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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Subpicosecond Ring Opening of 7-Dehydrocholesterol Studied by Ultrafast Spectroscopy
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Subpicosecond Ring Opening of 7-Dehydrocholesterol Studied by Ultrafast Spectroscopy

机译：超快光谱研究7-脱氢胆固醇的亚皮秒开环

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摘要

The photoinitiated electrocyclic ring opening of 7-dehydrocholesterol (DHC) to form previtamin D_3 was studied in methanol solution with femtosecond transient absorption spectroscopy. The molecules were excited at 265 nm and probed at 20 wavelengths ranging from 261 to 650 nm. A sizable absorption signal decaying on a 0.95 +- 0.10 ps time scale was seen at all probe wavelengths from 470 to 650 nm. This absorption is assigned to an S_1 -> S_n transition that peaks near 470 nm. Thus, the S_1 state has a lifetime of 0.95 ps, while the initially prepared S_2 state has a lifetime <= 0.1 ps. Data obtained with probe wavelengths in the ultraviolet yielded a subpicosecond (0.4-1.0 ps) component corresponding to the lifetime of the S_1 state. This data set also exhibits a 1-5 ps wavelength-dependent component assigned to vibrational cooling, and a 100 +- 20 ps component assigned to rotational isomerization of the photoproduct to achieve a thermal distribution of rotamers. No further dynamics are seen for time delays of up to 1 ns. Agreement between the static difference spectrum and the absorption difference observed in the kinetic data at 300 ps indicates that there are no significant longer-time dynamics.
机译：用飞秒瞬态吸收光谱法研究了甲醇溶液中7-脱氢胆固醇（DHC）光引发的电开环以形成维生素原D_3。分子在265 nm处激发，并在261至650 nm的20个波长处进行探测。在从470至650 nm的所有探针波长下，都看到了以0.95±0.10 ps时标衰减的可观吸收信号。该吸收被分配给峰值在470 nm附近的S_1-> S_n跃迁。因此，S_1状态的寿命为0.95 ps，而初始准备的S_2状态的寿命为<= 0.1 ps。用紫外线中的探针波长获得的数据产生了相当于S_1状态寿命的亚皮秒（0.4-1.0 ps）分量。该数据集还表现出分配给振动冷却的1-5 ps波长相关分量，以及分配给光产品的旋转异构化以实现旋转异构体热分布的100 + -20 ps分量。对于高达1 ns的时间延迟，没有进一步的动态。在300 ps的动力学数据中观察到的静态差异谱与吸收差异之间的一致性表明没有明显的较长时间的动力学。

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