Nanosecond photo-fusion of microcrystals on a polymer film observed with time-resolved ultramicroscopy

A time-resolved ultramicroscope was developed to monitor the process of pul se-laser-induced melting of 9,10-dicyanoanthracene microcrystals on poly(et hyl methacrylate) films. The microscope can detect sub-micrometer particles , but with an image (spacial) resolution of 2 mum and a time resolution of 400 ps. Quantitative comparison between images obtained at a certain time d elay after an excitation laser pulse and a pre-irradiated image is achieved using a correlation function. It was found that crystals having a size of about 10 mum turn in hot liquid droplets within the nanosecond excitation p ulse after which 80% of the droplets solidify with a time constant of 9 ns. The residual 20% of hot liquid droplets appear to have either evaporated o r fused and dispersed into the polymer with a time constant of 15 ns. (C) 2 001 Elsevier Science B.V. All rights reserved.