Ames New Interstellar Simulation Chamber
Cavity Ring Down Spectroscopy of Interstellar Organic Materials

Farid Salama, Ludovic Biennier, Robert Walker, Lou Allamandola, Jim Scherer, and Anthony O'Keefe

A major milestone has just been achieved at Ames. 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 -that 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 principle 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. To achieve this requires generating and maintaining large carbon-containing molecules and ions under interstellar-like conditions while simultaneously measuring their spectra under these conditions (i.e., 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 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 comprises thus 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 expended 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 K respectively are achieved this way as shown in Figure 1). Recording the cavity ring down signal is a direct measurement of the absolute absorption by the seeding molecules and ions. This is illustrated in Figure 2 that 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 (SBIR) 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 (DIBs). These bands that contribute to the global interstellar extinction were discovered eighty 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.

Figure 1: The figure shows the 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.

Figure 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.