Update collision docs

* Added a reference for the forward scattering theory section
* Included excitation and forward scattering as possible scattering
processes for ions in the usage section

Docs/source/refs.bib

@@ -35,6 +35,17 @@ @ARTICLE{Birdsall1991
 year = {1991}
 }
 
+@misc{Janssen2016
+author = {Janssen, J. F. J. and Pitchford L. C. and Hagelaar G. J. M. and van Dijk J.},
+doi = {10.1088/0963-0252/25/5/055026},
+journal = {Plasma Sources Science and Technology},
+number = {5},
+pages = {055026},
+title = {{Evaluation of angular scattering models for electron-neutral collisions in Monte Carlo simulations}},
+volume = {25},
+year = {2016}
+}
+
 @misc{Lim2007,
 author = {Lim, Chul-Hyun},
 issn = {0419-4217},

Docs/source/theory/multiphysics/collisions.rst

@@ -136,6 +136,8 @@ This process operates in two ways:
 1. If an excitation energy cost is provided, the energy cost is subtracted from the particle energy and no scattering is performed.
 2. If an excitation energy cost is not provided, the particle is not scattered and the velocity is unchanged (corresponding to a scattering angle of :math:`0` in the elastic scattering process above).
 
+See :cite:t:`b-Janssen2016` for a recommended use of this process.
+
 Benchmarks
 ----------
 

Docs/source/usage/parameters.rst

@@ -2167,7 +2167,7 @@ Details about the collision models can be found in the :ref:`theory section <mul
 
 * ``<collision_name>.scattering_processes`` (`strings` separated by spaces)
     Only for ``dsmc`` and ``background_mcc``. The scattering processes that should be
-    included. Available options are ``elastic``, ``back`` & ``charge_exchange``
+    included. Available options are ``elastic``, ``excitationX``, ``forward``, ``back``, and ``charge_exchange``
     for ions and ``elastic``, ``excitationX``, ``ionization`` & ``forward`` for electrons.
     Multiple excitation events can be included for electrons corresponding to
     excitation to different levels, the ``X`` above can be changed to a unique