Temperature-dependent rate coefficients for the reactions of the hydroxyl radical with the atmospheric biogenics isoprene, alpha-pinene and delta-3-carene

Pulsed laser methods for OH generation and detection were used to study atmospheric degradation reactions for three important biogenic gases: OHCisoprene (Reaction R1), OH+α-pinene (Reaction R2) and OH+Δ- 3-carene (Reaction R3). Gas-phase rate coefficients were characterized by non-Arrhenius kinetics for all three reactions. For (R1), k1 (241-356 K)= (1:93±0:08)× 10-11 exp{(466±12)/T} cm3 molecule-1 s-1 was determined, with a room temperature value of k1 (297 K)= (9:3± 0:4)×10-11 cm3 molecule-1 s-1, independent of bath-gas pressure (5-200 Torr) and composition (MDN2 or air). Accuracy and precision were enhanced by online optical monitoring of isoprene, with absolute concentrations obtained via an absorption cross section, αisoprene = (1:28±0:06)× 10-17 cm2 molecule-1 at λ = 184:95 nm, determined in this work. These results indicate that significant discrepancies between previous absolute and relative-rate determinations of k1 result in part from σ values used to derive the isoprene concentration in high-precision absolute determinations. Similar methods were used to determine rate coefficients (in 10-11 cm3 molecule-1 s-1/ for (R2)-(R3): k2 (238-357 K)= (1:83±0:04) ×exp{(330±6)/T } and k3 (235-357 K)= (2:48±0:14) ×exp{(357±17)/T }. This is the first temperature-dependent dataset for (R3) and enables the calculation of reliable atmospheric lifetimes with respect to OH removal for e.g. boreal forest springtime conditions. Room temperature values of k2 (296 K)= (5:4±0:2) ×10-11 cm3 molecule-1 s-1 and k3 (297 K)= (8:1±0:3)×10-11 cm3 molecule-1 s-1 were independent of bathgas pressure (7-200 Torr, N2 or air) and in good agreement with previously reported values. In the course of this work, 184.95 nm absorption cross sections were determined: σ = (1:54±0:08) ×10-17 cm2 molecule-1 for α-pinene and (2:40±0:12)×10-17 cm2 molecule-1 for 1-3-carene

Temperature-(208-318 K) and pressure-(18-696Torr) dependent rate coefficients for the reaction between OH and HNO3

Abstract. Rate coefficients (k5) for the title reaction were ob- tained using pulsed laser photolytic generation of OH cou- pled to its detection by laser-induced fluorescence (PLP– LIF). More than 80 determinations of k5 were carried out in nitrogen or air bath gas at various temperatures and pres- sures. The accuracy of the rate coefficients obtained was en- hanced by in situ measurement of the concentrations of both HNO3 reactant and NO2 impurity. The rate coefficients show both temperature and pressure dependence with a rapid in- crease in k5 at low temperatures. The pressure dependence was weak at room temperature but increased significantly at low temperatures. The entire data set was combined with se- lected literature values of k5 and parameterised using a com- bination of pressure-dependent and -independent terms to give an expression that covers the relevant pressure and tem- perature range for the atmosphere. A global model, using the new parameterisation for k 5 rather than those presently ac- cepted, indicated small but significant latitude- and altitude- dependent changes in the HNO 3 / NO x ratio of between − 6 and + 6 %. Effective HNO 3 absorption cross sections (184.95 and 213.86 nm, units of cm 2 molecule − 1 ) were ob- tained as part of this work: σ 213 . 86 = 4.52 + 0 . 23 − 0 . 12 × 10 − 19 and σ 184 . 95 = 1.61 + 0 . 08 − 0 . 04 × 10 − 17

Preparation of individual magnetic sub-levels of ⁴He(2³S₁) in a supersonic beam using laser optical pumping and magnetic hexapole focusing

