ELECTRONIC EXCITATIONS IN FINITE AND INFINITE POLYENES
ELECTRONIC EXCITATIONS IN FINITE AND INFINITE POLYENES
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DOI：
10.1103/physrevb.36.4337
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发表时间：
1987-09-15
影响因子：
3.7
通讯作者：
SCHULTEN, K
中科院分区：
文献类型：
--
作者：
TAVAN, P;SCHULTEN, K
我们研究长多烯中的电子激发，即一维强相关电子系统，它既不是无限的，也不是小的。在Hubbard和parier - parr - people （PPP）模型中，通过多参考双激励展开来描述激励[P]。Tavan和K. Schulten， J. Chem。物理学报，85,6602(1986)]。我们发现量子化的“跃迁”动量可以分配给有限链中的电子激励。这些动量将有限链的激发能与无限链的色散关系联系起来，也就是说，它们在有限系统和无限系统之间架起了桥梁。一个关键的结果是：含N个碳原子的多烯的激发能E可以用公式E β= Δ E 0 β+ α β k (N) q, q= 1,2，…式中，β表示激发类，Δ E 0 β表示无限系统中的能隙[α β k (N)> 0], k (N)表示基本跃迁动量。测定了交替和非交替多烯共价和离子激发的参数Δ E 0 β和α β。共价激发是三重态激发T的组合，即T， TT， TTT,....在无限多烯中，例如聚乙炔或聚二乙炔中，最低的单重态激发是TT态。现有的证据证明，这些状态可以分离成独立的三重态。TT态的键结构是中性孤子-反孤子对。长多烯中TT态的能级密度足够高，可以解离成单独的孤子。
We study electronic excitations in long polyenes, ie, in one-dimensional strongly correlated electron systems which are neither infinite nor small. The excitations are described within Hubbard and Pariser-Parr-Pople (PPP) models by means of a multiple-reference double-excitation expansion [P. Tavan and K. Schulten, J. Chem. Phys. 85, 6602 (1986)]. We find that quantized ‘‘transition’’momenta can be assigned to electronic excitations in finite chains. These momenta link excitation energies of finite chains to dispersion relations of infinite chains, ie, they bridge the gap between finite and infinite systems. A key result is the following: Excitation energies E in polyenes with N carbon atoms are described very accurately by the formula E β= Δ E 0 β+ α β k (N) q, q= 1, 2,..., where β denotes the excitation class, Δ E 0 β the energy gap in the infinite system [α β k (N)> 0], and k (N) the elementary transition momentum. The parameters Δ E 0 β and α β are determined for covalent and ionic excitations in alternating and nonalternating polyenes. The covalent excitations are combinations of triplet excitations T, ie, T, TT, TTT,.... The lowest singlet excitations in the infinite polyene, eg, in polyacetylene or polydiacetylene, are TT states. Available evidence proves that these states can dissociate into separate triplets. The bond structure of TT states is that of a neutral soliton-antisoliton pair. The level density of TT states in long polyenes is high enough to allow dissociation into separate solitons.