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Using HITRAN Files

Line lists in .par format from the HITRAN database (or line listings with the same format) are understood by PGOPHER and can be used in several different ways. At its simplest, the line positions and intensities can be overlaid as an "experimental" spectrum in an overlay, but the files can also be used as input to line position or intensity fits (PGOPHER can use the quantum numbers in the file) or imported as an external source allowing more general application of many of the features of PGOPHER, such as energy level plots, in addition to standard simulations.

The format of the HITRAN .par format is documented in "The HITRAN 2004 molecular spectroscopic database", J. Quant. Spectrosc. Radiat. Transfer 96, 139 (2005). The extension is typically .par. Both the older 100 character line and the newer 160 character line formats can be read. Lines that do not begin with a number are ignored, but all other lines must be exactly 100 or 160 characters in length. If using the new HITRAN online interface at select .par (160 chars) as the output format. (Alternative output formats may be handled by future versions.)

There are three different ways of using HITRAN files:

A. Loading the linelist as an overlay

To overlay a linelist simply load the .par file using File, Load Overlay, or simply drag and drop the .par file onto the PGOPHER window. (For this to work extension must normally be .par, as PGOPHER uses this to determine the file type.) This mode is the least flexible, as the only adjustment possible is the addition of a constant width to the lines if the Convolute option is set in the Experimental Plot.

B. Fitting to values in HITRAN files

The values in HITRAN files can also be used as input to line position or intensity fitting. To fit to energy levels, use the procedure for line position fitting with line lists in separate files, for which the .par file can typically be used without modification. For fitting the extension must be .par, as PGOPHER uses this to determine the file type. The quantum number format can normally be determined automatically, but a globalclass n directive can be added to the start of the file to specify the molecule type if necessary. The number corresponds to the entries in Table 3 of the "The HITRAN 2004 molecular spectroscopic database", J. Quant. Spectrosc. Radiat. Transfer 96, 139 (2005). upperstate/lowerstate and related directives described in Line List Input Format can be used to specify the states involved, if there is more than one possibility, or alternatively the states can be named following the global class quantum numbers. The names required are the "global" quanta for the state, with the fields separated by "_". The same procedure can be used to determine the transition dipole moments by fitting to the Einstein A coefficients in the .par file - select Intensity rather than Line in the log window to use this mode, and the IntensityUnits in the Simulation must be cm2WavenumberperMolecule.

C. Simulations using HITRAN files

For the most flexible use of HITRAN files, the .par files should be imported into PGOPHER - use File, Import, HITRAN File.... The resulting file will produce simulations in the normal way, allowing the temperature and linewidth to be adjusted and right clicking on any transition will give the underlying quantum numbers. Note however, that two extra steps will normally be required to produce correct results:
  1. If more than one isotopologue is present in the file the abundances for each will need to be entered manually. For example, importing the the HITRAN file for water will produce several HITRANMolecule objects named 11, 12, 13, ... corresponding to the HITRAN numbers. 11 corresponds to the main isotopologue, and the others can be deleted or the Abundance set to the correct value in the Molecule object. (Strictly the abundance should be set to 0.997317 rather than the default of 1 for the main isotopologue of water.)
  2. By default, the partition function is calculated by summing over the energy levels deduced from .par file. This can give a good starting point, but may not be correct. This is because relevant levels may have been excluded from the HITRAN linelist and also the process of deducing the effective energy level pattern from the linelist is not foolproof. (Any error this introduces will not affect relative intensities at a given temperature.) To correct this either:
    1. Specify the partition function as a table of values using an Interpolated Partition Function. To add this, right click on the HITRANMolecule object and select Add New..., Interpolated Partition Function and then right click on the Interpolated Partition Function object, select "Load..." and then select a file with the required values. There are various possible sources:
      1. The parsums.dat file available as part of the HITRAN 2008 distribution can be used here - you are prompted to select the right columns. Make sure only a single value is selected in the T and Q columns.
      2. HITRAN on the web makes available partition function tables for individual species under "Auxiliary data". The "show numerical data" link provides a simple text file with two columns (T and Q) which can be loaded as above. (The "Download gzipped data" link also produces usable files, though these must be decompressed before use.)
      3. HITRAN online makes partition function tables for individual species under "Documentation", "Isotopologues". A file qnn.txt is available for each isotopologue with two columns (T and Q) which can be loaded as above.
      4. For other sources of data all that is required is a text file with two columns - T and Q.
      The procedure must be repeated for each isotopologue.
    2. If this is not possible at a minimum check the simulation produces the right intensity at 296 K, as a check on the energy level pattern deduced from the par file. The Ediff setting can be important in obtaining correct results - it sets the threshold for level energies to be considered identical. (Some HITRAN linelists have inconsistent level energies.)

Example - HF spectrum

The examples below use 14_hit04.par containing the HF linelist from the 2004 HITRAN release. (Other versions should give similar results, though the 2012 version will show additional lines from DF.)

To display the linelist, simply drag and drop the .par file onto a PGOPHER window:

Import the file to allow more flexible usage: Use File, Import, HITRAN File..., and select the .par file. For higher temperatures the partition function should be taken from HITRAN. To do this right click on the HITRANMolecule object (141 for HF) and select Add New..., Interpolated Partition Function and then right click on the Interpolated Partition Function object and select the parsums.dat file from the HITRAN distribution. In the following dialog box, make sure HF is selected in the Q column, and no other rows. For some molecules the Abundance will need to be set; for HF there is only one isotopologue in the .par file but, for example, an HCl linelist would require 0.75 and 0.25 to be entered for the 35 and 37 isotopes of Cl.

The resulting spectrum will look the same as the above by default, but it is now possible to right click on a line to show the quantum numbers associated with the transition, and the temperature can be changed from the default. It is also possible to produce an energy level plot (View, Levels):

Note that HITRAN only provides partition functions staring at 70 K; for lower temperatures it is probably more accurate to delete the Interpolated Partition Function (or make it inactive), as the energy levels inferred from the HITRAN file are likely to be complete for lower temperatures.

Fitting to the HF data

The file HF.pgo is a standard PGOPHER linear molecule file, set up to simulate all the transitions in the .par file. Four vibrational levels are included in the model. The rotational constants (Origin, B, D and H) were determined by line position fitting using the .par file unmodified as the observations file. For this to work the vibrational states need to be named "0", "1", "2" and "3". The transition dipole moments were determined by a second fit to the same file, with the fit type set to Intensity rather than Line; the intensity units for the simulation need to be cm2WavenumberperMolecule. The partition function is calculated by a sum over all four vibrational levels rather than using an imported table; to allow the partition function values to be compared with HITRAN SymWt has been set to 4 to allow for the two spin ½ nuclei.