ICR Ion Trap Research Articles

A 4 K FT-ICR cell for infrared ion spectroscopy.

We present the design of the newly constructed cryogenic Fourier-transform ion cyclotron resonance (FT-ICR) ion trap for infrared ion spectroscopy. Trapped ions are collisionally cooled by the pulsed introduction of buffer gas into the cell. Using different buffer gases and cell temperatures, we record action spectra of weakly bound neutral gas-analyte complexes with an IR laser source. We show for the first time that ion-He complexes can be observed in an ICR cell at temperatures around 4 K. We compare the experimental vibrational spectra of Ag(PPh3)2+ obtained by tagging with different neutral gases: He, Ne, Ar, H2, and N2 to computed vibrational spectra. Furthermore, the conditions necessary for the formation of neutral tags within an ICR ion trap are studied.

40 years of Fourier transform ion cyclotron resonance mass spectrometry

This article reviews the development of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in several respects: (a) a strong static magnetic field serves to convert ion mass-to-charge ratio into cyclotron frequency. Because frequency is the most accurately measurable property, ICR MS inherently offers higher mass resolution and mass accuracy than any other mass analyzer. (b) Coherent excitation followed by induced charge detection yields a time-domain signal whose discrete Fourier transform produces a mass spectrum of ions spanning a wide m/z range simultaneously. By simple analogy to weighing objects with a mechanical balance, that “multiplex” advantage can be shown to be equivalent to “multichannel” detection by an array of individual single-channel detectors. (c) FT-ICR MS performance benefits from near-elimination of magnetic field inhomogeneity by inherent ion cyclotron rotation and ion axial oscillation; inherent nearly quadrupolar electrostatic trapping potential and nearly uniform rf electric field homogeneity near the center of the ICR ion trap (both improved even further by recent ICR cell designs); and theoretically optimal excitation and mass selection produced by stored-waveform inverse Fourier transformation (SWIFT). (d) External ion accumulation allows efficient coupling of atmospheric pressure continuous ionization sources (notably electrospray ionization) with pulsed high-vacuum FT-ICR MS excitation/detection, and injection of externally trapped ions through the “magnetic mirror” into the ICR ion trap has been optimized based on ion trajectory simulations. (e) MS/MS can be performed either inside (e.g., electron capture dissociation, infrared multiphoton dissociation) or outside (e.g., collision-induced dissociation, electron transfer dissociation) the ICR ion trap. (f) Finally, FT-ICR MS instrumentation and experimental event sequences have benefited from striking parallels to prior nuclear magnetic resonance spectroscopy developments. Similarly, non-ICR FT MS development (notably the orbitrap) has benefited from FT-ICR precedents.

Electron Capture Dissociation Implementation Progress in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Successful electron capture dissociation (ECD) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) applications to peptide and protein structural analysis have been enabled by constant progress in implementation of improved electron injection techniques. The rate of ECD product ion formation has been increased to match the liquid chromatography and capillary electrophoresis timescales, and ECD has been combined with infrared multiphoton dissociation in a single experimental configuration to provide simultaneous irradiation, fast switching between the two techniques, and good spatial overlap between ion, photon, and electron beams. Here we begin by describing advantages and disadvantages of the various existing electron injection techniques for ECD in FT-ICR MS. We next compare multiple-pass and single-pass ECD to provide better understanding of ECD efficiency at low and high negative cathode potentials. We introduce compressed hollow electron beam injection to optimize the overlap of ion, photon, and electron beams in the ICR ion trap. Finally, to overcome significant outgassing during operation of a powerful thermal cathode, we introduce nonthermal electron emitter-based electron injection. We describe the first results obtained with cold cathode ECD, and demonstrate a general way to obtain low-energy electrons in FT-ICR MS by use of multiple-pass ECD.

