Elles Christopher G, Rivera Christopher A, Zhang Yuyuan, Pieniazek Piotr A, Bradforth Stephen E
Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
J Chem Phys. 2009 Feb 28;130(8):084501. doi: 10.1063/1.3078336.
Two-photon absorption (2PA) spectroscopy in the range from 7 to 10 eV provides new insight on the electronic structure of liquid water. Continuous 2PA spectra are obtained via the pump-probe technique, using broadband probe pulses to record the absorption at many wavelengths simultaneously. A preresonance enhancement of the absolute 2PA cross section is observed when the pump-photon energy increases from 4.6 to 6.2 eV. The absorption cross section also depends on the relative polarization of the pump and probe photons. The variation of the polarization ratio across the spectrum reveals a detailed picture of the 2PA and indicates that at least four different transitions play a role below 10 eV. Theoretical polarization ratios for the isolated molecule illustrate the value of the experimental polarization measurement in deciphering the 2PA spectrum and provide the framework for a simple simulation of the liquid spectrum. A more comprehensive model goes beyond the isolated molecule picture and connects the 2PA spectrum with previous one-photon absorption, photoelectron, and x-ray absorption spectroscopy measurements of liquid water. Previously unresolved, overlapping transitions are assigned for the first time. Finally, the electronic character of the vertical excited states is related to the energy-dependent ionization mechanism of liquid water.
7至10电子伏特范围内的双光子吸收（2PA）光谱为液态水的电子结构提供了新的见解。通过泵浦-探测技术获得连续的2PA光谱，使用宽带探测脉冲同时记录多个波长处的吸收情况。当泵浦光子能量从4.6电子伏特增加到6.2电子伏特时，观察到绝对2PA截面的预共振增强。吸收截面还取决于泵浦光子和探测光子的相对偏振。整个光谱中偏振比的变化揭示了2PA的详细情况，并表明在10电子伏特以下至少有四种不同的跃迁起作用。孤立分子的理论偏振比说明了实验偏振测量在解读2PA光谱中的价值，并为液态光谱的简单模拟提供了框架。一个更全面的模型超越了孤立分子的情况，将2PA光谱与之前液态水的单光子吸收、光电子和X射线吸收光谱测量联系起来。以前未解决的重叠跃迁首次得到了归属。最后，垂直激发态的电子特性与液态水的能量依赖电离机制有关。
