J. P. Greene

Photoionization of the amidogen radical

The photoionization mass spectrum of NH2, prepared by the reaction H+N2H4, is presented. The adiabatic ionization potential is 11.14±0.01 eV (0.32 eV lower than reported by PES). A prominent autoionizing Rydberg series is observed, converging to the excited A 1A1 state at 12.445±0.002 eV. By extrapolation, NH2 should absorb strongly at ∼1150 A. From the threshold for formation of NH+ (NH2), we obtain ΔH0f 0(NH+)=396.3±0.3 kcal/mol. With auxiliary data, we compute ΔH0f 0(NH)=85.2±0.4 kcal/mol, ΔH0f 0(NH2)=45.8±0.3 kcal/mol, D0(H2N–H)=106.7±0.3, D0 (HN–H)=91.0±0.5, and D0 (N–H)=79.0±0.4 kcal/mol. Additional photoionization measurements on N2H4 and N2H3 are also included.

Photoionization mass spectrometric study and ab initio calculations of ionization and bonding in P–H compounds; heats of formation, bond energies, and the 3B1–1A1 separation in PH+2

The ion yield curves of PH+3, PH+2, and PH+ from photoionization of PH3 have been measured. The free radical PH2 has been generated by pyrolysis, and the ion yield curve of PH+2 (PH2) determined. These measurements yield directly I.P. (PH3)=9.870±0.002 eV, and I.P. (PH2)=9.824±0.002 eV. In addition, we deduce D0 (H2P–H)=82.46±0.46 kcal/mol, D0 (HP–H)=74.2±2 kcal/mol, D0 (P–H)=70.5±2 kcal/mol, I.P. (PH)=10.18±0.1 eV, and other thermochemically related quantities. The ionization energies of PH, PH2, and PH3 are computed by ab initio molecular orbital methods to fourth order in Mo/ller–Plesset theory, and are found to be in good agreement with experiment. The ground state of PH+2 is inferred to be 1A1, with the 3B1 state higher by ≥0.71 eV. This ordering is the reverse of that in CH2 and NH+2.

The ionization potentials of CH4 and CD4

The adiabatic ionization potential of CD4 is measured by photoionization mass spectrometry to be 12.658±0.015 eV, which is 0.05±0.02 eV higher than that of CH4. The difference is attributed to zero point energy differences, rather than different Jahn–Teller stabilization energies.

Bonding and ionization energies of N--F and P--F compounds

NF3, N2F4, NF2, PF3, P2F4, PF2, and PF2I have been studied by photoionization mass spectrometry. Modified or confirmatory values have been obtained for the heats of formation of NF+3, NF+2, and NF+ and the ionization potentials of NF3 and NF2. The stepwise bond energies are deduced to be D00(NF) =3.27±0.02 eV; D00(FN–F) =2.85±0.02 eV; and D00(F2N–F) =2.47±0.01 eV. Heats of formation at 0 K have been measured for PF+2 (89.6±0.5 kcal/mol), PF+ (≤214.8 kcal/mol), and PF2I (−142.3±1.0 kcal/mol). The directly measured ionization potential of PF2 is 8.85±0.01 eV. The stepwise bond energies are found to be D00(PF)

A photoionization study of SeH and H2Se

The photoion yield curve of SeH, prepared by the reaction H+H2Se is presented. The adiabatic I.P. is 9.845±0.003 eV, and autoionization structure is observed, from which higher I.P.’s are inferred. The photoion yield curves of H2Se+, SeH+, and Se+ from H2Se are also measured. The fragmentation thresholds, together with I.P. (SeH), enable one to infer the bond energies D0(HSe−H)=78.99±0.18 kcal/mol and D0(SeH)=74.27±0.23 kcal/mol. The adiabatic I.P. for H2Se (X 2B) is 9.886±0.003 eV.

Photoionisation of atomic bromine

The experimental relative photoionisation cross section of atomic bromine from ionisation threshold to approximately 900 AA is presented. Two autoionising Rydberg series are identified converging to 3P1, two to 3P0, three to 1D2 and two to 1S0. The spectrum bears a strong resemblance to that of atomic chlorine, including the additional sharp series converging to 1D2 unaccounted for in ab initio calculations. Complexities emerge in the 3P region, prefiguring the complex structure in atomic iodine.

Vibrational autoionization in PF3: Doing violence to the propensity rule

The photoionization spectrum of PF+3 in its threshold region displays two prominent progressions of autoionization peaks. When these are analyzed, together with earlier photoabsorption studies and a photoelectron spectrum, they lead to the conclusion that vibrational autoionization is occurring, with Δν≤−13. This conclusion stands in sharp contrast with the current theory of vibrational autoionization, which predicts a propensity rule Δν=−1. Other examples from the recent literature are summarized, to suggest that a more general theory of vibrational autoionization is required.

Photoionisation of atomic sulphur

The photoionisation spectrum of atomic sulphur prepared by the reaction of H atoms with H2S, is presented from its ionisation threshold to 925 AA. Autoionisation peaks assigned to quasidiscrete levels . . . (2D degrees )nd3S degrees , 3P degrees are sharp, whereas those designated . . . (2D degrees )nd 3D degrees are broad. This latter feature parallels the discontinuity observed between first-row and heavier elements in the noble gases and halogens. There is evidence for severe perturbation in the . . , (2D degrees )nd 3P degrees series, which could conceivably be attributed to a proximate 3s3p5 3P degrees state.

The barrier to inversion in NF+3

The first band in the He I photoelectron spectrum of NF3 has been obtained at sufficiently high resolution to reveal vibrational structure. The bifurcated nature of the band is interpreted as a transition from pyramidal NF3 to pyramidal NF+3, the latter having a lower barrier to inversion. The span of the Franck–Condon region in the transition is sufficient to surmount the barrier, whose magnitude can be estimated from the spectrum to be ∼0.75 eV. This is one of the very few cases for which an accurate experimental measurement of an inversion barrier is known.

Photoionisation of atomic phosphorus

The photoionisation spectrum of atomic phosphorus prepared by successive abstraction reactions involving H atoms and PH3, is presented from its ionisation threshold to 765 A. Prominent autoionisation structure is observed between the fine-structure limits3P2-3P0 near the ionisation threshold. In addition, a window resonance series is seen, involving inner s-shell excitation to . . .(5S)np 4P states. Some systematic behaviour of resonance profiles and quantum defects in a series of atoms is discussed.