Pressure Effects in Luminescence: Isobaric Experiments on NaI (Tl)

The effect of steady-state hydrostatic pressure on the properties of luminescent systems, with particular reference to the phosphorescence of NaI(Tl), is examined. It is shown that readily observable changes in decay mean life may be produced by very modest pressures. An interpretation of these effect is developed from a configurational coordinate model and also from thermodynamic considerations. Experimental results are in good agreement with these theoretical expectations. For a 0.66-ev trap in NaI(Tl), the change in center volume at the point of transition from metastable to emitting state is found to be +17.5×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^3$. Assuming a specific model for the center allows computation of the sign and magnitude of the critical displacement of ions required for trap collapse—in this case +0.19 A. The force constant for the metastable state is computed to be 5.8×${10}^5$ dynes/cm; and the vibrational frequency, assuming an effective mass equal to six ${\mathrm{I}}^{-}$ ions, is 3.4×${10}^{12}$ ${\sec }^{-1\mathrm{}}$ corresponding to a zero point energy for this trap of 0.007 ev and cross over at a vibrational quantum number of about 47. The transmission coefficient for cross-over to the emitting state appears to be of the order of 0.03%. Other possible experiments involving steady-state or transient pressure effects are outlined.