Measuring the Carrier Dynamics of Photocatalyst Micrograins Using the Christiansen Effect

The optical measurement of photocatalyst materials is subject to Mie scattering when the particle size is comparable to the wavelength of the probe light. A novel approach was developed to deal with this scattering problem in the transient spectroscopy of photocatalyst micrograins using the Christiansen effect because the probe light in the vicinity of the Christiansen frequency can be transmitted. Scattering theory was used to analyze the transient spectra of micrograins and estimate the extinction coefficient at the Christiansen frequency. The Drude-Lorentz model was used to calculate the complex refractive index considering the contributions from both phonons and free carriers. We found that the net photogenerated carrier density was linearly correlated with the absorbance at the Christiansen frequency. With the parameters obtained from Raman scattering measurements, the absolute net carrier density was also determined. We further demonstrated the versatility of this method by applying it to the photogenerated carrier dynamics of intrinsic 6H-SiC grains. The transient broadband mid-IR spectra were measured by the pump-probe technique, and the transient absolute net carrier density was estimated. The carrier relaxation dynamics was fitted with three components with lifetime constants that agreed well with those obtained for SiC by transient broadband THz conductivity spectroscopy. We predict that this method could be extended to other photocatalytic materials with suitable probe frequencies.