Size dependence of dipole and quadrupole plasmon resonances of light - induced sodium clusters

1. Size dependence of dipole and quadrupole plasmon resonances • of light-induced sodium clusters • AnastasiyaDerkachova, Krystyna Kolwas • Institute of Physics, Polish Academy of Sciences, • Al.Lotników 32/46, 02-668 Warsaw, Poland Abstract Experimental setup Scattered intensities : a) experimental; b) Mie theory We study the dependence of the optical properties of large sodium clusters as a function of their size. Sodium clusters are light induced [1,2] giving the unique opportunity of observing the smooth change of their sizes as a result of the spontaneous nucleation process in supersaturated vapor. Supersaturation is due to dissociation of Na2 molecules initiated by switching-on of the laser light at frequency resonant with some transition frequencies in sodium Na2 molecules. We measure the scattered intensities of the probe laser beam with polarization parallel and perpendicular the observation plane. The measurement is performed for several wavelengths of the probe light scattered by clusters growing with time. The full Mie scattering theory is used to describe intensities of light scattered by growing clusters in directions orthogonal to the direction of the incident light beam. The Drude dielectric function of free-electron metal is used to describe the optical properties of studied clusters. The method of finding the dependence of cluster radius and relative concentration is presented. We combine the formalism of the classical Mie scattering theory for arbitrary large size and the concept of the collective electron oscillations (plasmons) [3,4]. The maxima in normalized intensities of light scattered by single cluster we attribute to resonant excitation of dipole and quadrupole plasmon oscillation respectively. We show that plasmon resonances take place for different cluster sizes when excited with different wavelength. We compare these results with expectations resulting from solving the problem of eigenfrequencies of free-electron sphere with the dielectric function described within Lorentz-Drude model. This work was partially supported by the Polish State Committee for Scientific Research (KBN), grant No. 2 PO3B 102 22. References: 1. K. Kolwas, M. Kolwas, D. Jakubczyk, Appl. Phys. B 60, 173 (1995), 2. Demianiuk S, Kolwas K. J. Phys. B, 34, 1651 (2001), 3. K. Kolwas, S. Demianiuk, M. Kolwas, J. Phys.B, 29, 4761 (1996), 4. K. Kolwas, S. Demianiuk, M. Kolwas, J. Chem. Phys., 106(20), 8436 (1997) Mirror Polarizer 1 Induced beam source a) 12 457.9 514.5 10 Probe Beam source 568.2 647.1 Cell 8 6 Intensities [a.u.] Polarizer 2 4 I V 2 Shatter I H 0 0 2 4 6 8 10 12 14 SSD spectrometer Half-transmitting mirror Computer Time [s] Polarization geometry Mechanism of sodium clusters condensation incident light IV (I) Resonance excitation of dimers 488 nm Na2 Na2* Polarization of detected light Polarization of Collision assisted dimers dissociation collisions Na2* Na+Na* IH (III) Sodium vapour supersaturation Clusters condensation b) Looking for the best radius Evolution of cluster radius and concentration Identification of plasmons in scattered light 160 120 R(t) [nm] 80 40 0 0 2 4 6 8 10 12 14 Time [s] Plasmon resonances: a) experimental; b) Mie theory Summury Plasmon resonance frequencies 7 10 a) • The spontaneous nucleation process of clusters induced by light [1,2] gives the unique opportunity of observing the smooth change of cluster size up to the macroscopic droplet of radius of about 150nm. That enables the study of optical properties of free-electron clusters as a function of sizes in wide range up to hundreds of nanometers. • The presented experimental results seams to be the first observation of manifestation of: • the dipole plasmon resonance in clusters of so large sizes as few tens of nanometers, • the quadrupole plasmon resonance • The presented experimental results are not contradictory with theoretical predictions of the problem of eigenfrequencies of free-electron sphere with the dielectric function described within Lorentz-Drude model 5 10 Concentration [a.u.] experimental 3 10 Mie theory with Drude dielectric function 1 10 0 2 4 6 8 10 12 14 Time [s] b)