In situ nonlinear optical spectroscopy of electron–phonon couplings at alkali‐doped C₆₀/Ag(111) interfaces

Abstract

We use doubly resonant, infrared-visible sum-frequency generation spectroscopy (DR-SFG) to probe vibrational and electronic properties of C 60 and K-doped C 60 monolayers adsorbed on Ag(111) single crystal under ultra-high vacuum (UHV). We recorded the interface SFG spectra for five visible wavelengths. We observe a strong dependence of the SFG intensity of the totally symmetric A g (2) mode of the fullerene while scanning the visible wavelength, due to the DR-SFG phenomenon. The SFG intensity of the A g (2) mode is the strongest at 488 nm and at 532 nm for the pure and fully doped monolayers, respectively. These results demonstrate the occurrence of electron-phonon couplings at the C 60 /Ag(111) and saturated K/C 60 /Ag(111) interfaces. They enable us to determine the energy of the coupled electronic transition and to link the electronic resonance to the h u (HOMO) to t 1g (LUMO þ 1) transition of C 60 .

Tuning the Molecular Order of C60 Functionalized Phosphonic Acid Monolayers

“…The SFG spectrum of C 60 C 18 –PA is dominated by a narrow resonance at 1465 cm –1 that is attributable to the C 60 Ag(2) mode and has been reported before in SFG studies of C 60 functionalized monolayers and pristine C 60 layers. − Adsorption and attachment of functional molecules to a C 60 fullerene lifts its inversion symmetry and gives rise to a SFG active mode at 1465 cm –1 that serves as a clear signature of C 60 C 18 -phosphonic acids at the interface.…”

Sum-Frequency Generation Spectroscopy of Plasmonic Nanomaterials: A Review

“…For the sake of clarity, the case of molecules exhibiting electronic transitions in the visible spectral range will not be considered in this report. The interested reader will nevertheless find experimental results showing some adsorbed molecules which are resonant in both IR and visible spectral ranges, evidencing vibro-electronic SFG processes in various kinds of physical [ 45 , 46 , 47 ], chemical [ 33 ] and biological [ 48 ] systems. The description of the SFG processes arising from gold surfaces is a difficult task that has been extensively addressed experimentally and theoretically because it must take into account the non-linear contribution coming from the surface and the bulk of the specific electronic properties of this metal in the visible spectral range.…”

All-experimental analysis of doubly resonant sum-frequency generation spectra for Franck–Condon and Herzberg–Teller vibronic modes

“…20 DR-SFG setups with tunable visible lasers remain rare in spite of the variety of applications of such experiments available in the literature: chiral response of coupled oscillators [21][22][23][24][25][26][27] , vibrational and electronic structures of chromophores 16,[28][29][30][31][32][33] and conjugated polymers, 34,35 charge transfer states induced by molecular adsorption on metals. [36][37][38][39][40][41][42] One of the reasons for this sparseness may be that, as for resonant Raman scattering, data interpretation is made difficult by several aspects. i) The selection rules for double resonance of a vibration mode depend on the nature of the vibronic activity of this mode.…”