Large Molecular Ions Research Articles

Laser photoelectron spectroscopy of gas phase chlorobenzene- h5 and chlorobenzene- d5

We present here the first application of a “magnetic bottle” photoelectron spectrometer associated with a laser ionization source for studying the vibrational spectroscopy of large molecular ions. The instrument combines good resolution, very high and constant collection efficiency of electrons with up to 2 meV kinetic energy and ease of use. Photoelectron spectra of chlorobenzene- h 5 and chlorobenzene- d 5, after one- or two-color, two-photon ionization via various vibrational levels of the first excited neutral state 1B 2, have been investigated. This has enabled the assignment of 15 vibrational frequencies of the 2B 1 chlorobenzene- h 5 ion and one of its deuterated homologues. The vibrational state distribution of the generated ions has been determined very precisely providing new data for further spectroscopic studies.

Detection of large molecular ions by secondary ion and secondary electrom emission

Secondary ion and secondary electron emission has been investigated in a tandem time-of-flight mass spectrometer using incident molecular ions produced by laser desorption as primary ions. Measurements of the ratio of secondary electrons to secondary ions from a stainless steel target were made with a range of projectile masses and energies so that the separate dependenc on velocity and mass could be determined. The projectiles ranged in mass from about 300 to 6000 u, and in energy from 300 eV to 30 keV. Secondary accelerating voltages down to 20 V were used to reduce spurious contributions to the electron intensity caused by fragmentation of the projectiles or by grid effects. The electron-to-ion ratio is determined almost entirely by the velocity of the projectile; it is largely insensitive to the mass (for a given velocity). The ratio at first with decreasing velocity but appears to approach a constant value of about 0.1 below a velocity of ≈ 1.4 × 10 6 cm s −1 (≈1 eV u −1). A strong increase in the absolute electron yield and in the electron-to-ion ratio was observed for a fresh CsI target compared to a stainless steel target.

Surface secondary electron and secondary ion emission induced by large molecular ion impacts

This paper presents results on secondary ion and secondary electron yields under impact of large ions at 18 keV and with 100 < m/z < 70 000 on a CsI-coated surface. Ratios of secondary electron emission to secondary ion emission have been measured and it is shown that the negative secondary ion yield is much larger than the electron yield. Electrons are still emitted below an energy of impact per mass unit of 1 eV Da −1 (10 6 cm s −1). Effects of grids in the acceleration of secondary ions and secondary electrons are important.

Advances in flowing afterglow and selected-ion flow tube techniques

New developments in flowing afterglow and selected-ion flow tube (SIFT) techniques are briefly reviewed. Particular emphasis is given to the new chemical and physical information that can be obtained with use of the tandem flowing afterglow-triple quadrupole apparatus developed in the author's laboratory. Several outstanding recent achievements in the design and utilization of flowing afterglow and SIFT instruments in other laboratories are briefly highlighted that illustrate the power and flexibility of flow-tube-based methods. These include isotope tracer experiments with the tandem flowing afterglow-SIFT instrument in Boulder, studies of large molecular cluster ions with the variable temperature facility at Penn State, and gas-phase metal ion reactions with the laser ablation/fast flow reactor in Madison. Recent applications of the flowing afterglow-triple quadrupole instrument in our laboratory have made use of collision-induced dissociation (CID) as a tool for synthesizing novel ions and for obtaining new thermo-chemical information from threshold energy measurements. Collision-induced decar☐ylation of organic car☐ylate ions provides access to a variety of unusual and highly basic carbanions that cannot be generated with conventional ion sources. The formation and properties of saturated alkyl ions and studies of gas-phase reactions of the methyl anion are briefly described. We have developed a new method for carrying out “preparative CID” in a flowing afterglow with use of a mini-drift tube; some recent applications of this new ion source are presented. Measurement of CID thresholds for simple cleavage reactions of thermalized ions can provide accurate measures of bond strengths, gas-phase acidities and basicities, and heats of formation for ions and reactive neutral species. Applications of this approach in the thermochemical characterization of carbenes, benzynes and biradicals are described. Future prospects for the continued development of flow reactor-based methods are also briefly discussed.