Modification of Trapping Potential by Inverted Sidekick Electrode Voltage during Detection To Extend Time-Domain Signal Duration for Significantly Enhanced Fourier Transform Ion Cyclotron Resonance Mass Resolution

Applying an inverted voltage to the "sidekick" electrodes during ion cyclotron resonance detection improves both Fourier transform ion cyclotron resonance (FT-ICR) mass spectral signal-to-noise ratio (at fixed resolving power) and resolving power (at fixed signal-to-noise ratio). The time-domain signal duration increases by up to a factor of 2. The method has been applied to 7-T FT-ICR MS of electrosprayed positive ions from substance P and human growth hormone protein ( approximately 22 000 Da, m/Deltam50% 200 000), without the need for pulsed cooling gas inside the ICR trap. The modification can be easily adapted to any FT-ICR instrument equipped with sidekick electrodes. The present effects are shown to be comparable to electron field modification by injection of an electron beam during ICR detection, reported by Kaiser and Bruce (Kaiser, N. K.; Bruce, J. E. Anal. Chem. 2005, 77, 5973-5981.). Although the exact mechanism is not fully understood, computer simulations show that a flattening of the radial potential gradient along the magnetic field direction in the ICR trap may contribute to the effects. This study not only provides a way to enhance the quality of FT-ICR mass spectra but also offers insight into understanding of ion motions inside an ICR ion trap.

Stored waveform inverse Fourier transform (SWIFT) ion excitation in trapped-ion mass spectometry: Theory and applications

Stored waveform excitation produced by inverse Fourier transformation of a specified magnitude/phase excitation spectrum offers the most general and versatile means for broadband mass-selective excitation and ejection in Penning (FT-ICR) and Paul (quadrupole) ion trap mass spectrometry. Since the last comprehensive review of SWIFT excitation in 1987, the technique has been adopted, modified, and extended widely in both the ICR and quadrupole ion trap communities. Here, we review the principles, variations, algorithms, hardware implementation, and some applications of SWIFT for both ICR and quadrupole ion trap mass spectrometry. We show that the most desirable SWIFT waveform is that optimized to reduce both the time-domain SWIFT maximum amplitude and the amplitude near the start and end of the SWIFT waveform. We examine the “true” magnitude excitation spectrum, obtained by zero-filling and forward Fourier transforming the SWIFT time-domain waveform, in order to evaluate the trade-off between spectral magnitude uniformity and frequency (mass) selectivity. Apodization of the SWIFT waveform is optimally conducted by smoothing the excitation magnitude spectrum prior to generation of the SWIFT waveform by inverse FT. When (as for broadband ejection in a quadrupole ion trap) it is important that ions be excited near-simultaneously over a wide mass range, the phase spectrum (before inverse FT to generate the SWIFT waveform) may be overmodulated or randomly modulated (“filtered noise field”), with the recognition that very substantial non-uniformity in the “true” excitation magnitude spectrum will result.

Protonated water clusters and their black body radiation induced fragmentation

Protonated water clusters H +(H 2O) n , n = 5, …, 65 and their perdeuterated analogues were produced in a discharge sour and stored in an electromagnetic ICR-ion trap under collision-free conditions. The rates of their fragmentation or ‘evaporation’ due to absorption of the 300 K black body background radiation exhibit an overall τ ∼ 1 n dependence. Local deviations of some clusters (e.g. n = 21 or 55) from the overall trend are attributed to their higher stabilities. The fragmentation is modeled by extrapolating macroscopic water droplet evaporation to the microscopic clusters.

Ion traps for Fourier transform ion cyclotron resonance mass spectrometry: principles and design of geometric and electric configurations

Virtually all Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry experiments are based on ion confinement in a Penning trap: namely, a spatially homogeneous static magnetic field B, and a three-dimensional axial (along B) quadrupolar electrostatic trapping potential. In addition, it is desirable to provide an alternating azimuthal (i.e. in a plane perpendicular to B) spatially uniform dipolar electric potential to generate or detect ion cyclotron coherence and an alternating azimuthal quadrupolar potential in the presence of collisional cooling to axialize ions. In this paper, we show how each of these three potentials may be generated by infinitely extended electrode arrangements. We then examine the potentials actually produced by various (finite size) ICR ion trap geometric configurations. Finally, we show that by segmenting the electrodes of an ICR ion trap, and applying an appropriate potential to each segment, near-perfect potentials of all three types may be generated. The necessary principles and tools (Laplace's equation: harmonic potential; reciprocity theorem; V-vector method) are presented, followed by derivation and/or description and rationale for all of the principal ICR ion trap designs. We close by proposing a new “universal” trap consisting of six planar grids arranged in a cube: such a configuration is the first to promise near-perfect potentials of all three types while maintaining the symmetry and compact shape of a cubic Penning trap.

