This article describes recent progress on the development and application of pulsed high-intensity (∼0.1 mJ per pulse) vacuum-ultraviolet (VUV) radiation produced by commercial tabletop lasers for studies of gas phase chemical reaction dynamics involving polyatomic free radicals. Our approach employs near-triply resonant four-wave mixing of unfocussed nanosecond dye lasers in an atomic gas as an alternative to the use of synchrotron light sources for sensitive universal soft photoionization detection of reaction products using a rotatable source crossed molecular beams apparatus with fixed detector. We illustrate this approach in studies of the reactions of phenyl radicals with molecular oxygen and with propene. Future prospects for the use of tabletop laser-based VUV sources for studies of chemical reaction dynamics are discussed.

The reactions of phenyl radicals with propene have been studied at collision energies of 84 and 108 kJ/mol using the crossed molecular beams technique. The branching ratios between methyl radical elimination forming C8H8 and H-atom elimination forming C9H10 were found to be 10 ± 1:1 at 84 kJ/mol and 3 ± 1:1 at 108 kJ/mol. By using “soft” 9.9 eV vacuum ultraviolet photoionization for product detection, we were able to observe both product channels with negligible fragmentation of C9H10 to C8H8+. Our finding that CH3 elimination is dominant is consistent with conclusions from a recent study employing a pyrolysis molecular beam reactor using photoionization detection. However, our C8H8/C9H10 branching ratios are significantly larger than inferred from previous CMB experiments and RRKM calculations. For comparison, we have also studied the reactions of phenyl radicals with trans-2-butene at Ecoll = 97 kJ/mol. In this case, the symmetry of trans-2-butene makes both alkene addition sites chemically equivalent. The intermediate formed in the reaction with trans-2-butene is similar to the 2-carbon addition intermediate in the reaction with propene. We observed only methyl elimination in the reaction with trans-2-butene, with no evidence for H-atom elimination, consistent with conclusions that C–C bond fission is the most favorable channel in these systems. Analogies between phenyl radical reactions with propene and trans-2-butene are used to provide insight into the mechanisms in the propene reaction.

In an effort to make absorption spectrophotometry available to high school chemistry and physics classes, we have designed an inexpensive visible light absorption spectrophotometer. The spectrophotometer was constructed using LEGO blocks, a light emitting diode, optical elements (including a lens), a slide-mounted diffraction grating, and a photodiode detector. The photodiode detector was mounted on a rotatable arm for wavelength selection based on elementary laws of diffraction. This simple design demonstrates basic physical principles (such as diffraction and absorption of light) that are frequently lost in commercial “black box” instruments. The homemade spectrophotometer’s performance, as measured by comparison to a commercial spectrophotometer, was shown to be sufficiently quantitative to facilitate experiments in chemistry or physics classrooms.Keywords: Dyes/Pigments; First-Year Undergraduate/General; Hands-On Learning/Manipulatives; High School/Introductory Chemistry; Laboratory Equipment/Apparatus; Physical Chemistry; Quantitative Analysis; Spectroscopy; UV−Visible Absorption;

The photodissociation dynamics of CpCo(CO)2 was studied in a molecular beam using photofragment translational energy spectroscopy with 157 nm photoionization detection of the metallic products. At 532 and 355 nm excitation, the dominant one-photon channel involved loss of a single CO ligand producing CpCoCO. The product angular distributions were isotropic, and a large fraction of excess energy appeared as product vibrational excitation. Production of CpCO + 2CO resulted from two-photon absorption processes. The two-photon dissociation of mixtures containing CpCo(CO)2 and H2 at the orifice of a pulsed nozzle was used to produce a novel 16-electron unsaturated species, CpCoH2. Transition metal ligand exchange reactions, CpCoH2 + L → CpCoL + H2 (L = propyne, propene, or ammonia), were studied under single-collision conditions for the first time. In all cases, ligand exchange occurred via 18-electron association complexes with lifetimes comparable to their rotational periods. Although ligand exchange reactions were not detected from CpCoH2 collisions with methane or propane (L = CH4 or C3H8), a molecular beam containing CpCoCH4 was produced by photolysis of mixtures containing CpCo(CO)2 and CH4.

The reactions of neutral ground-state yttrium (Y) atoms with 1,3- and 1,4-cyclohexadiene (CHD) were studied using crossed molecular beams. Formation of YC6H6 + H2 and YH2 + C6H6 was observed for both isomers at collision energies (Ecoll) of 31.3 and 13.0 kcal/mol. Measured product branching ratios at Ecoll = 31.3 kcal/mol indicated that YH2 + C6H6 was the dominant channel, accounting for >97% of the products. An additional minor product channel, YC4H4 + C2H4, was observed for 1,3-CHD at the higher Ecoll. The reaction threshold for YC4H4 formation was determined to be 29.5 ± 2.0 kcal/mol based on fits to the data.

Phenyl radicals (C6H5) react with oxygen molecules (O2) to form phenylperoxy radicals (C6H5OO). These intermediates have been calculated to decompose either through O−O bond fission forming C6H5O + O or through a series of isomerization steps producing C5H5 + CO2, C5H5O + CO, or C6H4O2 + H. In this study, the reaction of phenyl radicals with molecular oxygen is investigated at a mean collision energy of 64 kJ/mol using the crossed molecular beams technique, employing detection via pulsed single-photon ionization at 9.9 eV. Here, we monitor the formation of phenoxy radicals (C6H5O) from the C6H5O + O channel, providing insight into the lifetimes of the C6H5OO intermediates. The measured distributions imply that the C6H5OO lifetimes (τ) are at least comparable to their rotational time scales, that is, τ ≥ 1 ps. Our lower limit for τ is at least 100 times longer than an upper limit inferred from a previous crossed beams experiment carried out at a higher energy.Keywords (keywords): crossed molecular beams; phenyl radical oxidation; radical beam generation; reaction dynamics; vacuum ultraviolet photoionization;

This article describes recent progress on the development and application of pulsed high-intensity (∼0.1 mJ per pulse) vacuum-ultraviolet (VUV) radiation produced by commercial tabletop lasers for studies of gas phase chemical reaction dynamics involving polyatomic free radicals. Our approach employs near-triply resonant four-wave mixing of unfocussed nanosecond dye lasers in an atomic gas as an alternative to the use of synchrotron light sources for sensitive universal soft photoionization detection of reaction products using a rotatable source crossed molecular beams apparatus with fixed detector. We illustrate this approach in studies of the reactions of phenyl radicals with molecular oxygen and with propene. Future prospects for the use of tabletop laser-based VUV sources for studies of chemical reaction dynamics are discussed.