F.M. Harris

A novel spectrometer, incorporating an electric sector and quadrupole mass filter, for studying dissociations of ions

Abstract If dissociating polyatomic ions have high velocities (which is usually the case in sectored mass spectrometers) the translational energies released in the dissociations are considerably amplified in the laboratory frame of reference. This gives rise to broad ion kinetic energy peaks for the fragment ions the shapes of which can be measured with high accuracy and from which accurate information on the distribution of translational energies released can be obtained. The broadness of the peaks results, however, in low resolution in fragment-ion spectra. A new type of spectrometer is described which overcomes this problem. Its main components are an electric sector and a quadrupole mass filter; the voltages applied to these are scanned under microcomputer control to give various types of spectra. Fragment-ion spectra are measured with unit-mass resolution and each of the dissociation reactions can be studied to obtain information on the translational energies released without interference from other reactions. Mass spectra are obtainable as well as constant daughter-ion and constant neutral-loss spectra. The spectrometer is described in detail and examples shown of each type of spectrum.

The effect of source temperature on the average internal energy of positive ions

Abstract The average internal energies of photoexcited ions have been varied by changing the temperature of the source in which the ions are formed and photoexciting them through constant photoexcitation energies in the field-free region of a double-focusing mass spectrometer. It is shown that the relative rates of competing dissociation reactions of photoexcited n -butylbenzene and n -pentylbenzene molecular ions are highly sensitive to source temperature. By comparing the results with those obtained by varying the photoexcitation energy at a fixed source temperature it is deduced that the increase in the average internal energy of ions entering the field-free region (i.e. prior to photoexcitation) is about 0.27 eV for a 100dgC increase in source temperature. The magnitude of the chance is confirmed in an entirely different type of experiment in which the translational energies released for various dissociation reactions of photoexcited molecular ions of t -butylbenzene. nitrobenzene and n -propylbenzene are measured as a function of source temperature. Calculations using known thermodynamic data show that the effects observed are related to the change in the average internal energy of the sample molecules with source temperature.

The variation of translational energy release in collision-induced dissociations of polyatomic ions with initial kinetic energy and with observation angle. I. Theoretical considerations

Abstract Angle-resolved mass spectrometry has hitherto focused upon variations in relative yields of fragment ions formed in collision-induced dissociations. This paper investigates the sequence of assumptions which must be made in order to interpret observed variations in peak widths, as the observation angle is varied, in terms of energy deposited in the projectile ion by the collision. Some of these assumptions, such as the nature of the collisional excitation mechanism, and the relationship of the energy thus deposited to the total internal energy of those ions sampled by the fragmentation process, are common to both types of experiment. Other assumptions, such as the manner in which the total internal energy of the reacting ion is partitioned into translational energy of the fragments, are peculiar to the peak width measurements. The latter, however, are free from ambiguities of interpretation arising from convolution of the collisional scattering with differences in angular distributions of fragment ions arising only from differences in translational energy release. For both types of experiment the situation is complex, and interpretation requires considerable care.

Translational energies gained by gases during collisional ionization with high-velocity neutral particles

Abstract A series of experiments are described in which a variety of monatomic and diatomic neutral species having kiloelectron volt translational energies enter an electrically floated collision gas cell where they ionize the target species. The target species are extracted and energy analysed to determine the translational energies gained by them in the collisions. The experimental results have been compared with those predicted by inelastic scattering theory and good agreement has been obtained.

The benzene dication: Energies of excited states determined by double-charge-transfer spectroscopy using OH+ and F+ projectile ions

Abstract Double-charge-transfer spectroscopy has been used to measure double-ionization energies of C 6 H 6 to electronically excited states of the dication. Since OH + and F + projectile ions were used, it is likely, because of the Wigner spin-conservation rule, that triplet states of the dication were populated. Previously computed energies of certain groupings of triplet states of C 6 H 6 2+ are in good agreement with the measured double ionization energies of 24.6±0.5, 27.3±0.5, 29.8±0.5, 31.1±0.5, 32.7±0.8, 34.5±0.8 and 37.2±1.0 eV. Because of the high density of singlet and triplet states, it is only certain that the spin-conservation rule applies to reactions in the present investigation which populate the states with the two lowest energies. Comparison with the calculated data suggests that 24.6 eV is the energy of the 3 A 2g state whilst 27.3 eV corresponds to the group of states 3 E 1u , 3 B 1g , 3 E 1g and 3 B 2g which have been calculated to be separated by no more than 0.34 eV.

