Liu Xinyao, Amini Kasra, Steinle Tobias, Sanchez Aurelien, Shaikh Moniruzzaman, Belsa Blanca, Steinmetzer Johannes, Le Anh-Thu, Moshammer Robert, Pfeifer Thomas, Ullrich Joachim, Moszynski Robert, Lin C D, Gräfe Stefanie, Biegert Jens
ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain.
Institute of Physical Chemistry, Friedrich-Schiller University, 07743 Jena, Germany.
J Chem Phys. 2019 Jul 14;151(2):024306. doi: 10.1063/1.5100520.
Observing changes in molecular structure requires atomic-scale Ångstrom and femtosecond spatio-temporal resolution. We use the Fourier transform (FT) variant of laser-induced electron diffraction (LIED), FT-LIED, to directly retrieve the molecular structure of HO with picometer and femtosecond resolution without a priori knowledge of the molecular structure nor the use of retrieval algorithms or ab initio calculations. We identify a symmetrically stretched HO field-dressed structure that is most likely in the ground electronic state. We subsequently study the nuclear response of an isolated water molecule to an external laser field at four different field strengths. We show that upon increasing the laser field strength from 2.5 to 3.8 V/Å, the O-H bond is further stretched and the molecule slightly bends. The observed ultrafast structural changes lead to an increase in the dipole moment of water and, in turn, a stronger dipole interaction between the nuclear framework of the molecule and the intense laser field. Our results provide important insights into the coupling of the nuclear framework to a laser field as the molecular geometry of HO is altered in the presence of an external field.
观察分子结构的变化需要原子尺度的埃（Å）和飞秒级的时空分辨率。我们使用激光诱导电子衍射（LIED）的傅里叶变换（FT）变体，即FT-LIED，在无需事先了解分子结构，也无需使用检索算法或从头计算的情况下，直接以皮米和飞秒分辨率获取HO的分子结构。我们识别出一种对称拉伸的HO场修饰结构，它很可能处于基电子态。随后，我们研究了一个孤立水分子在四种不同场强下对外部激光场的核响应。我们表明，当激光场强从2.5 V/Å增加到3.8 V/Å时，O-H键进一步拉伸，分子略有弯曲。观察到的超快结构变化导致水的偶极矩增加，进而导致分子的核框架与强激光场之间的偶极相互作用增强。我们的结果为在外部场存在的情况下HO的分子几何结构发生变化时，核框架与激光场的耦合提供了重要见解。
