A laboratory database of solid CO and CO₂ for ISO

Introduction

CO and, according to recent ISO observations, also CO₂ are important ices in dense clouds. Peak position and band width of the strong bands of CO and CO₂ indicate not only the polar and apolar character of the ice, but can also give an indication on temperature and radiation history of grains. The study of CO and CO₂ bands provides a powerful tool to determine the icy grain mantle composition, in particular the presence of solid O₂ and N₂, and the grain evolution in the cycle of interstellar clouds. We present laboratory measured infrared spectra of mixtures containing CO and CO₂ at temperatures between 10 and 80 K. Comparison of these laboratory data to astronomical spectra, taken by the ISO satellite and ground based facilities, is an essential tool to determine the composition and evolution of interstellar grain mantles.

This database contains 75 experiments on apolar ices and standard polar mixtures. The datafiles also contain the optical constants n and k. The changes in the profiles are simulated for different particle shapes such as spheres, ellipsoids and core/mantle particles. Using this database, observers can fit their observed profiles themselves, and assess the influence of any particle shape using the optical constants. Please refer to the present paper in the A & A special issue, when using datafiles from the database. We wish you fun and success! You can retrieve a ps file by clicking here. A complete paper with the physical interpretation "Infrared spectroscopy of apolar ice analogs" is submitted to A & A Main Journal and will be accessible here after the referee process. (Ehrenfreund et al. 1997).

Experimental

Ices were condensed as pure gas or gas mixture in a high vacuum chamber on the surface of a CsI window, cooled by a closed cycle He refrigerator to 10 K. Infrared transmission spectra were obtained with a BioRad FTS 40A spectrometer at a resolution of 1 cm^-1. Gases and gas mixtures have been prepared in a glass vacuum manifold. The purity of the used gases CO, CO₂, N₂ and O₂ was 99.9997 % (Messer Griesheim). The deposition rate and sample thickness growth rate were about 10¹⁵ molec cm^-2 s^-1 and 1 um hr^-1, respectively. Sample thickness of 0.05 to approximately 0.5 um have been estimated. A detailed description of the experimental setup has been given by Gerakines et al. 1995, A & A 296, 810.

Particle size and shape effects can affect the strong absorption features of solid CO and CO₂. Interactions of an electromagnetic radiation field with the molecules in small grains can change the absorption profile and peak position. It induces an electric charge near the surface of the grain, and therefore the oscillators in the grain are subjected to applied and induced electric field components. The strength of the induced component depends on the polarizability of the grain, which in turn depends on the grainshape and the dielectric or optical constants of the ice.

The optical constants of all our spectra were calculated using the standard Kramers-Kronig analysis (e.g. Hudgins et al. 1993, ApJS 86, 713). For our optical constants we find that particle shape effects influence the profile of the CO₂ stretch mode when the ice contains at least 10% CO₂. For CO this limit is 30% (Tielens et al. 1991, ApJ 381, 181). However, we stress that at present the optical constants obtained by different laboratory groups do not agree. Thus, the calculated cross sections for the different particle shapes are somewhat uncertain. We refer to the detailed discussion given in Ehrenfreund et al. 1997.

The grain shape calculations were done in the Rayleigh limit, which is an approximation of Mie theory for particles small compared to the wavelength. For the grain core we used the optical constants of astronomical silicate (Laor & Draine 1993, ApJ 402, 441). We have performed calculations for different particle shapes such as:

• ice spheres
• an ensemble of ice particles where each ellipsoidal shape is equally probable (CDE: `Continuous Distribution of Ellipsoids')
• core/mantle particles: silicate grain core and the ice mantle have equal volume
• core/mantle particles: silicate grain core size distribution (MRN) (Mathis, Rumple & Nordsieck 1977, ApJ 217, 425), with an ice mantle thickness of 0.01 um irrespective of the core size.

The table below lists all available datafiles

Data-file order

Data files are ordered as such: Two files are available for each mixture:

1. Code E1.NK - E75.NK contains the laboratory data and the optical constants.
   E1.NK - E75.NK files have 5 columns:
   • wavenumber
   • micron
   • absorbance
   • optical constant n
   • optical constant k
   The spectra are given between approximately 5000 and 500 cm^-1, according to the mixture and the presence of important overtone bands. All spectra are baseline corrected. Spikes between 1000 - 500 cm^-1 are intrinsic to the spectrometer-setup. Particularly strong spikes are present between 690--720 cm^-1, close to the CO₂ bending mode, and we removed these by linear interpolation. The remaining spikes have no influence on the optical constant or particle shape calculations of the CO and CO₂ profiles.
2. Code E1.PSC - E75.PSC contains the wavelength dependend absorption cross sections, normalized to grain (core + mantle) volume (`C_abs/V').
   E1.PSC - E75.PSC files have 5 columns:
   • micron
   • spheres
   • CDE
   • core mantle particles: volume 1:1
   • core mantle particles: MRN grain core size distribution mantle thickness 0.01 µm

Dynamical database

We want to state that this database is a dynamical system. Currently there are strong controversies concerning the measurements of optical constants of ices between various leading groups in the field. New laboratory measurements can always improve the accuracy to derive optical constants, therefore experiments might be added to this database or new measurements affecting optical constants and particle shape calculations may be implemented. We ask you therefore to check the "update" date.

P. Ehrenfreund, A. Boogert, P. Gerakines, D. Jansen

The database

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