Electron Impact Ionization in Helium Nanodroplets:  Controlling Fragmentation by Active Cooling of Molecular Ions

• Published in: Journal of the American Chemical Society Vol. 126; no. 36; pp. 11283 - 11292
• Main Authors: Lewis, William K, Applegate, Brian E, Sztáray, Judit, Sztáray, Bálint, Baer, Tomas, Bemish, Raymond J, Miller, Roger E
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.09.2004
• Subjects: Atomic and molecular physics; Exact sciences and technology; Mass spectra; Molecular properties and interactions with photons; Physics; Properties of molecules and molecular ions; Electron impact; Tertiary alcohol; Coincidence methods; Liquid helium; Time-of-flight method; Ionization potential; Fragmentation pattern; Mass spectroscopy; Droplets; Experimental study; Charge transfer; Organic compounds
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja030653q

Reported here is a study of the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. The molecules of interest are picked up by the helium nanodroplets as they pass through a low pressure oven. Electron impact ionization of a helium atom in the droplet is followed by resonant charge transfer to neighboring helium atoms. When the charge is transferred to the target molecule, the difference in the ionization potentials between helium and the molecule results in the formation of a vibrationally hot ion. In isolation, the hot parent ion would undergo subsequent fragmentation. On the other hand, if the cooling due to the helium is fast enough, the parent ion will be actively cooled before fragmentation occurs. The target molecule used in the present study is triphenylmethanol (TPM), an important species in synthetic chemistry, used to sterically protect hydroxyl groups. Threshold PhotoElectron PhotoIon COincidence (TPEPICO) experiments are also reported for gas-phase TPM to help quantify the ion energetics resulting from the cooling effects of the helium droplets.