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The vacuum-UV absorption and fluorescence spectroscopy of CF\(_2\)X\(_2\) (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, UK 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 ca. 3-80 ns have been measured. The peaks in the VUV absorption spectra of CF\(_2\)X\(_2\) are assigned to Rydberg transitions. For CF\(_2\)H\(_2\) below 11 eV, there is good agreement between the absorption and the fluorescence excitation spectra, whereas above 11 eV and for the whole range 8-22 eV for CF\(_2\)Cl\(_2\) and CF\(_2\)Br\(_2\) there is little similarity. This suggests that photodissociation to emitting states of fragment species represent minor channels. In the range 8-15 eV, emission is due mainly to CF\(_2\) A\(^1\)B\(_1\) - X\(^1\)A\(_1\)1 and weakly to CFX A\(^1\)A” - X\(^1\)A’. These products form by photodissociation of Rydberg states of CF\(_2\)X\(_2\), and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. For CF\(_2\)H\(_2\) below 11.8 eV CF\(_2\) A\(^1\)B\(_1\) can only form with H\(_2\), whereas for CF\(_2\)Cl\(_2\) and CF\(_2\)Br\(_2\) it is not possible to say whether the other products are 2X or X\(_2\). For energies above ca. 15 eV, emission is due to diatomic fragments ; CF B \(^2\Delta\) and A \(^2\Sigma\)\(^+\), CCl A \(^2\Delta\), CH B \(^2\Sigma\)\(^-\) and A \(^2\Delta\), Cl\(_2\) and Br\(_2\) D’ 2 \(^3\Pi_g\), and possibly CBr A \(^2\Delta\). From their appearance energies, there is evidence that, with the exception of CF B \(^2\Delta\) / CF\(_2\)H\(_2\) where the ground state of HF must form , the excited state of CF, CCl or CH forms in association with three atoms. Our results yield no information whether the three bonds in CF\(_2\)X\(_2\)* break simultaneously or sequentially. We suggest that the anomalous behaviour of CF\(_2\)H\(_2\), in forming H-H or H-F bonds in unimolecular photofragmentation processes, relates to the small size of the hydrogen atom, and hence the unimportance of steric effects in the tightly-constrained transition state. In no cases is emission observed from excited states of either the CF\(_2\) X free radical or the parent molecular ion, CF\(_2\)X\(_2\)X2\(^+\) .

Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

The vacuum-UV and visible spectroscopy of PF3 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following photoexcitation with monochromatized synchrotron radiation from the Daresbury, UK source in the energy range 9–20 eV with an average resolution of ∼0.015 eV. Transitions to the three lowest-energy bands in the Rydberg spectra show resolved vibrational structure, they are assigned to transitions to the (8a1)−1 4p, 5p, and 6p Rydberg states of PF3, and fluorescence is due to valence transitions in the PF2 radical. From a Franck–Condon analysis of the vibrational structure, it is shown that the FPF bond angle in PF3 increases by ∼14±1° upon photoexcitation. The use of optical filters shows that at least three excited electronic states of PF2 are responsible for the induced emission. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of 8 nm at the BESSY 1, Germany synchrotron s...

The formation of ArCO+ ions has been investigated in a photoelectron photoion coincidence experiment by dissociative ionization of Ar2CO clusters. The kinetic energy released (KER) in this dissociation has been measured as a function of the internal energy of the parent cluster ion and compared to the KER expected on the basis of statistical theory of unimolecular reaction, e.g., phase space theory. The latter calculations are based on high level ab initio calculations of the ArCO+ ion and the Ar2CO+ ion. The ab initio calculations show a nonlinear equilibrium geometry of the ArCO+ ion. The comparison of experimental and theoretical KER leads to an adiabatic ionization potential of the ArCO of 13.03 eV and a dissociation energy of the ArCO+ ion of 1.00 eV.

The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 a 3B1–X 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 A 1B1–X 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D 2A1 state of SiF4+ predomin...

The formation of (CO)+2 ion has been investigated by the dissociative ionization of Ar(CO)2 clusters in a threshold photoelectron photoion coincidence experiment. The kinetic energy released (KER) in the reaction Ar(CO)+2→(CO)+2+Ar has been measured as a function of the internal energy of the Ar(CO)+2 ion. The comparison between the experimental KER and the statistically expected KER allows one to extrapolate back to zero KER which corresponds to the thermochemical threshold for the formation of (CO)+2. The ionization potential (IP) of (CO)2 is determined to be 12.24±0.15 eV. This leads to a binding energy of the (CO)+2 of 1.80 eV one of the strongest known for any ionized van der Waals dimer. Our data are in line with a trans planar structure of the CO dimer ion as suggested by our ab initio calculations at the CI level.

The ArN+2 ion has been investigated by means of photoionization of an argon/nitrogen cluster beam in a threshold photoelectron photoion coincidence experiment. Two pathways for the formation of ArN+2 ions have been observed: (i) the nondissociative ionization of ArN2 neutrals and (ii) the dissociative ionization of Ar2N2. The two pathways are distinguished by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar2N+2→ArN+2+Ar has been measured as a function of the excitation energy. The comparison of the measured KER with the statistically expected KER allows us to extrapolate to the thermochemical threshold of the reaction under investigation. A consistent picture is obtained under two assumptions: (i) the ArN+2 ion is linear and (ii) the ionization potential of ArN2 is 14.486±0.05 eV. The former assumption is confirmed by high level ab initio calculations (QCISD/6‐311G*).

Total ion yield spectra of a supersonic beam of Ar clusters have been recorded in the Ar K‐shell (1s) spectral region using synchrotron radiation. The spectrum of the cluster component of beams containing argon clusters of mean size N≊400 atoms is similar to that of the x‐ray absorption spectrum of solid Ar. Analysis of the Ar 1s extended x‐ray absorption fine structure (EXAFS) indicates an average first shell Ar–Ar distance in the cluster similar to that of the solid, but there is an increased Debye–Waller (disorder/thermal motion) term as well as changes in the higher coordination shell signals.Time‐of‐flight mass spectra and photoion–photoion coincidence (PIPICO) spectra of argon cluster beams are reported and the charge separation mechanisms of multiply charged argon clusters are discussed. The results are compared to those from recent studies of the Ar 2p spectra of Ar clusters.

Total electron and total and partial ion yield spectra of Ar clusters (with average size up to 600±200) in the region of Ar 2p excitation have been measured using synchrotron radiation and time‐of‐flight mass spectrometry. As the average cluster size increases, the x‐ray absorption spectrum changes systematically from that of atomic Ar to that of solid Ar. The shape of the Ar 2p3/2→4s region is found to be a sensitive monitor of the cluster sizes present in a molecular beam of Ar clusters. Extended x‐ray absorption fine structure (EXAFS) is detected in the spectra of the larger clusters. There is a strong correlation between the intensity of the components of the Ar 2p3/2→4s signal associated with clusters and the intensity of the Fourier filtered first shell Ar 2p EXAFS signal. A low amplitude, high frequency fine structure is observed in the Ar 2p continuum of the heaviest clusters which corresponds closely to that observed in solid Ar. This signal develops with cluster size more slowly than the Ar 2p E...

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.