Secondary‐ion generation from large keV molecular primary ions incident on a stainless‐steel dynode

AbstractPositive and negative secondary ions from large (up to 40 KDa) peptide and protein primary ions incident at 5–40 keV collisional energies on a stainless‐steel (mesh) conversion dynode have been investigated in a tandem (reflectron) time‐of‐flight arrangement. Negative secondary‐ion spectra consisted mainly of H− and C2H−, C2H ions; positive secondary‐ion spectra were more complex and contained prominent signals due to H+, Na+ and K+, and further signals at m/z. 28, 41, 43, 45, 73 up to m/z 326. The secondary mass spectra were qualitatively very similar amongst the various peptide primaries and not different from those of other organic projectiles such as, for example, coronene or cyclodextrin. With increasing mass of the primary ion, ratios of the secondary‐ion abundances changed drastically in the negative secondary‐ion spectra (increase of higher m/z signals, decrease of H−). Since most of the positive secondary ions could be attributed to a silicon‐organic surface contamination of the dynode (vacuum grease), it is concluded that the formation of such secondary ions is due to sputtering rather than surface‐induced dissociation.

Radial velocity distributions of secondary ions ejected in electronic sputtering of organic solids

Initial radial velocity distributions of large organic molecular ions as well as low mass contaminant and fragment ions ejected from a solid by impact of 72.3 MeV 127I 13+ have been measured by deflecting the secondary ions by means of a pair of deflection plates placed in the field-free region of a time-of-flight mass spectrometer. The radial velocity distributions have been studied as a function of the angle of incidence of the primary ion for different masses and charge states of the secondary ions. The results indicate that different ejection mechanisms may be involved, applicable in different regions of the secondary ion mass range.

Sputtering of bio-organic samples

In recent years methods based on particle impact on solid and liquid surfaces have been exploited to extend mass spectrometry to high-molecular-weight organic compounds, like biomolecules. One of the most important aspects of these sputtering methods is that one is able to produce ions of whole organic molecules. So far mainly charged ejecta, as exploited in the mass spectrometry applications, have been studied. Since a couple of years, experimental electronic sputtering yields of neutral intact ejecta are available, showing a cubic dependence on deposited energy. Studies of radial velocity distributions of large organic molecular ions show that the ejection angle is related to the angle of incidence of the primary particle. These experimental findings have been reproduced in molecular dynamics simulations of electronic sputtering of large organic molecules. Also an analytical sputtering formula has been derived which describes sputtering by a particle producing a high energy density, in qualitative agreement with experimental data.

Internal energy effects in the collision‐induced dissociation of large biopolymer molecular ions produced by electrospray ionization tandem mass spectrometry of cytochrome c

AbstractThe effects of internal energy upon metastable and collision‐induced dissociation (CID) processes observed in the tandem mass spectrometry of multiply protonated molecules of horse‐heart cytochrome c (Mr 12360) are described. The internal energy of the multiply protonated (15H+) molecules (at m/z 825) formed by electrospray ionization (ESI) was manipulated using electric fields in the atmospheric pressure/vacuum interface region. Highly efficient CID is obtained for molecular ions collisionally activated in the ESI interface. The dissociation efficiency at 3.8 eV to 5.8 eV (center of mass) collision energies for internally “hot” molecular ions under multiple collision conditions in a triple quadrupole mass spectrometer is shown to be much greater than for “cold” ions. The CID spectra show dramatic effects due to internal energy, with charge stripping dominating for cold ions (where CID efficiency is very low) and extensive fragmentation observed for hot ions.

Compensation for non‐normal ejection of large molecular ions in plasma‐desorption mass spectrometry

AbstractBy introducing an Einzel lens in the field‐free region of a straight time‐of‐flight mass spectrometer, secondary ions of large mass can be detected more efficiently in plasma‐desorption mass spectrometry. This procedure compensates for the large radial velocities of large molecular ions recently observed experimentally. It is shown that large molecular ions like those of lysozyme can now be detected more efficiently, and larger molecular ions than observed before in plasma‐desorption mass spectrometry (like those of bovine ovalbumin, mol. wt 45 000) can now be studied.

Electronic sputtering of biomolecules

A review of results on electronic sputtering of biomolecules, induced by fast heavy ions like fission fragments, obtained by the Uppsala group the last two years is given. The readily determined yields of ions in electronic sputtering have been compared to total sputtering yields in a collector experiment. The results indicate that the neutral yield of the amino acid leucine is of the order of 10 4 times that of the ion yield. Studies of yield vs sample thickness, where samples were prepared with the Langmuir-Blodgett technique, show that intact molecular ions are sputtered from depths in the range 100–200 Å. These two results support the idea that craters of sizes of the order of 100 Å are formed in electronic sputtering induced by e.g. 90 MeV 127I ions. Furthermore, recent results indicate that the angular distribution of ejection of large molecular ions is peaked at an angle related to the direction of the incoming ion rather than being peaked perpendicular to the sample surface or being isotropic. Finally, all the mentioned results have been qualitatively demonstrated in a molecular dynamics simulation of the desorption process.

