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The mechanism of the thiol-ene reaction induced by 330-nm ultraviolet excitation of 1,2-di(quinolin-2-yl)disulfide (QSSQ) in the presence of methyl methacrylate (MMA) is investigated by sub-picosecond to microsecond transient absorption spectroscopy. The measurements, spanning more than seven orders of magnitude of time, directly reveal multiple radical reaction steps. The ground state quinoliene-2-thiyl radical (QS) is formed with a time constant of ~200 fs by photolysis of QSSQ, followed by (64 ± 1)% decay of the initially formed QS radical because of solvent cage induced geminate recombination and QS dimer formation with a rate coefficient of (3.4 ± 0.2) × 1010 M-1 s-1 in methanol solution. In MMA solution, the carbon centered radical QS-MMA forms with a bimolecular reaction rate coefficient of (2.8 ± 0.2) × 107 M-1 s-1. The distinct infrared band at 1653 cm-1 assigned to the C=O stretch mode of the QS-MMA radical decays rapidly in aerated solution, in contrast to observations in a solution purged of O2 by N2 bubbling. This decay is attributed to reaction of the QS-MMA radicals with molecular oxygen, producing peroxy radicals. Kinetic analysis of the intensity of the band at 1653 cm-1 reveals a bimolecular reaction rate coefficient of (3.3 ± 0.3) × 109 M-1 s-1 for the reaction of the QS-MMA radicals with molecular oxygen, and indicates that this reaction step is reversible.

This paper extends the knowledge of highly excited Rydberg states of HBr by presenting detailed measurements of the 3 + 1 resonance enhanced multiphoton ionisation (REMPI) spectrum of H81Br molecules obtained following excitation in the wavelength range 366–316 nm, i.e. at energies up to the first ionisation energy. Spectra have been recorded using both linearly and circularly polarised laser light. 34 vibronic bands have been analyzed, of which 13 are identified as new Rydberg origins (′ = 0) and 5 as new members of vibrational progressions (′≠0) originating from the promotion of a π electron into a σ, π or δ Rydberg orbital and correlating with the ground X2Π state of the HBr+ ion core. Symmetry assignments in terms of either (Λ, S) or (Ω, ω) coupling schemes for the various identified excited states are offered, based on band contour simulation and quantum defect considerations. The main result of the present work is the observation of new states and bands involving vibrationally excited levels which were not accessible in previous one and two-colour spectra. In particular, the three-photon selection rules allow observation of the ′ = 0 and 1 levels of the L1Φ3 state arising from the dδ Rydberg manifold, as well as three other Ω = 3 states within the dπ and dδ series which are reported here for the first time.

The spectroscopy and photochemistry of HI have been studied at excitation energies in the vicinity of H-atom Lyman–α (82259 cm−1) using the techniques of 1-photon vacuum UV photodissociation, resonance-enhanced multiphoton ionization (REMPI) spectroscopy and photofragment ion imaging. REMPI spectra, recorded using UV photons in the energy range 27350–27750 cm−1, exhibit complicated structure in channels corresponding to H+, I+ and HI+ ion formation. The former two channels require absorption of a minimum of 5 UV photons, whereas HI+ X 2Π formation requires 4 UV photons. The spectral features are assigned to resonances at the 3-photon energy involving six states with Ω = 0 and one state with Ω = 3. Ion images recorded in the same range of UV frequencies exhibit multiple velocity components for the H+ ionic fragment, and the excitation energy dependences of the ring radii reveal two mechanisms for H+ formation. The first route is 3 + 1 photon excitation to super-excited Rydberg states of neutral HI with [A 2Σ+] cores that autoionize to high vibrational levels (+15) of HI+ (X 2Π); subsequent 1-photon photolysis of these vibrationally excited ions makes H+. The second route is dissociation of 4-photon excited HI to H(n = 2) + I(2P3/2), followed by 1-photon ionization of the H(n = 2) atoms. Vacuum UV photolysis of HI at 121.6 nm combined with H-Rydberg atom photofragment translational spectroscopy reveals dissociation ia four different channels: H(n = 1) atoms are formed in association with I(5p5, 2P3/2), I(5p5, 2P1/2), I(5p4 6s1, 4P5/2) and I(5p4 6s1, 2P3/2). Anisotropy parameters for all these channels are positive but non-limiting, and photolysis propensities show a marked dependence on the initial rotational state of the HI. These observations are discussed in terms of excitation to predissociated Rydberg or ion-pair states identified in the REMPI spectra. A previously unreported Ω = 3 Rydberg state is identified ia HI+ formation following multiphoton absorption resonant at the 2 UV + 1 visible photon energy. A band with origin at 68326.18(73) cm−1, is assigned to excitation to the ′ = 0 level of a 3Δ3 state composed of a Rydberg dπ electron orbiting an HI+ X 2Π3/2 core.

