Spectroscopy of Trapped Ions

Ions can be con6ned in traps created by potential wells of electric and magnetic fields. Depending on the quality of the vacuum, the trapping time can be very long—hours, days, even months. Such an arrangement has several advantages compared to experiments with ion beams or ions in resonance cells  

Many experiments discussed below were motivated by the possibility of future frequency standards based on microwave measurements with trapped ions. 

Starting with the work of Dehmelt et high-precision experiments on trapped ions made use of various schemes of state selection and signal detection before lasers were introduced in ion trap spectroscopy. The first successful attempt of laser rf spectroscopy of trapped ions was made by Wineland and co-workers on singly ionized Mg, which is isoelectronic with the Na atom. The authors measured the hyperfine splitting of the 

The final result for the magnetic dipole hyperfine interaction constant as obtained from this experiment is 

A similar ion trap experiment was performed with Be ions. The ions were cooled via the 2s Si/2(M/ = -3/2, Mj = -1/2) - 2p /2( -3/2, -3/2) transition at A = 313 nm with a frequency doubled dye laser, and were additionally optically pumped in the ground state. Measurements of the axial (I z), magnetron (v ), and electric field shifted cyclotron (Vc)frequencies of the stored ions provide the free-space cyclotron 

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The past decade has seen an explosion in investigations of molecular ions using a variety of optical spectroscopic techniques in conjunction with trapping mass spectrometers. The mass selection and ion storage capabilities of instruments such as 3-D QITs and FT-ICR mass spectrometers provide valuable control over the ion population under investigation. Moreover, thanks to modem ion sources, the size of molecules is no longer a limitation for gas-phase ion spectroscopic studies. A number of spectroscopic techniques have been developed to probe gas-phase molecules that will be fruitful when applied to the spectroscopy of trapped ions. 

Polfer NC. Infrared multiple photon dissociation spectroscopy of trapped ions. Chem Soc Rev. 2011 40 2211-21. 

The electric mass filter is the basis for the electrodynamic trap used for studies of the spectroscopy of atomic ions that earned Paul and Dehmelt the 1989 Nobel Prize in Physics. A wide variety of electrode configurations can be used to trap particles, and a particularly simple design was proposed by Straubel (1956). His dc electrodes were flat plates, and the ac electrode was a simple torus or washer placed at the midplane between the endplates. 

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. 

Itano, W.M. Bergquist, J.C. Wineland, D.J. Laser spectroscopy of trapped atomic ions. 

Fluorescence spectroscopy is an alternative approach to spectroscopic characterization of trapped ions in coigunction with trapping MS. An electronically-excited ion, Af"+, is created by absorption of aUV or visible photon. Fluorescence emission, a radiative transition between the excited electronic state and ground state of the same spin state, is one pathway for de-excitation back to the ground electronic state (Equation 9.3). Other de-excitation pathways, which compete with fluorescence, are available, including internal conversion and fragmentation (PD). 

Photo-excitation of gas-phase ions may result in the photodetachment of an electron rather than photo-fragmentation. Coulombic considerations dictate that this process is more prevalent for anions than for cations. Electron photodetachment action spectroscopy of trapped anions has proved also to be a valuable source of molecular information. In some systems, electron photodetachment and PD compete. The mechanisms for these two processes in large molecules are yet to be understood fully consequently, their branching ratios in specific experimental conditions cannot be predicted as yet. One exciting possibility is the idea of using frequency and phase-shaped pulses to promote selected photochemical pathways. 

Wright, K.C. Blades, M.W. Fluorescence emission spectroscopy of trapped molecular ions, Pmc. 51st ASMS Conf. on Mass Spectrometry and Allied Topics, Montreal, Canada 2003. 

Bian, Q. Forbes, M.W. Talbot, F.O. Jockusch, R.A. An instrument for fluorescence excitation and emission spectroscopy of trapped, mass-selected gas-phase ions. In preparation. 

Part 3. Ion Spectroscopy. In Chapter 9, we return to the theme of ion photodissociation, which was included also in Volume IV, Part 6, in an exploration of trapped-ion photodissociation, electron photodetachment, and fluorescence. Trapped-ion fluorescence may offer an alternative approach for the elucidation of ion conformation. Whereas these spectroscopic experiments require high ion densities, much attention is directed to the spectroscopic study of single ions confined in an ion trap. Chapters 10 and 11 are illustrative of such studies, with the former devoted to the study of a single molecular ion in a linear ion trap and the latter to a single atomic ion in Paul-type ion traps. While both types of studies require extensive cooling of the subject ion, once such cooling has been achieved, the ions can remain confined for many hours. 

Kroto, H.W., Molecular Rotation Spectra, Dover Publications, Inc., New York, 1992. Roth, B., Koelemeij, J.C.J., Daerr, H., and Schiller, S., Rovibrational spectroscopy of trapped molecular hydrogen ions at millikelvin temperatures, Phys. Rev. A, 74,... 

D.J. Wineland, W.M. Itano, J.C. Bergquist, J.J. BoUinger Trapped Ions and Laser Cooling, NBS Technical Note 1086 (NBS, Washington, DC 1985) D.J. Wineland, W.M. Itano, R.S. VanDyck Jr. High-resolution spectroscopy of stored ions. In Advances in Atomic and Molecular Physics, Voi. 19, ed. by O.R. Bates, B. Bedersoii (Academic Press, New York 1983)... 

An inverted version of the messenger tagging technique for detecting ion absorption uses the fact that electronic and/or vibrational excitation of ions hinders formation of weakly-bound clusters. This effect, explored years ago in relation to laser isotope separation [82], has recently been demonstrated for spectroscopy of N2" ions, cooled to 10.6 K in a 22-pole trap by collisions with He and termed laser-induced inhibition of cluster growth (LIICG) [83]. An electronic spectrum is generated by monitoring the reduction of the steady-state concentration of ion-He complexes as a function of the excitation laser wavenumber. 

In a parallel effort, Wang and co-workers added a cold Paul trap to their electrospray ion source and used it for photoelectron spectroscopy of buffer-gas cooled anions [117, 118]. In this case, spectroscopic detection is achieved by collecting the photodetached electrons. While the trap could be cooled to 10 K, it is difficult to determine absolute ion temperatures because photoelectron spectra are inherently broader than IR or UV absorption spectra, preventing the resolution of low frequency hot bands. Although their focus was primarily on non-biological cluster ions of various types, elements of their machine design were later applied to the spectroscopy of biological ions [119]. 

Wetzel, D. M., 8c Brauman, J. I. (1987). Electron photodetachment spectroscopy of trapped negative ions. Chemical Reviews, 87, 607. 

In 1997, the controversial mechanism of the Biginelli reaction was reinveshgated by Kappe using NMR spectroscopy and trapping experiments [94], and the current generally accepted process was elucidated (see Scheme 9.23). The N-acyliminium ion 9-112 is proposed as key intermediate this is formed by an acid-catalyzed reaction of an aldehyde with urea or thiourea via the semiaminal 9-111. Intercephon of 9-112 by the enol form of the 1,3-dicarbonyl compound 9-113 produces the open-chain ureide 9-114, which cyclizes to the hexahydropyrimidine 9-115. There follows an elimination to give the final product 9-116. 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

Spectroscopic observations of alkylcarbonium ions in strong acid solutions, 4, 305 Spectroscopy, l3C NMR, in macromolecular systems of biochemical interest, 13,279 Spin alignment, in organic molecular assemblies, high-spin organic molecules and, 26,179 Spin trapping, 17, 1... 

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