Surface Composition of Free Mixed NaCl/Na2SO4 Nanoscale Aerosols Probed by X-ray Photoelectron Spectroscopy.

The local chemical surface composition of unsupported mixed solid NaCl/Na2SO4 aerosols ( d ∼ 70 nm) is studied by X-ray photoelectron spectroscopy. The solid aerosols are generated by drying aqueous droplets containing mixtures of the two salts in different mole fractions. The mole fraction of these salts is found to deviate at the solid aerosol surface significantly from the initial droplet composition. The minority species in the droplets are found to be enhanced at the surface of the solid mixed aerosols. This surface enhancement is rationalized in terms of the nucleation/crystallization process, where the salts evidently do not cocrystallize, rather than each salt forms pure crystal moieties. Characteristic variations of the surface ion concentration as a function of the mole fraction of the salts in the initial droplet are observed in the nanometer size regime. This is unlike core-shell architectures previously found in mixed micron salt aerosols, indicating that aerosol models derived from micron-sized aerosols are evidently not fully reliable to describe the surface composition of nanosized aerosols. Furthermore, surface enhancement of the minority component in mixed NaCl/Na2SO4 aerosols is also different from previous results on surface segregation of mixed NaCl/NaBr aerosols, where one of the anionic species is surface segregated for all mole fractions, which was explained in terms of the ability of the involved salts to cocrystallize and forming solid solutions. The present results rather indicate that mixed NaCl/Na2SO4 aerosols do not cocrystallize. Electron microscopy of deposited mixed salt aerosols reveals mostly a cubic structure of pure NaCl aerosols, whereas mixed salt aerosols are found to show a grainy structure composed of multiple small crystals which supports the present findings obtained from photoelectron spectroscopy.

Chirped pulse multiphoton ionization of nitrogen: control of selective rotational excitation in N(2)(+)(B 2Sigma(u)(+)).

We report on fluorescence spectra of N(2)(+)(B (2)Sigma(u)(+)) --> N(2)(+)(X (2)Sigma(g)(+)) obtained from multiphoton ionization of molecular nitrogen by 804 nm femtosecond laser pulses. The analysis of the fluorescence spectra reveals that the vibrational levels v = 0 and v = 4 in the B (2)Sigma(u)(+)-state of N(2)(+) are primarily populated. The rotational state distribution of N(2)(+)(B (2)Sigma(u)(+), v = 0) is determined from the rotationally resolved fluorescence spectra. It is demonstrated that the linear chirp of the 804 nm femtosecond laser pulse has a strong influence on the rotational state distribution of the vibrational ground state of the molecular cation N(2)(+)(B (2)Sigma(u)(+), v = 0). Possible mechanisms leading to the experimental results are discussed. The particular population of the vibrational levels as well as the linear chirp dependence of the fluorescence signal gives evidence for the importance of a resonant intermediate state. The N(2) a (1)Pi-state is likely involved in a resonant multiphoton excitation process. This permits to selectively control the rotational population of the cation that is formed via chirped pulse multiphoton ionization.