Vibrational spectroscopy of C60 and K-doped C60 on Ag(111) by sum-frequency generation

The C60/Ag(111) interface and the completely doped K/C60/Ag(111) interface were studied by infrared reflection absorption (IRAS) and infrared-visible sum-frequency generation (SFG) spectroscopies. The combination of both techniques provides a powerful means for the determination of the infrared and/or Raman character of the adsorbate vibrations. Accordingly, we show that the vibration observed by IRAS in the 1100 - 1500 cm-1 frequency range for the C60/Ag(111) interface is the Ag(2) "pentagonal pinch" mode of C60, which is Raman active and infrared silent in the free molecule. We also observe vibrational frequency and infrared intensity modification of the normally Raman active Ag(2) and infrared active T1u(4) modes upon K-doping.