Instrumental configuration for direct measurement of optical absorption of ion cyclotron resonance mass-selected trapped ions

Identification and structural analysis of gas-phase ions is presently based on methods similar to those used in condensed-phase chemistry 75 years ago: namely, breaking the ion apart and weighing the fragments, and/or using chemical reactions to identify groups or reactive centers. For example, dissociation of mass-selected parent ions by (e.g.) collision-induced dissociation [CID], photodissociation, electron impact dissociation, surface-induced dissociation, etc., yields a product ion mass spectrum from which parent ion structure and bonding are indirectly inferred.Optical spectroscopy, on the other hand, can reveal directly the structure of the absorbing species. Directly measured optical absorption spectra of ions have yielded structures of a few species, such as H3+. Most such experiments have been carried out in a discharge tube although a few mass selected ion spectra have been obtained in a fast ion beam. Here, we propose to conduct optical absorption experiments on mass-selected ions in an ICR ion trap; such experiments require that both optical absorption sensitivity and the maximum number of trapped ions be improved by an order of magnitude. To increase absorption sensitivity, we have chosen a newly developed cavity ringdown method which has previously been demonstrated for visible spectra of neutrals. By use of quadrupolar excitation and collisional cooling to axialize and mass-select ions in a multi-chamber trap, we hope to trap as many as 109 ions with an effective optical path length of 10 000 m, making it possible to detect ions of 10−16 cm2 absorption cross-section.

Observation, manipulation, and uses for magnetron motion in ion cyclotron resonance mass spectrometry

An ion of mass-to-charge ratio, m/q, moving under the combined influence of a static magnetic field, B, and an axial quadrupolar electrostatic potential field undergo three "natural" ("normal mode") motions: cyclotron rotation at frequency ω+, magnetron rotation at frequency ω−, and an axial "trapping" oscillation at frequency ωT. Ordinarily, magnetron motion is unobserved, and considered to be undesirable, because ion-neutral collisions result in radial diffusive loss of ions as they roll "downhill" on the magnetron potential surface. However, azimuthal quadrupolar excitation at the ion cyclotron frequency, ωc = qB/m, converts magnetron motion into cyclotron motion, and (for heavy ions) ion-neutral collisions then shrink the cyclotron orbit, leaving a compact ion packet at or near the center of the ICR ion trap. In this paper, we describe briefly the formal basis for such ion axialization, visualization of magnetron motion, the effect of quadrupolar rf excitation amplitude and duration, and how to achieve ion axialization over a wide range of ion m/q values. Applications of the axialization process include ion remeasurement with high (up to 99%) efficiency; prolonged and efficient ion trapping at high collision gas pressure for cooling of electronically, vibrationally, and/or rotationally "hot" ions; enhanced mass resolving power and peak height-to-noise ratio; and enhanced parent ion selectivity (first stage of MS/MS) and enhanced product ion S/N ratio and mass resolving power (second stage of MS/MS). Moreover, because ions may now be axialized, they need not be formed or injected on-axis, thereby making possible continuous injection of externally formed ions, and/or separation between ion and optical axes for photoionization, photodissociation, photodetachment, and optical spectroscopy of trapped ions.

Off-axis injection into an ICR ion trap: a means for efficient capture of a continuous beam of externally generated ions

Highly efficient ion injection into an ICR ion trap of ions generated outside the magnetic field is crucial for successful coupling of a high pressure external ion source, such as electrospray ionization, to an FT-ICR mass spectrometer. Any of several ion transport systems, including r.f.-only quadrupole, r.f.-only octupole, electrostatic einzel lenses, electrostatic ion guide, or use of a supersonic jet expansion to ensure that ions remain virtually on-axis during injection, can overcome the magnetic mirror effect and transport ions at high pressure through a magnetic field gradient toward a low pressure ICR ion trap. However, ions thus transported may exhibit a wide distribution in kinetic energy, making it difficult to trap them all. Gated trapping and pulsed-gas methods are useful for trapping ions from a pulsed source (e.g. laser desorption/ionization), but are not efficient for trapping of continuously injected ions (e.g. electrospray). Here, we introduce a novel method for trapping a continuously generated beam of ions, by injecting ions through a port held at a potential lower than that of the axial front trapping electrode and displaced laterally from the symmetry axis of the trap. Ions reflected from the back trap plate undergo magnetron rotation which in general prevents the ions from exiting back through their injection point. Ion/neutral collisions damp axial motion, and further reduce the ion's chance of escape. Supercomputer simulations of ion trajectories in the three-dimensional electrostatic potential, with collisions modeled as a frictional resistive force, predict efficient trapping of continuously injected ions up to mass-to-charge ratio m/z 50 000 over a wide range (>7 eV) of ion kinetic energy.

