Abstract
This paper reports gas-phase IR spectra of He-C-60(n+) (n = 1-3) ions in the range from 1100 to 1600 cm(-1). The spectra were recorded with a temperature-variable radio frequency ion trap instrument.
The experiments work with mass-selected ions confined in a linear wire quadrupole, where they form helium-tagged complexes following exposure to low-temperature (as low as 2.3 K) and high-density helium buffer gas. All measured lines are homogeneously broadened owing to a strong coupling between the vibrational states of the fullerene cage.
The short lifetimes of picoseconds or below rule out measurements of high-resolution IR spectra. Reproducible sidebands with distances between 5 and 20 cm(-1) enrich the spectra.
Tentative explanations are based on nonlinear and symmetry-breaking interactions or may be due to the influence of the attached He atom. Comparison of the He-C-60(+) spectrum with that of N-2-C-60(+) reveals shifts up to 6 cm(-1), while the IR spectra of He-C-60(3+) and He-32-C-60(3+) are very similar.
With the harmonic potential of the trap the ions are squeezed into the rather narrow laser beam, increasing the sensitivity and simplifying the determination of absolute photoabsorption cross sections. Disregarding shifts of several cm(-1), the gas-phase results for He-C-60(+) and He-C-60(2+) are in agreement with previous matrix results and with theoretical predictions.
The astrophysical relevance of our results is briefly mentioned.