Dielectric Functions

In order to assign the appropriate dipole polarizabilities, DDSCAT.5a must be given the dielectric constant of the material (or materials) of which the target of interest is composed. Unless the user wishes to use the dielectric function of either solid or liquid HO (see below), his information is supplied to DDSCAT through a table (or tables), read by subroutine DIELEC in file dielec.f, and providing either the complex refractive index m=n+ik or complex dielectric function as a function of wavelength . Since , or , the user must supply either m or , but not both.

The table formatting is intended to be quite flexible. The first line of the table consists of text, up to 80 characters of which will be read and included in the output to identify the choice of dielectric function. (For the sample problem, it consists of simply the statement m = 1.33 + 0.01i.) The second line consists of 5 integers; either the second and third or the fourth and fifth should be zero.

• The first integer specifies which column the wavelength is stored in.
• The second integer specifies which column Re(m) is stored in.
• The third integer specifies which column Im(m) is stored in.
• The fourth integer specifies which column Re is stored in.
• The fifth integer specifies which column Im is stored in.

If the second and third integers are zeros, then DIELEC will read Re and Im from the file; if the fourth and fifth integers are zeros, then Re(m) and Im(m) will be read from the file.

The third line of the file is used for column headers, and the data begins in line 4. There must be at least 3 lines of data: even if is required at only one wavelength, please supply two additional ``dummy'' wavelength entries in the table so that the interpolation apparatus will not be confused.

In the event that the user wishes to compute scattering by targets with the refractive index of either solid or liquid HO, we have included two ``built-in'' dielectric functions. If H2OICE is specified on line 10 of ddscat.par, DDSCAT\ will use the dielectric function of HO ice at T=250K as compiled by Warren (1984). If H2OLIQ is specified on line 10 of ddscat.par, DDSCAT\ will use the dielectric function for liquid HO at T=280K using subroutine REFWAT written by Eric A. Smith. For more information see http://atol.ucsd.edu/pflatau/scatlib/refr.html .