CO/pH(2): A molecular thermometer

We utilize reversible temperature dependent changes in the IR absorption sp ectrum of Co molecules isolated in solid parahydrogen (pH(2)) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm(-1) increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this tra nsition lies approximate to 12 K above the CO/pH(2) ground state. During th e deposition of similar to 100 ppm Co/pH(2) samples, we detect temperature gradients similar to 10 K/cm in similar to 0.1 cm-thick samples subjected t o heat loads similar to 10 mW/cm(2). The resulting estimated thermal conduc tivity (TC) is 3(+/-2) mW/cm-K, averaged over the 2 to 5 K region. This val ues is similar to 1000 times lower than the TC of single crystal solid pH(2 ), and similar to 10 times lower than previously measured for pH(2) solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov , and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnorm ally low TC to the known mixed fcc/hcp structure of the rapid vapor deposit ed solids.