Photoion Yield Curves Research Articles

ChemInform Abstract: OBSERVATIONS ON HOT BANDS IN THE MOLECULAR AND DISSOCIATIVE PHOTOIONIZATION OF ACETYLENE AND THE HEAT OF FORMATION OF THE ETHYNYL ION

Photoion yield curves in the vicinity of threshold are obtained for the molecular and the ethynyl ions of acetylene and acetylene‐d2 at ion source temperatures of 360, 298, and 130 °K. Weak ionization below the adiabatic threshold for C2H2+ and C2D2+ is ascribed to the ionization of molecules excited by one quantum of the bending vibrations, ν4 and ν5. Consideration of selection rules suggests a change in symmetry from the linear ground state molecule to a bent gound state ion. The 0 °K curves for C2H+ and C2D+ are estimated from the observed 130 °K data. Satisfactory agreement is obtained when the 298 °K data are compared with a curve calculated from the 0 °K curve by convolution with vibrational and rotational distributions. The 0 °K thresholds corrected for kinetic energy are used to calculate Δ H f0°(C2H+)=17.47 ± 0.01 eV (402.8 ± 0.2 kcal mol−1) and ΔH f °0(C2D+) = 17.43 ± 0.02 eV (402.0 ± 0.5 kcal mol−1). The ionization energy of the ethynyl radical is estimated to be 11.96 ± 0.05 eV.

Photoionization of carbon dioxide

The yields of photoions from CO2 cooled to 150 °K have been measured at a resolution of 0.22 Å for CO2+ and 0.4 Å for the O+ and CO+ fragments, in the photon energy region extending from onset for each species to approximately 20 eV. Most of the observed structure of the molecular ion yield curve recapitulates the well known features of the absorption spectrum, but a number of anomalies are reported. Autoionization peaks at photon energies just above the first molecular ionization limit of 13.773 ± 0.002 eV do not fit into a pattern characteristic of Rydberg series converging to excited vibrational levels of the ion. Although the Tanaka-Ogawa series and Henning's sharp series can be assigned as ns Rydberg series on the basis of quantum defects, the expected 3s members could not be observed in the present study. The most prominent feature of the O+ yield curve is a step at 19.39 eV, suggesting that the principal mechanism for production of this fragment is predissociation of CO2+ (C 2 Σg+) in its ground vibrational state. Completeness of this predissociation is inferred, although the proposed explanation involves doublet-quartet mixing at large bending angles. The structure of the O+ yield curve between 19.07 and 19.39 eV suggests that Rydberg states converging to CO2+ (C) also undergo predissociation, with spontaneous ionization, to produce this fragment ion. Evidence pertaining to production of CO+ fragments by predissociation of excited vibrational states of CO2+(C) is also presented.

Photon impact studies of C2HCN and CH3CN in the vacuum ultraviolet; heats of formation of C2H and CH3CN

A photodissociation process to produce CN B2Σ from C2HCN and CH3CN has been studied as a function of incident wavelength. Threshold photon energies required for the production of CN B2Σ from C2HCN and CH3CN are 9.41 ± 0.04 and 8.52 ± 0.03 eV, respectively, from which D0 (C2H–CN) ≤ 6.21 ± 0.04 eV and D 0(CH3–CN) ≤ 5.32 ± 0.03 eV are obtained. The photoionization yield curves have been measured for the C2HCN+ and C2H+ ions. Threshold photon energies obtained for the production of CN B2Σ, C2HCN+, and C2H+ from C2HCN lead to the following thermochemical values; I.P.(C2HCN) = 11.64 ± 0.01 eV, I.P.(C2H) = 11.96 ± 0.05 eV, Δ H f0°(C2HCN) ≥ 85 ± 1 kcal mol−1 (355 ± 4 kJ mol−1). Δ H f0°(C2H) = 127 ± 1 kcal mol−1 (531 ± 4 kJ mol−1) and D0 (C2H–H) = 5.38 ± 0.05 eV. Δ H f0°(C2H) obtained is in good agreement with the recent value obtained directly from a study of the high temperature reactions of graphite with hydrocarbons. Δ H f0°(CH2CN) ≥ 14 ± 1 kcal mol−1 (59 ± 4 kJ mol−1) derived from D0 (CH3–CN) agrees within the stated error limit with the value obtained recently by bomb calcorimetry. The fluorescence efficiency vs incident wavelength curves for C2HCN and CH3CN show several peaks corresponding to Rydberg states indicating that the process is predissociative. The absorption coefficient of C2HCN has been measured in the vacuum ultraviolet. The photoionization yield curve for C2HCN+ shows at least two Rydberg series converging to vibrationally excited C2HCN+ ions.

