Karpichev Boris, Edwards Laura W, Wei Jie, Reisler Hanna
Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.
J Phys Chem A. 2008 Jan 24;112(3):412-8. doi: 10.1021/jp077213w. Epub 2008 Jan 1.
The electronic spectroscopy and photodissociation dynamics of the CH3CHOH radical in the region 19,400-37,000 cm(-1) (515-270 nm) were studied in a molecular beam using resonance-enhanced multiphoton ionization (REMPI), photofragment yield spectroscopy, and time-of-flight (TOF) spectra of H and D fragments. The onset of the transition to the Rydberg 3s state, the lowest excited state, is estimated at 19,600 +/- 100 cm(-1). The 3s state dissociates fast, and no REMPI spectrum is observed. The origin band of the transition to the 3pz state, identified by 2 + 2 REMPI, lies at 32,360 +/- 70 cm(-1), and a vibrational progression in the C-O stretch is assigned. When exciting CH3CHOH near the onset of the unstructured absorption to the 3s state, only one peak is observed in the center-of-mass (c.m.) translational energy (Et) distribution obtained by monitoring H photofragments. The measured recoil anisotropy parameter beta = -0.7 +/- 0.1 is typical of a perpendicular transition. The O-H bond energy is determined to be D0 = 1.1 eV +/- 0.1 eV. At excitation energies >31,200 cm(-1) (3.87 eV) a second, low Et peak appears in the c.m. Et distribution with beta approximately 0. Its relative intensity increases with excitation energy, but its beta value does not change. In contrast, the beta value of the higher Et peak becomes monotonically less negative at higher excitation energies, decreasing to -0.2 +/- 0.1 at 35,460 cm(-1). By comparison of the TOF distributions of the isotopologs CH3CHOH, CH3CHOD, and CD3CHOH, it is concluded that two major product channels dominate the photodissociation, one leading to acetaldehyde and the other to vinyl alcohol (enol) products. There is no indication of isomerization to ethoxy. It appears that separate conical intersections lead to the observed channels, and the dynamics at the conical intersection and the exit channel deposit much of the available energy into internal energy of the products.
在分子束中，利用共振增强多光子电离（REMPI）、光碎片产率光谱以及H和D碎片的飞行时间（TOF）光谱，研究了CH₃CHOH自由基在19,400 - 37,000 cm⁻¹（515 - 270 nm）区域的电子光谱和光解离动力学。向最低激发态里德堡3s态跃迁的起始点估计为19,600 ± 100 cm⁻¹。3s态快速解离，未观察到REMPI光谱。通过2 + 2 REMPI确定的向3pz态跃迁的起始带位于32,360 ± 70 cm⁻¹，并归属了C - O伸缩振动的振动序列。当在无结构吸收向3s态的起始点附近激发CH₃CHOH时，通过监测H光碎片获得的质心（c.m.）平动能量（Et）分布中仅观察到一个峰。测得的反冲各向异性参数β = -0.7 ± 0.1是垂直跃迁的典型值。O - H键能确定为D₀ = 1.1 eV ± 0.1 eV。在激发能量>31,200 cm⁻¹（3.87 eV）时，在c.m. Et分布中出现第二个低Et峰，β约为0。其相对强度随激发能量增加，但β值不变。相反，较高Et峰的β值在较高激发能量下单调地变得不那么负，在35,460 cm⁻¹时降至-0.2 ± 0.1。通过比较同位素变体CH₃CHOH、CH₃CHOD和CD₃CHOH的TOF分布，得出两个主要产物通道主导光解离，一个导致乙醛，另一个导致乙烯醇（烯醇）产物。没有异构化为乙氧基的迹象。似乎不同的锥形交叉点导致了观察到的通道，并且锥形交叉点和出射通道处的动力学将大部分可用能量沉积到产物的内能中。
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