# Cross-Sections¶

The method is described in: C. Hill, S. N. Yurchenko, J. Tennyson, “Temperature-dependent molecular absorption cross sections for exoplanets and other atmospheres”, Icarus, 226, 1673-1677 (2013). See the paper here.

## Gaussian profile¶

Example (HWHM is the half-width at half-maximum):

```
Temperature 2000.0
Range 0.0 12000.0
Npoints 200001
absorption
Gaussian
HWHM 0.5 (cm-1)
threshold 1e-40 (to skip weak lines)
output abs_gauss_0.5_T2000.0
States "../all/ch4-50.states-all"
Transitions
a-03000.dat
a-03100.dat
a-03200.dat
a-03300.dat
a-03400.dat
a-03500.dat
a-03600.dat
a-03700.dat
a-03800.dat
a-03900.dat
end
```

## Doppler profile¶

Doppler is the effective mass of the molecule in amu.

Example:

```
Temperature 1500.0
Range 0.0 12000.0
Npoints 200001
emission
doppl
mass 16.0313
output dop_emiss_1500.0
States ch4-50.states-all
Transitions a-02100.dat
```

## Lorentzian profile¶

Here b is the normalization factor.

Example:

```
Temperature 300
Range 0.0 10000.0
Npoints 10001
absorption
Loren
HWHM 0.1 (cm-1)
output abs_lor_0.1_T300.0
States NaH.states
Transitions NiH.trans
```

## Stick spectrum¶

Example:

```
(ScH stick spectrum)
Temperature 1500.0
Range 0. 16000.0
Npoints 16001
absorption
stick
threshold 1e-29
output ScH_1500K_stick
States ScH.states
Transitions ScH.trans
```

## bin¶

is to produce average intensity per the wavenumber or wavelength interval as defined by Range/(Npoints-1). The wavelength is invoked by adding um to the range values.

Example:

```
(ScH bin spectrum)
Temperature 1500.0
Range 0. 16000.0
Npoints 16001
absorption
bin
output ScH_1500K_bin_stick
States ScH.states
Transitions ScH.trans
```

or:

```
(ScH bin spectrum)
Temperature 1500.0
Range 1. 100.0 um (or micron)
```

## Box¶

Is to plot the maximal transition intensity per wavenumber interval, which is a cheaper alternative for the stick spectrum

Example:

```
(ScH box spectrum)
Temperature 1500.0
Range 0. 16000.0
Npoints 16001
abundance 0.97
absorption
box
threshold 1e-29
output ScH_1500K_box_stick
States ScH.states
Transitions ScH.trans
```

## Line-width cut-offs¶

A line width cut-off can be defined using `cutoff`

or `line-cutoff`

```
cutoff 25 (cm-1)
```

```
line-cutoff 25 (cm-1)
```

where the cutoff value is in wavenumbers (cm^{-1}). The default value is 25 cm^{-1}. Alternatively,
one can define the cut-off in terms of the HWHM as follows:

```
cutoff 50 HWHM
```

## multi-grid¶

A multi-grid with regions of different resolutions can be defined using the following grid section:

```
grid
Range 0 100 Npoints 10000 cutoff 10
Range 100 1000 Npoints 1000 cutoff 25
Range 1000 10000 Npoints 100
end
```

The maximal number of sub-grids is 100. Currently this option only works with
simple sampling-type profiles, such as Voigt, Doppler Sampling, Gaussian Sampling or Bin.
The latter is commonly used to generate super-lines.
cutoff or line-cutoff is an optional keyword to allow region-dependent cutoffs for line profiles. If undefined, the value of the
global keyword cutoff the corresponding default value (25 cm^{-1}) is used.

## gf line list¶

A stick spectrum is produced with the gf-factors in place of the Einstein coefficients. Here is the example for the VALD format, where the columns are the wavelength in Angstrom, the lower state energy in eV, log10(gf), 0.0, the statistical weight 2J’+1 (upper state J’) and zero.

```
temperature 5000
Range 100. 16000.0
gf
vald
threshold 1e-29
output ScH_gf
States ScH.states
Transitions ScH.trans
```

## Using HITRAN .par with ExoCross¶

Here is an example of an ExoCross input file for computing absorption cross sections from a HITRAN .par with ExoCross:

```
Temperature 400
Range 0 8000
Npoints 800001
absorption
voigt
pf 274.56910 ref 1.74581257E+02
HITRAN
mass 18
iso 1 1
abundance 0.99734
pressure 1.0
transitions HTRAN_H2O_2020.par
species
air gamma 0.075 n 0.40 t0 296.0 ratio 0.70 delta 0.000000
self gamma 0.670 n 1.00 t0 296.0 ratio 0.30 delta 0.000000
end
output H2O_HITRAN_400K_voigt_1bar
```

It is important to provide two partition functions, for the target temperature (here 400 K) as well as for 296 K (HITRAN reference temperature). One also needs to define the air:self ratio as well as as the mass, isotopologe number etc.