Production Of Cl Atoms Research Articles

Observations of gas phase hydrochloric acid in the polluted marine boundary layer

AbstractShip‐based measurements of gas phase hydrochloric acid (HCl), particulate chloride (pCl−), and reactive nitrogen oxides (NOy) were made in the polluted marine boundary layer along the California coastline during spring 2010. These observations are used to assess both the rate of Cl atom production from HCl and the role of direct HCl emissions and subsequent partitioning as a source for pCl−. Observations of HCl made in coastal Southern California are broadly correlated with NOz (NOz ≡ NOy – NOx), peaking at 11 A.M. The observed median HCl mixing ratio in Southern California is 1.3 ppb (interquartile range: 0.53–2.7 ppb), as compared to 0.19 ppb (interquartile range: 0.10–0.38 ppb) measured along the Sacramento River between San Francisco and Sacramento. Concurrent measurements of aerosol ion chemistry indicate that aerosol particles sampled in Northern California are heavily depleted in Cl−, corresponding to a mean pCl− deficit of 0.05 ± 0.03 (1σ) ppb for sub‐10 µm aerosol particles. In comparison, aerosols measured in Southern California indicate that over 25% of particles showed an addition of Cl− to the particle population. Observations presented here suggest that primary sources of HCl, or gas phase chlorine precursors to HCl, are likely underestimated in the California Air Resource Board emissions inventory. These results highlight the need for future field observations designed to better constrain direct reactive halogen emissions.

Reprint of: Quantum yield of chlorine-atom formation in the photodissociation of chlorine peroxide (ClOOCl) at 308 nm

The production of Cl atoms in the laser flash photolysis of ClOOCl at 308nm has been investigated by time-resolved atomic resonance fluorescence at 235K. A value of ϕ=1.03±0.12 has been obtained for the primary quantum yield based on an absorption cross section ratio σ245/σ308=22 for ClOOCl at 245 and 308nm.

Chlorine-Catalyzed Ozone Destruction: Cl Atom Production from ClOOCl Photolysis

Recent laboratory measurements of the absorption cross sections of the ClO dimer, ClOOCl, have called into question the validity of the mechanism that describes the catalytic removal of ozone by chlorine. Here we describe direct measurements of the rate-determining step of that mechanism, the production of Cl atoms from the photolysis of ClOOCl, under laboratory conditions similar to those in the stratosphere. ClOOCl is formed in a cold-temperature flowing system, with production initiated by a microwave discharge of Cl(2) or photolysis of CF(2)Cl(2). Excimer lasers operating at 248, 308, and 352 nm photodissociate ClOOCl, and the Cl atoms produced are detected with time-resolved atomic resonance fluorescence. Cl(2), the primary contaminant, is measured directly for the first time in a ClOOCl cross section experiment. We find the product of the quantum yield of the Cl atom production channel of ClOOCl photolysis and the ClOOCl absorption cross section, (phisigma)(ClOOCl) = 660 +/- 100 at 248 nm, 39.3 +/- 4.9 at 308 nm, and 8.6 +/- 1.2 at 352 nm (units of 10(-20) cm(2) molecule(-1)). The data set includes 468 total cross section measurements over a wide range of experimental conditions, significantly reducing the possibility of a systematic error impacting the results. These new measurements demonstrate that long-wavelength photons (lambda = 352 nm) are absorbed by ClOOCl directly, producing Cl atoms with a probability commensurate with the observed rate of ozone destruction in the atmosphere.

A global (volume averaged) model of a chlorine discharge

A steady state global (volume averaged) model is developed for the chlorine discharge using a revised reaction set. Various calculated plasma parameters are compared with measurements found in the literature, showing a good overall agreement. The reaction rates for the various reactions are evaluated in the pressure range 1–100 mTorr. In particular, we explore the dissociation process as well as the creation and destruction of the negative ions Cl−. The discharge is highly dissociated throughout the pressure range explored, 1–100 mTorr, even when the absorbed power is low. The mechanism for Cl creation is complex. Although electron impact dissociation dominates with roughly 60–65% contribution, mutual neutralization of positive and negative ions and dissociative electron attachment are important contributors to the production of Cl atoms at high pressure. The electronegativity increases rapidly with decreasing dissociation fraction since the Cl− ions are created entirely by dissociative electron attachment, predominantly from Cl2(v = 0), but also up to 14% from Cl2(v > 0) at 100 mTorr. The negative ion Cl− is lost almost entirely through mutual neutralization with at high pressure while Cl+ has a significant contribution at low pressure.

