H.-W. Jochims

Charge separation in core excited argon clusters

Charge separation in core excited argon clusters is reported. Neutral argon clusters have been prepared in a supersonic molecular beam. Photoionization with monochromatized synchrotron radiation in the L3/L2 regime (240–260 eV) initiates various single and double ionization processes. The photoion–photoion‐coincidence (PIPICO) technique is applied to measure dissociative double ionization processes in core excited argon clusters. Three series of charge separation channels are observed: (i) Ar+/Ar+n, (ii) Ar+2/Ar+n, and (iii) Ar+3/Ar+n. Kinetic energy releases from charge separation reactions as well as the relative intensities of the PIPICO signals are discussed in relation to fragmentation mechanisms, resonant Auger spectra, and properties of cluster dications, such as cluster dication fragmentation energies and charge separation distances.

The photofragmentation of naphthalene and azulene monocations in the energy range 7–22 eV

Abstract Photoion mass spectrometry was used to study the fragmentation of naphthalene and azulene monocations over the excitation energy range 7–22 eV. Fifteen fragmentation processes in naphthalene and twelve in azulene have been examined in detail. The photoionization mass spectra at 20.58 eV are quasi-identical for the two isomers. This, and the constant value of the difference between the fragment appearance energies (AE) for naphthalene and azulene, equal to the difference in the heats of formations of the neutral parents, suggest that identical products are formed. The unimolecular dissociations fall mainly into (i) a “low energy” group, (AE 18 eV). The reactions in (i) have in common the bicyclic precursor C 10 H + 8 ion 18 which decays via rupture of one ring. The group (ii) reactions involve rupture of both rings to give an open chain precursor, the 1,6-bis-ethinyl-hexatriene radical cation 20 . Thermodynamic and mechanistic arguments are given to propose specific reaction pathways and product structures. Two general schemes rationalize the low-energy and high-energy ionic decompositions.

Vacuum-UV fluorescence spectroscopy of CF3X(X=F,H,Cl,Br) in the range 10–30 eV

Abstract The vacuum-UV and visible spectroscopy of CF3X(X=F,H,Cl,Br) using fluorescence excitation and dispersed emission techniques is reported. Fluorescence excitation spectra have been recorded following excitation using dispersed synchroton radiation in the energy range 10–30 eV at an average resolution of ∼0.06 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, assignments have been made for all the peaks in the Rydberg spectra of the titled molecules that photodissociate to a fluorescing state of a fragment. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Three distinct decay channels are observed: (a) CF3 fluorescence for photon energies in the range 10.5 to 13.5 eV, (b) CF2 fluorescence for photon energies in the range 14.7 to 15.7 eV, (c) for photon energies in the range 20–23 eV parent ion fluorescence in the case of CF4 and CF3H, predominantly CF emission in the case of CF3Cl and CF3Br. These assignments are confirmed by action spectra in which the energy of the vacuum-UV radiation is tuned for detection at a specific (dispersed) wavelength. These results confirm those reported in the previous paper from fluorescence coincidence experiments that parent-ion fluorescence is the major component of the emission for CF4 and CF3H excited in this latter energy range of 20–23 eV, whereas it represents only a minor component for CF3Cl and a negligibly-small component for CF3Br.

Core-level excitation in argon clusters

Abstract Argon clusters prepared in a supersonic-jet expansion are photoionized with monochromatized synchrotron radiation in the L 3 /L 2 regime (240–280 eV). The cluster cations and their fragments are mass selected in a quadrupole mass spectrometer. The photoionization-efficiency curves of small cluster ions already show similar features which are known from X-ray absorption of solid argon. These solid-like features become more dominant when the cluster-size distribution of the neutral precursors is increased. The spectra are analysed in terms of near-edge features (NEXAFS) and extended fine structure (EXAFS). The NEXAFS regime shows cluster-specific features which are different compared to the atom and the solid. In the energy regime above 260 eV, EXAFS-like structures have been identified. Possible fragmentation mechanisms for core-level-excited argon clusters are discussed.

Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV

The vacuum–ultraviolet (VUV) absorption and fluorescence spectroscopy of CF2X2 (X=H, Cl, Br) in the range 190–690 nm is reported. Tunable vacuum–UV radiation in the range 8–22 eV from synchrotron sources at either Daresbury, U.K. or BESSY1, Germany is used to excite the titled molecules. Fluorescence excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra at a resolution of 0.08 nm, and dispersed emission spectra with an optical resolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelength, were also recorded at BESSY1 with a resolution of 0.3 nm; appearance energies for production of a particular emitting state of a fragment are then obtained. Using the single-bunch mode of BESSY1, lifetimes of all emitting states that fall in the range ∼3–80 ns have been measured. The peaks in the VUV absorption spectra of CF2X2 are assign...

Photoionization mass spectrometry of six isomers of C7H8 in the 7–22 eV photon energy range

Photoion mass spectrometry in the 7–22 eV range was used to obtain the parent and fragment photoion yield curves and appearance energies for six isomers of C7H8: toluene, cycloheptatriene, norbornadiene, quadricyclane, spiro[2,4]hepta-4,6-diene and 1,6-heptadiyne. The apparent heats of formation of fragment ions m1+ were determined for various fragmentation pathways and compared with standard thermochemical values of ΔHf(m1+) in order to assign the fragmentation channels. Comparisons between the mass spectra of the six isomers obtained by photoelectron–photoion coincidence measurements at a photon excitation energy of 20 eV, and between the respective apparent ΔHf(m1+) values for these isomers, were used to discuss the possible formation of common isomers during the various dissociative ionization processes. Interconversion of norbornadiene and cycloheptatriene ions is suggested to occur prior to the formation of C7H7+ and other fragment ions in these two isomers. For toluene, isomerization does not appear to be necessary for the formation of low-energy fragment ions except for the tropylium form of C7H7+ and its sequential product C5H5+. The results also suggest that interconversion to an acyclic isomer common to all six species occurs in the formation of the three fragment ions C4H3+, C3H3+ and C2H3+ at high internal energies of the parent ion.

The vacuum UV photoabsorption spectrum of methyl bromide (CH3Br) and its perdeuterated isotopomer CD3Br: a Rydberg series analysis

The vacuum UV photoabsorption spectrum of CH3Br has been recorded between 6 and 25 eV. A large number of vibronic bands are observed. They were partly ascribed to vibrationless Rydberg transitions. In the high photon energy range of 12-25 eV, very weak diffuse bands are mostly assigned to transitions from the 3a1, le and (2a1 + la1) to 3s orbitals. In the 6-12 eV photon energy range, numerous weak to strong bands are observed. The sharpness is very variable over the entire spectral region. In a first step, the interpretation of the spectrum and the assignment of the Rydberg transitions is based on the simple Rydberg formula. The observed features are classified in two groups of four series, each converging to one of the two spin-orbit components of the X ~ E state of CH3Br . Rydberg series of nsa1, npa1, npe, nd and possibly nf characters are observed. The same measurements have been made for the first time on CD3Br in the 6-12 eV photon energy range. The observed features are classified into the same Rydberg series characterized by nearly the same δ values. Ionization energies for CD3Br X ~ 2 E3/2 at 10.565 eV and X ~ 2 E1/2 at 10.902 eV are deduced. In a second step, we fitted the experimental data to an energy expression taking into account both the exchange interaction and the spin-orbit coupling. Already states with n = 6 are found to correspond to Hunds case (c). Constant values of the quantum defects are deduced.

