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Ring-strain in cyclic organic molecules is well-known to influence their chemical reactivity. Here, we examine the consequence of ring-strain for competing photochemical pathways that occur on picosecond timescales. The significance of Norrish Type-I photochemistry is explored for three cyclic ketones in cyclohexane solutions at ultraviolet (UV) excitation wavelengths from 255–312 nm, corresponding to an π* ← n excitation to the lowest excited singlet state (S1). Ultrafast transient absorption spectroscopy with broadband UV/visible probe laser pulses reveals processes common to cyclobutanone, cyclopentanone and cyclohexanone, occurring on timescales of ≤1 ps, 7–9 ps and >500 ps. These kinetic components are respectively assigned to prompt cleavage of an α C–C bond in the internally excited S1-state molecules prepared by UV absorption, vibrational cooling of these hot-S1 molecules to energies below the barrier to C–C bond cleavage on the S1 state potential energy surface (with commensurate reductions in the energy-dependent α-cleavage rate), and slower loss of thermalized S1-state population. The thermalized S1-state molecules may competitively decay by activated reaction over the barrier to α C–C bond fission on the S1-state potential energy surface, internal conversion to the ground (S0) electronic state, or intersystem crossing to the lowest lying triplet state (T1) and subsequent C–C bond breaking. The α C–C bond fission barrier height in the S1 state is significantly reduced by the ring-strain in cyclobutanone, affecting the relative contributions of the three decay time components which depend systematically on the excitation energy above the S1-state energy barrier. Transient infra-red absorption spectra obtained after UV excitation identify ring-opened ketene photoproducts of cyclobutanone and their timescales for formation., Ultrafast spectroscopy of ring-opening in three cyclic ketones reveals how ring-strain affects Norrish Type-I α-cleavage mechanisms.

The dynamics of chemical reactions in liquid solutions are now amenable to direct study using ultrafast laser spectroscopy techniques and advances in computer simulation methods. The surrounding solvent affects the chemical reaction dynamics in numerous ways, which include: (i) formation of complexes between reactants and solvent molecules; (ii) modifications to transition state energies and structures relative to the reactants and products; (iii) coupling between the motions of the reacting molecules and the solvent modes, and exchange of energy; (iv) solvent caging of reactants and products; and (v) structural changes to the solvation shells in response to the changing chemical identity of the solutes, on timescales which may be slower than the reactive events. This article reviews progress in the study of bimolecular chemical reaction dynamics in solution, concentrating on reactions which occur on ground electronic states. It illustrates this progress with reference to recent experimental and computational studies, and considers how the various ways in which a solvent affects the chemical reaction dynamics can be unravelled. Implications are considered for research in fields such as mechanistic synthetic chemistry.

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.

Time resolved vibrational cooling towards equilibrium in perfluorinated and chlorinated solvents provides detailed insights into the transfer of energy between solute and solvent molecules., We present a systematic study of the mode-specific vibrational relaxation of NO2 in six weakly-interacting solvents (perfluorohexane, perfluoromethylcyclohexane, perfluorodecalin, carbon tetrachloride, chloroform, and d-chloroform), chosen to elucidate the dominant energy transfer mechanisms in the solution phase. Broadband transient vibrational absorption spectroscopy has allowed us to extract quantum state-resolved relaxation dynamics of the two distinct NO2 fragments produced from the 340 nm photolysis of N2O4 → NO2(X) + NO2(A) and their separate paths to thermal equilibrium. Distinct relaxation pathways are observed for the NO2 bending and stretching modes, even at energies as high as 7000 cm–1 above the potential minimum. Vibrational energy transfer is governed by different interaction mechanisms in the various solvent environments, and proceeds with timescales ranging from 20–1100 ps. NO2 relaxation rates in the perfluorocarbon solvents are identical despite differences in acceptor mode state densities, infrared absorption cross sections, and local solvent structure. Vibrational energy is shown to be transferred to non-vibrational solvent degrees of freedom (V-T) through impulsive collisions with the perfluorocarbon molecules. Conversely, NO2 relaxation in chlorinated solvents is reliant on vibrational resonances (V-V) while V-T energy transfer is inefficient and thermal excitation of the surrounding solvent molecules inhibits faster vibrational relaxation through direct complexation. Intramolecular vibrational redistribution allows the symmetric stretch of NO2 to act as a gateway for antisymmetric stretch energy to exit the molecule. This study establishes an unprecedented level of detail for the cooling dynamics of a solvated small molecule, and provides a benchmark system for future theoretical studies of vibrational relaxation processes in solution.

