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Abstract

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9–19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500–800) infrared photons in the 450–1800 cm^–1 spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C⁺₆₀ fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C⁺₃₂ is observed following the absorption of 1350–1400 cm^–1 as well as 1500–1530 cm^–1 photons while negligible fragmentation is observed when exciting around 520 cm^–1.