I don't speak French, but based on what you've written, "l'atome de l'esprit" is probably the artist's attempt to translate the Japanese 霊子 ("ryoushi"), an invented word some translate as "psycheon".
It's supposed to be the fundamental particle of spiritual power, much like atoms (原子, "genshi") were once thought to be for physical objects. I've also seen it translated as "spiritron", but I think psycheon sounds more pseudo-scientific.
