Stimulated infrared emission from rocks: assessing a stress indicator

To study the effect of stress-activated positive hole (p-hole) charge carriers on the infrared (IR) emission from rocks, we subjected a portion (~10 vol.%) of a large (30×60×7.5 cm3) block of anorthosite, a nearly monomineralic (Ca-rich feldspar) igneous rock, to uniaxial deviatory stress up to failure. We measured the IR emission from a flat surface ?40 cm from the stressed rock volume over the 800?1300 cm?1 (7.7?12.5 ?m) range. Instantly, upon loading, the emission spectrum and intensity change. At first narrow bands appear at 930 cm?1 (10.75 ?m), 880 cm?1 (11.36 ?m), 820 cm?1 (12.4 ?m) plus additional narrow bands in the 1000?1300 cm?1 (7.7?10.0 ?m) range. The 10.75?12.4 ?m bands are thought to arise from vibrationally excited O-O stretching modes, which form when p-hole charge carriers, which spread from the stressed rock volume into the unstressed rock, recombine at the surface. They radiatively decay, giving rise to "hot" bands due to transitions between excited states. Before failure the broad emission bands at 1170 cm?1 and 1030 cm?1 (8.7 and 9.7 ?m) also increase slightly in intensity, suggesting a small increase in temperature due to thermalization of the energy deposited into the surface through p-hole recombination. Stimulated IR emission due to hole-hole recombination and its follow-on effects may help understand the enhanced IR emission seen in night-time satellite images of the land surface before major earthquakes known as "thermal anomalies".