I.W. Griffiths

Comparison of photo-excitation of ions and collisional excitation using gases

Abstract The average increases in the internal energy of ions when excited by photons and in collisions with gas molecules have been compared. The positive molecular ions of n -butylbenzene and n -pentylbenzene were used with collisional excitation taking place in the second field-free region of a ZAB-2F mass spectrometer. The ratios of the intensifies of the peaks of the fragment ions C 7 H 7 + and C 7 H 8 + were measured and compared with similar data from photodissociation experiments in which the excitation energy is known to high accuracy. This procedure gives the average collisional excitation energies, i.e. the single-valued excitation energies which would be needed to produce the same dissociation pattern as is produced by the collision gas. These were investigated as a function of the nature of the collision gas, the gas pressure and translational energy of the ions entering the collision region.

Photodissociation of mass-selected ions investigated using a modified ZAB—2F spectrometer

Abstract A commerical reversed-geometry mass spectrometer, the ZAB—2F instrument, has been modified for studies of photodissociation of ions. A quartz window fitted to a special magnet flight-tube allows radiation from an argon-ion laser to pass into the second field-free region of the spectrometer. Photodissociation of mass-selected ions is studied by recording ion kinetic energy spectra of the photofragment ions, the currents being measured using the technique of phase-sensitive detection. The initial studies using the modified spectrometer have been of the molecular positive ions of n-butylbenzene and 4-nitrobenzonitrile. The results demonstrate the control over fragmentation pathways that can be achieved by changing the radiation wavelength and hence the degree of photoexcitation of the reacting ions.

Calculation of the abundance ratio [91+92+] from n-butylbenzene molecular ions as a function of internal energy

Abstract A simple form of the quasi-equilibrium theory (QET) is applied in the calculation of photodissociation fragment-ion currents for competing reactions of the positive molecular ion of n-butylbenzene. Previous experimental results have shown that dissociation of the molecular ion gives mainly fragment ions of masses 91 u and 92 u, the relative abundances being governed by the photoexcitation energy. Optimisation in the calculations of the frequency factor υ0 for the formation of 92+ and of the effective number of oscillators s leads to calculated values of the relative abundances of the photofragment ions which agree with the values determined experimentally. It is shown that only small changes in υ0 and s from their optimum values are needed to take the calculated relative abundances out of agreement with the experimental values. The outcome of the analysis is to give expressions for the rate constants as a function of the internal energy of the parent n-butylbenzene ion for the formation of the fragment ions 91+ and 92+.

Kinetic energies released on dissociation of photoexcited aromatic ions of high translational energy

Abstract An experiment is described in which the kinetic energy released upon photodissociation of organic ions in a modified, reverse-geometry mass spectrometer is measured. Linear dependence of the kinetic energy released on photon energy is observed for most of the ions studied, as predicted by statistical theories. The results indicate that only a fraction of the available degrees of freedom are involved. Non-linear dependence is observed for some of the ions studied and the significance of this is discussed. The kinetic energy release data for isomeric structures exhibit variation in their dependence upon photon energy, and it is suggested that the technique can be used for structural characterisation.

Disposition of excess energy into translational energy in laser photodissociation of mass-selected organic ions

Abstract The positive molecular ions of various alkyl benzenes have been photoexcited and the average translational energy,

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.

Dissociation rate calculations based on photodissociation ion kinetic energy spectra

Abstract The quasi-equilibrium theory (QET) in applied to the calculation of photodissociation daughter-ion currents. Treatments are given for a single reaction pathway from the molecular ion, and also for two competing reaction pathways. With particular reference to the photodissociation spectrum of the nitrobenzene molecular ion, which undergoes two competing reactions, it is found that good agreement can be achieved between calculated and observed ion kinetic energy spectra by treating as adjustable parameters only the frequency factor for NO− loss and the number of effective oscillators. It is shown that other factors which could influence the calculated results, such as the exact form of the internal energy distribution, are unimportant to first order. The fitting of calculated to experimental results leads to unique values for the frequency factor and the number of oscillators.

Unimolecular, collision-induced and photoinduced dissociation of CH3I+− and CD3+−

Abstract The properties of CH3I+− and CD3I+− ions have been studied using a mass spectrometer having facilities for collisional excitation and photoexcitation of mass-selected ions. Ion-kinetic-energy spectra for unimolecular dissociation of the metastable ions were measured and compared with those obtained following excitation of stable ions using the two techniques. For CH3+ or CD3+ fragment-ion formation, two distinct dissociation processes appear to occur for the metastable ions, but only one for ions excited in the spectrometer. One of the two processes is shown to depend on the energy of the electrons used to ionise the molecules in the source. Measurements of the translational energy released following photoexcitation shows it to decrease as the photoexcitation energy increases. This is explained in terms of the increasing importance at higher energies of the dissociation reaction giving I+ in an excited state. Similar data obtained for I+ fragment ions are described and discussed. An unusual feature of the I+ fragmentation peaks resulting from collision-induced dissociation is that they are composite, with a very broad component which relates to large releases of translational energy, of several electronvolts. In this broad component marked asymmetry is observed which resembles that found in the fragment-ion peaks resulting from collision-induced dissociation of smaller ions. This effect has been explained by Russek, and his prediction that the degree of asymmetry is dependent on the polarisability of the collision gas is verified in the present work.

Photodissociation of doubly-charged ions

Abstract Photodissociation of doubly-charged ion has been induced by radiation from an argonion laser and investigated using a sensitive double-focusing mass spectrometer of reversed geometry. Of the molecular ions studied, only those of benzene and benzonitrile were found to photodissociate. The kinetic energy release distribution f ( T ) for both ions was found to be independent of photoexcitation energy in the range 2.41–3.47 eV and with the same respective values of f ( T ) as for unimolecular dissociation. These results confirm that dissociation of doubly-charged ions is a process involving a predissociation mechanism with the major part of T being due to Coulombic repulsion of the charges on the fragment ions. However, the degree of dishing, which depends on several instrumental factors as well as on the translational energy spread of the fragment ions, was more marked in the case of photodissociation than for unimolecular dissociation. This is probably due to the good collimation of the laser beam and the fact that photodissociation occurs only for ions photoexcited in the volume of this beam. Thus, there is an effective pre-collimation of the photoexcited ions.

A combined MIKES and laser photodissociation study of dimethylsulphoxide (DMSO) and DMSO-d6

The mass-analyzed ion kinetic energy spectra (MIKES) of the molecular ion of dimethylsulphoxide (DMSO) and perdeuterated dimethylsulphoxide (DMSO-d6) have been obtained with a reversed-geometry mass spectrometer, ZAB-2F. The average (T22%) and T50% translational energy releases have been determined from the shapes of the major peaks in the ion kinetic energy spectrum. The dominant unimolecular processes of the metastable molecular ion result in the loss of either CH3· or OH· with almost equal frequency. Photodissociation with photons of 2.4 and 2.54 eV occurs almost exclusively by CH3· loss.