Ion spectroscopy: Where did it come from; where is it now; and where is it going?

Abstract

a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation "action spectroscopic" mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, highresolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions. (J Am Soc Mass Spectrom 2010, 21, 681-693) © 2010 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry T here are at least three perspectives guiding research in molecular spectroscopy of ions, which have given rise to rather distinct research communities. First is the goal of finding the quantum energy levels (electronic, vibrational, rotational) with high accuracy and energy resolution. Benefiting from experimental convenience, a vast body of results of this sort has been accumulated from photoionization and photoelectron spectroscopy experiments where the initial sample is the neutral molecule corresponding to an ion of interest.Second is the question of "what color" the ion is, that is to say, taking the ion itself as the sample, observing the wavelengths and transition intensities of the optical absorptions or emissions of the ion. These experiments are difficult, but have great interest for the study of ions found in nature, as for instance among the community of astronomers trying to sort out the telescope spectra corresponding to molecular species existing in extraterrestrial environments.Third is the use of spectra as a window to the identification and structural characterization of ionic molecules formed from interesting precursors by any of a variety of ionization, ion-molecule reaction, or ionextraction techniques. Combined with mass spectrometry, this branch of ion spectroscopy has developed into a new structural approach comparable to MS/MS, thanks to its ability to distinguish small conformational or structural differences among iso-mass ions. These studies have been dominated by the use of "action spectroscopy" (multi-photon and ion tagging dissociation techniques) to provide convenient, although indirect, access to the spectra of interesting ions.The three major parts of thi...

UV Photodissociation Action Spectroscopy of Haloanilinium Ions in a Linear Quadrupole Ion Trap Mass Spectrometer

“…Many mass spectrometry-based experiments, not just ion traps, have since exploited the use of coherent laser light sources for photodissociation spectroscopy -both UV and IR -as well as for photodissocation of molecules for structural characterization. [23][24][25][26][27] UV-vis PD action spectroscopy of ions is becoming increasingly prevalent due to advances and commercial availability of both ion trapping technologies and tunable laser sources. The linear quadrupole ion trap [28,29] is particularly well suited to PD experiments [30] as it provides a trapped ion ensemble with large number densities, located in a tight cylindrical ion packet confined around the ion-trap principal axis (z-axis), [31] other well-suited configurations include multipole traps [32,33] and storage rings.…”

Ion/Neutral, Ion/Electron, Ion/Photon, and Ion/Ion Interactions in Tandem Mass Spectrometry: Do We Need Them All? Are They Enough?

“…Ion spectroscopy has been actively pursued for many years with significant advances in bio-ion spectroscopy in recent years [27]. Ion/photon interactions are particularly attractive from the standpoint of the potential for obtaining wavelength dependent information and for the practical advantage of requiring no gases to be admitted into the vacuum system.…”

Infrared detection of the 1,3-dimethyl cyclopentadienyl cation, an isomer of protonated toluene

“…However, ion spectroscopy is problematic because of the low densities present, and the high temperatures produced by standard ion sources [1][2][3]. Recent developments in ion production and cooling methods, combined with new laser technology, make it possible to obtain higher quality infrared spectra of mass-selected ions via laser photodissociation [4][5][6][7][8]. We use these methods here to investigate a new isomer of the C 7 H 9 + cation.…”