Observations on hot bands in the molecular and dissociative photoionization of acetylene and the heat of formation of the ethynyl ion

Photoion yield curves in the vicinity of threshold are obtained for the molecular and the ethynyl ions of acetylene and acetylene-d2 at ion source temperatures of 360, 298, and 130 °K. Weak ionization below the adiabatic threshold for C2H2+ and C2D2+ is ascribed to the ionization of molecules excited by one quantum of the bending vibrations, ν4 and ν5. Consideration of selection rules suggests a change in symmetry from the linear ground state molecule to a bent gound state ion. The 0 °K curves for C2H+ and C2D+ are estimated from the observed 130 °K data. Satisfactory agreement is obtained when the 298 °K data are compared with a curve calculated from the 0 °K curve by convolution with vibrational and rotational distributions. The 0 °K thresholds corrected for kinetic energy are used to calculate Δ H f0°(C2H+)=17.47 ± 0.01 eV (402.8 ± 0.2 kcal mol−1) and ΔH f °0(C2D+) = 17.43 ± 0.02 eV (402.0 ± 0.5 kcal mol−1). The ionization energy of the ethynyl radical is estimated to be 11.96 ± 0.05 eV.

Photoionization of acetylene near threshold

Photoion yield curves for C 2H 2 and C 2D 2 cooled to 118 K are remeasured at 0.2 Å intervals from ionization threshold to about 0.6 eV above threshold. Clearly defined autoionization features are observed superimposed on vibrational step structure. The latter observation suggests molecular predissociation occurring in competition with the autoionization process. A simple means of estimating Franck-Condon factors for the direct ionization in the presence of autoionization gives values in agreement with photoelectron spectroscopy data. Comparisons of observed and calculated Rydberg levels converging to vibrationally excited states of the molecular ion are discussed briefly.

Effect of hot bands on the ionization threshold of some diatomic halogen molecules

Photoion-yield curves are obtained for the molecular ions of the homonuclear diatomic halogens and for iodine monochloride and iodine monobromide. Excepting iodine, ionization thresholds of ground-state and vibrationally-excited molecules are separately identified by measurements made at several ion-source temperatures. The adiabatic ionization energies (in eV) are: I(F 2 = 15.69, I(Cl 2) = 11.48, I(Br 2) = 10.52, I(ICl) = 10.07, and I(IBr) = 9.79. A preferred value of I(I 2) = 9.37 eV is proposed.

Photoionization Study of Chlorine Monofluoride and the Dissociation Energy of Fluorine

Mass spectra and photoionization yield curves are obtained for the molecular and atomic ions of chlorine monofluoride. The atomic ions are formed both by ion-pair and by dissociative ionization processes, although the F+ ion formed by the latter process is too weak for quantitative measurements. The ClF+ curve exhibits a weak onset at 12.55 eV, ascribed to a hot band. An intense onset at 12.65 eV is ascribed to the (0, 0) transition. The observed thresholds for the Cl+ ion support the spectroscopic determination of D(ClF) = 2.558 eV and, combined with a recent determination of the heat of formation of ClF, support the previously reported photoionization value of D0°(F2) = 1.34 eV.

Photoionization study of the dimethyl compounds of zinc, cadmium, and mercury

Photoionization yield curves from onset to 600 Å and ionization threshold values have been obtained for the ions M(CH 3) 2 +, CH 3M + and M + of the dimethyl compounds of zinc, cadmium, and mercury, and the CH 3 + from dimethyl-mercury. Bond dissociation energies and heats of formation of the ions are also reported.

