Cold ion spectroscopy

The most common RF ion trap is a Paul trap [42], a 3-D quadrupole device in which ions are confined in a small volume of typically a few tens of millimeters [2] between a hyqterbolically shaped inner surface of a ring electrode and two end-cap electrodes, also of hyperbolic shape (Fig. 1). Elach end-cap electrode has a central hole for loading and ejection of irais. As these traps are compact, commercially available, and allow mass-selection of stored ions, they have become an increasingly popular technically simple solution for cryogenic ion spectroscopy. Paul traps have several drawbacks for cold-ion spectroscopy, however inefficient ion injection an intrinsically limited ability to cool ions low storage volume and inconvenient optical access to the ions by laser beams. 

Fig. 12 Helium droplet machine for cold-ion spectroscopy, constructed in at the Fritz Haber Institute by von Helden and coworkers. Reproduced with permission from [59]...

Figure 21 compares the calculated structure of the isolated molecule, validated by cold ion spectroscopy, with that of the condensed-phase structure, solved by X-ray diffraction. The structures are similar, each containing a characteristic antiparallel beta-sheet motif. The near-symmetrical (C2) structure of the isolated. 

Fig. 21 Three-dimensional views of the [GS-h2H] structures (a) determined by cold-ion spectroscopy for the lowest-energy coirformer of the gas-phase species, and (b) obtained by X-ray diffraction for crystallized species (reconstructed from the data of [8]). Reprinted from [15], Copyright (2011), with permission from John Wiley and Sons...

Nagomova NS, Guglielmi M, Doemer M, Tavemelli I, Rothlisberger U, Rizzo TR, Boyarkin OV (2011) Cold-ion spectroscopy reveals the intrinsic structure of a decapeptide. Angew Chem Int Ed 50 5383-5386... 

At least three different fragmentation detection techniques are possible (1) observation of the change of shape of the cold ion crystal as fragments (lighter than the atomic coolant) are sympathetically cooled into the center of the crystal, (2) extraction and counting of the parent ions and of the fragmentation products, (3) motional resonance mass spectroscopy. 

Wang X-B, Woo H-K, Wang L-S. (2005) Vibrational cooling in a cold ion trap VibrationaUy resolved photoelectron spectroscopy of cold C q Anions. J. Chem. Phys. 123 051106-1-051106-4. 

Asvany O, Ricken O, Muller HSP, Wiedner MC, Giesen TF, Schlemmer S (2008) High-resolution rotational spectroscopy in a cold ion trap H2D and D2H. Phys Rev Lett 100 233004... 

Along similar lines, Choi et al. combined a cold Paul trap with a TOFMS, first cooling it to 150 K [135] and later to 10 K [136]. WhUe coupling a quadmpole irai trap with TOFMS was not new, having first been demonstrated by Lubman and coworkers in the early 1990s [137], Choi et al. were the first to do this with a cold ion trap and use it for photofragment spectroscopy [138]. Using protonated tyrosine as a benchmark [46], they estimated the internal temperature of their ions to be on the order of 50 K. 

Figure 8 Multiphoton excitation and ion spectroscopy Bottom spectrum cold, mass selected UVspectmm (S, <- So transition) of fluorobenzene, providing wavelengths for efficient and selective ionization. Middle spectra photoelectron spectra induced by UV-resonance enhanced two-photon absorption. Choosing different intermediate states [S,(0,0) or S,(6b )] results in different populations of the final fluorobenzene radical cations. Top spectrum spectroscopy of the excited ionic state of the fluorobenzene radical cation measured by muKiphoton dissociation spectroscopy. The ions have been prepared via the neutral 0°o transition. A special excitation scheme has been used to optimize cation spectroscopy (for further details see text).

The compounds resulting from the reaction of 748 were characterized by HRMS directly coupled to the reactor. The stable products 750 and 751 were analysed by GC and 111 NMR spectroscopy. The formation of the cyclodisilazane 750 is explained by dimerization of the unstable silanimine 749 only in the cold trap, as the reaction is carried out under high dilution conditions (equation 247). It was also shown that the hydrogen chloride elimination did not occur in the ion source of the mass spectrometer. 

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