Far-IR Experiments with Water

Reggie L. Hudson
Code 691, Astrochemistry Laboratory
NASA Goddard Space Flight Center
Greenbelt, MD 20771 USA
reggie.hudson@nasa.gov

The spectra were recorded in transmission mode (no reflection measurements) with a silicon substrate. The spectral range was approximately 100 - 500 cm-1 (20 - 100 µm) with a spectral resolution of 4 cm-1. The number of scans was 700 in cases where the signal-to-noise ratio was low but 60 otherwise. The data were converted to ascii (wavenumber,absorbance) values with absorbance, A, defined as log(Io/I), and "log" is to base 10. The data points are spaced at intervals of about 0.24 cm-1.

Data Set #1 (deposition at 14 K and warming)

Water was deposited for 39 minutes at 14 K. Far-IR absorbance spectra were recorded at 14, 40, 80, 100, 120, 140, and 160 K. Spectral baselines were adjusted to give an absorbance of 0 at 500, 400, 350, and 100 cm-1. The spectrum at 14 K had a maximum absorbance of 0.37 at 218 cm-1. Using the optical constants of Hudgins et al. (Ap. J. Suppl. Ser., 1993, 86, 713), this absorbance gives an ice thickness of 7.2 µm. Thus the deposition rate was 11 µm hr-1.

Data Set #2 (deposition at 160 K and cooling)

Water was deposited for 60 minutes at 160 K. Far-IR absorbance spectra were recorded at 160, 140, 120, 100, 80, 40, and 14 K. Spectral baselines were adjusted to give an absorbance of 0 at 500, 400, 350, and 100 cm-1. The spectrum at 160 K had a maximum absorbance of 0.75 at 225 cm-1. Using the optical constants of Bertie et al. (J. Chem. Phys., 1969, 50, 4501), or comparing with the measurements of Smith et al. (MNRAS, 1994, 271, 481), a thickness of about 5 µm was estimated for the ice. Thus the deposition rate was 5 µm hr-1.

Data Set #3 (depositions at 14 K < T < 160 K)

Water was deposited at 140, 120, 100, 80, and 40 K, and far-IR absorbance spectra were recorded at each temperature. Spectral baselines were adjusted to give an absorbance of 0 at 500, 400, 350, and 100 cm-1. The maximum absorbance of each spectrum was compared to the measurements of Smith et al. (MNRAS, 1994, 271, 481) to estimate ice thicknesses and, in turn, the deposition rates. These are summarized below:

          Deposition    Maximum     Estimated   Deposition 
     T       Time      Absorbance   Thickness      Rate 
             
   140 K    100 min       0.96        6.9 µm     4.1 µm hr-1
   120 K     75 min       0.71        5.1 µm     4.0 µm hr-1
   100 K    115 min       0.47        7.6 µm     4.0 µm hr-1
    80 K     75 min       0.46        7.4 µm     6.0 µm hr-1
    40 K     36 min       0.35        5.6 µm     9.3 µm hr-1

This page was last modified on: July 14, 2003 by Reggie L. Hudson.