Department of Chemistry, National University of Singapore, Singapore 117543.
J Phys Chem A. 2011 Apr 28;115(16):3820-6. doi: 10.1021/jp108176h. Epub 2011 Jan 11.
The electron dynamics of gold nanorods were systematically studied by using femtosecond transient absorption experiments. Two different excitation wavelengths (400 and 800 nm) have been used as the excitation sources to selectively excite transverse and longitudinal modes. The transient absorption spectra were found to be strongly dependent on the excitation wavelength and fluence. Laser pulses of 800 nm excite the longitudinal mode directly, which cause an increase in the electronic temperatures and subsequent broadening and bleaching of both the longitudinal and transverse modes. Pulses of 400 nm excite both the transverse and longitudinal modes simultaneously. At low excitation fluences, the energy is distributed into two modes according to their steady state extinction coefficients, under which the transient spectra are similar to those under excitation at 800 nm. However, as the excitation fluence exceeds a threshold, the bleaching of the longitudinal plasmon band saturates and the input energies mainly flow to the transverse mode. As a result, the bleaching of the transverse mode increases rapidly. The electron-phonon dynamics show a strong correlation with the bleaching amplitude. We have tried to explain the results with a consistent picture: the bleaching amplitude and electron-phonon relaxation time are directly related to energy distribution into different modes, which are excitation wavelength and fluence dependent. Our studies help to clarify the seemingly inconsistent results in the previous studies by different research groups.
金纳米棒的电子动力学通过飞秒瞬态吸收实验进行了系统研究。使用两个不同的激发波长（400nm 和 800nm）作为激发源，分别选择性地激发横向和纵向模式。瞬态吸收光谱强烈依赖于激发波长和光强。800nm 的激光脉冲直接激发纵向模式，导致电子温度升高，从而导致纵向和横向模式的展宽和漂白。400nm 的脉冲同时激发横向和纵向模式。在低激发光强下，能量根据其稳态消光系数分布到两个模式中，在此条件下，瞬态光谱与在 800nm 激发下的光谱相似。然而，随着激发光强超过一个阈值，纵向等离子体带的漂白饱和，输入能量主要流向横向模式。因此，横向模式的漂白迅速增加。电子-声子动力学与漂白幅度表现出很强的相关性。我们尝试用一致的图像来解释结果：漂白幅度和电子-声子弛豫时间与不同模式的能量分布直接相关，而能量分布又与激发波长和光强有关。我们的研究有助于澄清先前不同研究小组的研究中看似不一致的结果。
