How would I call a surface of equal energy?
There are already several terms I know to denote surfaces on which a certain quantity is constant:
- equipotential surface: a surface of equal potential energy
- isobaric surface: a surface of equal pressure
How would I denote a surface of equal energy without falling back on equi-energy surface?
Wiktionary lists isoenergetic as "Having the same, or constant, energy". You might peripherally introduce a definition, e.g. "... the diagram illustrates quantum-mechanical energy isosurfaces, or isoenergy surfaces ...".
The terms isoline and isosurface are used generically for 2D and 3D equal-quantity cases, respectively. After a phrase like "... the diagram illustrates quantum-mechanical energy isosurfaces ..." has introduced your terminology, references thereafter to isosurfaces would be understood as isoenergetic surfaces.
If only certain units of energy are used in the particular problem, tack iso on the front and see if it works. That seems to be what was done for isobel.
Look up the etymology of energy. It's Greek: ἐνέργεια. The rule is to use a Greek prefix with a Greek root, so you want Gk iso- rather than Latin equi- to mean "equal".
Putting iso- and energeia together and adding a little ordinary derivational morphology gives isoenergetic, which should work in context.