Broadband transient infra-red absorption measurements are reported that contrast the dynamics of reaction of Cl atoms with 2,3-dimethylbut-2-ene and n-pentane in chlorinated solvents. In both cases, H-atom transfer produces HCl and a hydrocarbon radical, but the energy release in the former reaction is greater because of the formation of a resonance-stabilized allylic radical. The reaction of Cl atoms with n-pentane in solution in CH2Cl2 forms HCl exclusively in its lowest vibrational level (v = 0), and our measured rate coefficient agrees with a prior report by Sheps et al., J. Phys. Chem. A, 2006, 110, 3087. The time-dependence of the growth of intensity in the HCl fundamental absorption band shows two domains of reaction: a prompt rise is associated with reaction of the photolytically generated Cl atoms with n-pentane molecules lying within the first solvent shell, whereas a slower further rise is attributed to reaction following diffusion through the solution. For the reaction of Cl atoms with 2,3-dimethylbut-2-ene, these two domains of reaction are also observed, and fitting to a kinetic model incorporating these components gives bimolecular rate coefficients for formation of HCl of (1.7 ± 1.4) × 1010 M−1 s−1 in CDCl3 and (3.4 ± 1.2) × 1010 M−1 s−1 in CCl4. However, an additional transient absorption is observed ∼115 cm−1 lower in wavenumber than the fundamental HCl absorption band and is assigned to the v = 2 ← v = 1 hot band of HCl. The absorption by the nascent HCl(v = 1) peaks after ∼20 ps and subsequently decays to baseline levels because of vibrational relaxation, which is shown to be enhanced by energy transfer to 2,3-dimethylbut-2-ene. In solution in CDCl3, the fraction of HCl formed initially in v = 1 is determined to be 0.24 ± 0.04 and in CCl4 it is 0.15 ± 0.02. The branching to HCl(v = 1) for these reactions in solution is significantly lower than the 0.48 ± 0.06 fraction reported for reaction of Cl atoms with propene in the gas phase [Pilgrim and Taatjes, J. Phys. Chem. A, 1997, 101, 5776]. This comparison between similar bimolecular reactions in solution and in the gas phase therefore identifies changes to the dynamics caused by interaction with the solvent. Solvent friction experienced by the separating products of the reaction is considered more likely to be responsible for the lower vibrational excitation of the nascent HCl than is solvent modification of the topology of the potential energy surface in the vicinity of the transition state. Possible consequences of addition of Cl to the unsaturated bond in 2,3-dimethylbut-2-ene are also discussed.

Absorption cross sections are reported for several rotationally resolved lines in the QQ-branch of the ν5 + ν9 combination vibrational band of ethene (ethylene) at wavelengths around 1.625 μm. The measurements were made using cavity ring-down spectroscopy with a near infra-red diode laser. In the strongest region of the band, a line-integrated absorption cross section for the QQ4(4) feature of 5.08 ± 0.38 × 10−22 cm2 molecule−1 cm−1 is obtained, with a peak cross section at line centre of 3.23 ± 0.03 × 10−20 cm2 molecule−1. The effects of addition of pressures of up to 200 Torr of air on spectral linewidths are analysed to obtain an air broadening coefficient (HWHM) of γ = 0.1060 ± 0.0011 cm−1 atm−1. In 1 atm of air, the line centre absorption cross sections in the QQ4 branch are reduced by a factor of ∼8 because of line broadening. The consequences of these data for direct monitoring of ethene in the atmosphere using sensors based on diode laser cavity ring-down spectroscopy are discussed.

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) is demonstrated for the detection of tropospheric methane. Spectra of the 2ν3 overtone band of methane at wavelengths near 1.65 μm have been recorded in both laboratory air and under reduced pressures, and show quantitative agreement with absorption intensities and pressure-broadened line widths reported in the HITRAN 96 database. Experiments demonstrate a minimum detectable absorption coefficient that is currently αmin = 1.5 × 10−8 cm−1, equivalent to a number density of CH4 at ambient temperature of 2.9 × 1011 molecules cm−3 in a low pressure environment where Doppler broadening dominates the spectral lineshapes. The effects of pressure broadening reduce the detection limit of CH4 in 1 atm of air to 1.3 × 1012 molecules cm−3, corresponding to 52 parts per billion by volume. This is well below the 1.72 parts per million by volume average methane concentration in the lower troposphere.

