Matrix Isolation

Our oldest and most versatile setup employs a combination of mass selection with matrix isolation spectroscopy. In these experiments reactive molecular ions are produced in an appropriate source, guided through a mass-selective lens system and frozen in a noble gas matrix at low temperatures. Electronic absorption spectra of many different species can be studied, from small clusters such as B3 and Si4 to long linear carbon chains like C17 and HC15H. The main limitation in matrix isolation spectroscopy are the matrix effects, namely that noble gas atoms have a perturbing influence on the species they surround. In absorption spectroscopy this leads to shifts and line broadening, making comparison between laboratory and astronomical observations difficult. However, matrix data provide a good starting point for high-resolution, gas-phase measurements.

Below is a schematic of the experiment. Generated ions are extracted from the source and guided through a differentially pumped vacuum system involving electrostatic lenses, a 90o deflector and a quadrupole mass filter. Ions with a chosen m/z are deposited with excess of neon onto a rhodium-coated sapphire plate that is mounted on a copper piece, which is cooled by a closed-cycle helium cryostat to temperatures around 6 K. To obtain detectable concentration of the ions, several hours of deposition is required. The detection system for the recording of electronic spectra consists of a broadband light source and a spectrograph, equipped with three rotatable gratings and two range-specific CCD cameras. Besides ions, neutrals may also be investigated after generation by a standard photobleaching technique. A custom-modified IR spectrometer complements the UV/Vis measurements.

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