Amaral G A, Ausfelder F, Izquierdo J G, Rubio-Lago L, Bañares L
Departamento de Quimica Fisica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.
J Chem Phys. 2007 Jan 14;126(2):024301. doi: 10.1063/1.2409925.
The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).
通过对CH₃碎片的速度成像技术，研究了在CH₃SH光解离的CH₃(X²A₁)+SH(X²Π)通道中，在第一个¹¹A″←X¹A′和第二个²¹A″←X¹A′吸收带的几个波长下的情况。在所研究的所有光解波长的图像中，都观察到了快速的高度各向异性（β=-1±0.1）的CH₃(X²A₁)信号，这与母体分子中C-S键断裂导致的从电子激发态直接解离过程一致。通过对CH₃图像的分析，提取了SH(X²Π)反碎片的振动布居。在第二个吸收带中，由于在从束缚的²¹A″第二激发态到非束缚的¹¹A″第一激发态的跃迁过程中作用的力，SH碎片以反转的振动分布形成。随着光解波长的减小，SH自由基的内能增加。在通过第一激发态进行光解离的情况下，CH₃的直接产生使得SH反碎片几乎没有内部激发。此外，在对应于激发到¹¹A″态的较长光解波长下，观察到了一个较慢的各向异性CH₃通道（β=-0.8±0.1），这与两步光解离过程一致，其中第一步对应于通过CH₃SH中S-H键的断裂产生CH₃S(X²E)自由基，随后新生的CH₃S自由基光解离产生CH₃(X²A₁)+S(X³P₀,₁,₂)。
