J. Berkowitz

Photoionization of HCN: The Electron Affinity and Heat of Formation of CN

A re‐examination of the photoionization of HCN has revealed two processes, HCN + hν→H + +CN−, and HCN + hν→H + +CN+e, not previously reported. The difference in threshold values for the two processes leads directly to the electron affinity for CN, 3.82 ± 0.02 eV. The threshold for the second process leads to ΔHf0°(CN) = 105.5 ± 2 kcal/mole, but ΔHf°(HCN) remains a major source of uncertainty.

Photoionization of Ethane, Propane, and n‐Butane with Mass Analysis

Relative photoionization cross‐section curves have been determined for parent and fragment ions of ethane, propane, and n‐butane over the photon‐energy range from the ionization potential to 14.0 eV. The data have been treated to give experimental breakdown curves which are in good qualitative and fair quantitative agreement with the predictions of the statistical theory of mass spectra. The results are also compared with data obtained by charge exchange, electron impact, and photoelectron energy analysis by other workers. Heats of formation of some ions have been obtained more accurately and the ionization potential of the C2H5 radical is confirmed to be 8.4 eV. Ion‐pair formation of H− is shown to occur at least for ethane and n‐butane and very probably for propane.

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.

Interpretation of Photoionization Threshold Behavior

Analytical expressions are derived for treating the threshold regions of photoionization efficiency curves. The effects of slitwidth, the initial thermal energy of the molecular system, and Franck–Condon factors are considered. In the dissociative ionization of small molecules such as F2 and HCN, discrepancies can arise from failure to take these factors into account. It is also shown that for large polyatomic molecules, the use of a Watanabe‐type plot to determine the adiabatic ionization potential can lead to large errors. The paper concludes with detailed applications to HF, O2, S2, Se2, Te2, S6, and S8, and some general observations applicable to the diatomic halogens, hydrogen halides, and alkali halides.

Electron Affinities of Halogen Diatomic Molecules as Determined by Endoergic Charge Transfer

Ion‐pair formation by photon absorption at threshold wavelengths has been used to prepare I−, Br−, and F− ions with approximately room‐temperature thermal energies, as verified by retarding‐potential measurements. The primary ions were accelerated and their reactions with halogen molecules studied at laboratory kinetic energies from 0.0 to about 4.0 eV. Thresholds were determined for endoergic reactions of the type X−+Y2→X+Y2 −, where X may be the same as Y. At least two reactions were used in determining each electron affinity. The agreement was good in all cases. The values of electron affinity obtained are 3.08±0.1 eV for F2, 2.38±0.1 eV for Cl2, 2.51±0.1 eV for Br2, and 2.58±0.1 eV for I2. Interhalogen molecular ions such as IBr− were also observed, and measurement of the threshold for formation gave the value 2.7±0.2 eV for the electron affinity of IBr. The retarding‐potential measurements of F− from F2 strongly support a value for the dissociation energy of F2 in the neighborhood of 1.6 eV.

Vacuum ultraviolet photoionization mass spectrometric study of C60

Gaseous C60 has been studied by photoionization mass spectrometry between the ionization threshold and 40.8 eV. An adiabatic threshold of 7.57±0.01 eV is observed, which may be slightly low due to hot bands. The energy derivative of the photoion yield curve is in rough agreement with the He i photoelectron spectrum on the positions of some peaks, but others are weak or absent. The discrepancy is not attributed to autoionization, but rather to selection rules governing the ejection of low energy electrons into high angular momentum waves. C++60 is observed at higher energies, and becomes ∼0.6 as intense as C+60 at 40.8 eV. The photoion yield curve of C++60, approximately linear well above threshold, appears to exhibit curvature near threshold, thwarting an attempt to make a distinction between two alternative values of the second ionization potential. Fragmentation to form C+58 is only observed at the highest energy, 40.8 eV. The unimolecular decay is modelled by quasiequilibrium theory. In this model, the...

Photoelectron spectroscopy of phthalocyanine vapors

The 21.2 eV photoelectron spectra of metal‐free phthalocyanine and some metal‐containing phthalocyanines MPc (M=Mg,Fe,Co,Ni,Cu, and Zn) have been obtained for the gaseous molecules. Comparison of these spectra reveals that the uppermost occupied orbitals are ringlike, and not metal‐3d‐like, in all cases. Identification of some features in the 1487.6 eV photoemission spectra of thin films of phthalocyanines with similar features in the gas phase spectra enable one to establish an absolute energy scale for the thin film work, and to locate the ionization energies of the 3d‐like orbitals. The experimental results and inferences are compared with recent ab initio calculations, and indicate that the Xα local density method is a promising one for describing the electronic structure of these large molecules and the generically related biological systems, chlorophyll and hemoglobin. In some spectra, phthalonitrile was identified as an impurity, and verified by obtaining the 21.2 and 40.8 eV photoelectron spectra ...

High‐Resolution Photoionization Study of the H2 Molecule near Threshold

Relative photoionization and absorption cross sections of H2 (ordinary and para) have been measured from 745–810 A at 300 and 78°K, with a resolution width of 0.04 A. The photoionization data show the presence of extensive structure due to autoionization of vibrationally excited Rybderg states. Analysis of the data on the D–X(6, 0) band in the region of the ionization threshold leads to the conclusion that the ionization potential of H2 lies between 124 418.2 and 124 393.5 cm−1 and is probably very near the former value. Analysis of the data on the B″–X(4, 0) band requires either that the ionization potential lies below 124 407.2 cm−1 (which is rather unlikely) or that autoionization of the R(2) and R(3) lines occurs in violation of one of the autoionization selection rules proposed by Beutler and Junger. Some Rybderg states that lie just below their ionization limits and do not autoionize spontaneously can be made to ionize in weak electric fields and by collision with unexcited H2 molecules. Direct ioni...

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.

Photoionization mass spectrometric study of formaldehyde H2CO, HDCO and D2CO

Photoionization efficiency curves for H2CO+, HDCO+, D2CO+ and HCO+, DCO+ and CO+ fragment ions are obtained. The ionization potentials of H2CO, HDCO, and D2CO thus determined are, respectively, 10.868, 10.879, and 10.882 eV. The appearance potential of HCO+ fragment is observed at 11.92±0.01 eV, from which the dissociation energy of H2CO+, the proton affinity of CO, and the ionization potential of HCO are inferred. The heat of formation ΔHf00 (HCO+) =196.2±0.5 kcal/mole is obtained, a value lower than any one previous determination. From the abundance ratios measurements the variation of the mass spectrum with photon energy is determined, and hydrogen isotope effects are measured. The observation of a metastable transition (D2CO+)m→DCO++D with a threshold some 2 eV above the dissociation limit is discussed along with the dissociation mechanisms with the help of correlation diagrams. Finally, the finer structure in the photoionization efficiency curves is analyzed in terms of preionized Rydberg series conv...