Continuum Lowering -- A New Perspective

What is meant by continuum lowering and ionization potential depression (IPD) in a Coulomb system depends upon precisely what question is being asked. It is shown that equilibrium (equation-of-state) phenomena and non-equilibrium dynamical processes like photoionization are characterised by different values of the IPD. In the former, the ionization potential of an atom embedded in matter is the difference in the free energy of the many-body system between states of thermodynamic equilibrium differing by the ionization state of just one atom. Typically, this energy is less than that required to ionize the same atom in vacuo. Probably, the best known example of such an IPD is that of Stewart and Pyatt (SP). However, it is a common misconception that this formula should apply directly to the energy of a photon causing photoionization, since this is a local adiabatic process that occurs in the absence of a response from the surrounding plasma. To achieve the prescribed final equilibrium state, additional energy, in the form of heat and work, is transferred between the atom and its surroundings. This additional relaxation energy is sufficient to explain the discrepancy between recent spectroscopic measurements of IPD in dense plasmas and the predictions of the SP formula. This paper provides a detailed account of an analytical approach to calculating thermodynamic and spectroscopic (adiabatic) IPDs in multicomponent Coulomb systems of arbitrary coupling strength. The ramifications are carefully examined in order to elucidate the roles of the various IPD forms. A formulation in terms of free energy leads to an analytical equation of state (EoS) that is thermodynamically self-consistent, provided that the bound and free electrons are dynamically separable. Of the various proposed formulae, the Spectroscopic (adiabatic) IPD gives the most consistent agreement with spectroscopic measurements.Comment: 80 pages 3 figures. S1. Expanded intro incl: summary of experiments; outline of ionization process & basis of local non-equilibrium hypothesis; revised para on connection with microfield. S2. New para on connection with self-energy; outline of basic continuum-lowering model used to illustrate the new ideas. S3. Rearranged text. S6. Revised & retitled. References: expanded. Minor changes throughou

High and low frequency dominated boundary layer transition through widely spaced discrete suction perforations

Boundary layer or wall-suction is a drag reduction technology with application to civil aircraft wings. Drag is reduced by increasing the extent of laminar flow across the boundary layer. Laminar flow has lower friction and form drag compared to turbulent flow. Wall-suction works by accelerating the air towards the surface of the wing, which has a stabilising effect on disturbances in the boundary layer; it also reduces the boundary layer thickness and thus the Reynolds number. In practice, this accelerated air must permeate through discrete pores in the surface. This creates local three-dimensional flow structures, which have been known to destabilise the flow and reduce the extent of laminar flow over wings, typically when the suction mass flow-rate is high. When this occurs the wall suction is said to be in a state of ‘over-suction’. In this work a set of experimental studies were performed on the problem of ‘over-suction’ for large arrays of suction perforations (in a widely spaced configuration), and for an isolated single suction perforation. It was found that the suction introduced a low frequency disturbance, this disturbance was found to grow (in amplitude) proportional to the suction rate. Furthermore, it was found that this low frequency was attenuated down-stream of the suction perforations, unless a pre-established N-type transition front was located close to the suction array. In this case the low-frequency was seen to interact with the N-type transition front, causing it to move upstream when at high suction rates.The amplitude of the low frequency also appeared to be Reynolds number independent, suggesting it may be an inflectional instability. The low frequency was correlated with inflection points in the stream-wise velocity‘u’profile across the span ‘z’. It has also been correlated with inflection points in the span-wise velocity ‘w’ profile across the wall-normal ‘y’. In the absence of any pre-established transition front, a high frequency disturbance was found to dominate the ‘over-suction’ process. This high frequency was found to only appear when the suction mass flow rate was sufficiently high, whereas the low frequency was found at all suction rates, and grew proportionally the non-dimensional suction mass flow-rate. This high frequency was also correlated with inflection points in the stream-wise velocity ‘u’ profile across the span‘z’. It has also been correlated with inflection points in the stream-wise velocity ‘u’ profile across the wall-normal ‘y’

Sulfur isotope fractionation during oxidation of sulfur dioxide: gas-phase oxidation by OH radicals and aqueous oxidation by H2O2, O3 and iron catalysis

The oxidation of SO[subscript 2] to sulfate is a key reaction in determining the role of sulfate in the environment through its effect on aerosol size distribution and composition. Sulfur isotope analysis has been used to investigate sources and chemical processes of sulfur dioxide and sulfate in the atmosphere, however interpretation of measured sulfur isotope ratios is challenging due to a lack of reliable information on the isotopic fractionation involved in major transformation pathways. This paper presents laboratory measurements of the fractionation factors for the major atmospheric oxidation reactions for SO2: Gas-phase oxidation by OH radicals, and aqueous oxidation by H[subscript 2]O[subscript 2], O[subscript 3] and a radical chain reaction initiated by iron. The measured fractionation factor for [superscript 34]S/[superscript 32]S during the gas-phase reaction is α[subscript OH] = (1.0089±0.0007)−((4±5)×10[subscript −5]) T(°C). The measured fractionation factor for [superscript 34]S/[superscript 32]S during aqueous oxidation by H[subscript 2]O[subscript 2] or O[subscript 3] is α[subscript aq] = (1.0167±0.0019)−((8.7±3.5) ×10[superscript −5])T(°C). The observed fractionation during oxidation by H2O2 and O3 appeared to be controlled primarily by protonation and acid-base equilibria of S(IV) in solution, which is the reason that there is no significant difference between the fractionation produced by the two oxidants within the experimental error. The isotopic fractionation factor from a radical chain reaction in solution catalysed by iron is αFe = (0.9894±0.0043) at 19 °C for [superscript 34]S/[superscript 32]S. Fractionation was mass-dependent with regards to 33S/32S for all the reactions investigated. The radical chain reaction mechanism was the only measured reaction that had a faster rate for the light isotopes. The results presented in this study will be particularly useful to determine the importance of the transition metal-catalysed oxidation pathway compared to other oxidation pathways, but other main oxidation pathways can not be distinguished based on stable sulfur isotope measurements alone

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