorbed spectra. This resolution clearly resolves the molecular lines from the atomic lines, thus motivating the search for a molecular signature in astrophysical x-ray spectra. The experimental setup enables different gas compositions and column densities. K-shell spectra of CO2, N-2, and O-2 reveal a plethora of absorption lines and photoelectric edges measured at molecular column densities between similar to 10(16) and 10(18) cm(-2) typical of GMCs. We find that the population of excited states, contributing to the edge, increases with gas density.