Heating of interstellar gas by large molecules or small grains

The heating of the interstellar medium by photoelectric emission from large molecules or small grains is explored. Photodetachment of large negative ions may be a significant heat source in diffuse clouds. For an abundance of large molecules relative to hydrogen greater than 2 x 10 to the -7th, the heating rate from the photoelectrons produced in the photoionization of large molecules and the photodetachment of large molecular negative ions exceeds the standard grain-heating rate. Theoretical models have been used to infer the abundances of large molecules from the C(+)/C abundance ratios in the interstellar clouds toward Zeta Oph and Zeta Per. 33 references.

Sputtering of biomolecules by fast heavy ions

The ejection of large intact molecular ions from a sample of biomolecules when bombarded with fast heavy ions is discussed. The mass spectrometric technique based on this effect and time of flight mass analysis is called plasma desorption mass spectrometry (PDMS). Recently small proteins and mixtures of such molecules have been studied by this method. The analytical potential of PDMS motivates the search for a deeper understanding of the mechanisms involved. A strongly nonlinear behaviour of the yield of large molecular ions versus energy deposited by the fast primary ion has been experimentally observed. A secondary electron multihit ion track model will be used to analyze the data. Furthermore new experimental data on sputtering yields of neutral intact molecules as well as data on the influence of sample film thickness on secondary ion yields will be presented.

Sputtering of large molecular ions by low energy particle impact

Some principal aspects of molecular ion formation from thermally labile compounds by particle impact in the keV energy range are briefly discussed and results of a recent study on the role of the liquid matrix in molecular ion formation at high fluences of incident particles are reported. It is shown that a renewal of the liquid layer surface during sputtering is accomplished by the sputtering process itself removing most of the radiation damage formed by the collision cascade. A contribution to molecular ion emission by supply of molecules from the bulk to the surface is found for surface active compounds such as peptides. Vibrationally cold molecular ions are formed by desolvation of cluster ions ejected in a spraying type of sputtering mechanism involving gasification of matrix/sample molecules. The charges of the cluster ions are mainly supplied by solvated ions present in the liquid.

Biomolecule mass spectrometry by fast heavy ion induced desorption

Fast heavy ions like fission fragments from a 252Cf-source induce desorption of molecular and fragment ions when they hit the surface of a bioorganic solid. This effect can be exploited in the ion-source of a mass spectrometer. As compared to other ionization methods available in mass spectroscopu this method is particularly powerful for large thermally labile biomolecules, e.g. proteins. Recently quasi-molecular ions from a protein of molecular weight 13 980 amu were observed in our laboratory with the use of fast ion induced desorption and the time-of-flight technique. A new area of research is rapidly developing. It involves studied of mechanisms for fast ion induced desorption and biomolecule ion formation and the development of experimental tools to determine the mass of these large molecular ions. Both 252Cf fission fragments and beams from several heavy ion accelerators are used in this research. The field will be briefly reviewed and results obtained recently at Uppsala will be discussed in some detail.

High resolution mass spectrometry of liquid metal ion sources

The mass spectra of In, Ga, and Au liquid metal ion sources (LMIS) exhibit large molecular ion clusters with the maximum observed number of atoms/cluster ranging from 6 for Ga to 12 for In. Au ion sources emit a large percentage of its ions in the form of clusters while the ratios of ion clusters to atomic ions for Ga and In sources are one part in 103 and 104, respectively. Emission characteristics (intensity, energy distributions, and deficits) of the emitted ions of different charge and mass states were measured as a function of ion source temperature, current and emission angle. The effects of these parameters on the emitted ion velocity distribution may yield information on the mechanism of ion formation and the observed energy spread. In all of the sources tested the (dI/dΩ) distribution of the molecular clusters was broad at low energy deficits and narrow at high deficits indicating different regions of formation. However, the atomic ion angular distributions for high and low ion energy deficits are nearly equal in width, but show distinct differences in shape. Models of ion formation are presented.

Two-color multiphoton spectroscopy: detection of n = 12-41 Rydberg states in diazabicyclooctane

Two-color multiphoton adsorptions to very high lying (n = 12-41) Rydberg states in diazabicyclooctane and azabicyclooctane are analyzed. The problem of spectral congestion close to the ionization limit is overcome by choosing an appropriate two-photon intermediate electronic state, the lowest-lying 3s Rydberg, to introduce vibrational selectivity in the sequential adsorption process. The subsequent one-photon absorption to the higher lying Rydberg preserves the vibrational selectivity leading to extremely simple, uncongested spectra. The sequential absorptions are monitored by production of ions during subsequent collisions in a low-pressure cell. Analyses of the extended Rydberg series have provided experimental values of ionization potentials to particular vibrationally excited cores to +/-5-cm/sup -1/ resolution. Several series, with differing quantum defects, are seen to converge to each limit. These series are correlated with different symmetry molecular orbitals. Finally, these results suggest that large molecular ions may be produced optically in specific vibrationally excited states by using modifications of this technique.