Off-axis injection into an ICR ion trap: a means for efficient capture of a continuous beam of externally generated ions

Highly efficient ion injection into an ICR ion trap of ions generated outside the magnetic field is crucial for successful coupling of a high pressure external ion source, such as electrospray ionization, to an FT-ICR mass spectrometer. Any of several ion transport systems, including r.f.-only quadrupole, r.f.-only octupole, electrostatic einzel lenses, electrostatic ion guide, or use of a supersonic jet expansion to ensure that ions remain virtually on-axis during injection, can overcome the magnetic mirror effect and transport ions at high pressure through a magnetic field gradient toward a low pressure ICR ion trap. However, ions thus transported may exhibit a wide distribution in kinetic energy, making it difficult to trap them all. Gated trapping and pulsed-gas methods are useful for trapping ions from a pulsed source (e.g. laser desorption/ionization), but are not efficient for trapping of continuously injected ions (e.g. electrospray). Here, we introduce a novel method for trapping a continuously generated beam of ions, by injecting ions through a port held at a potential lower than that of the axial front trapping electrode and displaced laterally from the symmetry axis of the trap. Ions reflected from the back trap plate undergo magnetron rotation which in general prevents the ions from exiting back through their injection point. Ion/neutral collisions damp axial motion, and further reduce the ion's chance of escape. Supercomputer simulations of ion trajectories in the three-dimensional electrostatic potential, with collisions modeled as a frictional resistive force, predict efficient trapping of continuously injected ions up to mass-to-charge ratio m/z 50 000 over a wide range (>7 eV) of ion kinetic energy.

Linear excitation and detection in Fourier transform ion cyclotron resonance mass spectrometry

We present the first Fourier transform ion cyclotron resonance (FT-ICR) ion trap designed to produce both a linear spatial variation of the excitation electric potential field and a linear response of the detection circuit to the motion of the confined ions. With this trap, the magnitude of the detected signal at a given ion cyclotron frequency varies linearly with both the number of ions of given mass-to-charge ratio and also with the magnitude-mode excitation signal at the ion cyclotron orbital frequency; the proportionality constant is mass independent. Interestingly, this linearization may be achieved with any ion trap geometry. The excitation/detection design consists of an array of capacitively coupled electrodes which provide a voltage-divider network that produces a nearly spatially homogeneous excitation electric field throughout the linearized trap; resistive coupling to the electrodes isolates the a.c. excitation (or detection) circuit from the d.c. (trapping) potential. The design is based on analytical expressions for the potential associated with each electrode, from which we are able to compute the deviation from linearity for a trap with a finite number of elements. Based on direct experimental comparisons to an unmodified cubic trap, the linearized trap demonstrates the following performance advantages at the cost of some additional mechanical complexity: (a) signal response linearly proportional to excitation electric field amplitude; (b) vastly reduced axial excitation/ejection for significantly improved ion relative abundance accuracy; (c) elimination of harmonics and sidebands of the fundamental frequencies of ion motion. As a result, FT-ICR mass spectra are now more reproducible. Moreover, the linearized trap should facilitate the characterization of other fundamental aspects of ion behavior in an ICR ion trap, e.g. effects of space charge, non-quadrupolar electrostatic trapping field, etc. Furthermore, this novel design should improve significantly the precision of ion relative abundance and mass accuracy measurements, while removing spectral artifacts of the detection process. We discuss future modifications that linearize the spatial variation of the electrostatic trapping electric field as well, thereby completing the linearization of the entire FT-ICR mass spectrometric techniques. Suggested FT-ICR mass spectrometric applications for the linearized trap are discussed.

Radiative and collisional association of mesitylene ion with parent neutral at 196 K

The dimerization of mesitylene (1,3,5-trimethylbenzene) ion was measured at 196 and 300 K in the ICR ion trap. Radiative association at 196 K had a rate constant 7×10 −12 cm 3 molecule −1 s −1. Analysis gave a collisional efficiency of 0.006, an IR photon emission rate of 29 s −1 and a complex redissociation rate of 5×10 3 s −1. The three-body association rate was 2.9×10 −22 cm 6 molecule −2 s −1 at 196 K and 9×10 −25 at 300 K. Mesitylene ion radiative association kinetics at 196 K should be a good analog for benzene ion association at interstellar cloud temperatures around 50 K.

