Thermal diffusivity and nonradiative relaxation rate of the Q₁(0) + S₂(0) transition in solid parahydrogen measured by transient thermal lensing spectroscopy

We have measured the thermal diffusivity D_th and the nonradiative relaxation rates Γ for the double transitions Q₁(0) + S₂(0) and Q₂(0) + S₁(0) in solid parahydrogen in the temperature range from 5.2 K to 12.2 K by the technique of transient thermal lensing spectroscopy (TTLS). We have derived a formula which reproduces the temporal evolution of the TTLS signal. In this formula, we have defined a characteristic wave-vector magnitude, with which we can examine the achievement of local thermal equilibrium in the system. The characteristic wave-vector magnitude also enables quantitative comparisons between the TTLS and the transient thermal grating spectroscopy, which is a similar technique to the TTLS. We have obtained the temperature dependences of D_th and Γ by least-squares fit of the derived function to the observed signals without employing any other temperature-dependent parameters. At relatively low temperatures, the temperature dependence of D_th has been found to be in good agreement with that in the previous measurements, while at relatively high temperatures the values of D_th are slightly smaller than those in the previous measurements. We have found that Γ is almost temperature independent in the investigated temperature range as has been theoretically predicted. The value of Γ obtained is 4.3 × 10⁴ s⁻¹, which is substantially slower than the value 3.0 × 10⁵ s⁻¹ reported by Kuo et al. [Phys. Rev. Lett. 53, 2575 (1984)].