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.

Cavity ring-down spectroscopy (CRDS) is a laser-based absorption spectroscopy technique that is starting to find extensive application as a consequence of the very high sensitivity of the method compared with more traditional absorption spectroscopy techniques. We describe the experimental implementation of CRDS and its application to a number of areas of research including laser diagnostics of hostile environments, reaction kinetics and spectroscopy, with particular emphasis on our ongoing studies of the fast (sub-nanosecond) predissociation of electronically excited states of small molecules and radicals.

The IO A2Π3/2–X2Π3/2 (v′,0) bands with v′ = 0–5 and the (1,1), (2,1) and (3,1) hot bands have been investigated in absorption using cavity ring-down spectroscopy. Analysis of the spectra gives refined band origins and rotational constants for the v′ levels and reveals strongly v′-dependent predissociation rates for the A2Π3/2 state. Fitting of spectral lineshapes for the rotationally resolved (2,0) band shows that v′ = 2 undergoes a rotation-induced predissociation, most probably via coupling to a 2Σ− state, with lifetimes for the rotational levels that range from ca. 1 ns at J′ = 1.5 to 15 ps at J′ = 50.5. In contrast, however, the (0,0) and (3,0) bands, which are also rotationally structured, exhibit apparently J′-independent predissociation rates. The (1,0), (4,0) and (5,0) bands are sufficiently lifetime-broadened that no rotational structure is evident. Fits to the band contours give average homogeneous (FWHM) linewidths for the various vibrational bands of 0.30 ± 0.03 cm−1 for v′ = 0, 6 ± 1 cm−1 for v′ = 1, 0.80 ± 0.05 cm−1 for v′ = 3, 9 ± 2 cm−1 for v′ = 4 and 60 ± 10 cm−1 for v′ = 5. The dominant predissociation mechanism for v′ = 0,1,3,4, and 5 is attributed to spin–orbit coupling between the A2Π3/2 state and one or more Ω = 3/2 repulsive states.

The formation of bromine atoms in their ground 2P3/2o (Br) and spin–orbit excited 2P1/2o (Br*) states as a result of photolysis of Br2 has been investigated at 260 nm and at 22 different wavelengths in the range 360–580 nm using the technique of photofragment ion imaging. Image analysis yields both the angular distributions and spin–orbit branching ratios of the photofragments at the various wavelengths, which serve to confirm and extend previously reported measurements of these quantities.Only ground state Br atoms are observed following excitation in the wavelength range 360–430 nm. The distribution of recoil velocity vectors for this Br+Br product channel is found to be well described by an anisotropy parameter β=-1.0±0.1, consistent with previous interpretations of the intense parent absorption at these wavelengths in terms of a perpendicular (ΔΩ=±1) transition from the X1Σg+(0g+) ground state to an excited state of 1Πu(1u) symmetry. Images recorded following excitation within the wavelength range 430–500 nm display a second ring attributable to the Br+Br* product channel. The angular anisotropy of this inner ring is described by β≈1.5 at excitation wavelengths ≈450 nm, but this declines to β≈0.6 at the longest excitation wavelength. The sign and the variation in magnitude of these β values suggest that parallel (ΔΩ=0) excitation to the B3Πu(0u+) state is the principal route to Br+Br* products at the shorter wavelengths, but that an alternative perpendicular route to these products involving initial excitation to the A3Πu(1u) state and subsequent transfer to a repulsive 1u state may play a role at the longer wavelengths. The relative intensities of the two velocity sub-groups in the ion images provide a direct measure of the Br+Br versus Br+Br* channel branching ratio: the latter makes only a minor contribution to the total product flux at λphot=430 nm, but this increases to ≈79% at λphot=500 nm. None of the images show any evidence of contributions from the possible Br*+Br* product channel. In accord with thermodynamic expectation, only ground state Br+Br products are observed following excitation at wavelengths λ>520 nm. These display a recoil anisotropy consistent with formation via transitions(s) of perpendicular character: the product flux is attributed to dissociation following excitation to the repulsive inner limb of the A3Πu(1u) potential curve.

