双光子过程探测原子团簇中的电子四极态
Exploration of electronic quadrupole states in atomic clusters by two-photon processes
摘要：
我们分析了特殊的双光子过程作为探索自由小变形原子团簇中电子四极态的可能手段。分析是在含时局域密度近似（TDLDA）。结果表明，直接双光子布居（DTP）和非共振受激拉曼（ORSR）散射可以有效地用于激发四极子态的高频（四极等离子体激元）和低频（红外）区域，分别。在ORSR中，孤立的偶极粒子-空穴态以及偶极等离子体激元的尾部可以作为中间态。结合DTP和ORSR同时研究低频和高频四极杆是最有效的。飞秒脉冲强度$I = 2\cdot 10^{10} - 2\cdot 10^{11} W/cm^2$和脉冲持续时间$T = 200 - 500$ fs被认为是最佳的。由于低位四极子态是由单个电子-空穴对控制的，它们的能量与空穴态的光电子数据相结合，可以得到费米能级以上的电子谱，从而大大扩展了我们对团簇单粒子谱的认识。此外，所开发的计划允许估计的四极子状态的寿命。
英文摘要：
We analyze particular two-photon processes as possible means to explore electronic quadrupole states in free small deformed atomic clusters. The analysis is done in the time-dependent local density approximation (TDLDA). It is shown that the direct two-photon population (DTP) and off-resonant stimulated Raman (ORSR) scattering can be effectively used for excitation of the quadrupole states in high-frequency (quadrupole plasmon) and low-frequency (infrared) regions, respectively. In ORSR, isolated dipole particle-hole states as well as the tail of the dipole plasmon can serve as an intermediate state. A simultaneous study of low- and high-frequency quadrupoles, combining DTP and ORSR, is most effective. Femtosecond pulses with intensities $I = 2\cdot 10^{10} - 2\cdot 10^{11} W/cm^2$ and pulse durations $T = 200 - 500$ fs are found to be optimal. Since the low-lying quadrupole states are dominated by one single electron-hole pair, their energies, being combined with the photoelectron data for hole states, allow to get the electron spectrum above the Fermi level and thus greatly extend our knowledge on the single particle spectra of clusters. Besides, the developed schemes allow to estimate the lifetime of the quadrupole states.
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