Molecular clustering about a positive ion. Structures, energetics, and vibrational frequencies of the protonated hydrogen clusters H+3, H+5, H+7, and H+9

The positive hydrogen clusters H+(H2)n for n=1, 2, 3, 4 have been studied via nonempirical molecular electronic structure theory. Using double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets, wave functions are reported at both the self-consistent field (SCF) and configuration interaction including all single and double excitations (CISD) levels of theory. In each case analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies. The effects of electron correlation are shown to be greater for these loose molecular complexes than for ordinary molecules. Although H+5 in its lowest energy conformation is not qualitatively described as H+3⋅H2, the larger molecular ions do fit the qualitative picture H+3(H2)n, with H+3 as a nucleating center. Of special interest here are the ‘‘new’’ normal modes of these clusters, i.e., those modes having no counterpart in the isolated H+3 or H2 species. There are 15 such vibrational degrees of freedom for H+9, and the resulting harmonic vibrational frequencies range from 775 cm−1 all the way down to 63 cm−1. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Kebarle.

The laser spectroscopy of cool, cold, and very cold molecular ions

Molecular ions have long presented a challenge to spectroscopists. Because of the extreme difficulty in generating molecular ions in high concentrations, sensitivity limitations have restricted work in this field. The technique of laser-induced fluorescence (l.i.f.) has provided us with a means to partially overcome this limitation. L.i.f. spectra of small molecular ions, e.g. N 2 +, CO+ and CO 2 +, show Doppler-limited resolution. Besides their spectroscopic information content, such spectra also serve as quantum-state selective ion detectors, by which dynamical processes involving such ions can be followed. L.i.f. spectra have also been recorded for larger organic ions, e.g. olefinic, acetylenic and benzenoid cations. The large molecular ions’ spectral congestion, caused by the large number of levels populated at room or elevated temperatures, can severely limit the resolution and interpretability of the spectra. As a means of surmounting this problem, we have studied many of the cations at reduced temperatures. The techniques employed include liquid N 2 and free jet expansion cooling in the gas phase, and isolation in solid Ne matrices. The combination of these different spectra lead to a wealth of information about electronic structure, vibrational frequencies, Jahn-Teller effects, etc., in organic molecular ions.

Molecular Synthesis in Interstellar Clouds: Recent Laboratory Studies of Ionic Reactions

Abstract The existence of complex molecules in the harsh environment of interstellar diffuse and dense clouds presents the challenging problem as to their mode of synthesis. Currently, this is believed to be predominantly via many parallel and sequential gas phase ion-neutral reactions which build-up large molecular ions which are then neutralized to give the observed molecules. The obJ.ctive of this paper is to review the current ideas of the most important ion-chemical routes to the observed molecules and to discuss the most recent laboratory data which have supplemented and extended earlier ideas. To this end, the physical conditions within the various types of interstellar clouds and their chemical compositions are first described. The basic ion-chemical models and some of the more important ionic reactions involved in them are then described as they variously apply to both dense and diffuse clouds. Reference is then made to the data obtained recently for the rate coefficients and product ion distributions for a large number of relevant exoergic binary ion-molecule and ion-atom reactions. Attention is then directed towards the process of ion-molecule radiative association which for some time was thought to occur only between C+ ions and H atoms and H2 molecules, but which is now thought to occur in many more ion-molecule interactions, including many involving minority neutral species. The very interesting process of isotope exchange in near-thermoneutral ion-molecule reactions is then considered since it has been invoked to explain the apparent fractionation of rare isotopes into some of the interstellar molecules. Finally, the various neutralization processes which are required to convert product ions into neutral molecules are discussed briefly, and the paper is ended with some concluding remarks.

On ion formation in laser desorption mass spectrometry with LAMMA

The laser microprobe LAMMA has been used to obtain laser desorption mass spectra of a variety of compounds from different chemical classes. Mass spectra of selected samples are presented and discussed which demonstrate the characteristic influence of the chemical nature of the compounds on the types of ions occurring in the spectra. It is suggested that molecular-ion formation from organic compounds and inorganic salts by laser irradiation is primarily a thermal process. Only in the case of formation of molecular ions M +− from aromatic compounds is an electronic process evident. The high heating-rate provided by pulse lasers possibly enhances the probability of desorption of large molecular ions.