Photoionization Study of Fe(CO)5 and Ni(CO)4.

Photoionization yield curves from onset to 600Å, and ionization threshold values have been obtained for the ions , , , , , Fe+ and CO+ from iron penta-carbonyl, and for , , , , Ni+, and CO+ from nickel tetracarbonyl. From these curves, information on the ionization-fragmentation processes of Fe(CO)5 and Ni(CO)4 under photon impact, have been obtained. Differences in the threshold energies as determined by photon and by electron impact methods increase with the fragmentation. The appearance potentials and the heats of formation of the metal-ions, agree within 0.1 eV, with those calculated from thermochemical data. The average bond dissociation energy of the neutral molecule agrees within 0.02 eV, with the thermochemical values. The kinetic shift for the formation of Fe+ and Ni+ ions is very small. The Ni ion is most likely formed directly from the molecule ion rather than by a cascade fragmentation process.

Absorption Cross Section and Photoionization Yield of I2 between 1050 and 2200 Å

The total absorption cross section and photoionization yield of molecular iodine were measured from 1050 to 2200 Å. Two thresholds were observed in the photoionization yield curve. The first was associated with the ion-pair production I+ + I− and the second to direct ionization of I2. From the appearance potential for ion-pair production and the accurate ionization potential of I2 it was possible to calculate the value of the electron affinity of iodine, namely, 3.073 ± 0.014 eV.

Photoionization Study of the Dissociation Energy of Fluorine and the Heat of Formation of Hydrogen Fluoride

Mass spectra and photoionization yield curves are obtained for the molecular and atomic ions of fluorine and hydrogen fluoride. The atomic ions of both molecules are formed by ion-pair and by dissociative ionization processes. The F2+ curve has a weak onset at 15.58 eV, ascribed to a hot band, followed by an abrupt, intense onset at 15.69 eV, ascribed to the 0–0 transition. Several broad bands of autoionization are observed. The onset of the dissociative ionization process occurs at 18.76 eV, from which we calculate D°0(F2) = 1.34 ± 0.03 eV (30.9 kcal mole−1). The only observed ion-pair process is spin forbidden, producing F+(3P) + F−(1S), for which the thermodynamic threshold is not reached. The HF+ curve has a slightly tailing onset at 15.92 eV. No F+ ion is observed. The dissociative ionization process yielding H+ occurs at 19.34 eV, permitting the calculation of D°0(HF) = 5.74 eV, and ΔHf°0(HF) = −2.83 ± 0.03 eV (−65.3 kcal mole−1). Identical ion-pair curves have a threshold at 15.87 eV resulting in EA (F) = 3.47 eV. The indicated uncertainties are estimated limits of error.

Mass-Spectrometric Study of Photoionization. XI. Hydrogen Sulfide and Sulfur Dioxide

Photoionization-yield curves are obtained for the molecule and fragment ions of H2S and SO2 from onsets of ionization to 600 Å. Structural features of the curves are discussed. Ionization thresholds are tabulated and heats of formation of ions and radicals are calculated. Derived quantities (in kilocalories per mole) include D0(HS–H) = 89.3, D0 (SH) = 83.2, ΔHf0°(S) = 65.0, D0(S2) = 99.4, and D0(OS–O) = 129.1. The ionization energy, I(SO) = 10.21 eV, is obtained by indirect means.

Mass-Spectrometric Study of Photoionization. IX. Hydrogen Cyanide and Acetonitrile

Photoionization yield curves are obtained from threshold to 600 Å for the HCN+ and CN+ ions of HCN and for the principal ions of CH3CN. The first ionization energy of HCN is 13.59 eV. The threshold for the CN+ ion permits calculation of ΔHf0°(CN) = 100 kcal mole−1, D0(CN) = 182.5 kcal mole−1, D0(H–CN) = 119.2 kcal mole−1, and D0(NC–CN) = 126.6 kcal mole−1. Dissociative photoionization processes in acetonitrile do not produce methyl or cyanogen radical ions in sufficient abundance for direct verification of the above. Nevertheless, threshold values of the principal photoions of CH3CN are tabulated, and some thermodynamic values are calculated, including ΔHf0°(CH2) = 94 kcal mole−1.

