G. Hagenow

The Photoionization of Ammonia revisited. The Vibrational Autoionization of NH3 and its three Isotopomers in the 10-12 eV photon energy range.

Abstract The photoionization efficiency curves of NH 3 and its three isotopomers have been investigated in the photon energy range of the first ionized state. From the analysis of the corresponding vibrational structure, wavenumbers (ω e ) and anharmonicity constants (ω e χ e ) are deduced. The detailed investigation of the abundant autoionization structure tends to show the adiabatic ionization energy to be 10.072 ± 0.010 eV for NH 3 , NH 2 D and NHD 2 and 10.083 ± 0.010 eV for ND 3 . All autoionization features were classified in vibrational progressions (ν 2 bending mode) belonging to n sa 1 (or n d) and n pe ( n = 5, 6, 7) Rydberg series. Vibrational autoionization occurs through Δν transitions up to −9. A qualitative analysis of the intensity distribution of these series strongly supports that transitions involving odd Δν values are favoured. This observation can be understood by applying group theoretical considerations to the theory of vibrational autoionization.

The threshold-photoelectron spectrum of NH3

Abstract Using synchrotron radiation, we have investigated the threshold-photoelectron spectrum of NH3 between 10 and 30 eV photon energy. In contrast to the He(I)-photoelectron spectrum, the threshold-photoelectron spectrum shows three bands between 10 and 21 eV photon energy. The first band is dominated by the vibrational autoionization of the 5sa1, the 6sa1 and the 6pe Rydberg series. Between the first and second band, a new contribution, starting at 13.26 eV, is assigned to the autoionization of (a) member(s) of Rydberg series converging to the e−1 ionization continuum. Its (their) autoionization populates the upper vibrational levels of the NH+3 ( X 2A1) state. From about 14.5 eV the A 2E band structures are identified. However, differences between the A 2E band shapes in the He(I)- and the threshold-photoelectron spectra suggest that very likely autoionization contributions are involved in the latter spectrum. These phenomena could play a role in the production of the NH+2 and NH+ fragment ions. The last very weak band, at about 25 eV in the photoelectron spectrum, is assigned to the NH+3 ( B 2A1) state.

Vibronic coupling effects observed in the high-resolution He(I) photoelectron spectra of BF3

Abstract The He(I) photoelectron spectrum of BF 3 has been measured with an electron energy resolution better than 15 meV. The 1A 2 ′ PE band (IP vert =15.92 eV) reveals vibrational fine structure with intervals of 0.025 eV. In the case of the 2E′ PE band (IP vert =20.08 eV) an additional third progression with intervals of 0.092 eV is observed. The results are discussed on the basis of vibronic interaction between electronic states using symmetry arguments.

The photoabsorption of BF3 in the energy range 6.5-20 eV

Abstract The absorption spectrum of BF 3 has been measured in the photon energy range 6.5–20 eV using monochromatized synchrotron radiation. Assignments have been made on the basis of term value and symmetry arguments. The π→π* (1e″→2a″ 2 ) transition is centered at 13.13 eV, whereas a second excitation to π* (2a′→2a″ 2 ) is located at 17.52 eV. Another valence-type transition (1a′ 2 →5a′ 2 ) is connected with an excitation energy of 15.64 eV. All final states of the valence-type transitions have term values between 3.5 and 4.0 eV. The first member of an n -type Rydberg series converging to the 2E′ ionization energy is assigned to a transition to the Rydberg mixed-valence final orbital 3s+σ* (5a′ 1 ). Members of n p and n d type Rydberg series converging to the 1A′ 2 ionization energy are located in the energy range 13.7⩽ hv ⩽ 15.4 eV. The dense vibrational features centered around 18.5 eV consist of five Rydberg series excited from 2E′. Shape resonance effects in the valence shell spectrum of BF 3 are discussed.

Photoionization mass spectrometry of kinetic energy-selected ions. An ion retarding potential difference method applied to NO + and O + from N2O

Abstract A new method is described to obtain photoionization efficiency curves of kinetic energy-selected fragment ions. The method is tested and discussed in the case of the dissociative photoionization yields of NO + and O + from N 2 O. The attention is focussed on the dissociative autoionization processes in the threshold region, in the Franck-Condon region of the N 2 O (A 2 Σ + ) state and in the Franck-Condon gap between the A 2 Σ + and B 2 π states.

Core excited states in BF2Cl and BFCl2

Abstract The mixed boron halides BF 2 Cl and BFCl 2 exist in a statistical equilibrium with BF 3 and BCl 3 and cannot be isolated pure. This work has shown that inner-shell excitation processes in these unsymmetrical halides can be studied using specific mass selected fragment ions. The photoion yield spectra have been recorded by means of monochromatized synchroton radiation in the energy range 190–210 eV. Electronic transitions have been assigned on the basis of estimated B(1s) ionization energies (BF 2 Cl:201.8 eV; BFCl 2 :200.8 eV) and a correlation with core excited states in BF 3 and BCl 3 . In agreement with the MO theory, BF 2 Cl and BFCl 2 show four resonances. Two of them, in the case of BF 2 Cl, are shape resonances with term values −2.7 and −4.0 eV, respectively.