Development of Autoionization-Detected Infrared Spectroscopy for Molecular Cations

研究了超声速射流中分离裸分子离子的红外光谱分析新技术。采用双色双共振激励，选择泵浦高里德堡态，其离子核与相应的裸离子具有相同的结构。通过检测振动自电离产生的分子离子来测量离子核的红外吸收。通过使用这种被称为“自电离探测红外（ADIR）光谱”的技术，获得了以下结果：(a)观察到氟苯酚和甲氧基苯酚阳离子的羟基伸缩振动，为分子阳离子中的分子内氢键提供了第一个直接证据。(b)观察甲酚阳离子的羟基伸缩振动，发现羟基频率明显依赖于邻甲基的取向。这一结果表明在羟基和甲基之间存在一种新型的分子内氢键。(c)首次观察到芳香阳离子中CH的伸缩振动。在乙基酚阳离子中观察到芳香CH和烷基CH的伸展，芳香CH在电离时表现出高频位移。

A new technique for infrared spectroscopy of isolated bare molecular ions in a supersonic jet was developed. High Rydberg states, of which ion core has the same structure as the corresponding bare ion, were state-selectively pumped by two-color double resonance excitation. Infrared absorption of the ion core was measured by detecting molecular ions produced by vibrational autoionization. By using this technique, which is called "autoionization-detected infrared (ADIR) spectroscopy", the following results have been obtained ;(a) The OH stretching vibrations of fluorophenol and methoxyphenol cations were observed, providing the first direct evidence for intramolecular hydrogen bonds in molecular cations.(b) The OH stretching vibration of the cresol cation was observed, and it was found that the OH frequency showed clear dependence on the orientation to the neighboring methyl group. This result indicates the presence of a new type of intramolecular hydrogen bonding between the hydroxyl and methyl groups.(c) The first observation of CH stretching vibrations in aromatic cations were performed. The aromatic and alkyl CH stretches in the ethyiphenol cation were observed, and the aromatic CH showed high-frequency shifts upon the ionization.