1 Stimulated IR Emission from the Surface of Rocks during DeformationFriedemann T. Freund1,2, Dimitar Ouzounov3, Yulin Zhang4, Qincheng Zhang4, Rachel Post2, John Keefner5, Joshua Mellon2, Akthem Al-Manaseer6 3) NASA Goddard Space Flight Center/SSAI MS 902, , Greenbelt, MD 20771 4) UC Santa Barbara, ICESS 5) NASA Ames Astrobiology Academy 2003 6) Department Civil & Environmental Engineering San Jose State University, San Jose, CA Acknowledgments: Work supported by a grant from the 2003 NASA Ames Director’s Discretionary Fund. 1) NASA Ames Research Center, Earth System Science and Technology Branch Moffett Field, Ca, 2) Department of Physics, San Jose State University, San Jose, CA ABSTRACT Studying the IR emission from rocks placed under high levels of stress may lead to a better understanding of pre-earthquake "thermal anomalies", observed in mid-IR satellite images. We report on laboratory experiments to test the theory that positive hole charge carriers, activated during plastic deformation, travel through unstressed rock, reach the rock surface and recombine, leading to stimulated emission of mid-IR photons. Monitoring the IR emission from the front face of large blocks of anorthosite, a nearly monomineralic Ca-rich feldspar rock, cm from where the rocks were loaded, we observe near-instantaneous changes in the IR emission spectrum. The spectral signatures include new bands at 955, 908, and 860 cm-1 (10.5–11.6 mm). These bands suggest the radiative de-excitation of vibrationally highly excited O–O oscillators that are predicted to form at the rock surface when positive hole charge carriers arrive and recombine to peroxy links. Theory Positive holes (p-holes are defect electrons in the O2- sublattice of igneous minerals. They are normally dormant as positive hole pairs (PHP), chemically equivalent to peroxy links, Si-OO-Si. Dislocations sweeping through the mineral grains during plastic deformation break the PHPs and momentarily generate highly mobile p-holes. These p-holes spread through the unstressed portion of the rock and arrive at the surface as depicted in Figure 5 (left). They lead to a positive surface charge. We predict that the p-holes recombine at the surface, returning to their PHP state. The recombination event releases substantial energy (estimated ≤2.4 eV). Hence, the O-O bond of the newly formed PHP is born in a vibrationally highly excited state as depicted in Figure 5 (right). Some of this vibrational excess energy will be channeled into adjacent Si-O and Al-O bonds. We predict emission of IR photons at the characteristic frequencies of the O-O bond and the Si-O/Al-O bonds. We predict that these spectroscopic features will become clearer the lower the rock surface temperature. . Experimental Blocks (60 x 30 x 7 cm3) of anorthosite, a monomineralic Ca-rich feldspar rock, were loaded between two pistons (11.25 cm Ø), using a SATEC press (250 t) and a BOMEN FT-IR spectrometer (7-14 mm), equipped with two black body sources (ambient and 60°C) for calibration. The imaged spot size on the rock surface was ~5 cm Ø. The load was applied at a constant rate up to failure. Typical runs lasted for min. Figure 5 (right): Recombination of the two p-holes leads to a highly excited PHP, which radiates off some excess energy and channels the rest into adjacent Si-O bonds. Figure 5 (left): Section of an SiO2 surface where two p-holes arrive on adjacent O2- sites. Figure 1: IR emission experiment First Attempt at Confirming the Theory We recorded the IR emission from an anorthosite slab cooled to -10°C and allowed to warm up enough to that its surface has stopped frosting over. We wiped off the condensing water film. Figure 6 compiles all IR spectra taken during loading up to failure. Figure 7 shows how the IR intensities for Boxes I, II, III evolve during loading: initial increase followed by (weak) fluctuations. Figure 8 shows the difference “Files 2-5 minus preload Files” etc. for the spectral windows where the O-O, Si-O and Al-O stretching modes are expected to occur: indeed, new bands seem to appear. Note: fine points and fine lines show original data, bold line smoothened data. Results The room temperature IR emission spectrum of the anorthosite rock is typical of Ca-rich feldspar labradorite. It is dominated by a broad maximum at 9.7 mm and a secondary maximum at 8.5 mm, both believed to be due primarily to Si–O & Al–O stretching modes. Figure 2 shows the IR emission spectrum recorded during the last minutes before beginning to load the rock. However, as soon as we began to apply load to the rock cm away from the surface spot analyzed, the IR emission spectrum changes (Figure 3). During the run the ambient temperature continued to drift. Plotting the content of the different spectral windows as a function of the file number reveals fluctuating intensity variations (Figure 4), overprinting the temperature drift. Figure 6: Overall IR emission Run #22 Figure 2: 10 files at 25 scans each (red lines) and average (black line). Slight drift in room temperature. Figure 7: Intensity during loading Figure 8: Evidence for vibrational de-excitation bands from newly formed PHPs. Preliminary Conclusions The appearance of distinct IR emission bands at the frequencies of the O-O, Si-O and Al-O vibrations, including hot band transitions for highly excited states, suggests the predicted radiative de-excitation of vibrationally highly excited O-O bonds plus a non-radiative energy transfer onto Si-O/Al-O bonds. Figure 3: 45 files in groups of 3 (75 scans) recorded during loading. The boxes outline spectral windows. Figure 4: Contents of Box II plotted as function of file number (time sequence), incl the pre-run files. National Aeronautics and Space Administration