An electrostatic ion guide for efficient transmission of low energy externally formed ions into a Fourier transform ion cyclotron resonance mass spectrometer

A new method for transmitting externally formed ions into an ICR ion trap is demonstrated. In an electrostatic ion guide, a potential difference is applied between a conductive cylinder and a rigid wire suspended along the central axis of the cylinder. The cylinder is then positioned between an ion source located outside the bore of superconducting solenoidal magnet and an ion trap located at or near the center of the solenoid. simion simulations predict that low-energy ions entering the ion guide will spiral around the central wire and pass through the fringe of the magnet to reach the ICR ion trap. The theoretical predictions are borne out by experiments in which Na + and K + ions from a thermionic emitter are transmitted with high efficiency through the fringe field of the magnet to the ICR ion trap. Mass resolving power of 285 000 for K + is shown. The electrostatic ion guide offers the advantages that: (a) a wide range of low-energy external sources (e.g., fast-atom on fast-ion bombardment, electrospray, glow discharge, etc.) may be used; (b) prior acceleration of the ions along the magnetic field direction (and subsequent deceleration to slow the ions on entry into the ICR ion trap) is not required; (c) ions are focused along magnetic field lines once the ions have passed through the magnetic fringe field; and (d) ions formed initially off axis are efficiently captured and transmitted by the ion guide without additional focusing.

Fast neutral beam Fourier transform ion cyclotron resonance mass spectrometry for analysis of insulating and conductive materials

The use of an autoneutralizing fast neutral SF 6 beam (FNB) for secondary ion quadrupole mass spectrometry of refractive samples has been demonstrated previously. Here, we demonstrate the FNB technique for generation and high-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometric detection of ions from a wide variety of samples. The fast neutral beam generates abundant secondary ions from both conductive and insulating samples for detection in an open-ended ICR ion trap, as demonstrated by representative mass spectra of methyl stearate, fluorocarbon polymers, gramicidin S, and fullerenes. Optimization of the FNB-FT-ICR-MS experiment is discussed in detail, with particular emphasis on analysis of insulating materials which are not readily analyzed by ion-bombardment secondary ion mass spectrometry.

Circularly polarized quadrature excitation for Fourier-transform ion cyclotron resonance mass spectrometry

A quadrature method for producing circularly polarized excitation for selective detection of positive or negative ions has been proposed for Fourier-transform ion cyclotron resonance mass spectrometry. The new technique achieves the same post-excitation ion cyclotron orbital radius at half the excitation rf voltage of conventional linearly polarized (dipolar) excitation, and is especially useful for exciting and detecting dynamically trapped ions of one charge state in a mixture of simultaneously trapped positive and negative ions. Circularly polarized single-frequency resonant excitation is accomplished experimentally by simultaneous application ofa sinusoidal rf voltage to one pair of electrodes of a cubic ICR ion trap and a 90° or 270° phase-shifted voltage of the same frequency to a second orthogonal pair of electrodes, followed by conventional dipolar detection. Selective excitation of 35C1+ or 35CI− ions from a dynamically trapped mixture of 35C1+ and 35C1− ions is demonstrated. Circularly polarized broadb excitation by use of frequency-sweep or stored-waveform (SWIFT) excitation is proposed and discussed.

Quadrupolar excitation and collisional cooling for axialization and high pressure trapping of ions in Fourier transform ion cyclotron resonance mass spectrometry

