Can we Observe Gas Phase Nucleation at the Molecular Level?

Abstract

We propose and discuss an experiment for the study of neutral gas phase nucleation on a molecular level using propane as the condensable gas. The experiment combines a uniform Laval expansion with soft mass spectrometric detection. The uniform Laval expansion allows nucleation experiments under welldefined conditions while the mass spectrometric detection provides molecularlevel information on the molecular aggregates formed. It is discussed how one could observe the onset of nucleation and retrieve the size of the critical nucleus from the mass spectra.

Water nucleation at extreme supersaturation

“…12 Furthermore, to avoid saturation effects caused by the monomer in the cluster measurements, the monomer ion was selectively deflected by a pulsed plate located in front of the MCP detector for the cluster measurements. 10,11,13…”

“…The present work reports the first laboratory measurements of water nucleation to provide direct molecular level information such as critical cluster sizes at temperatures below 87 K. This is achieved by monitoring cluster size distributions during nucleation in the uniform postnozzle flow of a Laval expansion [10][11][12][13] with mass spectrometry after single-photon vacuum ultraviolet (VUV) ionization. [14][15][16][17] It has recently been shown that in contrast to electron ionization, singlephoton vacuum ultraviolet ionization close to the threshold is a soft ionization method even for weakly bound molecular clusters.…”

“…To account for this, we choose to refer to experimental conditions where S > 1 but nucleation is not yet observable as subcritical conditions, while supercritical conditions refer to conditions for which nucleation has already taken place in the experiment. [11][12][13] Nucleation is very sensitive to changes in the actual conditions (p, T, and S). Our nucleation studies are performed in the uniform postnozzle flow of a Laval expansion which ensures working under constant and well-known experimental conditions.…”

“…Figure 1 shows a simplified scheme of the experimental setup, which has been described in detail in previous publications. [10][11][12][13] Two feeding valves (not shown) with opening times of 6 ms supplied the gas mixture (water vapor, carrier gas, and internal standard) to the stagnation volume of the Laval nozzle at a stagnation pressure and temperature of p 0 and T 0 , respectively. Mass flow controllers were used to regulate the flows of the carrier gases (argon, Pan-Gas 5.0 or nitrogen, PanGas 5.0) and the internal standard FIG.…”

Overview: Homogeneous nucleation from the vapor phase—The experimental science

“…By applying high ion extraction voltages, they are able to observe cluster distributions [253][254][255] that include the monomer, dimer, trimer, etc., through to particles with diameters between 10 and 20 nm. As illustrated in Fig.…”