## 1 Introduction
The work [29] theoretically predicted that individual substons could coagulate with one another in water and aqueous salt solutions to form clusters of substons; the theoretical aspects of bubston coagulation with the formation of the cluster phase are presented in [30]. The existence of separate bubstons was proved in experimental works [31; 32], and the existence of the subston-cluster phase was confirmed in [33; 34; 35]. In these works, substons and bubston clusters were studied by using the technique of dynamic light scattering, as well as in experiments on the study of the angular dependences of the scattering matrix elements by using polarization scatterometry. In addition to the techniques based on light scattering, there also exists an alternative technique based on the excitation of optical breakdown. This technique is applicable because substons and bubston clusters are the centers of laser breakdown, which was recognized in a recent review on optical breakdown and plasma formation in water, see Ref. [36]. Recently, we have shown [37; 38; 39] that the technique of optical breakdown of liquids transparent to laser radiation can be used to study certain characteristics of individual substons. In particular, it turned out that the optical breakdown threshold in water containing the bubston phase is close by order of magnitude to the excitation threshold of various nonlinear optical processes, for example, stimulated Raman and hyper-Raman scattering. Note that in Refs. [37; 38; 39] the optical breakdown was studied under the assumption that the breakdown centers are separate substons. At the same time, no experiments have been carried out in which the specific features of optical breakdown on the bubston clusters as breakdown centers were examined. The aim of this work is to study the peculiarities of optical breakdown on bubston clusters, induced by nanosecond and picosecond laser pulses at wavelength \(\lambda\) = 1064 nm. As is known [40], the absorption coefficient in water at this wavelength at room temperature is 0.12-0.13 cm\({}^{-1}\); i.e., obviously, the heating, associated with the absorption, can be neglected. Various regimes of optical breakdown in water at \(\lambda\) = 1064 nm for pulses of different durations are described in detail in Ref. [13]. As noted in this work (see also numerous articles cited in this review), the breakdown mechanisms of water, free of solid impurities,are multiphoton absorption, as well as cascade ionization. It was shown in [13] that the breakdown due to cascade ionization occurs in two steps: (i) cascade initiation through the creation of seed electrons and (ii) cascade buildup to the substantially high density of free electrons. Cascade initiation requires at least one or more free electrons to be present in the focal volume at the beginning of the laser pulse. These'seed' electrons in an impure liquid are most likely to come from the ionized impurities in the focal area prior to the laser pulse. In a pure liquid (free of external solid or liquid particles), \"seed\" electrons can be produced only due to the ionization of liquid molecules. Since we consider water samples, which were preliminary purified from foreign particles, the initial density of \"impurity\" electrons in the bulk of water must be equal to zero; that is, seed electrons in our case can appear only as a result of the multiphoton ionization of molecules in the liquid bulk, which requires the very high energy of a laser pulse. Indeed, as shown in review [13], the optical breakdown threshold due to the multiphoton ionization of water molecules at a wavelength of 1064 nm for the pulses with a duration of 10 ns (this wavelength and pulse-width were used in our experiments, see below) is about 10\({}^{11}\) W/cm\({}^{2}\). As will be shown below, cluster coalescence is induced at the intensities of about 10\({}^{6}\)-10\({}^{7}\) W/cm\({}^{2}\), i.e., at much lower intensities.