Spectral and temporal characteristics of Ba I emission in femtosecond laser-induced breakdown on the surfaces of aqueous solutions

Temporal behavior of the intensity of Ba I emission lines (413.24 and 553.55 nm) in plasma of optical breakdown induced by femtosecond laser pulses (800 nm, 45 fs, 0.82 mJ) on the surface of BaCl$_2$ aqueous solution was experimentally studied as dependent on delay time $t_{d}$ relative to the breakdown onset. The maximum intensity of the Ba I (413.24 nm) line was observed at $t_{d}$ = 20 ns, while the intensity of the Ba I (553.55 nm) line reached a maximum at $t_{d}$ = 40 ns. At $t_{d}$ = 10 ns, the intensity of the Ba I (413.24 nm) line was almost three times as large as that of Ba I (553.55 nm) line (despite the about two orders of magnitude lower probability of spontaneous transition for the former line), which is explained by the recombination cascade. It is established that, in the subsystem of 5$d$6$p^{3}D^\circ$ and 6s6$p^{1}P^\circ$ levels, the Boltzmann distribution is not valid and the local thermodynamic equilibrium is absent.