title: Infrared multiphoton ionization of superhot C60: experiment and model calculations.
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 (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) 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).
authors.affiliation: Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel.
authors.forename: Anatoly
authors.initials: A
authors.lastname: Bekkerman
authors.forename: Eli
authors.initials: E
authors.lastname: Kolodney
authors.forename: Gert
authors.initials: G
authors.lastname: von Helden
authors.forename: Boris
authors.initials: B
authors.lastname: Sartakov
authors.forename: Deniz
authors.initials: D
authors.lastname: van Heijnsbergen
authors.forename: Gerard
authors.initials: G
authors.lastname: Meijer