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable 4He in the 23S1 level in a supersonic beam, namely, magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of v = 830 m/s, we deduce from a comparison with a particle trajectory simulation that up to 99% of the metastable atoms are in the MJ″ = +1 sub-level after magnetic hexapole focusing. Using laser optical pumping via the 23P2–23S1 transition, we achieve a maximum efficiency of 94% ± 3% for the population of the MJ″ = +1 sub-level. For the first time, we show that laser optical pumping via the 23P1–23S1 transition can be used to selectively populate each of the three MJ″ sub-levels (MJ″ = −1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific MJ″ sub-levels than magnetic hexapole focusing

Extreme Ultraviolet Wave Packet Interferometry of the Autoionizing HeNe Dimer

Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant interatomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and high spectral resolution

Zeeman deceleration of metastable nitrogen atoms

Raw data, simulations and analysis code for the evidence presented in the paper "Zeeman deceleration of metastable nitrogen atoms" by Katrin Dulitz, Jutta Toscano, Atreju Tauschinsky and Timothy P Softley published in J. Phys. B: At. Mol. Opt. Phys. 49 (2016) 075203 (6pp

Zeeman deceleration of metastable nitrogen atoms

Raw data, simulations and analysis code for the evidence presented in the paper "Zeeman deceleration of metastable nitrogen atoms" by Katrin Dulitz, Jutta Toscano, Atreju Tauschinsky and Timothy P Softley published in J. Phys. B: At. Mol. Opt. Phys. 49 (2016) 075203 (6pp

Temperature-(208–318 K) and pressure-(18–696 Torr) dependent rate coefficients for the reaction between OH and HNO₃

Rate coefficients (k5) for the title reaction were obtained using pulsed laser photolytic generation of OH coupled to its detection by laser-induced fluorescence (PLP-LIF). More than eighty determinations of k5 were carried out in nitrogen or air bath gas at various temperatures and pressures. The accuracy of the rate coefficients obtained was enhanced by in-situ measurement of the concentrations of both HNO3 reactant and NO2 impurity. The rate coefficients show both temperature and pressure dependence with a rapid increase in k5 at low temperatures. The pressure dependence was weak at room temperature but increased significantly at low temperatures. The entire dataset was combined with selected literature values of k5 and parameterised using a combination of pressure dependent and independent terms to give an expression that covers the relevant pressure and temperature range for the atmosphere. A global model, using the new parameterisation for k5 rather than those presently accepted, indicated small but significant latitude and altitude dependent changes in the HNO3 / NOx ratio of between −6 % and +6 %. Effective HNO3 absorption cross sections (184.95 and 213.86 nm, units of cm2 molecule−1) were obtained as part of this work: σ213.86 = 4.52+0.23−0.12 × 10−19 and σ184.95 = 1.61+0.08−0.04 × 10−17

Zeeman deceleration beyond periodic phase space stability

In Zeeman deceleration, time-varying spatially-inhomogeneous magnetic fields are used to create packets of translationally cold, quantum-state-selected paramagnetic particles with a tuneable forward velocity, which are ideal for cold reaction dynamics studies. Here, the covariance matrix adaptation evolutionary strategy (CMA-ES) is adopted in order to optimise deceleration switching sequences for the operation of a Zeeman decelerator. Using the optimised sequences, a 40% increase in the number of decelerated particles is observed compared to standard sequences for the same final velocity, imposing the same experimental boundary conditions. Furthermore, we demonstrate that it is possible to remove up to 98% of the initial kinetic energy of particles in the incoming beam, compared to the removal of a maximum of 83% of kinetic energy with standard sequences. Three-dimensional particle trajectory simulations are employed to reproduce the experimental results and to investigate differences in the deceleration mechanism adopted by standard and optimised sequences. It is experimentally verified that the optimal solution uncovered by the evolutionary algorithm is not merely a local optimisation of the experimental parameters { it is a novel mode of operation that goes beyond the standard periodic phase stability approach typically adopted