Pulse heating of water at the surface of gold nanoparticles: Femtosecond laser spectroscopy of energy relaxation of aqueous colloid of plasmonic nanoparticles under strong excitation conditions

Abstract

The plasmon peak of gold nanoparticles (AuNP) is due to resonance of plasma oscillations of conduction electrons of the metal. The frequency of the AuNP plasmon band peak is determined by the parameters at which Frohlich's resonance occurs [1]. For spherical Au nanoparticles, the Frohlich condition is achieved when + where = + is the dielectric permittivity of AuNP and ε d is the dielectric permittivity of the medium surrounding the AuNP. The AuNP plasmon resonance peak can be used as an optical sensor, since its position depends on the dielectric constant of the metal and the medium, which in turn depend on the temperature/pressure. Extinction, absorption, and scattering cross sections σ ext, abs, scat of the plasmon peak are usually expressed through nanoparticle geometric section πr 2 and factor Q ext, abs, scat : σ ext, abs, scat = Q ext, abs, scat πr 2 . Near the plas mon resonance peak, Q abs > 1, which means strong absorption of light by AuNP. At a high energy density of a femtosecond laser excitation pulse, temperature and pressure jumps in the AuNP and the adjacent nanolayer of surrounding water can be expected because of the strong absorption by AuNP [2]. There are several AuNP heating modes: (1) the AuNP lattice temperature Т does not exceed 100°C, the boiling(2) Т does not exceed the critical temperature of water of 373.95°C (pressure of 22.06 MPa), T vap < T < T cr ; (3) Т does not exceed the melting point of the metal 1063°C, T cr < T < T AuM ; and (4) Т does not exceed the boiling point of the metal T AuM < T < T AuB . The relaxation dynamics of the energy absorbed in the AuNP plasmon reso nance substantially affects the dynamics of heating and pressure rise in the surrounding layer of water. Under femtosecond pulse excitation, the light energy absorption/relaxation processes are considered in terms of the following model: absorption of a quantum of light and plasmon excitation → plasmon decay yielding hot (nonthermalized) electron → thermaliza tion of the electron and electron gas heating in the metal conduction band to a temperature T e → metal lattice heating to a temperature T L via electron-lattice relaxation → energy transfer to the environment [3]. Mathematically, the dynamics of these processes are modeled using the Anisimov two temperature model and numerical solution of the Navier-Stokes equa tions [4]. The problem is that the numerical solutions of these equations in the case of strong temperature and pressure gradients, first, are hampered by techni cal difficulties of computation and, second, ignore sig NANOSTRUCTURED SYSTEMS AND MATERIALSAbstract-The dynamics of differential absorption spectra of gold nanoparticles has been studied by femto second laser spectroscopy over a wide range of excitation energy density. The position of the plasmon reso nance peak strongly depends on the dielectric properties of the medium, thereby allowing the use of plas monic nanoparticles as an optical probe. It has been shown that femtosecond laser spectroscopy techniques, involving analysis of different...