Carbajo Paula Gorrotxategi, Smith Shona C, Holloway Anne-Louise, Smith Carina A, Pope Francis D, Shallcross Dudley E, Orr-Ewing Andrew J
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom.
J Phys Chem A. 2008 Dec 4;112(48):12437-48. doi: 10.1021/jp8070508.
Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, Phi(HCO), have been measured for the tropospherically relevant range of wavelengths (lambda) between 300 and 330 nm. The HCO photoproduct was directly detected by using a custom-built, combined ultra-violet (UV) absorption and cavity ring down (CRD) detection spectrometer. This instrument was previously employed for high-resolution (spectral resolution approximately 0.0035 nm) measurements of absorption cross-sections of HCHO, sigma(HCHO)(lambda), and relative HCO quantum yields. Absolute Phi(HCO) values were measured at seven wavelengths, lambda = 303.70, 305.13, 308.87, 314.31, 320.67, 325.59, and 329.51 nm, using an independent calibration technique based on the simultaneous UV photolysis of HCHO and Cl(2). These Phi(HCO) measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for Phi(HCO). The absolute Phi(HCO)(lambda) determinations and previously measured sigma(HCHO)(lambda) were used to scale an extensive set of relative HCO yield measurements. The outcome of this procedure is a full suite of data for the product of the absolute radical quantum yield and HCHO absorption cross-section, Phi(HCO)(lambda)sigma(HCHO)(lambda), at wavelengths from 302.6 to 331.0 nm with a wavelength resolution of 0.005 nm. This product of photochemical parameters is combined with high-resolution solar photon flux data to calculate the integrated photolysis rate of HCHO to the radical (H + HCO) channel, J(HCO). Comparison with the latest NASA-JPL recommendations, reported at 1 nm wavelength resolution, suggests an increased J(HCO) of 25% at 0 degrees solar zenith angle (SZA) increasing to 33% at high SZA (80 degrees). The differences in the calculated photolysis rate compared with the current HCHO data arise, in part, from the higher wavelength resolution of the current data set and highlight the importance of using high-resolution spectroscopic techniques to achieve a complete and accurate picture of HCHO photodissociation processes. All experimental Phi(HCO)(lambda)sigma(HCHO)(lambda) data are available for the wavelength range 302.6-331.0 nm (at 294 and 245 K and under 200 Torr of N(2) bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of Phi(H(2)+CO)(lambda)sigma(HCHO)(lambda) for the molecular (H(2) + CO) photofragmentation channel, produced using the measured Phi(HCO)(lambda) sigma(HCHO)(tau) values, are also provided at 0.1 and 1.0 nm resolution.
已测量了甲醛（HCHO）光解产生自由基（H + HCO）通道的绝对量子产率Phi（HCO），测量范围为对流层相关的波长（λ）300至330nm。通过使用定制的紫外（UV）吸收和腔衰荡（CRD）检测组合光谱仪直接检测HCO光产物。该仪器先前用于高分辨率（光谱分辨率约为0.0035nm）测量HCHO的吸收截面σ（HCHO）（λ）以及相对HCO量子产率。使用基于HCHO和Cl₂同时紫外光解的独立校准技术，在七个波长λ = 303.70、305.13、308.87、314.31、320.67、325.59和329.51nm处测量了绝对Phi（HCO）值。这些Phi（HCO）测量结果显示，作为波长函数的变化比美国国家航空航天局 - 喷气推进实验室（NASA - JPL）当前对Phi（HCO）的建议更大。绝对Phi（HCO）（λ）测定值和先前测量的σ（HCHO）（λ）用于缩放一组广泛的相对HCO产率测量值。该程序的结果是在波长范围302.6至331.0nm、波长分辨率为0.005nm时，得到了一套完整的绝对自由基量子产率与HCHO吸收截面的乘积Phi（HCO）（λ）σ（HCHO）（λ）数据。该光化学参数乘积与高分辨率太阳光子通量数据相结合，以计算HCHO在自由基（H + HCO）通道的积分光解速率J（HCO）。与以1nm波长分辨率报告的最新NASA - JPL建议相比，在太阳天顶角（SZA）为0°时，J（HCO）增加了25％，在高SZA（80°）时增加到33％。与当前HCHO数据相比，计算出的光解速率差异部分源于当前数据集的更高波长分辨率，并突出了使用高分辨率光谱技术以全面准确地了解HCHO光解离过程的重要性。所有实验性的Phi（HCO）（λ）σ（HCHO）（λ）数据在波长范围302.6 - 331.0nm（在294和245K以及200托N₂浴气下）作为支持信息提供，波长分辨率分别为0.005、0.1和1.0nm。还以0.1和1.0nm分辨率提供了使用测量的Phi（HCO）（λ）σ（HCHO）（τ）值生成的分子（H₂ + CO）光解离通道的等效数据集Phi（H₂ + CO）（λ）σ（HCHO）（λ）。
