Load HITRAN 2004+ Data

Imports HITRAN molecular absorption data as raw and absorbance data.

Loads 2004+ HITRAN format files, and applies a Voigt lineshape to each line within the desired range. Returns data in a structure containing the raw data as well as the absorption coefficient spectrum.
* Usage: *
hitran_struct = load_hitran(file_name, wavenumber_1_per_cm, pressure_atm, partial_pressure_atm, path_length_cm, isotopologues_, weight_array_amu)

* Input format: *
** file_name - String of HITRAN file path to load
** wavenumber_1_per_cm - Array of wavenumber samples on which to
calculate the lineshape and absorption
** pressure_atm - Total pressure of gas.
** partial_pressure_atm - Pressure of gas if all gas but the molecular species
of interest were evacuated.
** path_length_cm - Path length of the cell.
** isotopologues_ - Array of isotopologue codes to keep in
the hitran_struct. Meaning is *loosely* defined in
L. S. Rothman et al., "AFGL atmospheric absorption
line parameters compilation: 1982 edition,"
Appl. Opt. 22, 2247 (1983).
As far as I can tell, if the molecule is linear,
you take the ones digit of the nuclear weight of
each atom in order to get the isotopologue code.
** molecular_weight_amu - Array of molecular weights of the isotopologues given
in isotopologues_.

* Output format: *
** hitran_struct - A structure containing the following fields:
*** absorption_coefficient_1_per_cm
- An array of absorption coefficients
as a function of wavenumber_1_per_cm
*** molecule_number
- HITRAN "Molecule number"
*** isotopologue_number
- HITRAN "Isotopologue number"
*** line_center_wavenumber_1_per_cm
- HITRAN "Vacuum wavenumber"
*** line_strength_at_reference_temperature_cm_per_molecule
- HITRAN "Intensity"
*** A_coefficient_1_per_s
- HITRAN "Einstein A coefficient"
*** air_broadened_half_width_1_per_cm_per_atm
- HITRAN "Air-broadened half-width"
*** self_broadened_half_width_1_per_cm_per_atm
- HITRAN "Self-broadened half-width"
*** air_pressure_induced_line_shift_1_per_cm_per_atm
- HITRAN "Air pressured-induce line shift"
*** voigt_half_width_1_per_cm
- The Voigt profile as a function of inputs at 296 K,
as calculated using the formulae in:
M. Gharavi and S. Buckley, "Single Diode Laser
Sensor for Wide-Range H2O Temperature
Measurements," Appl. Spectrosc. 58, 468 (2004).
*** peak_absorbance_
- The peak absorbance of each line, as calculated using the
formulae in:
M. Gharavi and S. Buckley, "Single Diode Laser
Sensor for Wide-Range H2O Temperature
Measurements," Appl. Spectrosc. 58, 468 (2004).

* Reference for format:
L. S. Rothman et al., "The HITRAN 2004 molecular spectroscopic database," J. Quant. Spectrosc. Radiat. Transfer 96, 139 (2005).

* Reference for definition of isotopologue number:
L. S. Rothman et al., "AFGL atmospheric absorption line parameters compilation: 1982 edition," Appl. Opt. 22, 2247 (1983).

* Reference for formulae:
M. Gharavi and S. Buckley, "Single Diode Laser Sensor for Wide-Range H2O Temperature Measurements," Appl. Spectrosc. 58, 468 (2004).

WARNING: This code does *not* take into account the temperature dependence of the transitions. Not only does that change several of the formulae below, but implementation requires calculation of the molecular partition function (think statistical mechanics), which is very involved. Here, we assume the reference temperature of 296 K.