Observation of Electric Monopole Transitions in Tetracyanoquinodimethane

Abstract

The first direct observation of electric monopole transitions in solids is reported for tetracyanoquinodimethane using an inelastic electron scattering experiment. Good agreement is obtained with a semiempirical self-consistent molecular-orbital calculation which predicts dipole allowed transitions near 3 and 7 eV and optically forbidden monopole transitions near 5 eV. We find an anomalous momentum dependence in the shape of the first dipole transition, possible due to local field effects.High-energy inelastic electron scattering has long been considered a useful supplement to optical spectroscopy since with a single spectrometer one can obtain the dielectric response function over a range of energies extending from the infrared to the soft x-ray region. 1 However, with the exception of extensive measurements of plasmon dispersion, 2 comparatively little use has been made of the ability to transfer momentum in the scattering event in solid-state studies. Momentum transfer produces nonvertical transitions between energy bands and gives rise to new selection rules which greatly expand the information available through optical spectroscopy. In low-energy electron scattering experiments optically forbidden transitions have been observed for atoms and simple molecules in the gas phase, 3 ' 4 but in solids they are only observed through the apparent breakdown of selection rules at some suitably low energy. 5 In this paper we report results of a high-energy electron scattering experiment which yields unambiguous identification of monopole transitions in solid tetracyanoquinodimethane (TCNQ) as well as an anomalous momentum dependence of the first dipole transition possibly due to local field effects.Within the Born approximation the differential scattering cross section per unit energy loss per unit solid angle for high-energy electron scattering is 6whereto and 1^ are the initial-and final-state wave functions with energies E 0 and£ /7 respectively, and q is the momentum transferred in the inelastic event. For q <l/r c (where r c is a characteristic size of Jhe wave function) one can write the exponential e i(X ' T~l +iq-r+i(eq # r) 2 . The first term does not contribute because of the orthogonality of initial and final states, while the second term, which dominates at very small q, contributes only dipole-allowed transitions. In terms of spherical harmonics the third term contains Y 0 and Y 2 causing both monopole and quadrupole transitions. Thus, for dipole-allowed transitions the energy-loss probability is proportional to q~2, whereas it is constant for monopole and quadrupole transitions. In general, the monopole and dipole terms will be comparable when qr c~l which for large molecular wave functions (r c~2 A) implies q~0.5 A" 1 . A great deal of interest has been shown in charge-transfer salts based on the molecule TCNQ. While numerous calculations have been performed on the molecular orbitals of neutral and charged molecules 7 only one measured peak 8 in the optical absorption spectrum of the neutral molecule has...

Some effects of electron channeling on electron energy loss spectroscopy

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