Investigation of Ultramarine Pigment Excited State Dynamics by Pump-Probe Microscopy and Spectroscopy

Abstract

First mined in Afghanistan nearly 6,000 years ago, lapis lazuli is a blue pigment also known as ultramarine, a material that was highly prized (and corresponding highly priced) by Western painters during the Middle Ages and the Italian Renaissance. Both the mineral lapis lazuli and the synthetic ultramarine can undergo degradation in paintings and other works of cultural heritage, which presents challenges to the preservations of these works. Due to the limitations of many modern analytical techniques, art conservators and conservation scientists often still need to remove a sample of paint in order to understand the layering of pigments in a painting. Femtosecond transient absorption (also called pump-probe) spectroscopy and imaging are here used to explore the effects of depth, polarization, and power on the ultrafast excited state dynamics of ultramarines both natural and synthetic, in order to further understand the photophysics and potential photo-degradation pathways of ultramarine pigments in paintings. Both lapis lazuli and synthetic ultramarine undergo identical forms of photo-induced transformation in the context of these experiments, where it appears that either the lazurite chromophore or the sodalite cage structure of ultramarine is destroyed.