Identification of nascent products formed in the laser photolysis of CH 3OCl and HOCl at 308 nm and around 235 nm. Total Cl-atom quantum yields and the state and velocity distributions of Cl( 2P j)

Nascent products formed in the laser photolysis of the two hypochlorites CH 3OCl and HOCl at wavelenths close to the maxima in their absorption spectra have been determined by resonantly enhanced multiphoton ionisation (REMPI) using a time-of-flight (TOF) mass analyser. Cl-atoms in the states 2 P 3 2 and 2P 1 2 were found to be the predominant photodissociation products for both hypochlorites. In the case of HOCl, the nascent OH was also recorded with rotational resolution. Formation of other possible photofragments from CH 3OCl i.e. HCl, CH 3 and ClO was clearly excluded to < 10 −2 and < 10 −3, respectively. The absolute quantum yields for Cl-atom production in the photolysis at the atmospherically relevant wavelength of λ = 308 nm were found to be φ = 0. 95 ± 0.05 and φ = 1.00 ± 0.05 for CH 3OCl and HOCl, respectively. Relative spin-orbit state population of the nascent chlorine ( 2P j) atoms varied strongly with photolysis wavelength. The kinetic energy released to the Cl-atoms in the photolysis at two wavelengths was determined for CH 3OCl and HOCl for the first time. In the photolysis at λ = 308 nm, Cl-atoms are formed with E trans of about 1 eV. The atmospheric implications of this finding are briefly outlined.

UV laser photodissociation of CF 2ClBr and CF 2Br 2 at 298 K: quantum yields of Cl, Br, and CF 2

In the photodissociation of CF 2ClBr, the quantum yields for Cl atom production were measured to be 1.03 ± 0.14 0.27 ± 0.04, and 0.18 ± 0.03, respectively, at 193, 222, and 248 nm, while those for Br atom were 1.04 ± 0.13 at 193 nm, 0.86 ± 0.11 at 222 nm, and 0.75 ± 0.13 at 248 nm. Errors are 95% confidence limits and include estimated systematic errors. The CF 2 quantum yields in 193 nm photolysis were measured to be 1.15 ± 0.30 for CF 2Br 2 and 0.91 ± 0.30 for CF 2ClBr. These results indicate the quantum yield for photodissociation of CF 2ClBr is unity.

Experimental studies of Cl-atom reactions at high temperatures: Cl+H2→HCl+H from 291 to 1283 K

A method is presented for producing Cl atoms for kinetic experiments above 750 K. Combined with alow temperature Cl-atom production technique, rate coefficients for the title reaction have been obtained over a wide temperature range. The Cl atoms were generated by flash photolysis of CCl 4 for the lowtemperature measurements and by pulsed laser photolysis of NaCl for the high-temperature measurements. The relative Cl-atom concentrations were monitored by time-resolved resonance fluorescence. The data are well fitted by the expression k 1 (291–1283 K)=4.4×10 −11 exp(−2568 K/ T ) cm 3 molecule −1 s −1 , with 2σ precision limits of ±9 to ±16%, depending on temperature, and corresponding 2σ accuracy limits of about ±26%. There is good agreement between the rate coefficients measured here and those from previous low-temperature studies, leading to the recommendation k 1 (200–1283 K)=2.3×10 −16 ( T /K) 1.63 exp(−1592 K/ T ) cm 3 molecule −1 s −1 , with a suggested 2σ accuracy limit of ±28% for the combined data. A transition state theory calculation based on a semiempirical London-Eyring-Polanyi-Sato (LEPS) potential energy surface gives excellent agreement with the combined experimental results. Extension of our previous recommendation for the reverse reaction to lower temperatures leads to k 2 (200–1200 K)=6.6×10 −16 ( T /K) 1.44 exp(−1241 K/ T ) cm 3 molecule −1 s −1 , with 2σ accuracy limits within ±25%. There is excellent agreement between the present data and the predicted expression from that reaction and equilibrium data.

