Softening of the symmetric breathing mode in gold particles by laser-induced heating
Softening of the symmetric breathing mode in gold particles by laser-induced heating
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DOI：
10.1021/jp0276092
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发表时间：
2003-07-31
影响因子：
3.3
通讯作者：
Sader, JE
中科院分区：
文献类型：
--
作者：
Hartland, GV;Hu, M;Sader, JE
利用时间分辨光谱技术研究了不同强度泵浦激光脉冲作用下球形金粒子的对称呼吸模式。结果表明，当抽运激光功率为2-3 μ J/pulse时，呼吸模的周期随抽运激光功率的增加而增加。这归因于由于激光诱导加热而导致的颗粒的弹性性质的软化。在泵浦激光功率大于类似于3uJ/脉冲时，周期与强度的数据变平。这最有可能是由于在高泵浦强度下样品吸收饱和引起的。在这些实验中研究的颗粒相对较大-直径在50和100 nm之间。选择大颗粒是因为它们具有较慢的散热时间，这意味着颗粒中的温度在实验过程中更好地定义。粒子的温度估计从激光功率密度，金的热容量，并在泵浦波长的吸收假设样品服从比尔定律。这使我们能够将实验结果与基于已知的黄金温度依赖性弹性常数的周期与温度的计算进行比较。实验和计算的周期是非常一致的黄金的熔点，这是预测发生在类似3 μ J/脉冲（大约相同的点，周期与强度数据变平）。在较高的功率下，测得的周期明显短于熔融金颗粒的预测。这意味着我们可以接近金的熔点，但我们不能完全熔化粒子。拍频信号的阻尼分析表明，我们可以在高激光功率下形成固核/液壳粒子。
The symmetric breathing mode in spherical gold particles has been examined by time-resolved spectroscopy using different intensity pump laser pulses. The results show that the period of the breathing mode increases as the pump laser power increases, up to pump laser powers of 2-3 muJ/pulse. This is attributed to softening of the elastic properties of the particles due to laser-induced heating. At pump laser powers greater than similar to3uJ/pulse the period versus intensity data flatten off. This most likely arises from saturation of the sample absorption at high pump intensities. The particles studied in these experiments were relatively large-between 50 and 100 nm in diameter. Large particles were chosen because they have slower heat dissipation times, which means that the temperature in the particles is better defined during the course of the experiment. The particle temperatures were estimated from the laser power density, the heat capacity of gold, and the absorption at the pump wavelength-assuming that the samples obey Beer's law. This allows us to compare the experimental results to calculations of the period versus temperature, which are based on the known temperature-dependent elastic constants of gold. The experimental and calculated periods are in excellent agreement up to the melting point of gold, which is predicted to occur at similar to3 muJ/pulse (approximately the same point where the period versus intensity data flatten off). At higher powers the measured periods are significantly shorter than those predicted for molten gold particles. This implies that we can approach the melting point of gold, but we cannot completely melt the particles. Analysis of the damping of the beat signal indicates that we may form solid-core/liquid-shell particles at high laser powers.