Photoacoustic spectroscopy is a standout technique widely used for absorption measurements of atmospheric aerosols. Here we investigate the relative humidity dependence of photoacoustics and its implication for evaporation kinetics.

The mass accommodation coefficient, α M, describes evaporation and condensation kinetics at the liquid–vapor interface. In spite of numerous experimental efforts, reliable values of α M are still not available for many substances. Here, we present a novel experimental technique, photothermal single-particle spectroscopy (PSPS), that allows for a robust retrieval of mass accommodation coefficients from three simultaneous independent measurements. PSPS combines resonant photoacoustic absorption spectroscopy with modulated Mie scattering measurements on single particles. We study the mass transport of water on organic aerosol droplets that are optically trapped using counter-propagating tweezers. We find the mass accommodation coefficient of water on a pure model organic that is fully miscible with water to be 0.021 at 296 K and to decrease by more than an order of magnitude when the temperature increases to 309 K. The experimentally observed temperature dependence of α M shows an Arrhenius behavior. Furthermore, the water content of the droplets is found to have a profound effect on α M. No concentration dependence of α M is observed at low water concentrations, while at elevated water concentrations, we observe a 5-fold increase in α M. The technique presented in this work has the potential to become a reliable method for the retrieval of α M values at liquid–vapor interfaces, which are essential for accurate global climate and pharmaceutical aerosol inhalation modeling, to mention but a few.

The photocycle of photoactive yellow protein (PYP) begins with small-scale torsional motions of the chromophore leading to large-scale movements of the protein scaffold triggering a biological response. The role of single-bond torsional molecular motions of the chromophore in the initial steps of the PYP photocycle are not fully understood. Here, we employ anion photoelectron spectroscopy measurements and quantum chemistry calculations to investigate the electronic relaxation dynamics following photoexcitation of four model chromophores, para-coumaric acid, its methyl ester, and two analogues with aliphatic bridges hindering torsional motions around the single bonds adjacent to the alkene group. Following direct photoexcitation of S at 400 nm, we find that both single bond rotations play a role in steering the PYP chromophore through the S/S conical intersection but that rotation around the single bond between the alkene moiety and the phenoxide group is particularly important. Following photoexcitation of higher lying electronic states in the range 346-310 nm, we find that rotation around the single bond between the alkene and phenoxide groups also plays a key role in the electronic relaxation from higher lying states to the S state. These results have potential applications in tuning the photoresponse of photoactive proteins and materials with chromophores based on PYP.