Unraveling the Ultrafast Photochemical Dynamics of Nitrobenzene in Aqueous Solution.

Nitroaromatic compounds are major constituents of the brown carbon aerosol particles in the troposphere that absorb near-ultraviolet (UV) and visible solar radiation and have a profound effect on the Earth's climate. The primary sources of brown carbon include biomass burning, forest fires, and residential burning of biofuels, and an important secondary source is photochemistry in aqueous cloud and fog droplets. Nitrobenzene is the smallest nitroaromatic molecule and a model for the photochemical behavior of larger nitroaromatic compounds. Despite the obvious importance of its droplet photochemistry to the atmospheric environment, there have not been any detailed studies of the ultrafast photochemical dynamics of nitrobenzene in aqueous solution. Here, we combine femtosecond transient absorption spectroscopy, time-resolved infrared spectroscopy, and quantum chemistry calculations to investigate the primary steps following the near-UV (λ ≥ 340 nm) photoexcitation of aqueous nitrobenzene. To understand the role of the surrounding water molecules in the photochemical dynamics of nitrobenzene, we compare the results of these investigations with analogous measurements in solutions of methanol, acetonitrile, and cyclohexane. We find that vibrational energy transfer to the aqueous environment quenches internal excitation, and therefore, unlike the gas phase, we do not observe any evidence for formation of photoproducts on timescales up to 500 ns. We also find that hydrogen bonding between nitrobenzene and surrounding water molecules slows the S1/S0 internal conversion process.

Real-time monitoring of aerosol particle formation from sulfuric acid vapor at elevated concentrations and temperatures.

In the present study, time-resolved aerosol particle formation from sulfuric acid vapor is examined with special attention to the stabilization of molecular clusters in the early phase of unary nucleation. An important factor governing this process is the amount of condensable acid vapor. Here it is produced from fast gas-phase reactions in a batch-type reaction cell for which we introduce modifications enabling real-time monitoring. The key component for size- and time-resolved detection of ultrafine particles is a new 1 nm-SMPS. With this new tool at hand, the effect of varying the precursor concentration over two orders of magnitude is investigated. We demonstrate the ability to tune between different growth scenarios as indicated by the size-resolved particle traces which exhibit a transition from sigmoidal over quasi-stationary to peak-like shape. The second key parameter relevant for nucleation studies is the temperature-dependent cluster evaporation. Due to a temperature rise during the mixing stage of the experiment, evaporation is strongly promoted in the early phase. Therefore, the present study extends the T-range used in, e.g., smog chambers. We investigate this temperature effect in a kinetic simulation and can successfully combine simulated and measured data for validating theoretical evaporation rates obtained from DLPNO-CCSD(T0)-calculations.

Ion imaging of spatially inhomogeneous nanoplasmas in NaCl particles.

Studying photoemission from free, unsupported aerosol particles is a powerful method for gaining insight into light-matter interactions at the nanoscale. We used single-shot velocity map imaging to experimentally measure kinetic energy and angular distributions of ions emitted following interaction of sub-micrometer NaCl particles with femtosecond pulses of near infrared (NIR, 800 nm) and ultraviolet (UV, 266 nm) light. We combined this with time-dependent simulations of light propagation through the particles and a rate equation approach to computationally address the origin of the observed ion emission. For both NIR and UV pulses, ion emission is caused by the formation of an under-dense nanoplasma with similar densities, although using an order of magnitude weaker UV intensities. Such conditions result in remarkably similar ion fragments with similar kinetic energies, and no obvious influence of the plasma formation mechanism (photoionization or collisional ionization). Our data suggests that Coulomb explosion does not play a significant role for ion emission, and we discuss alternative mechanisms that can lead to material ablation from under-dense nanoplasma. Finally, we show how finite size effects play an important role in photoemission through generation of spatially inhomogeneous nanoplasmas, which result in asymmetric ion emission that depends on particle size and laser wavelength. By utilizing the single-particle information available from our experiments, we show how finite size effects and inhomogeneous nanoplasma formation can be exploited to retrieve the size and orientation of individual submicrometer aerosol particles.