Iron Complexes as Potential Carriers of Diffuse Interstellar Bands: The Photodissociation Spectrum of Fe+(H2O) at Optical Wavelengths

The fullerene ion
C60+ is the
only carrier of diffuse interstellar bands (DIBs)
identified so far. Transition-metal compounds feature electronic transitions
in the visible and near-infrared regions, making them potential DIB
carriers. Since iron is the most abundant transition metal in the
cosmos, we here test this idea with Fe+(H2O).
Laboratory spectra were obtained by photodissociation spectroscopy
at 80 K. Spectra were modeled with the reflection principle. A high-resolution
spectrum of the DIB standard star HD 183143 served as an observational
reference. Two broad bands were observed from 4120 to 6800 Å.
The 4120–4800 Å band has sharp features emerging from
the background, which have the width of DIBs but do not match the
band positions of the reference spectrum. Calculations show that the
spectrum arises from a d–d transition at the iron center. While
no match was found for Fe+(H2O) with known DIBs,
the observation of structured bands with line widths typical for DIBs
shows that small molecules or molecular ions containing iron are promising
candidates for DIB carriers.

富勒烯离子C60+迄今为止是唯一已知的散布星际带（DIBs）的载体。过渡金属化合物在可见光和近红外区域具有电子跃迁，使其成为潜在的DIB载体。鉴于铁是宇宙中最丰富的过渡金属，本研究以Fe+(H2O)为对象，检验了此观点。在80 K的温度下，通过光解光谱法获得了实验室光谱。光谱通过反射原理进行了建模。DIB标准星HD 183143的高分辨率光谱作为观测参照。在4120至6800 Å范围内观察到了两个宽波段。4120–4800 Å波段中，背景中出现了尖锐的特征，其宽度与DIBs相当，但与参考光谱的波段位置不匹配。计算表明，该光谱起源于铁中心的d-d跃迁。虽然未发现与已知DIBs相匹配的Fe+(H2O)，但观察到具有典型DIBs线宽的结构化波段，表明含有铁的小分子或分子离子是DIB载体的有利候选。