Ionization of gold nanoparticles in solution by pulse laser excitation as studied by mass spectrometric detection of gold cluster ions

We prepared gold nanoparticles in an aqueous solution of sodium dodecyl sulfate (SDS), which were introduced inside a vacuum as a continuous liquid flow (liquid beam). When the particles in the liquid beam were irradiated with a nanosecond laser pulse, ions were ejected from the liquid beam into the vacuum. We analyzed massto-charge ratios of the ions by time-of-flight mass spectrometry. Gold cluster ions, Au-n(+), and their hydrated ions, Au-n(+)(H2O)(m) (n, m = 1, 2, 3,...), were observed. The action spectrum of the cluster ions, showing the abundance of the ions against wavelength of the excitation laser pulse, does not accord with the optical absorption spectrum of the gold nanoparticles but agrees well with the component of the interband transition. This finding indicates that gold nanoparticles were ionized as a result of repetitive photoexcitation-relaxation cycles.