Hickson Kevin M, Keyser Leon F
Earth and Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
J Phys Chem A. 2005 Aug 11;109(31):6887-900. doi: 10.1021/jp051176w.
Absolute rate data and product branching ratios for the reactions Cl + HO2 --> HCl + O2 (k1a) and Cl + HO2 --> OH + ClO (k1b) have been measured from 226 to 336 K at a total pressure of 1 Torr of helium using the discharge flow resonance fluorescence technique coupled with infrared diode laser spectroscopy. For kinetic measurements, pseudo-first-order conditions were used with both reagents in excess in separate experiments. HO2 was produced by two methods: through the termolecular reaction of H atoms with O2 and also by the reaction of F atoms with H2O2. Cl atoms were produced by a microwave discharge of Cl2 in He. HO2 radicals were converted to OH radicals prior to detection by resonance fluorescence at 308 nm. Cl atoms were detected directly at 138 nm also by resonance fluorescence. Measurement of the consumption of HO2 in excess Cl yielded k1a and measurement of the consumption of Cl in excess HO2 yielded the total rate coefficient, k1. Values of k1a and k1 derived from kinetic experiments expressed in Arrhenius form are (1.6 +/- 0.2) x 10(-11) exp[(249 +/- 34)/T] and (2.8 +/- 0.1) x 10(-11) exp[(123 +/- 15)/T] cm3 molecule(-1) s(-1), respectively. As the expression for k1 is only weakly temperature dependent, we report a temperature-independent value of k1 = (4.5 +/- 0.4) x 10(-11) cm3 molecule(-1) s(-1). Additionally, an Arrhenius expression for k1b can also be derived: k1b = (7.7 +/- 0.8) x 10(-11) exp[-(708 +/- 29)/T] cm3 molecule(-1) s(-1). These expressions for k1a and k1b are valid for 226 K < or = T < or = 336 and 256 K < or = T < or = 296 K, respectively. The cited errors are at the level of a single standard deviation. For the product measurements, an excess of Cl was added to known concentrations of HO2 and the reaction was allowed to reach completion. HCl product concentrations were determined by IR absorption yielding the ratio k1a/k1 over the temperature range 236 K < or = T < or = 296 K. OH product concentrations were determined by resonance fluorescence giving rise to the ratio k1b/k1 over the temperature range 226 K < or = T < or = 336 K. Both of these ratios were subsequently converted to absolute numbers. Values of k1a and k1b from the product experiments expressed in Arrhenius form are (1.5 +/- 0.1) x 10(-11) exp[(222 +/- 17)/T] and (10.6 +/- 1.5) x 10(-11) exp[-(733 +/- 41)/T] cm3 molecule(-1) s(-1), respectively. These expressions for k1a and k1b are valid for 256 K < or = T < or = 296 and 226 K < or = T < or = 336 K, respectively. A combination of the kinetic and product data results in the following Arrhenius expressions for k1a and k1b of (1.4 +/- 0.3) x 10(-11) exp[(269 +/- 58)/T] and (12.7 +/- 4.1) x 10(-11) exp[-(801 +/- 94)/T] cm3 molecule(-1) s(-1), respectively. Numerical simulations were used to check for interferences from secondary chemistry in both the kinetic and product experiments and also to quantify the losses incurred during the conversion process HO2 --> OH for detection purposes.
利用放电流动共振荧光技术结合红外二极管激光光谱，在总压力为1托氦气的条件下，于226至336K温度范围内测量了反应Cl + HO₂ → HCl + O₂（k₁a）和Cl + HO₂ → OH + ClO（k₁b）的绝对速率数据及产物分支比。对于动力学测量，在单独实验中使用伪一级条件，两种试剂均过量。HO₂通过两种方法产生：通过H原子与O₂的三分子反应以及F原子与H₂O₂的反应。Cl原子通过He中Cl₂的微波放电产生。在通过308nm共振荧光检测之前，HO₂自由基被转化为OH自由基。Cl原子也通过138nm的共振荧光直接检测。测量过量Cl中HO₂的消耗得到k₁a，测量过量HO₂中Cl的消耗得到总速率系数k₁。以阿伦尼乌斯形式表示的来自动力学实验的k₁a和k₁值分别为(1.6±0.2)×10⁻¹¹ exp[(249±34)/T]和(2.8±0.1)×10⁻¹¹ exp[(123±15)/T] cm³ molecule⁻¹ s⁻¹。由于k₁的表达式仅对温度有微弱依赖性，我们报告k₁的与温度无关的值为(4.5±0.4)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹。此外，还可推导出k₁b的阿伦尼乌斯表达式：k₁b = (7.7±0.8)×10⁻¹¹ exp[-(708±29)/T] cm³ molecule⁻¹ s⁻¹。这些k₁a和k₁b的表达式分别在226K≤T≤336K和256K≤T≤296K范围内有效。所引用的误差为单标准偏差水平。对于产物测量，向已知浓度的HO₂中加入过量的Cl，并使反应完全进行。通过红外吸收测定HCl产物浓度，得到236K≤T≤296K温度范围内的k₁a/k₁比值。通过共振荧光测定OH产物浓度，得到226K≤T≤336K温度范围内的k₁b/k₁比值。随后将这两个比值都转换为绝对值。以阿伦尼乌斯形式表示的来自产物实验的k₁a和k₁b值分别为(1.5±0.1)×10⁻¹¹ exp[(222±17)/T]和(10.6±1.5)×10⁻¹¹ exp[-(733±41)/T] cm³ molecule⁻¹ s⁻¹。这些k₁a和k₁b的表达式分别在256K≤T≤296K和226K≤T≤336K范围内有效。动力学和产物数据的结合得出k₁a和k₁b的以下阿伦尼乌斯表达式分别为(1.4±0.3)×10⁻¹¹ exp[(269±58)/T]和(12.7±4.1)×10⁻¹¹ exp[-(801±94)/T] cm³ molecule⁻¹ s⁻¹。数值模拟用于检查动力学和产物实验中二级化学的干扰，并量化检测过程中HO₂→OH转化过程中产生的损失。