Ab-initio calculations at the CASSCF and MRCI level have been performed on the valence doublet and quartet electronic states of the ClO radical in order to explore the predissociation dynamics of the A2ΠΩ state (Ω=1/2 or 3/2). The ab-initio calculations neglect spin–orbit coupling. Large basis sets with diffuse orbitals were employed to ensure the calculated X2Π and A2Π potentials match closely the available experimental data. In total, the potential energies of fifteen valence states were computed at a series of internuclear separations in order to map the potential energy curves. The X2Π, A2Π and a4Σ- potentials are bound; though the latter has yet to be observed experimentally. All the other states are repulsive, and the lowest ten cross the A2Π3/2 component at energies below the v′=6 vibrational level. Using arguments based on electronic configurations, candidate states for the predissociation of the A2ΠΩ state can be selected. Simulations of the predissociation were performed by Fermi golden rule calculations for each vibrational level, using the ab-initio repulsive potentials and RKR curves for the A2Π3/2 and A2Π1/2 states. The vibrational dependence of the predissociation rate in the A2Π3/2 component was used to derive the magnitude of the individual couplings with the repulsive potentials. Comparison between the lifetimes of the vibrational levels in the A2Π3/2 and A2Π1/2 components restricts the principal players in the predissociation to three repulsive states: 14ΣΩ+, 24ΣΩ- and 32ΠΩ. The possibility of coupling between the A2Π3/2 component and the 12Δ state is also suspected. A marked similarity is revealed for the variation of predissociation rates with v′ for the A2Π3/2 and A2Π1/2 components despite a spin–orbit state dependence of the absolute rates for individual v′ levels. A repulsive 22Σ+ state is identified which is likely to be involved in the UV photodissociation of ClO. Similar ab-initio calculations on FO have revealed no bound excited valence 2Π state correlating diabatically to the Cl(2P)+O(1D) dissociation limit.

The UV absorption of ClO at wavelengths between 285 and 320 nm has been investigated using cavity ring-down spectroscopy. This wavelength region spans the (0,0) to (7,0) bands of the A2Π3/2–X2Π3/2 and A2Π1/2–X2Π1/2 transitions. The previously unobserved A2Π3/2–X2Π3/2 (0,0) and (1,0) absorption bands have been recorded with rotational resolution, and spectra of the (2,0) to (6,0) bands of the A2Π1/2–X2Π1/2 transition are shown for the first time. Analysis of the spectra gives refined band origins and rotational constants for the A2Π v′ levels and reveals a strong v′ dependence in the linewidths of rotational features. The lifetimes of the A2Π3/2v′=0–2 and A2Π1/2v′=2–4 levels are revised from previous estimates, and the lifetimes of A2Π1/2v′=5 and 6 levels have been determined. The deduced predissociation rates for A2Π3/2v′=3–7 confirm earlier studies. The lifetimes of vibrational levels of the two spin–orbit components of the A2ΠΩ state are markedly different. No evidence of J′-dependence in the predissociation is found, in contrast to the corresponding A2Π3/2 state of the IO radical.

The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201–210 nm has been investigated. Ground state hydrogen photofragments, H(2S), were detected using the H Rydberg atom time-of-flight (HRTOF) technique to obtain directly the relative yields of the available product channels: H(2S)+Cl(2P3/2) and H(2S)+Cl(2P1/2). The product branching fractions are reported and compared with recent experimental measurements and theoretical calculations. In addition, the two spin–orbit components of ground state chlorine photofragments formed by photolysis of HCl at 205.5 nm were monitored using (2+1) resonance enhanced multiphoton ionization (REMPI). The relative sensitivity of this detection method for Cl(2P3/2) and Cl(2P1/2) atoms is found by comparing the relative REMPI signal intensities with the product branching fraction determined by the HRTOF technique.

