The beat frequency they were after was in the THz range and this was in order to fit Hagelstein’s theory of optical phonons –
… and yes - small gain was seen. However, in the earlier similar work without beat frequencies – single laser only - much higher gain (order of magnitude more) has been reported by Letts/Cravens. The reproducibility was apparently better in the later experiments - but I do not think the lower result with the beat frequency is leading anywhere. From: H LV Beat frequencies of two lasers irradiating a surface appear in _Stimulation of Optical Phonons in Deuterated Palladium_ by Dennis Letts and Peter Hagelstein https://www.lenr-canr.org/acrobat/LettsDstimulatio.pdf Harry
