Reassessing the atmospheric oxidation mechanism of toluene.

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O2 to form the cresol proceeds with a smaller barrier than O2 addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol-Ar cluster near the ionization threshold.

The structure of the phenol-argon cluster (PhOH-Ar) in high-n Rydberg states is investigated by the newly developed technique of mass analyzed threshold ionization detected infrared (MATI-IR) spectroscopy. This method selectively measures IR spectra of molecular clusters in very high-n Rydberg states (n > 100) utilized in zero kinetic energy (ZEKE) photoelectron and MATI spectroscopy, whose ionic cores are essentially the same as the corresponding bare cation. The MATI-IR spectrum exhibits only the free OH stretching vibration (ν) when the π-bound cluster of the neutral ground electronic state (S0) is resonantly excited via the S1 origin to Rydberg states converging to its adiabatic ionization energy level, IE0(π). When Rydberg states converging to vibrationally excited levels of the local π-bound minimum are prepared, in addition to ν also the hydrogen-bonded OH stretching vibration (ν) of the H-bonded global minimum is observed in the MATI-IR spectra, even for vibrational excitation of only 14 cm(-1) above IE0(π). These results show that the π→ H site switching reaction of the Ar ligand from the aromatic ring to the OH group proceeds only from vibrationally excited states in the π-bound cation core with a small barrier of less than 14 cm(-1) from IE0(π). On the other hand, directly photoionized PhOH(+)-Ar shows both ν and ν in the IR spectra, even when it is just ionized to IE0(π). This result implies that the ionization-induced π→ H site switching occurs without excess energy in the H-bound or π-bound cations, in contrast to very high-n Rydberg states converging to levels of the π-bound cation. The different efficiencies of the site switching for the Rydberg ion core and the bare ion and the mechanism for the π→ H site switching are interpreted by direct ionization from the π-bound to the H-bound structures in addition to the conventional vertical ionization and transitions to high-n Rydberg states.

Ionization-induced π → H site-switching in phenol-CH4 complexes studied using IR dip spectroscopy.

IR spectra of phenol-CH4 complexes generated in a supersonic expansion were measured before and after photoionization. The IR spectrum before ionization shows the free OH stretching vibration (ν(OH)) and the structure of neutral phenol-CH4 in the electronic ground state (S0) is assigned to a π-bound geometry, in which the CH4 ligand is located above the phenol ring. The IR spectrum after ionization to the cationic ground state (D0) exhibits a red shifted ν(OH) band assigned to a hydrogen-bonded cationic structure, in which the CH4 ligand binds to the phenolic OH group. In contrast to phenol-Ar/Kr, the observed ionization-induced π → H migration has unity yield for CH4. This difference is attributed to intracluster vibrational energy redistribution processes.

Characterization of black carbon in fine aerosol particles using high lateral resolution TOF-SIMS.

Fine aerosol particles were analyzed by time-of-flight secondary ion mass spectrometry with high lateral resolution. After sulfate particles with a diameter of about 1 µm were sputtered by gallium primary ions (a gallium focused ion beam), solid materials with a diameter of about 100 nm were occasionally found inside the particles. Since the mass spectrum for the solid material was almost the same as that of graphite, we concluded that the solids were black carbon. It was also found that the black carbon located at the surface of the sulfate core, and they were usually surrounded by organic matter.

Imaging of polycyclic aromatic hydrocarbons by means of sputtered neutrals mass spectrometry using a diode-pumped solid-state laser.

Laser post-ionization of sputtered molecules by pulsed Ga focused ion-beam (Ga-FIB) bombardment was examined for the detection and imaging of polycyclic aromatic hydrocarbons (PAHs) on particles. As model samples, pyrene and pelyrene adsorbed on TiO2, blended regents of pyrene and n-heneicosan were used. The TiO2 particle size was selected to be several micro-meters. Laser light and Ga-FIB were synchronized with each other. The repetition rate synchronized with Ga-FIB was 1 kHz for pyrene analysis and 2 kHz for perylene, respectively. The laser wavelength was set to 266 nm. The wavelength was a generated fourth harmonic of a Nd:YAG DPSS (diode-pumped solid-state) micro-chip laser (UV microchip laser). By using a UV microchip laser, laser-SNMS (laser post-ionized sputtered neutral mass spectrometry) analysis and imaging were performed. The imaging of pyrene (m/z = 202, C16H10) and perylene (m/z = 252, C20H12) has been successful. Both the scanning ion microscopy image of TiO2 and the PAHs image in laser-SNMS analysis were well-fitted with each other.

Analysis of low-lying gerade Rydberg states of acetylene using two-photon resonance fluorescence excitation spectroscopy.

The np gerade Rydberg states of acetylene were analyzed using two-photon resonance fluorescence excitation spectroscopy in the 72,000-93,000 cm(-1) energy region. The npπ(1)Σ(g)(+) and npπ(1)Δ(g) Rydberg series (n = 3-5) were identified in the fluorescence excitation spectrum measured by monitoring the C(2) d(3)Π(g)-a(3)Π(u) Swan system. Some vibronic bands were assigned to the npπ(1)Δ(g)-X̃(1)Σ(g)(+) transition on the basis of rotational analysis. The 5pσ(1)Π(g) state was observed, which is the first such observation in an npσ(1)Π(g) series. Rotational analysis of the 5pσ(1)Π(g)-X̃(1)Σ(g)(+) transition showed e/f-symmetry dependent predissociation of acetylene in the 5pσ(1)Π(g) state. The 0(0)(0) band of the deuterated acetylene (C(2)D(2)) 4pπ(1)Σ(g)(+)-X̃(1)Σ(g)(+) transition exhibits an atypical structure, which was satisfactorily reproduced by a simple model of quantum interference between the discrete and quasi-continuum states. The predissociative lifetimes of the npπgerade Rydberg states were estimated from the spectral profiles. The predissociation mechanism of acetylene in the Rydberg states is discussed.

Vibrational signature of the conformers in tyramine studied by IR dip and dispersed fluorescence spectroscopies.

Dispersed fluorescence and IR dip spectra of seven conformers of tyramine were measured in a supersonic jet. Observed vibrational bands are assigned based on quantum chemical calculations. The vibrational frequency of out-of-plane CH bending in the benzene ring shows characteristic shifts according to the conformation of C(beta)-C(alpha), while that of CH stretching in the methylene chain reflects the conformation of N-C(beta). From these vibrational frequencies, assignments of the conformers observed in the S(1)-S(0) laser induced fluorescence spectrum are discussed.