Infrared spectroscopy and modeling of co-crystalline CO2·C2H2 aerosol particles. I. The formation and decomposition of co-crystalline CO2·C2H2 aerosol particles

Simulation of Cocrystal Formation in Planetary Atmospheres: The C6H6:C2H2 Cocrystal Produced by Gas Deposition

The formation of molecular cocrystals in condensed aerosol particles has been recently proposed as an efficient pathway for generation of complex organics in Titan's atmosphere. It follows that cocrystal precipitation may facilitate the transport of biologically important precursors to the surface to be sequestered in an organic karstic and sand environment. Recent laboratory studies on these planetary minerals have predominantly synthesized cocrystals by the controlled freezing of binary mixtures from the liquid phase, allowing for their structural and spectroscopic characterization. However, these techniques are perhaps not best representative of aerosol nucleation and growth microphysics in planetary atmospheres. Herein, we report the first synthesis of the known 1:1 C₆H₆:C₂H₂ cocrystal using vapor deposition methods onto a cryogenically cooled substrate. Subsequent transmission FTIR spectroscopy has confirmed the formation of the empirical C₆H₆:C₂H₂ cocrystal structure via the observation of diagnostic infrared spectral features. Predicted by periodic-DFT calculations, altered vibrational profiles depict a changing site symmetry of the C₆H₆ and C₂H₂ components after transition to the cocrystal unit cell geometry. The 80 K temperature of the cocrystal phase transition overlaps with the condensation curves obtained for both species in Titan's lower stratosphere, revealing that the cocrystal may act as an important environment for photo- and radio-lytic processes leading to the formation of higher order organics in Titan's atmosphere. Such solid-state astrochemistry can now be pursued in oxygen-free laboratory settings under (ultra)high vacuum using standard surface science setups.

Infrared spectra reveal box-like structures for a pentamer and hexamer of mixed carbon dioxide-acetylene clusters

Except for a few cases like water and carbon dioxide, identification and structural characterization of clusters with more than four monomers is rare. Here, we provide experimental and theoretical evidence for existence of box-like structures for a pentamer and a hexamer of mixed carbon dioxide-acetylene clusters. Two mid-infrared cluster absorption bands are observed in the CO2ν3 band region using a tunable diode laser to probe a pulsed supersonic jet. Each requires the presence of both carbon dioxide and acetylene in the jet, and (from observed rotational spacings) involves clusters containing about 4 to 7 molecules. Structures are predicted for mixed CO2 + C2H2 clusters using a distributed multipole model, and the bands are assigned to a specific pentamer, (CO2)3-(C2H2)2, and hexamer, (CO2)4-(C2H2)2. The hexamer has a box-like structure whose D2d symmetry is supported by observed intensity alternation in the spectrum. The pentamer has a closely related structure which is obtained by removing one CO2 molecule from the hexamer. These are among the largest mixed molecular clusters to be assigned by high-resolution spectroscopy.

Atmospheric processes on ice nanoparticles in molecular beams

This review summarizes some recent experiments with ice nanoparticles (large water clusters) in molecular beams and outlines their atmospheric relevance: (1) Investigation of mixed water–nitric acid particles by means of the electron ionization and sodium doping combined with photoionization revealed the prominent role of HNO₃ molecule as the condensation nuclei. (2) The uptake of atmospheric molecules by water ice nanoparticles has been studied, and the pickup cross sections for some molecules exceed significantly the geometrical sizes of the ice nanoparticles. (3) Photodissociation of hydrogen halides on water ice particles has been shown to proceed via excitation of acidically dissociated ion pair and subsequent biradical generation and H₃O dissociation. The photodissociation of CF₂Cl₂ molecules in clusters is also mentioned. Possible atmospheric consequences of all these results are briefly discussed.

From plasmon spectra of metallic to vibron spectra of dielectric nanoparticles

Light interacts surprisingly differently with small particles than with bulk or gas phase materials. This can cause rare phenomena such as the occurence of a "blue moon". Spectroscopic particle phenomena of similar physical origin have also spawned countless applications ranging from remote sensing to medicine. Despite the broad interest in particle spectra, their interpretation still poses many challenges. In this Account, we discuss the challenges associated with the analysis of infrared, or vibron, extinction spectra of small dielectric particles. The comparison with the more widely studied plasmon spectra of metallic nano-particles reveals many common features. The shape, size, and architecture of particles influence the band profiles in vibron and plasmon spectra in similar ways. However, the molecular structure of dielectric particles produces infrared spectral features that are more diverse and detailed or even unique to vibron spectra. More complexity means higher information content, but that also makes the spectra more difficult to interpret. Conventional models such as classical electromagnetic theory with a continuum description of the wavelength-dependent optical constants are often no longer applicable to these spectra. In cases where accurate optical constants are not available and for ultrafine particles, where the molecular structure and quantum effects become essential, researchers must resort to molecular models for light-particle interaction that do not require the prior knowledge of optical constants. In this Account, we illustrate how vibrational exciton approaches combined with molecular dynamics simulations and solid-state density functional calculations provide a viable solution to these challenges. Molecular models reveal two important characteristics of vibron spectra of small molecularly structured particles. The band profiles in vibron spectra are largely determined by transition dipole coupling between the molecules in a particle. Below a specific particle size limit, conventional models fail. Molecular models explain many other phenomena in particle spectra, such as size, shape, and mixing effects, providing the foundation for a better understanding of the interaction of solar radiation with aerosols and clouds and for the design of dielectric nanomaterials.