Correlation Effects in the Ionization of Molecules: Breakdown of the Molecular Orbital Picture

A toroidal grating monochromator designed for operation in the soft X-ray region at the synchrotron radiation source BESSY is described. With three in situ interchangeable gratings the instrument covers the spectral range 180 to 1150 eV. The current (source size-limited) resolution is 0.39, 0.7 and 2.6 eV at photon energies of 250, 400 and 800 eV, respectively. The photon flux is typically 8 × 10 11 photons s −1 (100 mA ) −1 (Δ h ω) −1 . Photoemission experiments on free molecules and on solid surfaces have been used to demonstrate the performance of the monochromator.

A high flux toroidal grating monochromator for the soft X-ray region

Electron energy loss spectra of O2 and N2 physisorbed on metallic substrates showing a series of high overtone losses have recently been reported. In the case of N2, the intense overtone excitation is credited to the formation of a well‐known temporary negative ion state with a resonance lifetime ∼10−15 s for gas phase N2−. The principal distinction between the gaseous and physisorbed molecule EELS spectrum is a significant depletion of the overtone intensity which has been attributed to a surface‐induced decrease in the resonance lifetime. In the present work, a time dependent quantum mechanical model applicable to vibrational excitation in resonance scattering is outlined which quantitatively accounts for the observed spectra and, in particular, the surface modifications to the gas phase results. The essential feature of the model is one in which the intramolecular dynamics of the intermediate state is characterized by nuclear propagation over a harmonic potential curve spatially displaced from the grou...

Shape resonances, overtones, and electron energy loss spectroscopy of gas phase and physisorbed diatomic molecules

Data have been taken on both the partial cross sections and angular distribution parameters for the first five bands in CS2 with a photon energy from 21 to 70 eV and for the first four bands of COS from 21 to 40 eV. The results are compared with calculations based on the multiple scattering method. In the case of photoionization of the 2πg orbital of CS2 and the 3π orbital of COS, the experimental results and calculations have been successfully explained in terms of the Cooper minimum. The behavior of the fifth band of CS2 is accounted for by assigning it to a satellite peak arising from photoionization of the 2πu orbital by way of configuration interaction. Comparison between the experimental results and calculations showed some problem areas, particularly with regard to the prediction of shape resonances, and suggestions are made for future improvement of the calculations.

Angle resolved photoelectron spectroscopy of CS2 and COS measured as a function of photon energy from 21 to 70 eV

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels of C2H2+X 2Πu have been measured in the range 13 eV⩽hν⩽25 eV using synchrotron radiation. Below hν∼16 eV, these data exhibit strong non‐Franck–Condon effects, namely, wavelength‐dependent vibrational branching ratios and vibrational‐state‐dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed below hν∼16 eV, in addition to the C–C stretch mode, which is the only mode readily observed in photoelectron spectra of C2H2 at shorter wavelengths, e.g., at the He I (21.2 eV) resonance line. The non‐Franck–Condon behavior is attributed to resonant photoionization processes, whose identification is discussed in the framework of several recent theoretical and experimental studies on acetylene and related molecules.

I. Khan

Papers

The effect of a resonance on vibrational structure in the photoelectron spectrum of acetylene