Ice analogs database

Introduction

This database contains the infrared spectra (4000 -- 400 cm-1) of laboratory analogs of interstellar ices. These analogs consist of mixtures of the molecules H2, H2O, NH3, CH4, CO, H2CO, CH3OH, O2, N2, and CO2. These mixtures can contain from 1 up to 3 of these components. For each sample spectra are available for a number of systematically increasing UV doses, and at a number of different temperatures.

Data format

The data is supplied as simple text files with 2 columns. The units are cm-1 and Absorbance (=(Optical depth)/ln10).

Experimental Procedures

A detailed description of the experimental procedures can be found in Gerakines et al. 1995 (Astronomy and Astrophysics 296, 810) and Gerakines et al. 1996. Only the directly relevant features are described here. Ice samples consisting of a mixture of gases are deposited on a CsI window inside a vacuum chamber at a temperature of 10 K. Subsequent irradiation is performed with a hydrogen discharge vacuum UV lamp which gives a nominal flux of 1.0E+15 photons cm-2 s-1 (Ephoton > 6 eV), with an uncertainty of about a factor 2. Irradiation doses are stepwise increased, and generally correspond to 5s, 15s, 1m, 3m, 10m or 12m, and 1h irradiation time. After the final irradiation step, gradual warm-up was performed, generally to 50, 80, 120, 160 K. In most cases also higher temperatures were studied, but not at standard values. A spectrum is obtained just before sample deposition (the background), and then at each of the above defined experimental stages. The final spectrum is made by ratioing the spectrum by the background. The thickness of the samples were choosen at 0.11-0.15 micron. This choice optimizes S/N on the obtained features, while still ensuring (near) optical thinness (~ 50 % of the UV radiation is absorbed by the sample). Resolution equals 1 cm-1 for the experiments involving pure compounds, and 2 cm-1 for the mixtures.

Summary of experiments

The table below summarizes the samples for which measurements have been performed. The first column gives the name of the experiment. The name was constructed as comp1[_comp2[_comp3]_number], where comp1, comp2 and comp3 are the molecular constituents, while, in case of 2 components, the number gives the abundance of component 2 relative to 1, or, for three components it is just a numerical identifier of this particular mixture.
  Experiment             Composition           Date   Ori.Name  Comment
                 comp.1 comp.2 comp.3 comp.4

    H2O           100                         080295    PE15
    NH3           100                         100295    PE17
    CH4           100                         160295    PE21
    CO            100                         260195    PE07
    CO2           100                         010295    PE10
   H2CO           100                         240195    PE06
   CH3OH          100                         030395    PE25    10K spectra
                  100                         060395    PE26    > 10K
    O2            100                         030295    PE12
    N2            100                         060295    PE13

H2O_NH3_20        100     20                  251295    B08
H2O_CH4_33        100     33                  010196    B10
H2O_CO_33         100     33                  141195    B05
H2O_O2_20         100     20                  251295    B07
H2O_CH3OH_100     100    100                  071195    B03
NH3_CH3OH_50      100     50                  091195    B04
NH3_O2_100        100    100                  290296    B17
CO_H2O_10         100     10                  161195    B06
CO_NH3_100        100    100                  240296    B15
CO_O2_100         100    100                  120196    B12
O2_H2O_10         100     10                  110196    B11
O2_N2_100         100    100                  250296    B16

H2O_CO_H2_1       100     33     33           020296    B14   Sticking H2 < 100%
H2O_CO_NH3_1      100     23     21                     XCN10

Some warnings

A few isolated spurious features result from a few limitations in the procedure. First, due to the use of a sandwich of frozen layers of Ar to embed the actual sample (see Gerakines et al. 1996 for a description of this technique), lines are introduced of H2O molecules diluted in Ar caused by the presence of residual traces of water in the vacuum set-up. These lines can be found between 1580 and 1670 cm-1 and are often visible after 3m of irradiation (At earlier stages they can be corrected). Second, a number of sharp features appear during warm-up, for example at 1095 and 1235 cm-1. These features are due to the presence of some organic material absorbed on the CsI substrate. Normally the corresponding features are fully eliminated when the ratioing by the background takes place. However, the features are sometimes temperature dependent causing the presence of the sharp structure in the warm-up spectra. Finally a sharp strong peak is usually found at ~700 cm-1 which is related to the vibration of the set-up caused by the cryogenic system.

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Last modified: Mon Jan 8 13:33:52 GMT 1996