Rise profile of the thermal lens signal: Contribution of the temperature lens and the population lens

Time profiles of the rise parts of variations of a probe beam light densities under the thermal lens (TL) experimental condition are measured with nanosecond pulsed laser excitation. The rise profile in benzene can be explained well by the probe beam expansion only by a refractive index lens due to the density fluctuation, which is induced by heat from the radiationless transition. In water, the signal is fitted well by the sum of two contributions of the refractive index change; i.e., due to the density (the thermal lens) and temperature (the temperature lens) variations. The coefficient of the temperature variation (∂n/∂T)ρ is evaluated from the relative signal intensity. The contribution of the population lens is clearly observed in the rise curve of the ‘‘TL signal’’ after the photoexcitation of C60 in benzene.