Bessel beams were used to create a counter-propagating optical trap for capturing and manipulating aerosol particles. Aerosol droplets were characterized through measurement of the elastic scattered light at three wavelengths; the trapping wavelength of 532 nm was used in conjunction with two probe beams at 405 nm and 633 nm to reduce the uncertainty in estimating droplet radii of 1 μm or less. Control of the aerosol size distribution sampled by the counter-propagating trap was demonstrated by varying the trapping beam core diameters and intensities. Smaller droplet sizes were preferentially selected with a 1.7 μm core diameter compared to cores of 2.7 μm and 4.5 μm. Further, an increase in core intensity was shown to broaden the range in droplet sizes that were optically trapped. The possibility of using such an approach to isolate and analyze the properties of single accumulation mode aerosol particles is discussed.

Transient, broadband infra-red absorption spectroscopy with picosecond time resolution has been used to study the dynamics of reactions of CN radicals with tetrahydrofuran (THF) and d(8)-THF in liquid solutions ranging from neat THF to 0.5 M THF in chlorinated solvents (CDCl3 and CD2Cl2). HCN and DCN products were monitored via their v(1) (C equivalent to N stretching) and v(3) (C-H(D) stretching) vibrational absorption bands. Transient spectral features indicate formation of vibrationally excited HCN and DCN, and the onsets of absorption via the fundamental bands of HCN and DCN show short (5-15 ps) delays consistent with vibrational relaxation within the nascent reaction products. This interpretation is confirmed by non-equilibrium molecular dynamics simulations employing a newly derived analytic potential energy surface for the reaction in explicit THF solvent. The rate coefficient for reactive formation of HCN (as determined from measurements on both the 1(0)(1) and 3(0)(1) fundamental bands) decreases with increasing dilution of the THF in CDCl3 or CD2Cl2, showing pseudo-first order kinetic behaviour for THF concentrations in the range 0.5-4.5 M, and a bimolecular rate coefficient of (1.57 +/- 0.12) x 10(10) M-1 s(-1) is derived. Simultaneous analysis of time-dependent HCN 1(0)(1) and 3(0)(1) band intensities following reaction of CN with THF (3.0 M) in CD2Cl2 suggests that C-H stretching mode excitation is favoured, and this deduction is supported by the computer simulations. The results extend our recent demonstration of nascent vibrational excitation of the products of bimolecular reactions in liquid solution to a different, and more strongly interacting class of organic solvents. They serve to reinforce the finding that dynamics (and thus the topology of the reactive potential energy surface) play an important role in determining the nascent product state distributions in condensed phase reactions.

Quantum yields, Phi(NO(2)), are reported for the NO(2) formation channel (RO + NO(2)) following photodissociation of methyl and isopropyl nitrate (CH(3)ONO(2) and C(3)H(7)ONO(2)) at the tropospherically relevant wavelengths of 308-320 nm. Measurements were made using ultra-violet (UV) laser absorption and cavity ring down spectroscopy (CRDS) detection of the NO(2) photoproducts. Analysis of CRDS data to obtain NO(2) quantum yields required account to be taken of the spatially inhomogeneous NO(2) distribution resulting from photolysis with a laser beam with a Gaussian intensity profile. Photodissociation quantum yields were measured to be unity (withor=10% uncertainties (2SD)) for both methyl and isopropyl nitrate at wavelengths of 308, 315 and 320 nm in the presence of N(2) bath gas at pressures up to 700 Torr, and at a temperature of 294 K. An apparent decrease in the NO(2) yields, observed at bath gas pressures below 500 Torr, may result from photochemical formation of internally excited NO(2) which is not thermalized by collisions on the timescale of the measurement, or from diffusion of the photoproducts. Quantum yield values from the current work are in agreement with previously reported values at 308 and 248 nm photo-excitation wavelengths.

LAB-frame velocity distributions of Cl-atoms produced in the photoinitiated reaction of CH(3) radicals with HCl have been measured for both the ground Cl ((2)P(3/2)) and excited Cl* ((2)P(1/2)) spin-orbit states using a DC slice velocity-map ion imaging technique. The similarity of these distributions, as well as the average internal excitation of methane co-products for both Cl and Cl* pathways, suggest that all the reactive flux proceeds through the same transition state on the ground potential energy surface (PES) and that the couplings which promote nonadiabatic transitions to the excited PES correlating to Cl* occur later in the exit channel, beyond the TS region. The nature of these couplings is discussed in light of initial vibrational excitation of CH(3) radicals as well as previously reported nonadiabatic reactivity in other polyatomic molecule reactions. Furthermore, the scattering of the reaction products, derived using the photoloc method, suggests that at the high collision energy of our experiment (E(coll) = 22.3 kcal mol(-1)), large impact parameter collisions are favoured with a reduced kinematic constraint on the internal excitation of the methane co-product.