2D Study of Nitrogen Cluster Growth by Rayleigh Scattering

CHRISTEN W. ; M. CHUNG; N. NIKMARAM; VALLE SEIJO, M F.

Berlin

Conferencia; Bunsen-Tagung 2023 - Physical Chemistry of the Energy Transition; 2023

German Bunsen Society for Physical Chemistry

Our experimental capabilities allow the routine generation of many atmospherically relevant particles such as (CH4)n, (C3H8)n,(CO)n, (CO2)n, (H2O)n, (N2)n, (O2)n, etc. An important parameter here is particle size, which we can tune continuously from the individual molecule up to nanoparticles with a diameter of about 20 nm, using a pulsed supersonic jet obtained by the adiabatic expansion of a gas under very high pressure into vacuum. Until recently, we had to estimate the mean size of the generated particles using empirical scaling laws. These may not be accurate, valid at high source densities, or available at all for the system of interest. Hence, we have extended the experiment by an optical setup for laser-induced Rayleigh scattering.In this contribution we present a comprehensive experimental investigation of the generation of nitrogen particles by a pulsed high-pressure supersonic jet expansion of the pure gas through a parabolic nozzle into vacuum. In this study source pressure is varied between 2 MPa and 11 MPa and source temperature between 230 K and 310 K. The mean particle size is characterized by time and space resolved Rayleigh scattering measurements, in combination with time resolved residual gas pressure readings. One of the goals of this investigation is to estimate the distance where particle growth ceases. Because of the small scattering cross section Rayleigh signals typically are weak and thus in almost all previous studies the supersonic jet is probed at very shortdistances where particles still might grow. Hence, in this study Rayleigh data are obtained for different nozzle-laser distances. The dependence of the mean particle size on stagnation pressure is discussed with respect to the prediction by Hagena (Rev. Sci. Instrum. 1992, 63, 2374).Because the experimentally observed Rayleigh scattering signal is proportional to both the number of monomers and the mean particle size, the background corrected Rayleigh signal is normalized by the number of particles ejected in a jet pulse. This is accomplished by a simultaneous time-resolved reading of the pressure gauge output.The result of this study, beyond the Hagena parameters, is that particles still seem to grow in a distance of 3 mm from the nozzle. Additional experiments are in progress to confirm this conclusion.