Collision-induced reactions of the triply-charged triatomic cation CS23+.

Abstract The ion translational energy spectroscopy of CS23+. ions was investigated. No unimolecular dissociation processes were observed. Collision-induced dissociation processes fell into two categories, those proceeding directly from CS23+. and those proceeding via CS22+ or CS2+. formed by electron transfer from collision-gas molecules. A preliminary discussion of the observed stability of CS23+., in terms of molecular orbitals established by photoelectron spectroscopy, is included.

Calibration of energy deposition in collision-induced dissociations of the butylbenzene molecular ion

Abstract Calibration of the intensity ratios for the fragment ions C7H7+ and C7H8+. formed by collision-induced dissociations, in terms of the increase in internal energy of the C10H14+. (butylbenzene) precursor ion, has been achieved in photodissociation experiments or by using charge-exchange ionization. The results of the various calibration experiments show large discrepancies between one another. The nature of these discrepancies is investigated. It is shown that a major contributing factor is the internal energy content of the C10H14+. precursor ions prior to excitation. Another important factor is the efficiency of detection of the fragment ions. Also, such calibrations of increases in internal energy should be applied only to results of collision-induced dissociations of ions which were prepared in the same way as for the calibration experiment, and which were studied in an identical apparatus.

A laser-ion beam study of the photodissociation of CS2+·(D2Σu+) to S2+·(2Πg) + C(3Pg)

Abstract The photodissociation of CS 2 +· ions, generated by electron impact on CS 2 gas, has been studied in the 514.5–458 nm wavelength region. The translational energy spectra of the photofragment S 2 +· ions from which the translational energies released were calculated indicate absorption from one excited state of CS 2 +· . This state is tentatively identified as B ( 2 Σ u + ).

Photodissociation of positive ions trapped within the space charge of a magnetically confined electron beam

Abstract Photodissociation of molecular ions of substituted benzene compounds has been induced by laser radiation admitted directly into the source of a mass spectrometer. The degree of photodissociation could be varied from 3 to 65% depending upon the conditions within the source and the duration of a mechanically produced pulse of laser radiation at 514.5 nm. Positive ions could be trapped for periods of up to several milliseconds within the space charge of a magnetically confined electron beam under optimum conditions of pressure, electron energy and repeller voltage. Average residence times and relative photodissociation cross-sections were obtained for molecular ions of m-bromotoluene over a wide range of source conditions using a simple kinetics scheme by which the temporal variation of observed ion currents could be simulated. The photodissociation cross-section was observed to vary from 8.3×10−21 to 1.2×10-19 cm2 photon−1 over the range of conditions employed. Laser-induced photodissociation is shown to be a useful technique for characterizing ion trapping potential wells within a conventional source of a mass spectrometer.

Metastable ions and photon-induced reactions

Abstract The ways of studying the reactions of ions that fragment spontaneously between the source and detector of a mass spectrometer are described. The masses, velocities and directions of the product ions formed as a result of fragmentation of these metastable ions can all be investigated and, in addition, information can be obtained on the detailed course of the reactions (mechanism and rate as a function of energy). The partitioning of the energy of the transition state into translational energy and internal energy of the products can be studied in detail. The fragmentation of small ions provides data on their electronic, vibrational and rotational energy levels. Predissociation is an important effect in this case. A convenient method of adding a known amount of electronic energy to the ions is to cause them (either within the source or during their passage through the spectrometer) to absorb a photon. The complementary information to that obtained from studies of metastable ions that is then obtainable is discussed.