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The dissociative photoionization of vinyl bromide (C2H3Br) has been investigated by photoionization mass spectrometry (PIMS) and time-of-flight threshold photoelectron-photoion coincidence (TOF-TPEPICO) spectrometry using synchrotron radiation. The photoionization efficiency curves of the three most abundant ions, i.e. C2H3Br+, C 2 H 3 + and C 2 H 2 + , are measured and analyzed in detail. A C 2 H 3 + / Br - photoion-pair formation process has been detected for the first time. Some arguments are provided in favor of the electronic excitation of the C 2 H 3 + fragment and of the isomerization of the C 2 H 2 + ions. The breakdown diagram for these ions in the 9.8–21 eV photon energy range, derived from TOF-TPEPICO mass spectra, is discussed. The involvement of the successive C2H3Br+ ionic states and the role of autoionization of C2H3Br Rydberg states are emphasized. For the two minor HBr+ and Br+ fragment ions only the breakdown diagrams are measured and appearance energies are derived.

The fine structure of the vacuum UV photoabsorption spectrum of CH3Br and CD3Br has been analyzed in the 6.9–10.2 eV photon energy range. A large number of lines have been observed, classified and assigned to the vibrational excitation accompanying a series of Rydberg transitions. The effects of the Jahn–Teller distortion and of the spin–orbit splitting of the ground electronic state of the ion have been considered. The former effect has been evaluated by ab initio calculations, showing that the 2E state (in the C3v symmetry group) splits into 2A′ and 2A″ states in the CS symmetry group. Even though the energy difference of about 1 meV is extremely small, the 2A′ state is energetically the lowest component whereas the 2A″ is found to be a transition state. The Jahn–Teller stabilization energy and the wavenumbers associated with all vibrational modes have been calculated. Experimentally, the entire fine structure could be described in terms of three vibrational modes, i.e., hcω4 = 146 ± 6 meV (1178 ± 48 cm−1), hcω5 = 107 ± 6 meV (863 ± 48 cm−1) and hcω6 = 71 ± 4 meV (572 ± 32 cm−1), respectively, as resulting from an average over all analyzed Rydberg states. In CD3Br the corresponding energies are hcω4 = 104 ± 3 meV (839 ± 24 cm−1), hcω5=82 ± 4 meV (661 ± 32 cm−1) and hcω6 = 62 ± 4 meV (500 ± 32 cm−1). These values are in fairly good agreement with those predicted by the present ab initio calculations for the ionic ground state. The experimental isotopic ratio ρi = [ω/ωisot]i is 1.15 ± 0.14, 1.31 ± 0.14 and 1.39 ± 1.0 and is predicted to be 1.04, 1.34 and 1.36, respectively, for ω6, ω5 and ω4. On the basis of the present study an alternative assignment of the CH 3 Br + ( X ˜ 2 E ) photoelectron band structure is proposed.

The threshold photoelectron spectroscopic (TPES) and the photoionization mass spectrometric (PIMS) study of methyl chloride (CH3Cl) in the 8–20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The detailed analysis of the TPES shows numerous features in the Jahn–Teller split ground X 2 E ( 2 A ′ – 2 A ′′ ) state of CH3Cl+ and a new continuous band peaking at 13.4 eV. These were assigned to both direct ionization and autoionizing transitions. The transitions to the A 2 A and to the B 2 E states undergo a large enhancement ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak signals in the 19–26 eV photon energy range were mainly assigned to 2a1−1 ionization and to double excitations described essentially by the 2e−2, 4a11 and 1e−12e−14a11 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH3Cl+ into CH2+, CH3+, CHCl+ and CH2Cl+ from threshold up to 20 eV photon energy. At the onset, the CH3+, CHCl+ and CH2Cl+ fragment ion production is correlated to the ground state of CH3Cl+ and all fragment ions have to appear through dissociative autoionization from the 2e → 3p Rydberg state. This interpretation is supported by the photoabsorption spectrum (PAS) measured recently in the same photon energy range. At higher energies, beside direct dissociation of the A 2 A 1 and B 2 E states of CH3Cl+ autoionization also contributes to the fragmentation in all decay channels. For CH3+ the photoion-pair process is analyzed and detailed assignments are proposed on the basis of recent PAS data.

In this paper, we report the photoabsorption and the constant ionic state spectroscopy of vinyl bromide (C 2H3Br). The photoabsorption spectrum was measured using synchrotron radiation and was investigated in detail between 5.0 and 12.0 eV photon energy revealing many previously unobserved structures. These features were analyzed in terms of valence to virtual valence transitions and Rydberg series. The examination of the three Rydberg series converging towards the first ionization threshold (2a" 1 ns, 2a" 1 np and 2a" 1 nd) leads to wave numbers of 335 ± 30, 690 ± 30 and 1305 ± 30 cm -1 . The vibrational wave numbers of the progressions belonging to Rydberg series converging towards the second ionization threshold are 485 ± 30 and 1145 ± 50 cm 1 . Ab initio calculations helped the assignment of the valence transitions and of the observed vibrational wave numbers in the Rydberg series. Constant ionic state spectra were recorded for vibronic states corresponding to the first and the second electronic states of the ion. Their fine structures are assigned to the autoionization of Rydberg states. The autoionization decay of these Rydberg states is analyzed qualitatively in terms of the vibrational nature of the final ionic state.

