Shortt B J, Van Der Burgt P J M, Giammanco F, Slevin J A, Lanigan W
Department of Experimental Physics, National University of Ireland, Maynooth, County Kildare, Ireland.
Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Oct;66(4 Pt 2):046411. doi: 10.1103/PhysRevE.66.046411. Epub 2002 Oct 16.
This paper presents the results of an experimental investigation into collective effects in the transient plasma formed by multiphoton ionization of atomic deuterium with a pulsed laser. The laser wavelength is varied in a narrow range around 243 nm, so that the photoionization is resonant with the metastable 2S(1/2) state. The ion yield, the ion time-of-flight spectra, and the yield of Lyman-alpha photons have been measured as a function of laser intensity (from 1 to 340 MW/cm(2)) and laser detuning around the 1S(1/2)-2S(1/2) two-photon resonance. During and shortly after the laser pulse, collective effects resulting from the mutual interaction of the photoelectrons and the ions affect the spatial and temporal distribution of the ions. Because of the near-degeneracy of the 2S(1/2), 2P(1/2), and 2P(3/2) states, the resonant multiphoton ionization is affected by the Stark mixing of these states in the collective field. As a result, the time-dependent yields of ions and of Lyman-alpha photons are modulated by the interplay of the multiphoton ionization of the atoms and the collective effects in the plasma. From the measurements it is deduced that collective effects are important above a critical charge density of 3x10(8) ions/cm(3). An asymmetry is observed in the line profile of the total ion yield as a function of laser detuning. This asymmetry is interpreted to be due to the effect of the collective field upon the intermediate resonant 2S(1/2) state of the photoionization process.
本文介绍了一项实验研究的结果，该研究针对用脉冲激光对原子氘进行多光子电离所形成的瞬态等离子体中的集体效应。激光波长在243nm附近的狭窄范围内变化，以使光电离与亚稳2S(1/2)态共振。测量了离子产率、离子飞行时间谱以及莱曼α光子产率与激光强度（从1到340MW/cm²）和围绕1S(1/2)-2S(1/2)双光子共振的激光失谐的函数关系。在激光脉冲期间及之后不久，光电子与离子的相互作用所产生的集体效应会影响离子的空间和时间分布。由于2S(1/2)、2P(1/2)和2P(3/2)态近乎简并，共振多光子电离会受到这些态在集体场中的斯塔克混合的影响。结果，离子和莱曼α光子的时间相关产率会受到原子多光子电离与等离子体中集体效应相互作用的调制。从测量结果推断，在临界电荷密度3×10⁸离子/cm³以上，集体效应很重要。观察到总离子产率的线轮廓随激光失谐存在不对称性。这种不对称性被解释为是由于集体场对光电离过程的中间共振2S(1/2)态的影响所致。
