Impulsive solvent heating probed by picosecond x-ray diffraction
Impulsive solvent heating probed by picosecond x-ray diffraction
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DOI：
10.1063/1.2176617
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发表时间：
2006-03-28
影响因子：
4.4
通讯作者：
Ihee, H
中科院分区：
文献类型：
--
作者：
Cammarata, M;Lorenc, M;Ihee, H
激光激发溶液的时间分辨衍射信号有三个主要成分：纯溶质项、溶质-溶剂交叉项和纯溶剂项。最后一项对主体溶剂的热力学状态非常敏感，在化学反应过程中，由于吸收光的溶质分子向周围的溶剂分子传递能量，以及随后与散射体积周围的环境达到平衡的弛豫，主体溶剂的热力学状态可能会发生变化。液体的体积膨胀系数α通常类似于1x10(-3)K-1，大约是固体的1000倍。因此，溶剂散射是一种非常灵敏的在线温度计。到目前为止，散射X射线信号的分解一直得到了分子动力学(MD)模拟的帮助，分子动力学模拟是一种能够模拟溶剂响应以及用于数据分析的溶质项和溶质/溶剂交叉项的方法。在这里，我们提出了一种适用于大多数含氢溶剂的实验方法，它可以直接测量溶剂对瞬时温升的响应。用波长为1.5和1.7微米的近红外飞秒激光脉冲激发了甲醇溶液中OH伸缩和CH3不对称伸缩的泛音模式，并用100ps同步加速器的X射线脉冲探测了激发的CH3OH*子集向本体的热传递和随后的热膨胀所引起的流体动力学。时间分辨数据使我们能够提取两个关键差异：恒定密度下温度变化引起的溶剂衍射变化，类似于100ps的非常短的时间延迟，以及恒温下密度变化引起的项。后一术语在以后的时间变得相关，类似于1亩S，当液体体积膨胀以适应其在常压下的新温度时。这两个项是流体动力学状态方程的主要组成部分，它们是化学反应期间溶剂反应的自洽重建所必需的。我们将实验溶剂项与分子动力学模拟得到的溶剂项进行了比较。当应用于C2H4I2在甲醇中溶解的时间分辨数据时，使用实验确定的溶剂差大大提高了全局拟合的质量。
The time-resolved diffraction signal from a laser-excited solution has three principal components: the solute-only term, the solute-solvent cross term, and the solvent-only term. The last term is very sensitive to the thermodynamic state of the bulk solvent, which may change during a chemical reaction due to energy transfer from light-absorbing solute molecules to the surrounding solvent molecules and the following relaxation to equilibrium with the environment around the scattering volume. The volume expansion coefficient alpha for a liquid is typically similar to 1x10(-3) K-1, which is about 1000 times greater than for a solid. Hence solvent scattering is a very sensitive on-line thermometer. The decomposition of the scattered x-ray signal has so far been aided by molecular dynamics (MD) simulations, a method capable of simulating the solvent response as well as the solute term and solute/solvent cross terms for the data analysis. Here we present an experimental procedure, applicable to most hydrogen containing solvents, that directly measures the solvent response to a transient temperature rise. The overtone modes of OH stretching and CH3 asymmetric stretching in liquid methanol were excited by near-infrared femtosecond laser pulses at 1.5 and 1.7 mu m and the ensuing hydrodynamics, induced by the transfer of heat from a subset of excited CH3OH* to the bulk and the subsequent thermal expansion, were probed by 100 ps x-ray pulses from a synchrotron. The time-resolved data allowed us to extract two key differentials: the change in the solvent diffraction from a temperature change at constant density, seen at a very short time delay similar to 100 ps, and a term from a change in density at constant temperature. The latter term becomes relevant at later times similar to 1 mu s when the bulk of liquid expands to accommodate its new temperature at ambient pressure. These two terms are the principal building blocks in the hydrodynamic equation of state, and they are needed in a self-consistent reconstruction of the solvent response during a chemical reaction. We compare the experimental solvent terms with those from MD simulations. The use of experimentally determined solvent differentials greatly improved the quality of global fits when applied to the time-resolved data for C2H4I2 dissolved in methanol.