Saturation spectroscopy of vibronic transitions in single molecules

The technique of saturation spectroscopy has been applied to study the individual vibronic spectra of single impurity molecules embedded in a low-temperature solid matrix (see Fig. 1). Homogeneous linewidths and vibrational frequencies for two lower vibronic levels of the excited electronic state S1 have been determined for several terrylene single molecules in a naphthalene crystal; noticeable distribution of these parameters has been observed. A certain correlation can be pointed out between the vibrational frequency and the purely electronic transition frequency of a terrylene molecule. This study has been performed in cooperation with the SMS group @ ETH Zurich (laboratory of Prof. U.P.Wild) [23].

Fig.1. The vibronic spectra of three terrylene single molecules in naphthalene, obtained by saturation spectroscopy measurements. The probe laser was scanned over the regions of the first and the second vibronic bands of terrylene in naphthalene. The vertical scale corresponds to the fluorescence intensity in thousands of counts per second; the zero detuning of the probe laser frequency corresponds to the absolute frequency of 17410 cm^-1. Black solid lines represent the fluorescence excitation spectra with the pump laser being switched off (background of ~ 7⋅10^4 counts/s is subtracted). Grey lines represent the same spectra, but with the pump laser being tuned into resonance with the purely electronic lines of the molecules A, B, and C, respectively; ~10^5 counts/s are subtracted, an increase of the background is caused by the pump laser excitation. Black dots represent the difference between the spectra, taken without and with the application of the pump laser; the resulting spectra correspond to the individual vibronic spectra of the molecules A, B, and C, respectively. These vibronic spectra are fitted with a sum of a Lorentzian curve and an additional background, slightly changing due to a slow drift of the pump laser frequency during the scan.