Cryogenic 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. 

One source of such constraints could be the orientationally averaged cross section obtained from imi mobility. At the end of Sect. 3.2.2 we discussed the use of drift tube ion mobdity as a conformational filter for cryogenic ion spectroscopy but didn t mention the information content obtained from IMS. The cross section determined from the drift time in such an experiment could, in principle, be used as a filter in the conformatimial search procedure by comparing the cross sections of candidate structures to the measured value. While this would require a fast calculation of the cross section, for use as a conformational filter this could be done with reduced accuracy. 

Various groups have now implemented messenger spectroscopy in different forms using a variation of MS hardware. Instruments based on triple-quadrupole (-like) geometries were developed [117, 120-122] (see Fig. 4) and have, among others, led to the successful application of messenger spectroscopy in cryogenic ion... 

Biomolecular spectroscopy on frozen samples at cryogenic temperatures has the distinct disadvantage that the biomolecules are in a state that is not particularly physiological. Recall that EPR spectroscopy is done at low temperatures to sharpen-up spectra by slowing down relaxation, to increase amplitude by increasing Boltzmann population differences, and to decrease diamagnetic absorption of microwaves by changing from water to ice. Certain S = 1/2 systems, notably radicals and a few mononuclear metal ions, have sufficiently slow relaxation, and sufficiently limited spectral anisotropy to allow their EPR detection in the liquid phase at ambient temperatures, be it in aqueous samples of reduced size. 

Within the last one and a half decades, it became possible to perform experiments directly on the atomic and molecular level. This came with the improvement of existing experimental techniques such as electron microscopy, where the resolution was increased to make single atoms visible [1] high-resolution spectroscopy of single ions or atoms trapped in a radio frequency field or in focused laser beams [2-4] and the spectroscopic isolation of single molecules in solids at cryogenic temperatures [5-7], which evolved from spectral hole-burning spectroscopy. 

Wang, X.B. Wang, L.S. Development of a low-temperature photoelectron spectroscopy instrument using an electrospray ion source and a cryogenically controlled ion trap. Rev. Set Instrum. 2008, 79, 173108. 

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