The growth of gold nanoparticles by reduction by citrate and ascorbic acid has been examined in detail to explore the parameter space of reaction conditions. It is found that gold particles can be produced in a wide range of sizes, from 9 to 120 nm, with defined size distribution, following the earlier work of Turkevich and Frens. The reaction is initiated thermally or in comparison by UV irradiation, which results in similar final products. The kinetics of the extinction spectra show the multiple steps of primary and secondary clustering leading to polycrystallites.

Intense nonequilibrium femtosecond laser excitation of gold nanoparticles in water leads to a transient heating of the nanoparticles, which decays via heat transfer to the water phase. It is shown that the water temperature rises to near the critical temperature and the water undergoes an explosive evaporation in the subnanosecond range. The formation of vapor bubbles shows a threshold dependence on laser fluence. The nascent nanoscale vapor bubbles change the heat dissipation drastically. The nanoscale structure is resolved directly with a combination of x-ray scattering methods sensitive to the particle lattice expansion and the change in the water structure factor.

Pulsed x-ray scattering is used to examine the lattice dynamics in gold nanoparticles in water following excitation with intense femtosecond laser pulses. At lower excitation power the initial lattice heating is followed by cooling on the nanosecond time scale. The decay can be described by solving the heat transfer equations including both the bulk conductivity in water and a finite thermal boundary resistance at the particlewater interface. The lattice expansion rises linearly with excitation power, up to an excitation power corresponding to a lattice temperature increase of 529 K. At higher temperatures the lattice shows a loss of long-range order due to pre-melting of the particles. At the bulk melting temperature, complete melting occurs within the first 100 ps after laser excitation.

hort-pulse laser ablation is promising owing to the low threshold for material removal from surfaces. In the laser-ablation process, solid material transforms into a volatile phase initiated by a rapid deposition of energy. Explosive boiling can be one of the mechanisms in which matter is heated close to the critical point. Other pathways of non-thermal excitation will be present for very short laser pulses 1. Here we observe a different channel of ablation from gold nanoparticles, which takes place below the particle melting point. This process is induced by the optical near-field, a subwavelength field enhancement close to curved surfaces, in particular. Using picosecond X-ray scattering, we can track the temporal and energetic structural dynamics during material ejection from the nanoparticles. This effect will limit any high-power laser manipulation of nanostructured surfaces, such as surface-enhanced Raman measurements 2 or plasmonics with femtosecond pulses. Ablation can be characterized as a thermal process, where the material is allowed to thermalize to form a superheated liquid, before ejection sets in 3,4. The limit of superheating, which determines the ablation threshold, is related to the spinodal temperature, at which no barrier for vaporization persists. Thermalization can be reached within times exceeding the electron-phonon and phonon-phonon scattering times 5. For laser pulses shorter than this limit, the explosive boiling will be independent of the pulse length. In addition to the thermal regime, other channels of non-thermal structure modification exist. These will become important for femtosecond excitation. A well-studied example is plasma formation, an important mechanism for transparent media 1. The so-called dielectric breakdown is caused by multiphoton absorption over the bandgap of dielectrics. Other non-thermal processes are observed in highly excited semiconductors 6. Both pathways of material disintegration and ablation will be established at threshold laser fluences considerably above the limits of reversible interaction and material melting (0.4-1.4 J cm −2 for 100 fs to 1 ns pulses 1 , 0.25 J cm −2 from theoretical calculations 7 , both ablation thresholds on gold films; or 2 J cm −2 for dielectric breakdown in fused silica 1). The comparison to values of bulk gold is possible in the present case owing to the relative insensitivity

The ultrafast excitation of gold nanoparticle sols causes a strong nonequilibrium heating of the particle lattice and subsequently of the water shell close to the particle surface. Above a threshold in laser fluence, which is defined by the onset of homogeneous nucleation, nanoscale vapor bubbles develop around the particles, expand and collapse again within the first nanosecond after excitation. We show the existence of cavitation on the nanometer and subnanosecond time scale, described within the framework of continuum thermodynamics.