1. Caglioti, G.: From perception to thought-a nonequilibrium dynamic instability implying a symmetry breaking, in Katachi ∪ Symmetry. In: Ogawa, T., Miura, K., Masunari, T., Nagy, D. (eds.), pp. 355-361. Springer, Tokyo (1996)
2. and Caglioti, G.: Optical art: "illusions" and paradoxes of symmetry and quantum mechanics, in Symmetry 2000. In: Hargittai, I., Laurent, T.C. (eds.) Part 2, pp. 457-466. Portland Press, London (2002)
3. Bertolotti, M.: The history of the Laser. Taylor & Francis (2004). ISBN 978-0-7503-0911-0
4. Feynman, R., Leighton, R.B., Sands, M.: The Feynman Lectures of Physics, vol III, pp. 8–12. Addison–Wesley Publishing Company (1965). This well known textbook offers an over-simplified picture in which the N atom tunnels from one side to the other of the triangle made of the three H atoms. If so no isotopic effect would be observed. The tunneling motion is actually a coherent one of all atoms so as to keep the center of mass fixed. In this motion the light H atoms are mostly displaced, which yields the large isotopic effect observed on the replacement of the H atoms with one or more D atoms. Note that the transition between the two specular configurations can also be obtained with a rigid rotation of the molecule. Rotational excitation frequencies are however in the THz range, much above the 24 GHz tunneling oscillation and with a much smaller isotope effect
5. Herbauts, I.M., Dunstan, D.J.: Quantum molecular dynamics study of the pressure dependence of the ammonia inversion transition. Phys. Rev. A 76, 062506 (2007); for deuterated ammonia ND3 the microwave frequency is as low as 1.6 GHZ, as compared to 24 GHz for NH3. While the chemical barrier to tunneling is about the same for the two isotopes, the tunneling frequency is an exponentially decreasing function of the isotope mass