Department of Chemistry and Kenneth S. Pitzer Center for Theoretical Chemistry, University of California, Berkeley, California 94720, USA.
J Phys Chem A. 2011 Jun 30;115(25):7220-33. doi: 10.1021/jp2003003. Epub 2011 Jun 8.
We present a theoretical analysis of the electronic absorption spectra of tetracene in (4)He droplets based on many-body quantum simulations. Using the path integral ground state approach, we calculate one- and two-body reduced density matrices of the most strongly localized He atoms near the molecule surface and use these to investigate the helium ground-state quantum coherence and correlations when tetracene is in its electronic ground and excited states. We identify a trio of quasi-one-dimensional, strongly localized atoms adsorbed along the long axis of the molecule that show some quantum coherence among themselves but far less with the remaining solvating helium. We evaluate the single-particle natural orbitals of the localized He atoms by diagonalization of the one-body density matrix and use these to construct single- and many-particle solvating helium basis states with which the zero-phonon spectral features of the tetracene-(4)He(N) absorption spectrum are then calculated. The absorption spectrum resulting from the three-body density matrix for the strongly bound trio of helium atoms is in very good agreement with the experimental data, accounting quantitatively for the anomalous splitting of the zero-phonon line [Hartmann, M.; Lindinger, A.; Toennies, J. P.; Vilesov, A. F. Chem. Phys. 1998, 239, 139; Krasnokutski, S.; Rouillé, G.; Huisken, F. Chem. Phys. Lett. 2005, 406, 386]. Our results indicate that the combination of strong localization and the quasi-one-dimensional nature of trios of helium atoms adsorbed along the long axis of tetracene leads to a quantum coherent, yet highly correlated ground state for the helium density closest to the molecule. The spectroscopic analysis shows that this feature accounts quantitatively for the anomalous splittings and hitherto unexplained fine structure observed in the absorption spectra of tetracene and suggests that it may be responsible for the corresponding zero-phonon splittings in other quasi-one-dimensional planar aromatic molecules.
我们基于多体量子模拟，对四并苯在（4）He 液滴中的电子吸收光谱进行了理论分析。我们使用路径积分基态方法，计算了分子表面附近最强烈局域化的 He 原子的单粒子和双粒子约化密度矩阵，并利用这些矩阵研究了四并苯处于电子基态和激发态时氦原子的基态量子相干和关联。我们确定了一组沿分子长轴吸附的准一维、强烈局域化的原子，它们之间表现出一定的量子相干性，但与其余溶剂化氦原子的相干性要小得多。我们通过对角化单粒子密度矩阵来评估局域化 He 原子的单粒子自然轨道，并利用这些轨道构建单粒子和多粒子溶剂化氦基态，然后计算四并苯-（4）He（N）吸收光谱的零声子谱特征。通过对强烈束缚的三原子氦对的三体密度矩阵进行计算，得到的吸收光谱与实验数据非常吻合，定量地解释了零声子线的异常分裂[Hartmann, M.; Lindinger, A.; Toennies, J. P.; Vilesov, A. F. Chem. Phys. 1998, 239, 139; Krasnokutski, S.; Rouillé, G.; Huisken, F. Chem. Phys. Lett. 2005, 406, 386]。我们的结果表明，强烈局域化和沿四并苯长轴吸附的三原子氦对的准一维性质导致了最接近分子的氦密度的量子相干但高度关联的基态。光谱分析表明，这种特征定量地解释了四并苯吸收光谱中观察到的异常分裂和迄今未解释的精细结构，并表明它可能是其他准一维平面芳香族分子中相应的零声子分裂的原因。
