In Search of the Molecular Triplet of C 60 using Low Temperature Raman Spectroscopy
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In Search of the Molecular Triplet of C60 using Low Temperature Raman Spectroscopy. G. Chambers, A.B. Dalton, and H.J. Byrne, Facility for Optical Characterisation And Spectroscopy (FOCAS)/ School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland.
ABSTRACT The excited state properties of C60 thin films have been probed in the temperature range 77273K using Raman spectroscopy. The change in the Raman, 2Ag mode of C60 (whose position is largely independent of temperature) was monitored as a function of the excitation intensity at 514.5nm. This mode normally positioned at 1469cm-1, was seen to shift reversibly to a lower Raman frequency with increasing laser intensity. Two excited state species have been identified. The first, at 1466cm-1 has been associated with the molecular triplet of C60. The second species at 1463cm-1, has been speculated to be an excited state co-operative involving two or more excited states in the solid and is seen to be intrinsic to solid state C60 below the phase transition.
INTRODUCTION A considerable understanding of the photophysical and photochemical properties of C60 both in solution and solid has evolved over the past decade [1]. However there remains a number of important questions regarding the nature of the excited states in the solid. In solution, optical excitation leads to a population of the long lived triplet state, via the singlet state. The process has been well characterised by transient absorption measurement [2] and a strong triplet-triplet absorption at 750nm is observed [3]. However this strong feature is not observed in the solid and indeed the excited state photodynamics differ considerably from those in solution [4] leading to questions as to whether the molecular triplet exists in the solid. Further to transient absorption measurements the Raman spectroscopic signature of the molecular triplet in solution has recently been associated with a positioning of the 2Ag mode at 1466cm-1 at room temperature [5]. Similar measurements however at room temperature in the solid are hampered by the much documented photopolymerisation of C60 [6] which occurs via a 2+2 cycloaddition. In this paper it is proposed that below this phase transition temperature, it should be possible to isolate the intermediate excited state species associated with the photopolymerisation and thereby identify the Raman signature of the molecular triplet of C60 and any differences, which exist between the excited states of isolated molecule and solid state. A temperature dependent study of the excited states will also be used to help elucidation the nature of the excited states in the solid.
EXPERIMENTAL Gold grade C60 soot obtained from Hoechst was vacuum sublimed at a pressure of 10-6 mbar onto glass substrates using an Edwards A305 coating system. The average thickness of the W1.10.1
resulting films was ~0.8µm. Raman measurements were taken using an Instruments S.A. Labram 1B spectroscopic microscope (x10 objective, spot radius ~10µm) equipped with an external Argon ion la
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