(1 + 1) Resonance-enhanced multiphoton ionization (REMPI) spectroscopy has been used to record spectra of jet-cooled C2H2, C2HD and C2D2 simultaneously in a time-of-flight mass spectrometer (TOF/MS) over the wavenumber range 46 400–48 400 cm–1. The spectra for C2H2 and C2D2 are compared with previous absorption spectra recorded for room-temperature samples and laser-induced fluorescence spectra of jet-cooled molecules. For C2D2, the spectra are assigned beyond 47 860 cm–1, thereby extending the known spectroscopy of this molecule. For C2HD, assignments are suggested for the entire wavelength range recorded, and the spectrum is seen to be dominated by long progressions in the ν′2, ν′3 and ν′6 vibrational modes. Hot bands originating from molecules with one or two quanta of the trans- bending ν″4 vibrational mode are observed because of the favourable Franck–Condon factors for excitation from these vibrational levels to the A state (A1 Au for C2H2 and C2D2, A1A″ for C2HD). To assist in the spectral assignment and remove possible ambiguities between hot bands and transitions from the zero-point vibrational level of the X1Σg+(X 1Σ+ for C2HD) electronic state, we carried out photolysis experiments on jet-cooled C2HD and C2D2. The photodissociation is performed by a laser tuned to different vibrational features of the REMPI spectrum, and the kinetic energies of the resulting hydrogen or deuterium atoms are determined by TOF measurements. This technique distinguishes clearly between photolysis of vibrational ground state and vibrationally excited molecules. The vibrational constants obtained from fits to the REMPI spectral data agree well with the literature values. We suggest some reassignments of spectral features of C2D2 in light of the results of our photodissociation experiments.

Cavity ring-down spectroscopy of HNO in the range 17 540–17 590 cm–1 reveals that lifetime broadening contributes to the linewidths of the A1A″–X 1A′(110)–(000)K′= 4–K″= 3 subband rotational transitions. The deconvoluted predissociation-induced widths for the rotational levels of the upper state depend on the value of J′, with the deduced life-times decreasing from 310 ± 160 to 40 ± 6 ps for J′= 4–16.

Rotationally inelastic scattering of methyl radicals (CD3 and CH3) in collisions with helium is examined by a combination of velocity map imaging experiments and quantum scattering calculations. In the experiments a beam of methyl radicals seeded in Ar intersects a beam of He atoms at 90 degrees at a collision energy of 440 +/- 35 cm(-1) (CD3 + He) or 425 +/- 35 cm(-1) (CH3 + He). The methyl radicals are prepared photolytically in a gas expansion that cools them to 15 K, giving a distribution over a small number of initial (low) rotational angular momentum states. By resonance-enhanced multi-photon ionization detection, we obtain velocity map images which are specific to a single rotational angular momentum quantum number n' of the methyl radicals, but averaged over a small subset of the projection quantum number k'. We extract resolved angular scattering distributions for n' = 2-9 (for CD3). We compare these to predictions of scattering calculations performed based on a recent potential energy surface [P. J. Dagdigian and M. H. Alexander, J. Chem. Phys. 2011, 135, 064306] in which the methyl radical was fixed at its equilibrium geometry. The fully (n, k) -> (n', k') resolved differential cross sections obtained from the calculations, when combined in weighted sums over initial (n, k) levels corresponding to the 15 K experimental radical temperature, and final k' levels that are not resolved in the spectroscopic detection scheme, show excellent agreement with the experimental measurements for all final states probed. This agreement gives confidence in the calculated dependence of the scattering on changes in both the n and k quantum numbers.

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.

Two frequency chirped continuous wave diode lasers operating in the near infrared (IR) at wavelengths of lambda approximately 1.535 microm and lambda approximately 1.520 microm have been used to measure acetylene concentrations using the P(17) and R(9) rotational lines of the (nu1 + nu3) vibrational combination band. The diode lasers were frequency chirped by applying an electrical current pulse to the laser driver at a repetition rate of greater than 1 kHz. As the laser is operated at high repetition rates, more than 1000 spectra per second can, in principle, be acquired and summed, allowing fast accumulation of data, rapid averaging and consequent improvement of the signal to noise ratio and detection limit. Experiments were performed using a single-pass cell with a path length of 16.4 cm, and also an astigmatic multi-pass absorption cell aligned to give a path length of 56 m. Detection limits corresponding to minimum detectable absorption coefficients, alpha(min), of 5.6 x 10(-5) and 7.8 x 10(-8) cm(-1), respectively, were obtained over a 4 s detection bandwidth. These detection limits would correspond to mixing ratios of 21 parts per million by volume (ppmv) and 59 parts per billion by volume (ppbv) of acetylene at 1 atm in air, with the deleterious effects of pressure broadening accounted for. The single-pass cell was used to perform breakthrough volume (BTV) experiments for the low volume adsorbent traps used to pre-concentrate organic compounds in air, taking advantage of the capability of the system to measure concentrations in real time.

