New Interstellar Simulation Chamber Cavity Ring-Down Spectroscopy of Interstellar Organic Materials
Farid Salama, Ludovic Biennier, Robert Walker, Lou Allamandola, Jim Scherer, Anthony O'Keefe
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A major milestone has just been achieved at Ames Research Center: a new facility has been developed to directly simulate gaseous molecules and ions at the low temperature and pressure conditions of interstellar space. This laboratory facility--which is unique within NASA--combines the techniques of supersonic free-jet expansion spectroscopy (JES) with the techniques of cavity ring-down absorption spectroscopy (CRDS). The principal objective is to determine the spectroscopic properties of large interstellar aromatic molecules and ions under conditions that precisely mimic interstellar conditions. The aim of this research is to provide quantitative information to analyze astronomical spectra in support of NASA's Space Science and Astrobiology missions, including data taken with the Hubble Space Telescope.
Understanding the origin, physical properties, and distribution of the most complex organic compounds in the universe is a central goal of Astrophysics and Astrobiology. Achieving this understanding requires the generation and maintenance of large carbon-containing mole-cules and ions under interstellar-like conditions with simultaneous measurement of their spectra under these conditions (that is, in the gas phase at very low densities and at very low temperature). As an aside, these organic structures are those that constitute the building blocks of carbon nanotubes. This process has been accomplished by combining three advanced techniques: free supersonic jet expansion, low-temperature plasma formation, and the ultrasensitive technique of cavity ring-down spectroscopy. The new facility thus comprises a pulsed-discharge, supersonic slit-jet source mounted in a high-flow vacuum chamber and coupled to a cavity ring-down spectrometer. Under these experimental conditions, a beam of argon or helium gas seeded with polycyclic aromatic hydrocarbon molecules (PAHs) is expanded in the gas phase into the cavity ring-down chamber. When the expanding beam is exposed to a high-voltage ionizing electronic discharge, positively charged ions are formed that are characterized by very low, interstellar-like, rotational and vibrational temperatures (temperatures of the order of 10 and 100 kelvin (K), respectively, are achieved this way, as shown in figure 1). The cavity ring-down signal is a direct measurement of the absolute absorption by the seeding molecules and ions. This fact is illustrated in figure 2, which shows the spectrum of the PAH naphthalene ion (C10H8+). This unique experimental facility has been developed in collaboration with Los Gatos Research through a Small Business Innovative Research contract.
The data shown in figure 2 can now be used to analyze the astronomical spectra. For example, the absorption band of the PAH ion (C10H8+) shown in figure 2 can be directly compared to the absorption spectrum of the diffuse interstellar bands. These bands, which contribute to the global interstellar extinction, were discovered 80 years ago and remain an enigma to this day.
For the first time, the absorption spectrum of large organic molecules and ions can be measured under conditions that mimic entirely the interstellar conditions.
Point of Contact: F. Salama
(650) 604-3384
fsalama@mail.arc.nasa.gov
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Fig. 1. Location of the "zone of silence" in a supersonic free jet expansion. The physical conditions within the boundaries of the "zone of silence" approach interstellar conditions.
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Fig. 2. The CRDS absorption spectrum of the naphthalene cation (C10H8+) under simulated interstellar space conditions. The spectrum is obtained when an argon free jet expansion seeded with naphthalene is exposed to a high-voltage discharge. Note the absorption line of metastable argon that is used for internal wavelength calibration.
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