Articles | Volume 16, issue 22
Research article
23 Nov 2016
Research article |  | 23 Nov 2016

A Monte Carlo approach for determining cluster evaporation rates from concentration measurements

Oona Kupiainen-Määttä

Abstract. Evaporation rates of small negatively charged sulfuric acid–ammonia clusters are determined by combining detailed cluster formation simulations with cluster distributions measured in the CLOUD experiment at CERN. The analysis is performed by varying the evaporation rates with Markov chain Monte Carlo (MCMC), running cluster formation simulations with each new set of evaporation rates and comparing the obtained cluster distributions to the measurements. In a second set of simulations, the fragmentation of clusters in the mass spectrometer due to energetic collisions is studied by treating also the fragmentation probabilities as unknown parameters and varying them with MCMC. This second set of simulations results in a better fit to the experimental data, suggesting that a large fraction of the observed HSO4 and HSO4 ⋅ H2SO4 signals may result from fragmentation of larger clusters, most importantly the HSO4 ⋅ (H2SO4)2 trimer.

Short summary
The article introduces a new method for determining the evaporation rates of small molecular clusters based on cluster concentration measurements. Evaporation rates cannot be measured directly or computed very accurately from any classical or quantum theory, so the presented method will give new understanding about molecular clusters. In addition, it can give some information about what happens to clusters during the measurement process.
Final-revised paper