He I photoelectron spectroscopy of four isotopologues of formic acid: HCOOH, HCOOD, DCOOH and DCOOD

Abstract He I photoelectron spectra of four isotopologues of formic acid, HCOOH, HCOOD, DCOOH and DCOOD have been measured, mainly with an electron kinetic energy resolution of 15 meV. Quantum chemical calculations of geometries, vibrational mode frequencies and the potential energy distributions characterising each normal mode were made for the neutral 1 1 A ′ ground state, the cation ground state 1 2 A ′ and first excited ion state 1 2 A ″ of the four isotopologues. The results were used to analyse observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region and this analysis provided values for several vibrational mode frequencies of the ion states. The calculated structure and internal dynamics of the 1 2 A ′ ground state of the ion provide satisfactory agreement with experiment but for the 1 2 A ″ excited state a more refined theoretical treatment, permitting greater structural flexibility, is required. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of the four isotopologues. Analysis is also made of PES features concerning the higher energy states of the ions between 12.2 and 21 eV. The adabatic ionization energies of the 2 2 A ′ , 2 2 A ″ , 3 2 A ′ and 4 2 A ′ states in this energy region were determined. Vibrational frequencies were obtained for the 3 2 A ′ ion state of the isotopologues and new assignments were made concerning the energy and structure of the 4 2 A ′ ion state, whose adiabatic energy was found to lie about 60 meV below the 3 2 A ′ state.

Fluorescence of Ar(2p)-excited argon clusters

Abstract Fluorescence of argon and argon clusters is investigated in the Ar(2p)-excitation regime (240–310 eV). Fluorescence excitation spectroscopy shows a different evolution as a function of the average neutral cluster size compared with total electron and ion yields as well as partial ion yields of mass-selected cluster ions. These differences are rationalized in terms of various multiple ionization processes occurring in argon clusters. The dispersed fluorescence shows a broad continuum between 165–290 nm which is interpreted as emission of the “third continuum” of argon. The results suggest that the process leading to fluorescence of clusters in the Ar(2p) regime is due to the radiative decay of doubly charged clusters.

Vacuum-UV fluorescence spectroscopy of CCl3F, CCl3H and CCl3Br in the range 8–30 eV

The fluorescence spectroscopy of CCl3X (X=F, H, Br) in the range 200–700 nm is reported, using vacuum-UV radiation in the range 8–30 eV from a synchrotron as a tunable photoexcitation source. Excitation spectra, with undispersed detection of the fluorescence, have been recorded at the Daresbury UK source with a resolution of 0.1 nm, corresponding to an average energy resolution of ca. 0.015 eV. Dispersed emission spectra in the range 200–700 nm have been recorded at the BESSY 1 Germany source with an optical resolution of 8 nm, following photoexcitation at the energies of the peaks in the excitation spectra. Action spectra, in which the vacuum-UV energy is scanned with detection of the fluorescence at a specific wavelength, have also been recorded at BESSY 1 with a resolution of 0.3 nm; thresholds for production of a particular excited state of a fragment are then obtained. Using single-bunch mode, lifetimes of all the emitting states that fall in the range ca. 3–100 ns have been measured. For photon energies in the range 8–12 eV, emission is due to both CCl2 A1B1–1A1 and CXCl A1A″–1A′. These products form by photodissociation of low-lying Rydberg states of CCl3X, and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. It is not possible to say whether the other products form as two halogen atoms or a diatomic molecule. For energies in the range 13–17 eV, emission is due to diatomic fragments; CCl A2Δ, CF B2Δ, CH B2Σ- and A2Δ, CBr A2Δ, and Cl2 D′ 23Πg. From their threshold energies, there is now accumulated evidence that the excited state of CCl or CX forms in association with three isolated atoms. Our results yield no information on whether the three bonds in CCl3X* break simultaneously or sequentially. In the range 13–17 eV, Cl2* almost certainly forms in conjunction with ground-state CX+Cl. This ion-pair state of Cl2 also forms at higher excitation energies around 20 eV, probably with atomic products C+X+Cl. In no cases is emission observed from excited states of either the CCl3 radical or the parent molecular ion, CCl3X+.