Krohn J Lippe M Li C Signorell R 2020 We investigate homogeneous gas-phase nucleation of CO2 and C3H8 in the uniform postnozzle flow of Laval expansions in the temperature range of 31.2 K to 62.9 K and 32.0 K to 42.1 K, respectively. Time-dependent cluster size distributions are recorded with mass spectrometry after single-photon ionization with vacuum ultraviolet light. Net monomer-cluster forward rate constants and experimental nucleation rates J are retrieved from the time-dependent cluster size distributions. The comparison of experimental enhancement factors derived from these net forward rates with calculated enhancement factors provides an indication for the transition from barrier-limited to barrierless nucleation. Our data suggest such a transition for CO2, but not for C3H8. The values of J lie in the range from 9 × 1014 cm-3 s-1 to 6 × 1015 cm-3 s-1. For CO2, the comparison of J with a modeled nucleation rate JQM based on quantum chemical calculations of the free energy barrier also hints at a transition from barrierless condensation to barrier-limited nucleation. Furthermore, we address the influence of the carrier gas pressure on the nucleation rate. https://sonar.ch/global/documents/3575 eng info:eu-repo/semantics/altIdentifier/doi/10.1039/d0cp01771j info:eu-repo/semantics/altIdentifier/pmid/32632423 Physical chemistry chemical physics : PCCP. - 2020 Physical and Theoretical Chemistry General Physics and Astronomy Carbon dioxide and propane nucleation: the emergence of a nucleation barrier. http://purl.org/coar/resource_type/c_6501