Sprecher D, Beyer M, Merkt F
Laboratory of Physical Chemistry, ETH Zurich, Wolfgang-Pauli-Str. 10, CH-8093 Zürich.
Chimia (Aarau). 2013;67(4):257-61. doi: 10.2533/chimia.2013.257.
Recent experiments are reviewed which have led to the determination of the ionization and dissociation energies of molecular hydrogen with a precision of 0.0007 cm(-)1 (8 mJ/mol or 20 MHz) using a procedure based on high-resolution spectroscopic measurements of high Rydberg states and the extrapolation of the Rydberg series to the ionization thresholds. Molecular hydrogen, with only two protons and two electrons, is the simplest molecule with which all aspects of a chemical bond, including electron correlation effects, can be studied. Highly precise values of its ionization and dissociation energies provide stringent tests of the precision of molecular quantum mechanics and of quantum-electrodynamics calculations in molecules. The comparison of experimental and theoretical values for these quantities enable one to quantify the contributions to a chemical bond that are neglected when making the Born-Oppenheimer approximation, i.e. adiabatic, nonadiabatic, relativistic, and radiative corrections. Ionization energies of a broad range of molecules can now be determined experimentally with high accuracy (i.e. about 0.01 cm(-1)). Calculations at similar accuracies are extremely challenging for systems containing more than two electrons. The combination of precision measurements of molecular ionization energies with highly accurateab initio calculations has the potential to provide, in future, fully reliable sets of thermochemical quantities for gas-phase reactions.
本文回顾了近期的实验，这些实验通过基于对高里德堡态的高分辨率光谱测量以及将里德堡系列外推至电离阈值的程序，以0.0007厘米⁻¹（8毫焦/摩尔或20兆赫兹）的精度确定了分子氢的电离能和解离能。分子氢仅由两个质子和两个电子组成，是能够研究化学键所有方面（包括电子相关效应）的最简单分子。其电离能和解离能的高精度值为分子量子力学和分子中的量子电动力学计算的精度提供了严格检验。这些量的实验值与理论值的比较使人们能够量化在进行玻恩 - 奥本海默近似时被忽略的对化学键的贡献，即绝热、非绝热、相对论和辐射修正。现在可以通过实验高精度地（即约0.01厘米⁻¹）测定多种分子的电离能。对于包含两个以上电子的系统，以类似精度进行计算极具挑战性。分子电离能的精确测量与高精度的从头计算相结合，未来有可能为气相反应提供完全可靠的热化学量集。
