Light-Induced Frequency Shift in Chemical Spirals

The light-sensitive Belousov-Zhabotinsky (BZ) reaction with a ruthenium-based catalyst is a convenient system to study the effect of external perturbations on dynamical behavior of chemical reactions. The Ru(bpy)3Cl2 complex was first suggested as a luminescent indicator for the BZ reaction by Demas and Diemente.1 Later the reaction was found to be sensitive to visible light by Gaspar et al.,2 and several studies have elucidated the mechanism of the photosensitivity.3,4 The light-sensitive BZ reaction can also be used in spatially extended reactors where light can alter the spatiotemporal behavior of the system. Kuhnert et al. suggested that the light-sensitive BZ reaction could be used for image processing.5 Steinbock et al. used periodic light perturbations to stimulate meandering motion of the spiral core.6 These studies were, however, conducted in a limited range of concentrations where the reaction is inhibited by the light. We have examined the behavior of spirals in an open reactor for a wide range of bromate concentrations. Examples of system behavior for different bromate concentrations are shown in Figure 1. Projection of blue light with intensity 20 mW/cm2 on the region inside the dashed circle results in the suppression of the wave propagation (Figure 1a), the decrease of the wave speed (Figure 1b), the increase of the wave speed (Figure 1c), and the formation of a target pattern (Figure 1d). The BZ reaction is famous for its ability to sustain spiral waves.7 We present here a study of the primary bifurcations of BZ spirals as a function of malonic acid and bromate concentrations and the illumination intensity. The important characteristics of the spirals are their frequency and wavelength. These properties are governed by the spiral core, which is a self-sustained wave generator. In the ruthenium-catalyzed BZ reaction, light with the wavelength around 450 nm changes the underlying chemical reaction, resulting in a change in the frequency of the oscillations. We measure the spiral frequencies and wavelengths to obtain the dispersion relation as a function of bromate concentration.