Lowest Ionization Potentials of Some Nitrogen Heterocyclics

The photionization potentials of the highest occupied orbital of pyrazine (9.29 eV), pyrimidine (9.35 eV), pryidazine (8.71 eV), naphthalene (8.15 eV), quinoline (8.62 eV), isoquinoline (8.55 eV), quinoxaline (9.02 eV), cinnoline (8.95 eV), and phthalazine (9.22 eV) are determined. In addition, estimates of the second ionization potentials of cinnoline (∼9.1 eV) and phthalazine (∼9.3 eV) are obtained from the analyses of the spectral characteristics of the photoionization yield curves. A discussion of the observed and estimated values of the ionization potentials in terms of the type of the ionized electron (π vs nonbonding) is given. It seems that for all the molecules studied, except pyridazine and phthalazine, the π electron has a lower ionization potential than the nonbonding electron. The interchange of the type of the ionized electron in pyridazine and phthalazine is caused mainly by the strong repulsive interaction between the two lone pairs of electrons on the adjacent nitrogen atoms in these molecules as well as by the stabilization of the π electrons in these systems.

Photoionization studies by total ionization measurements. I. Benzene and its monohalogeno derivatives

The photoionization yield curves for C 6H 6, C 6D 6, and C 6H 5X (X = F, Cl, Br, I) have been determined by the total ionization technique, up to 13.5 eV. A comparison has been made between the results obtained by the photoionization technique and the photoelectron analysis method. It appears from this comparison that the latter technique is not able in all cases to correctly detect all electronic excited states of the ions if only the very high photon energy of the He resonance line is used. An entirely new assignment of the molecular orbital levels in C 6H 6 and C 6H 5F has been made on the basis of our results and of recent theoretical calculations. This assignment shows that two σ levels are present, in C 6H 6, between the two well-known π levels. On the basis of this orbital level scheme, tentative interpretations of the levels observed for the C 6H 5Cl, C 6H 5Br and C 6H 5I molecules are given.

Mass-Spectrometric Study of Photoionization. VIII. Dicyanogen and the Cyanogen Halides

Photoionization-yield curves are obtained for the molecule and selected radical ions of C2N2, FCN, ClCN, BrCN, and ICN from threshold to 600 Å. Vibrationally excited states of ions and autoionization of Rydberg levels in the molecules are observed and discussed briefly. The X+ ion thresholds in the heavier cyanogen halides are used to obtain: ΔHf0o (CN) = 101.5 kcal mole−1, ΔHf0o (CN+) = 430.0 kcal mole−1, and I (CN) = 14.2 eV. These are applied to compute ΔHf0o (FCN) = 5.6 kcal mole−1, D (F–CN) = 5.0 eV, I (C2) = 12.15 eV, and other thermodynamic properties. The formation of CN+ from dicyanogen apparently includes about 0.6 eV excess energy near the threshold.

Ionization Potentials and the Structures of the Photo-ionization Yield Curves of Ethylene and its Halogenoderivatives

THE problem of interpreting the differences between ionization potentials obtained by electron (E.I.) and by photon impact (P.I.) has led us to a careful examination of the structures occurring in the photo-ionization yield curves of cis and trans dichloro- and dibromoethylenes1–3. The regularly spaced structures found in these curves have been interpreted in terms of vibrational progressions involving the C—C valence vibrations of the ions.

Pre-ionization Phenomena in the Appearance of Some Polyatomic Molecular Ions

IT has been shown elsewhere1–4 how the photo-ionization yield curves of ethylene and of some halogenated derivatives of the ethylene molecule can be interpreted by means of step functions resulting from vibrationally electronically excited states of the molecular ions.