The vacuum UV photoabsorption spectrum of CH 3 F has been recorded between 7 and 24 eV and has been analyzed in detail. Broad and structureless peaks are observed over the entire photon energy range. They are all assigned to transitions to Rydberg states, members of series converging successively to the 2e −1 , 5a −1 and 1e −1 ionization limits. These features are underlying very long series of narrow and weak structures in the range of 10–13.2 eV. Through a continuum subtraction procedure, about 70 lines could clearly be identified. These have been assigned to long vibrational progressions belonging to Rydberg states corresponding to the 2e→3p and 2e→6s/5d configurations. These states are split by a Jahn–Teller distortion. Ab initio calculations lead to a Jahn–Teller stabilization energy of about 0.84 eV. The main features observed in the two bands are assigned to one single long vibrational progression of hcω ≃0.160 eV (1 290 cm −1 ) related to the C–F Jahn–Teller inactive stretching vibration. The remaining structure is assigned to the low excitation of the Jahn–Teller active vibrational modes, i.e. the bending modes ν 5 and ν 6 characterized by hcω 5 ≃0.120 eV (970 cm −1 ) and hcω 6 ≃0.082 eV (660 cm −1 ). Leaning on the present results and assignments previously reported photoelectron spectroscopic data on CH 3 F + [L. Karlsson, R. Jadrny, L. Mattsson, F.T. Chau, K. Siegbahn, Phys. Scripta 16 (1977) 225] have been reconsidered and reinterpreted.

In this paper, we report the He(I) photoelectron spectrum (PES) and the threshold-photoelectron (TPES) spectrum of C2H3Br. The fine structure observed in the first two ionic states in the He(I) spectrum is assigned to progressions belonging partially to previously unobserved vibrational normal modes. The TPES has been measured between 9.0 and 25.0 eV, and the photon energy range of 9.8–12.0 eV has been investigated in more detail. Extensive calculations with the gaussian set of programs have been performed to help in the assignment of the observed features. Furthermore, a conical intersection between the A 2 A ′′ and the B 2 A ″ states was found to take place along the C–Br stretching coordinate. Intramolecular dynamics of the B 2 A ′′ state, initially prepared in the Frank–Condon region, was probed by the Fourier transform of the spectrum. The B 2 A ′′ state is almost readily depleted, most probably due to a very effective internal conversion through the conical intersection.

Single-electron capture (SEC) by NH 3 2+ with rare gases (He, Ne, Ar, Kr, Xe) and benzene, NH 3 2+ + G → NH 3 + + G + , has been investigated using a forward geometry double-focusing mass spectrometer. The SEC cross-section goes through a maximum within a reaction window centered at an exothermicity of approximately 4.5 eV. This correspondes to a target-projectile distance for the charge transfer of about 3.2 A. The average internal energy of NH 3 + -ions resulting from SEC has been obtained both via translational energy gain measurements and via an analysis of the daughter ion spectra. The agreement between both methods is satisfactory and shows that the lower the ionization energy of the target, the more probable the excitation of NH 3 + to the A 2 E state. With benzene (ionization energy = 9.25 eV) as the target gas, however, the excess energy is converted partly to internal energy of C 6 H 6 + , leading most probably to the D 2 B 2u , Ẽ 2 B 1u and F 2 A 1g states of C 6 H 6 + . A model based on the Landau-Zener non-adiabatic formalism, including one entrance channel (NH 3 2+ + G) and two exit channels (NH 3 + X 2 A 2 ″ + G + and NH 3 + A 2 E + G + ) accounts qualitatively for the observed results in the case of exothermic single-electron capture. This model is not appropriate for slightly exothermic or endothermic charge transfer, i.e., in the present case, for the Ne and He target gases.

In the framework of a research project on the collisional mechanisms involved in tandem mass spectrometry, the collisional activation processes of NH + 3 with different target gases have been investigated. The experiments have been performed on an upgraded AEI-MS9 sector instrument equipped with two collision cells and include daughter ion spectra, ion kinetic energy spectra, determination of kinetic energy release distributions (KERD) and appearance energy measurements. We have shown that collisionally activated dissociation (CAD) of NH + 3 into NH + +[H, H] depends on the internal energy of NH + 3 . This internal energy effect is also observed for the NH + 2 +H channel, when Xe is the target gas; with He, such an effect could not be detected. The analysis of the KERD shows that production of NH + 2 upon CAD takes place from the NH + 3 ground electronic state by rotational predissociation; two components are observed, one which corresponds to a near-threshold process (mean kinetic energy release 〈 T 〉=0.13 eV), and one characterized by 〈 T 〉=0.56 eV corresponding to an excess energy of 2.1 eV above the dissociation threshold. NH + fragments can be formed by two processes: the first one occurs at threshold and leads to NH + (X 2 Π) + H 2 (X 1 Σ + g ); the second one is characterized by large KER values (up to 7 eV) and leads to either electronically excited NH + or dissociation of H 2 .