Comparative Study Of Hole-phonon Coupling Structure Of Monolayer And Bilayer Rubrene Thin Film On HOPG Surface

The main topic of our research is the hole-phonon coupling in rubrene bilayer,which can be derived from UPS spectra.We adopted He*light source to irradiate vacuum-sublimed rubrene thin film on HOPG substrate,we gradually enhanced the film thickness to obtain thickness dependent ultraviolet photoelectron spectroscopy(UPS)and metastable atomic electron spectroscopy(MAES),also we adapted the emission angel to obtain angle resolved ultraviolet photoelectron spectroscopy(ARUPS),then we superimposed ARUPS of different emission angle to obtain angle-integrated UPS spectrum,and we deconvoluted the spectrum based on electron-phonon coupling theory by single mode analysis(SMA),revealing the vibrational fine structure of rubrene bilayer,also we deconvoluted ARUPS of different emission angel and recorded the angle dependent variation of HOMO intensity,to obtain photoemission angular distribution(PAD),from PAD we can infer that moecuar orientation adopted a different orientation in bilayer with respect to monolayer,demonstrating a tilted shoulder to shoulder arrangement,and flat lying molecules in transient monolayer structure totally transform into tilted molecules in close-packed monolayer.We reveal the fitting parmetres by two perspectives,one is the comparison with gas-phase rubrene molecule to demonstrate the impact of conformation on electron-phonon coupling,two is the comparison with monolayer result,which reveal the change of electron-phonon coupling structure with increased film thickness.The comparison shows that twisted-conformed rubrene molecule has a larger vibration energy and smaller reorganization energy than planar-cinformed rubrene molecule,which is consistant with theoretical result,and bilayer rubrene has a larger vibration energy and reorganization energy than monolayer rubrene,which is relevant with the change in molecular orientation,in monolayer molecule lies flat,the vibration of phenyl side rings are hindered by substrate,leading to a smaller vibration energy,the close-packed shoulder to shouder arrangement in bilayer enhanced the intermolecular Interaction,thus lead to a larger reorganization energy.Moreover,an additional peak D is found in bilayer vibration structure,with intensity mismatch with other two series.The shape of PAD from peak D is different with monoayer PAD,thus peak D is not a residual signal of monolayer molecule.These are the main results of the paper,although most of the data has been satisfactorily explained,the subject can achieve more progress,for the origin of peak D lacks of a reliable explanation,attempts can be made to research the quasi-dynamic behavior of peak D,by using IR pump.