Search

Environmental air-water interfaces are often covered by thin films of surface-active organic substances that play an important role for air-sea gas exchange and aerosol aging. Surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy has been widely used to study the static structure of organic monolayers serving as simple model systems of such films. Probably due to the difficulties to correlate the SFG signal intensity with the surface concentration, corresponding time-resolved studies of surface reactions are scarce. In this study, quantitative time-resolved measurements have been performed on the oleic acid monolayer ozonolysis, which is considered a benchmark system for investigating the reactivity and fate of unsaturated natural organics. Surface concentration calibration data have been obtained by combining the pressure-area isotherm and VSFG spectra acquisition such that the 2D phase behavior of the oleic acid film could be properly taken into account. In contrast to literature reports, surface-active oxidation products were found to be negligible and do not interfere with the VSFG measurements. A pseudo-first-order kinetic analysis of the time-resolved data yielded a bimolecular rate constant of k2(oleic acid + O3 → products) = (1.65 ± 0.64) × 10(-16) cm(3) molecules(-1) s(-1), corresponding to an uptake coefficient of γ = (4.7 ± 1.8) × 10(-6). This result is in very good agreement with most recent monolayer measurements based on alternative methods and underlines the reliability of the time-resolved VSFG approach.

The potential of the thermal decomposition of cyanogen azide (NCN3) as a high-temperature cyanonitrene (NCN) source has been investigated in shock tube experiments. Electronic ground-state NCN(3Σ) radicals have been detected by narrow-bandwidth laser absorption at overlapping transitions belonging to the Q1 branch of the vibronic 3Σ+−3Π subband of the vibrationally hot 3Πu(010)−3Σg−(010) system at = 30383.11 cm(-1) (329.1302 nm). High-temperature absorption cross sections σ have been directly measured at total pressures of 0.2−2.5 bar, log[σ/(cm2 mol(-1))] = 8.9−8.3 × 10(-4) × T/K (±25%, 750 < T < 2250 K). At these high temperatures, NCN(3Σ) formation is limited by a slow electronic relaxation of the initially formed excited NCN(1Δ) radical rather than thermal decomposition of NCN3. Measured temperature-dependent collision-induced intersystem crossing (CIISC) rate constants are best represented by kCIISC/(cm3 mol(-1) s(-1)) = (1.3 ± 0.5) × 1011 exp[−(21 ± 4) kJ/mol/RT] (740 < T < 1260 K). Nevertheless, stable NCN concentration plateaus have been observed, showing that NCN3 is an ideal precursor for NCN kinetic experiments behind shock waves.

The overall rate constant for the radical-radical reaction C2H5 + HO2 --> products has been determined at room temperature by means of time-resolved mass spectrometry using a laser photolysis/flow reactor combination. Excimer laser photolysis of gas mixtures containing ethane, hydrogen peroxide, and oxalyl chloride was employed to generate controlled concentrations of C2H5 and HO2 radicals by the fast H abstraction reactions of the primary radicals Cl and OH with C2H6 and H2O2, respectively. By careful adjustments of the radical precursor concentrations, the title reaction could be measured under almost pseudo-first-order conditions with the concentration of HO2 in large excess over that of C2H5. From detailed numerical simulations of the measured concentration-time profiles of C2H5 and HO2, the overall rate constant for the reaction was found to be k1(293 K) = (3.1 +/- 1.0) x 10(13) cm3 mol(-1) s(-1). C2H5O could be confirmed as a direct reaction product.