Department of Chemistry, The Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
J Chem Phys. 2012 Jul 14;137(2):024704. doi: 10.1063/1.4733720.
Electronic excitation of metal by intense laser pulses stimulates nuclear motions of adsorbates through nonadiabatic coupling, resulting in diffusion and desorption of adsorbates. These processes take place via precursor states: adsorbates whose vibrational modes with respect to substrate are highly excited. This paper reports the dynamics of precursor states of CO on Pt(111) probed by use of infrared-visible sum frequency generation with phase-sensitive detection, which allows us to obtain the second-order nonlinear susceptibility and thus the vibrational response function. Without pump pulses at 400 nm, the inverse Fourier transformation of the vibrational response function reveals a free induction decay of vibrational polarization of C-O stretching created by a short infrared pulse. The free induction decay is perturbed when an intense 400-nm pump pulse following the infrared pulse is irradiated, because diffusion and desorption of CO are induced by the pump pulse. The time evolution of instantaneous C-O stretching frequency retrieved from the perturbed free induction decay shows a redshift followed by a rapid reverse shift when the fluence of pump pulse is high enough to desorb CO. This indicates that the frustrated modes of CO is first substantially excited and then the parallel momentum of CO is converted to the normal one through mutual collisions, leading to substantial excitation of the external stretching mode of CO.
强激光脉冲对金属的电子激发通过非绝热耦合刺激吸附物的核运动，导致吸附物的扩散和脱附。这些过程通过前驱态发生：吸附物的相对于衬底的振动模式被高度激发。本文通过使用相位敏感红外可见和频产生探测，报告了 Pt(111)上 CO 的前驱态动力学，这允许我们获得二阶非线性极化率，从而获得振动响应函数。在没有 400nm 泵浦脉冲的情况下，红外脉冲产生的 C-O 伸缩振动极化的自由感应衰减的傅里叶逆变换揭示了自由感应衰减。当紧随红外脉冲的强 400nm 泵浦脉冲照射时，自由感应衰减受到干扰，因为 CO 的扩散和脱附是由泵浦脉冲诱导的。从受扰的自由感应衰减中检索到的瞬时 C-O 伸缩频率的时间演化表明，当泵浦脉冲的强度足以脱附 CO 时，会出现红移，然后迅速反向移动。这表明 CO 的受挫模式首先被显著激发，然后 CO 的平行动量通过相互碰撞转化为正常动量，导致 CO 的外部伸缩模式的显著激发。
