Many scientists were also reminded of the Mössbauer effect, a process involving nuclear transitions in a solid.
Furthermore, nonlinearity of the oscillating cloud would produce both spatial and temporal harmonics, so nuclear transitions of higher multipolarity could also be driven at multiples of the laser frequency.
NAA is significantly different from other spectroscopic analytical techniques in that it is based not on electronic transitions but on nuclear transitions.
The emission and absorption of X-rays by gases had been observed previously, and it was expected that a similar phenomenon would be found for gamma rays, which are created by nuclear transitions (as opposed to X-rays, which are typically produced by electronic transitions).
In general, gamma rays are produced by nuclear transitions from an unstable high-energy state to a stable low-energy state.