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α-pinene is the most abundant monoterpene that represents an important family of volatile organic compounds. Molecular identification of key transient compounds during the α-pinene ozonolysis has been proven to be a challenging experimental target because of a large number of intermediates and products involved. Here we exploit the recently developed hybrid instruments that integrate aerosol mass spectrometry with a vacuum ultraviolet free-electron laser to study the α-pinene ozonolysis. The experiments of α-pinene ozonolysis are performed in an indoor smog chamber, with reactor having a volume of 2 m3 which is made of fluorinated ethylene propylene film. Distinct mass spectral peaks provide direct experimental signatures of previously unseen compounds produced from the reaction of α-pinene with O3. With the aid of quantum chemical calculations, plausible mechanisms for the formation of these new compounds are proposed. These findings provide crucial information on fundamental understanding of the initial steps of α-pinene oxidation and the subsequent processes of new particle formation.

Spectroscopic characterization of clusters is crucial to understanding the structures and reaction mechanisms at the microscopic level, but it has been proven to be a grand challenge for neutral clusters because the absence of a charge makes it difficult for the size selection and detection. Infrared (IR) spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser (VUV-FEL) has recently been developed in the lab. The IR-VUV depletion and IR+VUV enhancement spectroscopic techniques open new avenues for size-selected IR spectroscopies of a large variety of neutral clusters without confinement (i.e., an ultraviolet chromophore, a messenger tag, or a host matrix). The spectroscopic principles have been demonstrated by investigations of some neutral water clusters and some metal carbonyls. Here, the spectroscopic principles and their applications for neutral clusters are reviewed.

Infrared photodissociation spectroscopy of mass-selected [MO(CO2)n]+ (M=Sc, Y, La) complexes indicates that the conversion from the solvated structure into carbonate one can be achieved by the ScO+ cation at n=5 and by the YO+ cation at n=4, while only the solvated structures are observed for the LaO+ cation. These findings suggest that both the ScO+ and YO+ cations are able to fix CO2 into carbonate. Quantum chemical calculations are performed on [MO(CO2)n]+ to identify the structures of the low-lying isomers and to assign the observed spectral features. Theoretical analyses show that the [YO(CO2)n]+ complex has the smallest barrier for the conversion from the solvated structure into carbonate one, while [LaO(CO2)n]+ exhibits the largest conversion barrier among the three metal oxide cations. The present system affords a model in clarifying the effect of different metals in catalytic CO2 transformation at the molecular level.

The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, [(CH $_3$ NH $_2$ )(H $_2$ O) $_n$ ] $^+$ ( $n$ =1 $-$ 5), have been studied using quantum chemical calculations at the MP2/6-311+G(2d,p) level. The results reveal that the formation of proton-transferred CH $_2$ NH $_3$ $^+$ ion core structure is preferred via the intramolecular proton transfer from the methyl group to the nitrogen atom and the water molecules act as the acceptor for the O $\cdots$ HN hydrogen bonds with the positively charged NH $_3$ $^+$ moiety of CH $_2$ NH $_3$ $^+$ , whose motif is retained in the larger clusters. The CH $_3$ NH $_2$ $^+$ ion core structure is predicted to be less energetically favorable. Vibrational frequencies of CH stretches, hydrogen-bonded and free NH stretches, and hydrogen-bonded OH stretches in the calculated IR spectra of the CH $_2$ NH $_3$ $^+$ and CH $_3$ NH $_2$ $^+$ type structures are different from each other, which would afford the sensitive probes for fundamental understanding of hydrogen bonding networks generated from the radiation-induced chemical processes in the [(CH $_3$ NH $_2$ )(H $_2$ O) $_n$ ] $^+$ complexes.

Structural characterization of neutral water clusters is crucial to understanding the structures and properties of water, but it has been proven to be a challenging experimental target due to the difficulty in size selection. Here, we report the size-specific infrared spectra of confinement-free neutral water nonamer (H2O)9 based on threshold photoionization, using a tunable vacuum ultraviolet free-electron laser. Distinct OH stretch vibrational fundamentals in the 3200–3350 cm−1 region are observed, providing unique spectral signatures for the formation of an unprecedented (H2O)9 structure evolved by adding a ninth water molecule onto a hydrogen bond-unbroken edge of the (H2O)8 octamer with D2 d symmetry. This nonamer structure coexists with the five previously identified structures that can be viewed as derived by inserting a ninth water molecule into a hydrogen bond-broken edge of the D2 d/ S4 octamer. These findings provide key microscopic information for systematic understanding of the formation and growth mechanism of dynamical hydrogen-bonding networks that are responsible for the structure and properties of condensed-phase water.

The mass-selected infrared photodissociation (IRPD) spectroscopy was utilized to investigate the interactions of cationic cobalt with carbon dioxide molecules. Quantum chemical calculations were performed on the [Co(CO $_2$ ) $_n$ ]+clusters to identify the structures of the low-lying isomers and to assign the observed spectral features. All the [Co(CO $_2$ ) $_n$ ]+ ( $n$ =2 $-$ 6) clusters studied here show resonances near the CO $_2$ asymmetric stretch of free CO $_2$ molecule. Experimental and calculated results indicate that the CO $_2$ molecules are weakly bound to the Co+ cations in an end-on configuration via a charge-quadrupole electrostatic interaction. The present IRPD spectra of [Co(CO $_2$ ) $_n$ ]+ clusters have been compared to those of Ar-tagged species ([Co(CO $_2$ ) $_n$ ]+-Ar), which would provide insights into the tagging effect of rare gas on the weakly-bounded clusters.

This paper describes an experimental technique for studying neutral metal complexes using infrared + vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free-electron laser (VUV-FEL). The preliminary IR spectroscopy results of mass-selected nickel tetracarbonyl are reported in this work. The results demonstrate that the tunable VUV-FEL light allows the selective ionization of a given neutral cluster free of confinement along with the recording of well-resolved IR spectra. As the ionization energies of many neutral clusters are accessible by a broadly tunable VUV-FEL (50-150 nm) and near-threshold ionization can be readily achieved, the proposed experimental method offers unique possibilities for the size-specific study of a wide variety of confinement-free neutral clusters.