Historical research guide · 0816 · 2008
Thermal Changes in Palladium Deuteride Induced by Laser Beat Frequencies
Dennis Letts · Dennis Cravens · Peter L. Hagelstein
In one minute
Dennis Letts and Dennis Cravens run experiments in Austin, Texas; Peter Hagelstein is a theorist at MIT. This chapter of the American Chemical Society’s Low-Energy Nuclear Reactions Sourcebook reports what happened when the three combined their trades. Letts and Cravens shine two ordinary low-power diode lasers on a single spot of a palladium cathode loaded with deuterium in an electrochemical cell. Two lasers of slightly different colour produce a beat — a slow throb in the combined light, at the difference between the two frequencies — and that beat can be tuned across the terahertz range where palladium deuteride’s own lattice vibrations live. The cell then warms by far more than the laser light alone can deliver, and it does so only when the beat sits near 8, 15 or 20 terahertz. Those are the optical-phonon frequencies Hagelstein’s theory had picked out in advance. The heat signal is small, in the tens to hundreds of milliwatts — but it is a heat signal with a tuning dial on it.
Connection
Chapter 12 is about getting nuclei in a loaded metal lattice to react, and the argument turns on whether the lattice itself is doing the work. This is the experiment that gives the lattice a dial: excess heat that appears and disappears as a terahertz beat frequency is tuned onto and off the palladium deuteride phonon bands, which is exactly the handle an engineer needs to turn an anomaly into a controllable device.
From the source
Two lasers irradiating a deuterated palladium cathode at a single spot can induce significant thermal changes larger than that expected from laser heating alone. The lasers are low power — of order forty milliwatts — while the excess thermal signal runs from tens of milliwatts to a few hundred, measured in an isoperibolic calorimeter whose enclosure is held to within three hundredths of a degree and whose cell power is held to within twenty milliwatts.Publisher’s abstract, first sentence; companion ICCF-14 paper, section 2.1, Instrumentation and Calorimetry, Figures 1c to 1f
The effect is observed only when the lasers are tuned to a beat frequency around 8, 15 or 20 terahertz. The selectivity is sharp rather than gradual: in the first campaign a beat of 8.75 terahertz held for 114 minutes produced no thermal response at all, and retuning to 8.26 terahertz raised the cell temperature immediately, with cell resistance falling as it did.Publisher’s abstract, second sentence; companion ICCF-14 paper, section 2.2 and Figure 6, experiment 662G
These experiments support the notion that optical phonon modes may be involved in the excess heat process. The three responsive frequencies are the known optical-phonon band frequencies of palladium deuteride, and Hagelstein had predicted in advance that the band edges — where compressional phonon modes have low group velocity — would be the places to stimulate. Mapping the responsive bands gives centres at 8.3, 15.3 and 20.5 terahertz with widths of roughly two terahertz each, and the correlation between excess power and beat frequency on the rising side of each band is 0.82 to 0.85.Publisher’s abstract, third sentence; companion ICCF-14 paper, section 2.2.4, Figures 10b and 10c
The campaign includes its own null test. On 25 March 2007 the beat frequency was scanned from 3.12 to 6.8 terahertz in steps of about 0.7 terahertz, one hour at each step, stopping short of the 8 terahertz band. No excess power appeared anywhere in that scan, and the calorimeter stayed flat to about ten milliwatts over ten hours — so the laser light itself is not what the calorimeter is seeing.Companion ICCF-14 paper, section 2.2.1, The Null Experiment, experiment 662N, Figure 7
Dual-laser beat stimulation turned a famously temperamental experiment into a repeatable one. The authors report reproducibility above ninety per cent over the year to mid-2008, with cathodes made from several different palladium sources responding when stimulated at one of the three band frequencies, and two cells run a year apart giving nearly identical responses of about 175 milliwatts near the 20 terahertz mode. Raising cell temperature from 62 to 73 degrees Celsius raised excess power from 225 to 900 milliwatts.Companion ICCF-14 paper, section 3, Improved Reproducibility with Dual Laser Stimulation, Figures 11 to 13
What to watch: the authors name two open points and the measurements that would close them. The 20 terahertz response sits above the longitudinal-optical band edge of pure palladium deuteride, which the authors suggest may be a band edge belonging to hydrogen present as an impurity — testable by controlling the hydrogen content of the heavy water and by phonon-band calculations for mixed palladium deuteride-hydride. And the gold overlayer plated onto the cathode has not been ruled out as the site of the reaction, which the authors set as work for the following year.Companion ICCF-14 paper, section 5, Discussion
The way in
https://doi.org/10.1021/bk-2008-0998.ch015How to cite it
Dennis Letts, Dennis Cravens, Peter L. Hagelstein (2008) Thermal Changes in Palladium Deuteride Induced by Laser Beat Frequencies doi:10.1021/bk-2008-0998.ch015