A recently developed technique for axialization of ions in a Penning (i.e. hyperbolic) trap has been adapted for Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry in a dual cubic trap instrument. A resonant alternating electric azimuthal quadrupolar field at the combination frequency, ω c = ω + + ω −, converts ion magnetron motion into collision-damped cyclotron motion. The usual collision- induced drift of ions from the center of the trap under the influence of the radially outward-directed electric force of the trapping field is thereby effectively inverted, so that ions with off-axis ICR orbit centers may be driven back to on-axis low radius ion cyclotron orbits. This effect is frequency-dependent and therefore mass-selective. The experiment requires only minor wiring modifications to an existing cubic, tetragonal, or cylindrical ICR ion trap. With this technique, we have been able to store benzene molecular ions for several seconds at high pressure (> 1 x 10 −5 mbar) in one half of a dual trap, as demonstrated by subsequent transfer of these ions to the other half of the dual trap for ICR excitation and detection. We also report simultaneous axialization and extended trapping of ions of different mass-to-charge ratios by use of multiple-frequency stored-waveform inverse Fourier transform excitation. Proposed applications include axialization and cooling for improved detection of product ions in MSMS experiments, ion trapping at high pressure for extended periods for measurement of slow ion/molecule reaction rates, and increased sensitivity for detection of externally injected ions.

Dyanmic ion trapping for Fourier‐transform ion cyclotron resonance mass spectrometry: Simultaneous positive‐ and negative‐ion detection

AbstractThe conventional static electric potential used to trap ions fro fourier‐transform ion cyclotron resonance (FTICR) mass spectrometry has been replaced by a low‐amplitude [as low as Vac≈︁0.5 V for N] alternating (17.5kHz) electric field applied to the end caps of an ICR ion trap. Ion z‐motion is then governed by a Mathieu equation, whose solution leads to a z‐stability diagram for which optimal results are obtained at z‐stability parameter, qz = 4qλV ac/(mω2)≈︁0.5, in which m/q is the ion mass‐to‐charge ratio, Ω is the RF frequency, and λ=2.7737/d2 for a cubic trap of edge length, d. A triangular waveform appears to be more effective than sinusoidal modulation. We demonstrate experimentally three major additional advantages of RF trapping for FTICR mass spectrometry: (a) both positive and negative ions may be trapped and detected simultaneously; (b) Magnetron motion is eliminated, along with the electrostatic radial field‐induced ICR frequency shift and sidebands; and (c) mass calibration follows a simpler law (m=a/v, in which a is a constant and v is the measured ICR orbital frequency) than for electrostatic trapping (m=a/v+b/v2, in which a and b are constants). All prior FTICR mass spectrometric capabilities are preserved, except that (as in an RF‐only quadrupole ion trap) optimal sensitivity is observed only for 0.4⩽qz⩽0.7, so that the mass‐to‐charge ratio range for a single FTICR mass spectrum is limited accordingly.

Fast neutral beam ion source coupled to a Fourier transform ion cyclotron resonance mass spectrometer

The coupling of an autoneutralizing SF−6 fast ion-beam gun to a Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometer is described. The fast neutral beam provides for secondary-ion-type FT/ICR mass analysis [e.g., production of abundant pseudomolecular (M+H)+ ions] of involatile samples without the need for external ion injection, since ions are formed at the entrance to the ICR ion trap. The design, construction, and testing of the hybrid instrument are described. The feasibility of the experiment (for both broadband and high-resolution FT/ICR positive-ion mass spectra) is demonstrated with tetra-butylammonium bromide and a Tylenol■ sample. The ability to analyze high molecular weight polymers with high mass resolution is demonstrated for Teflon■. All of the advantages of the fast neutral beam ion source previously demonstrated with quadrupole mass analysis are preserved, and the additional advantages of FT/ICR mass analysis (e.g., high mass resolving power, ion trapping) are retained.

Slow dissociations of thiophenol molecular ion. Study by TRPD and TPIMS (time-resolved, photodissociation and time-resolved photoionization mass spectrometry)

Dissociation of thiophenol molecular ion to yield competitive product ions at m/z 84 (C 4 H 4 S + ) and 66 (C 5 H 6 + ) was studied by several techniques. Time-resolved photodissociation at 308 nm (4.20 eV ion internal energy) in the ICR ion trap gave rate constants of 1.3×10 5 s −1 for m/z 84 formation and 8×10 4 s −1 for m/z 66 formation, and at 355 nm (3.66 eV ion internal energy) it gave 2.8×10 3 s −1 for m/z 84 formation, with m/z 66 not observed at this wavelength. Time-resolved photoionization mass spectrometry at 800 μs trapping time gave thresholds of 2.9 eV for m/z 84 formation and 3.2 eV for m/z 66 formation; the photoionization efficiency curves cross at approximately 4 eV internal energy so that m/z 66 si more abundant at energies above this; at 20-μs trapping a kinetic shift of approximately 0.2 eV was observed relative to 800-μs trapping