Monte Carlo-fluid model of chlorine atom production in Cl2, HCl, and CCl4 radio-frequency discharges for plasma etching

Chlorine chemistries are often used for the radio-frequency (rf) discharge plasma etching of compound semiconductors, metals, and silicon. A variety of gas mixtures are used as Cl atom donors, many of which have different electron transport coefficients. In this article we computationally investigate 13.56 MHz rf discharges sustained in He/Cl2, He/HCl, and He/CCl4 gas mixtures in the context of plasma etching. The study is performed using a Monte Carlo-fluid hybrid model of rf discharges. We find that the Cl atom production efficiency is surprisingly similar in these mixtures, while the details of the electron transport (sources of ionization, locations of attachment, electronegativity) dramatically differ. We also find that even at the low pressures of interest (0.25–1 Torr) attachment in He/HCl mixtures is dominated by vibrationally excited HCl, in analogy to high-pressure discharge devices.

Quantum yield of chlorine-atom formation in the photodissociation of chlorine peroxide (ClOOCl) at 308 nm

The production of Cl atoms in the laser flash photolysis of ClOOCl at 308 nm has been investigated by time-resolved atomic resonance fluorescence at 235 K. A value of φ = 1.03 ± 0.12 has been obtained for the primary quantum yield based on an absorption cross section ratio σ 245/σ 308 = 22 for ClOOCl at 245 and 308 nm.

Oscillator strengths of Cl(I) in the vacuum ultraviolet: The 2D- 2P transitions

Experimental oscillator strengths are reported in absorption for the 2 D j - 2 P j transitions of atomic chlorine using chemical titration for the quantitative production of Cl atoms. The closely spaced 2 D 5 2 , 3 2 - 2P 3 ,2 doublet at 118.9 nm was resolved in emission using a 6.65 m normal incidence vacuum spectrometer. Multiplet emission ratios are combined with line absorption measurements to obtain the following absolute oscillator strengths: 2 D 3 2 - 2P 3 2 (118.875 nm), 0.0050 ± 0.0010; 2 D 3 2 - 2P 1 2 (118.877 nm), 0.068 ± 0.007; 2 D 3 2 - 2P 3 2 (120.135 nm), 0.102 ± 0.010.

ChemInform Abstract: PHOTODISSOCIATION OF THIOPHOSGENE

The photolysis of thiophosgene produces CS(A 1π) below the incident wavelength (1270 A) from which ΔHf °0(SCCl2) ?7.92±1 kcal mol−1 (33.1±4 kJ mol−1) is derived. The quantum yield for the production of Cl atoms has been determined from the amounts of HCl produced from the photolysis of thiophosgene–heptane mixtures. At 2537 A the yield is unity over the pressure region 0.4–80 torr (0.05–10 kPa) of thiophosgene, indicating that the primary photochemical process is almost entirely SCCl+Cl. Both at 3660 and 4358 A the quantum yield of Cl production increases with a decrease in thiophosgene pressure and is 0.90±0.17 and 0.38±0.19, respectively, as the pressure approaches zero. From the pressure dependence of the quantum yield the lifetime of the excited state 1A2 is estimated to be about 55 nsec at 4358 A and about 6 nsec at 3660 A. The photolysis data of the 1A2 state of thiophosgene are correlated with the fluorescence lifetime measured previously. D°0(SCCl–Cl) =63.4±0.5 kcal mol−1 (265.3±2 kJ mol−1) and ΔH...

Photodissociation of thiophosgene

The photolysis of thiophosgene produces CS(A 1π) below the incident wavelength (1270 Å) from which ΔHf °0(SCCl2) ?7.92±1 kcal mol−1 (33.1±4 kJ mol−1) is derived. The quantum yield for the production of Cl atoms has been determined from the amounts of HCl produced from the photolysis of thiophosgene–heptane mixtures. At 2537 Å the yield is unity over the pressure region 0.4–80 torr (0.05–10 kPa) of thiophosgene, indicating that the primary photochemical process is almost entirely SCCl+Cl. Both at 3660 and 4358 Å the quantum yield of Cl production increases with a decrease in thiophosgene pressure and is 0.90±0.17 and 0.38±0.19, respectively, as the pressure approaches zero. From the pressure dependence of the quantum yield the lifetime of the excited state 1A2 is estimated to be about 55 nsec at 4358 Å and about 6 nsec at 3660 Å. The photolysis data of the 1A2 state of thiophosgene are correlated with the fluorescence lifetime measured previously. D°0(SCCl–Cl) =63.4±0.5 kcal mol−1 (265.3±2 kJ mol−1) and ΔHf °0(SCCl) =43±1 kcal mol−1 (180±4 kJ mol−1) are derived. The photochemistry of thiophosgene is compared with that of phosgene and formaldehyde. A new method of Cl isotopic enrichment is suggested.