Charged-phonon absorption in dopedC60

Abstract

We interpret the giant infrared resonances observed in A6C60 (A K, Rb) by Fu etal. as being due to a classic charged-phonon effect in which the Ti"molecular vibration modes of C60 acquire electronic oscillator strength via coupling to virtual t I" t I"electronic transitions. For the compounds 8 C60 (x =3, 4, and 6), we predict, for weak electron-molecular-vibration coupling, that the softenings of the Ti"modes are proportional to x, but that the integrated oscillator strengths of their absorption bands are proportional to the square of x. Fu eral. ' have recently measured the infrared (IR) spectrum of AsCso (A =K, Rb) and find that several of the T~" infrared-active modes that appear in the spectrum of undoped crystalline C6o have acquired in the fully doped material anomalously large oscillator strengths and undergone softening.The oscillator strength enhancement of two of the modes is of order 10 . In this paper, we interpret these enhancements as being due to a classic charged-phonon effect in which the T~"vibrational modes of molecular C6o acquire electronic oscillator strength via coupling to virtual electronic transitions between the C60 t~"and t~g molecular orbitals. For weak electron-molecular-vibration (EMV) coupling, the theory we introduce to describe this effect predicts for the compounds A"Csp (x 3, 4, and 6) that the softening of the T~"modes is proportional to x, but that the integrated oscillator strength of their absorption bands is proportional to the square of x. The values of the dimensionless EMV coupling constants 0 ") which we are able to tentatively deduce from the data of Fu eral. are indicative of weak individual EMV coupling (A, "-0. 02) but fairly strong total EMV coupling. In fact, we deduce that the static electronic polarizability of C6O contributed by t~" t~g transitions is enhanced by a factor of 1.67 due to EMV coupling. We stress that the charged-phonon effect promises to be a means of yielding key experimental data for testing theoretical calculations of the electronic and vibrational structure of ionic C60, as well as providing a monitor of the charge state of the Cso molecule in doped fullerenes.The charged-phonon effect, or the borrowing of electronic oscillator strength by a vibrational mode from an electronic transition to which the mode has become coupled, is well known in physical chemistry.It is more spectacularly known in the physics of low-dimensional solids, where the conditions required to produce the coupling often arise from symmetry-breaking effects. ' In discussing its theory for the A C60 compounds we will introduce, in the present paper, four simplifying assumptions. First, we take the widths of the electronic bands in A"C60 to be zero so that we may work entirely within a molecular picture. In this limit x electrons occupy the threefold degenerate ti"molecular orbitals of each C60 ion, while the higher-lying t tg orbitals remain empty in the ground-state configuration. Electron-energy-loss spectroscopy studies of Sohrnen, Fink, and Kratschmer indicate that t...

Optical spectroscopy on monomeric and polymeric 1:1 fulleride salts

“…If the distance between fulleride ions gets shorter, the modes should harden in absence of other perturbations; electron-phonon interaction, on the other hand, causes softening whose magnitude depends on the strength of the coupling. As shown both experimentally [4,5] and theoretically [6], the highestfrequency mode has a much larger electron-phonon coupling constant than the 1183 cm -1 mode. Therefore, the former is indicative of the coupling, while the latter reflects the changes in lattice constant.…”

“…There is growing evidence for an orthogonal phase (ortho-I) consisting of covalently bound polymers [1,2] and another one (ortho-II) of dimers [3]. Vibrational spectroscopy has been used to identify the valence state of anions in C60 salts [4,5] and it was shown early on that electron-phonon interactions play a significant role in the infrared properties of the salts [6]. In this study, we compare the infrared spectra of different phases of RbC60 to (Ph4)2C60I, a charge transfer salt containing C60 monoanions.…”

Infrared spectra of one- and two-dimensional fullerene polymer structures:RbC60and rhombohedralC60Iwasa1, Forró2 1997Phys. Rev. B36090View full textAdd to dashboardCite

“…[18][19][20] From the large shift and intensity enhancement in fullerides, the coupling constant was determined to be the largest of the four for this mode. 17 Surprisingly, our spectrum of rhombohedral C 60 shows an even larger softening from the F 1u position of 1429 cm Ϫ1 in neutral C 60 . The interball distance in RbC 60 in the chain direction is shorter than that in the rhombohedral network, thus the hardening in the one-dimensional chains could in principle be accounted for.…”

“…This coupling occurs in C 60 nϪ anions, to the t 1u →t 1g electronic transition involving the extra electrons. 17 Assuming a similar mechanism involving the band formed by the molecular t 1u and t 1g orbitals, intensity enhancement ͑and softening, see below͒ should occur for the F 1u modes. In rhombohedral C 60 , which consists of neutral units, the relative intensity of the infrared lines is determined solely by nuclear motion, giving a more uniform distribution of spectral weight.…”

Far-infrared vibrational properties of linearC60polymers: A comparison between neutral and charged materials

“…Previous attempts to assess the vibronic coupling of charged C 60 's have neglected this aspect by assuming that the T 1u modes retain their character in the charged material. 28,47 In the ''charged-phonon'' model proposed by Rice and Choi, 47 the novel behavior of the T 1u modes for C 60 nϪ molecules was explained in terms of the coupling of intramolecular vibrations to the t 1u →t 1g electronic excitations involving the single-particle orbitals. The IR intensity is predicted to increase quadratically with charge, and the mode frequencies are predicted to decrease linearly with charge.…”