Last Isidore, Jortner Joshua
School of Chemistry, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel.
J Chem Phys. 2004 Nov 1;121(17):8329-42. doi: 10.1063/1.1802554.
In this paper we present a theoretical and computational study of the temporal dynamics and energetics of Coulomb explosion of (CD4)(n) and (CH4)(n) (n=55-4213) molecular heteroclusters in ultraintense (I=10(16)-10(19) W cm(-2)) laser fields, addressing the manifestation of electron dynamics, together with nuclear energetic and kinematic effects on the heterocluster Coulomb instability. The manifestations of the coupling between electron and nuclear dynamics were explored by molecular dynamics simulations for these heteroclusters coupled to Gaussian laser fields (pulse width tau=25 fs), elucidating outer ionization dynamics, nanoplasma screening effects (being significant for I< or =10(17) W cm(-2)), and the attainment of cluster vertical ionization (CVI) (at I=10(17) W cm(-2) for cluster radius R(0)< or =31 A). Nuclear kinematic effects on heterocluster Coulomb explosion are governed by the kinematic parameter eta=q(C)m(A)/q(A)m(C) for (CA(4))(n) clusters (A=H,D), where q(j) and m(j) (j=A,C) are the ionic charges and masses. Nonuniform heterocluster Coulomb explosion (eta >1) manifests an overrun effect of the light ions relative to the heavy ions, exhibiting the expansion of two spatially separated subclusters, with the light ions forming the outer subcluster at the outer edge of the spatial distribution. Important features of the energetics of heterocluster Coulomb explosion originate from energetic triggering effects of the driving of the light ions by the heavy ions (C(4+) for I=10(17)-10(18) W cm(-2) and C(6+) for I=10(19) W cm(-2)), as well as for kinematic effects. Based on the CVI assumption, scaling laws for the cluster size (radius R(0)) dependence of the energetics of uniform Coulomb explosion of heteroclusters (eta=1) were derived, with the size dependence of the average (E(j,av)) and maximal (E(j,M)) ion energies being E(j,av)=aR(0) (2) and E(j,M)=(5a/3)R(0) (2), as well as for the ion energy distributions P(E(j)) proportional to E(j) (1/2); E(j)< or =E(j,M). These results for uniform Coulomb explosion serve as benchmark reference data for the assessment of the effects of nonuniform explosion, where the CVI scaling law for the energetics still holds, with deviations of the a coefficient, which increase with increasing eta. Kinematic effects (for eta>1) result in an isotope effect, predicting the enhancement (by 9%-11%) of E(H,av) for Coulomb explosion of (C(4+)H(4) (+))(eta) (eta=3) relative to E(D,av) for Coulomb explosion of (C(4+)D(4) (+))(eta) (eta=1.5), with the isotope effect being determined by the ratio of the kinematic parameters for the pair of Coulomb exploding clusters. Kinematic effects for nonuniform explosion also result in a narrow isotope dependent energy distribution (of width DeltaE) of the light ions (with DeltaE/E(H,av) approximately 0.3 and DeltaE/E(D,av) approximately 0.4), with the distribution peaking at the high energy edge, in marked contrast with the uniform explosion case. Features of laser-heterocluster interactions were inferred from the analyses of the intensity dependent boundary radii (R(0))(I) and the corresponding average D+ ion energies (E(D,av))(I), which provide a measure for optimization of the cluster size at intensity I for the neutron yield from dd nuclear fusion driven by Coulomb explosion (NFDCE) of these heteroclusters. We infer on the advantage of deuterium containing heteronuclear clusters, e.g., (CD4)(n) in comparison to homonuclear clusters, e.g., (D2)(n/2), for dd NFDCE, where the highly charged heavy ions (e.g., C4+ or C6+) serve as energetic and kinematic triggers driving the D+ ions to a high (10-200 keV) energy domain.
在本文中，我们对超强（(I = 10^{16} - 10^{19}) (W cm^{-2})）激光场中((CD_4)n)和((CH_4)n)（(n = 55 - 4213)）分子异质团簇的库仑爆炸的时间动力学和能量学进行了理论和计算研究，探讨了电子动力学的表现，以及核能量学和运动学效应对异质团簇库仑不稳定性的影响。通过分子动力学模拟研究了这些与高斯激光场（脉冲宽度(\tau = 25) fs）耦合的异质团簇中电子与核动力学之间的耦合表现，阐明了外层电离动力学、纳米等离子体屏蔽效应（对于(I \leq 10^{17}) (W cm^{-2}) 很显著）以及团簇垂直电离（CVI）的实现（对于团簇半径(R_0 \leq 31) Å，在(I = 10^{17}) (W cm^{-2}) 时）。对于((CA_4)n)团簇（(A = H, D)），核运动学效应通过运动学参数(\eta = q_Cm_A / q_Am_C)来控制，其中(q_j)和(m_j)（(j = A, C)）是离子电荷和质量。非均匀异质团簇库仑爆炸（(\eta > 1)）表现为轻离子相对于重离子的超越效应，呈现出两个空间分离的子团簇的膨胀，轻离子在空间分布的外边缘形成外层子团簇。异质团簇库仑爆炸能量学的重要特征源于重离子对轻离子驱动的能量触发效应（对于(I = 10^{17} - 10^{18}) (W cm^{-2}) 为(C^{4 +})，对于(I = 10^{19}) (W cm^{-2}) 为(C^{6 +})）以及运动学效应。基于CVI假设，推导了异质团簇（(\eta = 1)）均匀库仑爆炸能量学对团簇尺寸（半径(R_0)）的依赖关系的标度律，平均（(E{j, av})）和最大（(E{j, M})）离子能量的尺寸依赖关系为(E{j, av} = aR_0^2)和(E_{j, M} = \frac{5a}{3}R_0^2)，以及离子能量分布(P(E_j))与(E_j^{1/2})成正比；(E_j \leq E_{j, M})。这些均匀库仑爆炸的结果作为评估非均匀爆炸效应的基准参考数据，其中能量学的CVI标度律仍然成立，(a)系数的偏差随着(\eta)的增加而增大。运动学效应（对于(\eta > 1)）导致同位素效应，预测((C^{4 +}H_4^+)n)（(\eta = 3)）库仑爆炸的(E{H, av})相对于((C^{4 +}D_4^+)n)（(\eta = 1.5)）库仑爆炸的(E{D, av})增强（(9% - 11%)），同位素效应由一对库仑爆炸团簇的运动学参数之比决定。非均匀爆炸的运动学效应还导致轻离子的窄同位素依赖能量分布（宽度为(\Delta E)）（(\Delta E / E_{H, av} \approx 0.3)且(\Delta E / E_{D, av} \approx 0.4)），该分布在高能边缘处达到峰值，这与均匀爆炸情况形成鲜明对比。通过对强度依赖的边界半径((R_0)I)和相应的平均(D^+)离子能量((E{D, av})I)的分析推断了激光 - 异质团簇相互作用的特征，这为优化这些异质团簇在强度(I)下的团簇尺寸以实现由库仑爆炸驱动的(dd)核聚变产生的中子产率（NFDCE）提供了一种度量。我们推断与同核团簇（例如((D_2){n / 2})）相比，含氘异核团簇（例如((CD_4)_n)）对于(dd) NFDCE的优势，其中高电荷重离子（例如(C^{4 +})或(C^{6 +})）作为能量和运动学触发器将(D^+)离子驱动到高（(10 - 200) keV）能量域。
