Ultra-sensitive thermal lens spectroscopy of water

The accurate knowledge of the water absorption spectrum is of vital importance for many branches of science and technology. Although it was investigated by different techniques, particularly between 300 - 450 nm, there is still significant disagreement between various studies. From 3 to 0.3 mm, the water absorption coefficient drops 8 orders of magnitude, reaching a minimum at ~ 0.4. However, the wavelength and value of the minimum absorption is still unclear.This study presents the use of a dual-beam thermal lens configuration in measuring the absorption spectrum for pure water from 350 to 528 nm. The minimum linear absorption coefficient of- 1.5timesl0^-5 cm^-1 was found around 380 nm. This value is the lowest ever obtained for water and a good agreement was obtained with the literature in the blue-green range. At 380 nm, the absorption is about one order of magnitude smaller than the loss due to Rayleigh scattering and comparable to the Raman scattering.