F. Carnovale

Photoelectron spectroscopy of argon clusters: Evidence for an Ar13 ionization chromophore

Ultraviolet photoelectron spectra have been measured for argon clusters prepared in pulsed supersonic expansions of both pure and seeded mixtures of argon. The broad band spectra show structural features which relate to the degree of condensation and hence the mean cluster size obtained at different stagnation pressures. These are interpreted in terms of an ion–core hypothesis with diatomics‐in‐molecules calculations being used to simulate spectral features. The analysis shows that Ar+3 cores are formed in the initial ionization process of very small clusters, with the Ar+13 core becoming dominant for all larger clusters and for the condensed state of argon.

Photoelectron spectroscopy of the nitrogen dimer (N2)2 and clusters (N2)n: N2 dimer revealed as the chromophore in photoionization of condensed nitrogen

The He i photoelectron spectra of gas‐phase nitrogen dimer and nitrogen clusters have been measured in a pulsed cluster beam. The dimer (N2)2 is characterized by broad bands with vertical ionization energies which are 0.3±0.1 eV lower than for N2 monomer. The bands observed for a mixture of small clusters, estimated to be of average size N=10, are identical to the dimer bands except for further shifts of 0.3 eV to lower ionization energies. The clusters bandwidths and band shapes are virtually the same as measured for thin films of condensed N2, indicating that the nitrogen dimer (N2)2 is the ionization chromophore in each case. This offers support for Haberland’s hypothesis that ionization of any Mn cluster produces the ion M+2Mn−2 provided M is a closed‐shell atom or molecule. The theory of electronic relaxation polarization of the dielectric medium, which explains the gas‐to‐solid ionization energy shifts, is modified for the case of finite clusters and to account for dimer ion formation.

A photoelectron spectroscopic study of the formic acid dimer

The He (I) photoelectron spectrum of the gas‐phase dimer of formic acid, (HCOOH)2, has been measured using a high pressure nozzle inlet system operating at room temperature. Interpretation of the spectrum with the aid of molecular orbital calculations indicates that the dimer shows considerable interaction of the monomer‐based states of σ symmetry, but little interaction of π states. The σ splittings arise mainly from through‐space interactions with the bridging hydrogen atoms making a small contribution to the stability of the lower energy molecular orbital resulting in each case.

Photoelectron spectroscopy of the NO dimer

He i photoelectron spectra are presented for the NO dimer, (NO)2, and for a mixture of higher clusters, (NO)nn≥3. The NO dimer shows a weak second band associated with a shake‐up transition at a vertical I.E. of 10.8 eV, 1.9 eV above that of the first band. This disagrees with a theoretical estimate of 2.7 eV. The ratio of intensities of these two bands is measured as 1:0.18 in comparison with the theoretical estimate of 1:0.28. The spectrum of the mixture of higher clusters shows the analogous two bands at vertical I.E.s of 8.4 and 10.8 eV, respectively. The band separation of 2.4 eV is close to the value of 2.6 eV observed in the photoemission spectrum of condensed NO.

Ammonia dimer studied by photoelectron spectroscopy

The He i photoelectron spectrum of (NH3)2 is shown to have only a single broad unstructured ionization band near 10 eV. This demonstrates the equivalence or near‐equivalence of the nitrogen lone‐pair orbitals of each NH3 unit in the dimer structure. While theory has generally predicted that (NH3)2 has a classical structure with a near‐linear N‐‐H–N linkage, such a structure would be expected to show two ionization bands separated by about 1.5 eV. However analysis of the observed low ionization energy band suggests that a maximum possible band separation would be only 0.5 eV for ionization from the nitrogen lone‐pair orbitals. While this result concurs with the nonclassical structure for (NH3)2 proposed by Klemperer and co‐workers, it does not exclude the symmetric C2h structure as a likely equilibrium geometry for the dimer. On the contrary, it is argued that the photoelectron spectrum is supportive of the latter structure. Split‐valence basis Hartree–Fock calculations on (NH3)2 at various nonclassical ge...

Photoelectron spectroscopic studies of the monomers and dimers of acetic and trifluoracetic acids

Abstract Comparison of the He(I) and He(II) photoelectron spectra of acetic and trifluoracetic acids has clarified the assignments of their p -based ionization potentials. The He(I) spectra of the gas-phase homodimer of each molecule have been observed using a high-pressure nozzle inlet system operating at room temperature. Spectrum-stripping of the monomer—dimer mixed-spectra gives dimer spectra which are interpreted with the aid of molecular orbital calculations. Appropriate mixtures of the two acids have been analyzed to obtain the He(I) spectrum of the 1:1 heterodimer. Assignments of the three dimer spectra indicate that the inductive influences of the methyl and trifluoromethyl groups are transmitted across the hydrogen-bond bridges.

Photoelectron spectroscopy of krypton and xenon clusters

Ultraviolet photoelectron spectra have been measured for pulsed supersonic beams of krypton and xenon as dilute mixtures in helium. The spectra exhibit broad bands which are located at lower ionization energies relative to the monomer ion states, 2P3/2 and 2P1/2, with which they correlate. The structural features of the spectra, particularly in the first band group, become more complex with increasing condensation, apparently relating to the mean cluster size associated with each spectrum. The spectra are interpreted using the cluster‐size dependent core‐ion model developed to explain the analogous spectra of the argon clusters. The argon clusters spectra were interpreted as showing the presence of Ar+3, Ar+7, and Ar+13 core ions, with Ar+3 involved in the ionization of small neutral clusters, and Ar+13 produced by the dominant ionization mechanism in large clusters as well as in solid argon. The krypton and xenon clusters show variations of this behavior. The relatively large spin‐orbit splitting of the ...

Photoelectron spectroscopic assignment of the p-states of benzenethiol

Abstract He(I) anisotropy parameters and He(I)/He(II) relative band intensities have been determined and utilised to make a complete assignment of the benzenethiol p -electron-region photoelectron spectrum. The major S-atom contribution appears more so in the third band than in the first.

A Photoelectron Spectroscopic Study of [Rh(CO)2Cl]2.

Abstract The He(I) and He(II) vapor phase photoelectron spectra of the [Rh(CO)2Cl]2 dimer have been measured and interpreted with the aid of the results of a non-empirical pseudopotential calculation. The relative He(I)/He(II) band intensity ratios indicate that the spectrum is essentially divided into Rh 4d, Cl 3p and CO σ,π ionization regions. Ionization potential shifts between the free and complexed moieties suggest that there is only a small nett transfer of electron density from Rh to Cl and CO.

Photoelectron spectra of the gauche and trans conformers of 1,2-bromochloroethane

Abstract The HeI photoelectron spectra of the gauche and trans conformers of 1,2-bromochloroethane have been obtained by analysis of time-averaged spectra recorded at different temperatures. Variations in ionization potentials of the two conformers are consistent with data obtained from molecular orbital calculations. Analysis of a HEII spectrum and comparisons with the related molecules 1,2-dichloroethane and 1,2-dibromoethane shows that the ClBr non-bonded interactions include both through-bond and through-space contributions.