Continuous wave (cw) diode lasers are increasingly being used as light sources in the visible and near-IR regions of the spectrum for cavity ring-down spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS); the latter technique is also widely known as integrated cavity output spectroscopy (ICOS). The very high sensitivities to weak absorptions that are possible with cw CRDS and CEAS, coupled with the quantitative nature of the absorption measurements, are enabling a rapidly expanding range of applications. We review the benefits and practical implementation of these techniques; methods of data analysis for extraction of quantitative absorption data; the sensitivities of cw CRDS and CEAS, and how they might be optimised; and applications of cw CRDS and CEAS in molecular spectroscopy, atmospheric chemistry, plasma and flame chemistry, analytical science, and medical diagnosis via breath analysis. The development of CRDS and CEAS techniques exploiting cw diode lasers and, very recently, high luminosity light-emitting diodes, has stimulated a wealth of high-sensitivity measurements. Highlights include quantitative measurement of various ultra-trace gases such as: NO3, NO2 and ethene in ambient air samples; CO2 isotopologues, ethane and other organic compounds in human breath samples; and excited electronic states of N2 and O2 in plasmas and discharges. Exciting developments include wavelength extension into the mid-IR and UV regions, and use of novel locked-cavity techniques to increase data acquisition rates and sensitivities.

The performance of a continuous wave diode laser based cavity ring-down (CRD) spectrometer is demonstrated for retrieval of NO2 mixing ratios in ambient air. Measurements are made at wavelengths around 410 nm, a region free from other spectroscopic interferences, and the instrument is shown to be capable of quantitative and precise NO2 mixing ratio determinations without any requirement for calibration. The minimum concentration of NO2 that can be measured by the spectrometer is estimated to be 0.1 ppbv (parts per billion by volume) in 50 s at atmospheric pressure. Using dynamic dilution of a standard NO2 sample, the performance of the spectrometer has been validated for NO2 measurements over the range of 0.2 to 80 ppbv. NO2 measurements performed directly on laboratory air are compared with data obtained with a calibrated chemiluminescence instrument and shown to be in excellent agreement.

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) with near infra-red diode lasers is demonstrated for quantitative detection of trace levels of unsaturated volatile organic compounds (VOCs) at wavelengths that avoid overlapping absorptions by more abundant atmospheric constituents such as H2O and CO2. The current detection limit, with due allowance for pressure broadening by 1 atmosphere of air, is 6 parts per billion by volume (ppbv) for ethyne at an air wavelength of 1519.670 nm, and is sufficient for direct atmospheric detection of this molecule in many urban environments. Detection limits for alkenes are inferior, and, without incorporating the consequences of pressure broadening, include 78 ppbv for ethene and 900 ppbv for 1,3-butadiene. While the CRDS detection method offers several advantages over established gas chromatographic techniques for monitoring of small VOCs such as ethyne, it appears to be less well suited to study of larger organic compounds. Methods are discussed for improving the instrument to reach the sensitivities required to monitor the various alkenes and other C–H containing molecules in the troposphere.

Dynamical stereochemistry is the study of how chemical reactivity depends on the angle of approach of reagents to one another: the ‘chemical shape’ of molecules. This chemical shape has profound effects in a number of branches of chemistry, and current experimental and theoretical approaches to understanding dynamical stereochemistry are discussed. The experimental strategies employed have become very diverse in recent years and novel techniques are described, together with the limitations inherent in a number of experimental methods. Simple models for interpreting experimental data are discussed as are the results of more rigorous potential-energy surface and scattering calculations. The level of detail that can be achieved in understanding reaction dynamics and dynamical stereochemistry is illustrated for the reaction of chlorine atoms with methane.