J. H. D. Eland

Mass spectra and doubly charged ions in photoionization at 30.4 nm and 58.4 nm

Abstract Doubly charged ion abundances and photoionization mass spectra produced by He(I)α and He(Il)α radiation are reported for molecules from several classes of compound, and are compared to data from previous electron and photon impact investigations. More doubly charged ions relative to singly charged ions are produced by photon impact than by electron impact at the same energy, the contrast being most marked for small polyatomic molecules. Lifetimes of double charged ions are reported for CO 2 2+ , COS 2+ , CS 2 2+ , and DI 2+ . A compilation of existing data on doubly charged ions is presented, and an empirical formula for estimating the appearance potentials of doubly charged ions is proposed. It is concluded that at 30.4 nm initial double ionization is a substantial fraction of total ionization for all large molecules, and must appear in the photoelectron spectra.

Photoionization of N2O: Mechanisms of photoionization and ion dissociation

From high resolution photoionization mass spectrometric data on normal and isotopically substituted N2O it is concluded that the formation of NO+ below the A state of N2O+ but above the thermochemical threshold involves the calculated but hitherto unobserved 4A″ state. This state decays to NO+ from low vibrational levels, and mainly to O+ from higher levels. The O+ ion is a more important product of N2O photodissociation than previously thought. The B state of N2O+ is shown to decompose to NO+ and N2+ and the C state to NO+, N2+, N+, and O+. The branching in autoionization from the A, B, and C Rydberg series to states of different continua has been determined and is found to depend more on the type of series than on the principal quantum number. The shapes of the resonances are strikingly different in the different final channels. The relevance of these findings to ionospheric O++N2 reaction is discussed.

Formation and predissociation of CO+2(? 2Σ+g)

The discrepancy between photoionization mass spectrometry and photoelectron spectroscopy in the CO+/O+ ratio from CO2 at 584 A is resolved by new measurements. New features in the photoelectron spectrum of CO+2(? 2Σ+g) lead to a new vibrational analysis, with vibration frequencies close to those of CO2(?1 Σ+g). The observations are discussed in terms of a perturbation of the ? 2Σ+g state by the ionization continuum of either the ? 2Πg state or the 4Πu state of CO+2.

Dissociative photoionization of carbon disulphide and carbonyl sulphide

High resolution photoionization mass spectrometry shows that S+ from CS2 and COS, and CS+ from CS2 appear at the thermochemical dissociation limits with no kinetic or internal energy release to the fragments, accompanied by zero energy electrons. S2+ is formed from CS2 by a bimolecular reaction of CS2+(?) of very high cross section, as well as by unimolecular fragmentation. Five new Rydberg series of COS leading to the ? and ? states are identified, and empirical observations on resonance peak shapes which differ in different channels are presented.

Photoionization mass spectrometry of HI and DI at high resolution

The ionization potentials of HI and DI to the 2Π3/2 and 2Π1/2 states are redetermined by extrapolation of Rydberg series seen in photoionization, as: I.P.(HI), 2Π3/2=10.386±0.001 eV, 2Π1/2 =11.0495±0.001 eV; I.P.(DI), 2Π3/2=10.387±0.001 eV, 2Π1/2 =11.0505±0.001 eV The spin‐orbit parameter A(HI) =−0.664±0.002 eV. Several new Rydberg series have been identified and the first vibrationally excited levels have been located in three of the 2Π states. The spectra of HI and DI are discussed and are contrasted with the photoionization spectrum of atomic Xe. Changes in the relative shapes and intensities of Rydberg peaks as functions of n* are ascribed to changes in angular momentum coupling schemes.

Angular distributions, energy disposal, and branching studied by photoelectron–photoion coincidence spectroscopy: O2+, NO+, ICl+, IBr+, and I2+ fragmentation

It is shown that some levels of the A2Πu states of I2+, IBr+, and ICl+ are stable toward dissociation, and ought to decay by emission. The B2Σg+ state of I2+ is fully dissociated to ground state I++I, while the B2Σg+ states of IBr+ and ICl (probably) yield excited decay products. The c3Π and B1Π states of NO+ dissociate to ground state N++O fragments, while the c4Σg− and 2Πu states of O2+ near 24 eV dissociate to excited levels of O++O. The origin of anomalous time‐of‐flight peak shapes is discussed, and several examples are attributed to anisotropic fragment angular distributions.

Rates of unimolecular pyridine ion decay and the heat of formation of C4H4

A detailed analysis of rate constant and metastable ion data from photoionization mass spectrometry and photoelectron—photoion coincidence spectroscopy of pyridine shows that the dissociation C5H5N+ → C4H4+ + HCN is well described by the strong form of the quasi-equilibrium theory. By theoretical modelling of the observations we set limits of 277–282 kcal mol−1 on the heat of formation of C4H4+. The use of photoionization mass spectrometry data to obtain breakdown curves is thoroughly examined.

Fluorescence quantum yields of isotopic CO2+ ions

Abstract Fluorescence quantum yields of the A 2 Π u and B 2 Σ u + states of the isotopic ions 12 C 16 O 2 + , 13 C 16 O 2 + and 12 C 18 O 2 + , formed by 584 A photoionization of CO 2 , have been measured by a photon—photoion coincidence technique. The corresponding 584 A photoelectron spectra, measured for 12 C 16 O 2 and 13 C 16 O 2 , showed that the relative photoelectron branching ratios for forming the A 2 Π u and B 2 Σ u + states of CO 2 + are isotope independent. The fluorescence quantum yields are greater than unity for the A 2 Π u state and less than unity for B 2 Σ u + for the three isotopic ions but the values are isotopic dependent, the difference from unity correlating with the degree of spectral perturbation in the B 2 Σ u + − X 2 Π g 0 0 band. Calculations are made to identify the vibronic levels of A 2 Π u capable of interacting with the B 2 Σ u + (0, 0, 0) level. The discrepancy concerning the apparent population ratio of the A 2 Π u and B 2 Σ u + states of CO 2 + , formed by photoionization of CO 2 , between the results of photoelectron spectroscopy and those of CO 2 + , ion fluorescence intensities is explained as due to interelectronic state coupling. The effects of this discrepancy on the determination of cross sections for forming the B 2 Σ u + and A 2 Π u states by various excitation mechanisms are discussed.

Accurate photoionization thresholds by multiphoton ionization: Pyrrole

Multiphoton ionization spectroscopy has been used to measure ionization threshold for pyrrole.(AIP)

Vibrational level populations in the autoionization of oxygen

Autoionization from the I and I′ resonance series of O2 has been reexamined using high resolution for photons and electrons. Photoelectron spectra on the resonances are predicted from the upper potential energy surface parameters and photoionization yield curve alone, without reference to the uncertain line shapes and widths. Bond lengths are determined as 1.370±.005 A in I and 1.380±.005 A in I′ of O2.