In the photoelectron spectrum of N 2 the apparent ionization energy to form the B 2 ⌺ u + state increases linearly with the photon energy. Rotationally resolved measurements of the fluorescent decay of this state show a linear increase of rotational heating with increasing photon energy. These results are in quantitative agreement with the prediction of the theory of recoil-induced rotational excitation, indicating that the rotational heating that has been observed previously arises primarily from such recoil-induced excitation. Together with other results that have been reported they show that recoil-induced internal excitation is significant in many situations, including near threshold.When a photoelectron is ejected from an atom, molecule, or solid the remaining ion has a recoil momentum that is equal and opposite to that of the electron. Although the first discussions of this effect ͓1͔ were limited to translational recoil, it was recognized by Domcke and Cederbaum ͓2͔ that the recoil effect could lead to internal excitation of the ion. However, this prediction remained unverified until recent observations in core-electron photoelectron spectra of recoil excitation of vibrations in molecules ͓3,4͔ and phonons in solids ͓5͔. These experiments show that the recoil-induced internal excitation is quantitatively in accord with a model based on emission of the electron from a localized atom.Although a model based on emission from a localized atom may be appropriate for core ionization, it is not apparent that such a model is appropriate for valence ionization, where the electrons are delocalized. This question has been recently addressed by Takata et al. ͓6͔ who showed that at a photon energy of 8 keV there is a shift in the apparent position of the Fermi edge of aluminum that is consistent with the recoil being taken up by a single atom.The investigations mentioned above have been concerned with vibrational excitation. Here we consider recoil-induced rotational excitation during valence photoionization of N 2 . Thus we extend the previous investigations by considering a different type of internal excitation and by considering valence excitation in a distinctly different system ͑a small molecule rather than a solid͒. Specifically we investigate rotational excitation during photoionization to produce the B 2 ⌺ u + state of N 2 + . Using both photoelectron and fluorescence spectroscopy we show that there is recoil-induced rotational excitation in quantitative accord with a model based on emis-sion of the electron from a localized atom. Moreover, we note that this effect is observable even within 100 eV of threshold. Thus it becomes apparent that significant recoilinduced internal excitation is widespread in terms of both the physical system ͑molecule or solid͒ and the energy range.It has been previously noted that the distribution of rotational states produced during photoionization to form the B 2 ⌺ u + state of N 2 + depends on the photon energy ͓7,8͔. The distribution shifts to higher values of the rotational quan...

The gas-to-solid shift of benzene is reported in the C 1s-core level regime, where the C 1s → π*-transition is investigated between 284.0 eV and 286.5 eV. Simultaneous experiments on the gas phase and condensed species are used to determine the gas-to-solid shift within an accuracy of ±5 meV. Specifically, it is observed that the vibrationally resolved C 1s → π*-transition in solid benzene is red-shifted by 55 ± 5 meV relative to the transition of the isolated molecule. Contrary to previously reported experimental data and estimates this gas-to-solid shift is somewhat smaller than the gas-to-cluster shift. It is significantly smaller than that determined in previous work on gaseous and condensed benzene. These results are discussed in terms of structural properties of molecular clusters and solid benzene by involving ab initio calculations as well as processes leading to spectral shifts of core-excited variable size matter. Finally, changes in the shape of the C 1s → π*-band upon the formation of solid benzene and benzene clusters are discussed.

High resolution X-ray spectroscopic studies on free SF6 molecules and SF6 clusters near the S 2p ionization thresholds are reported. Spectral changes occurring in clusters for the intense molecular-like S 2p1/2,3/2 → 6a1g-, 2t2g-, and 4eg-resonances are examined in detail. Neither gas-to-cluster spectral shifts nor changes in peak shape are observed for the pre-edge 6a1g-band. Significant changes in band shape and distinct gas-to-cluster shifts occur in the S 2p1/2,3/2 → 2t2g- and 4eg-transitions. These are found in the S 2p-ionization continua. The quasiatomic approach is used to assign the experimental results. It is shown that a convolution of asymmetric and symmetric contributions from Lorentzian and Gaussian line shapes allows us to model the spectral distribution of oscillator strength for the S 2p1/2,3/2 → 2t2g-, and 4eg-transitions. The asymmetry is due to trapping of the photoelectron within the finite size potential barrier. The Lorentzian contribution is found to be dominating in the line shape of the S 2p → 2t2g- and 4eg-bands. The spectroscopic parameters of the spin-orbit components of both the 2t2g- and 4eg-bands are extracted and their gas-to-cluster changes are analyzed. The photoelectron trapping times in free and clustered SF6 molecules are determined. Specifically, it is shown that spectral changes in clusters reflected in core-to-valence-transitions are due to a superposition of the singly scattered photoelectron waves at the neighboring molecules with the primary and multiply scattered waves within the molecular cage.

The structures of mixed argon-nitrogen clusters of different compositions are investigated by analyzing core level shifts and relative intensities of surface and bulk sites in the Ar 2p(3/2) regime in soft X-ray photoelectron spectroscopy. These structures are confirmed by core level shift calculations taking induced dipole interactions into account, in which several model structures of the mixed clusters are considered by Monte Carlo simulations. These results suggest that the mixed argon-nitrogen clusters show partial core-shell structures, where an argon core is partially covered by nitrogen molecules.