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Abstract
Vibrational real-time spectra of poly-[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) were measured in a 5 fs pump–probe experiment simultaneously at 128 probe wavelengths with a multichannel detection system. The spectral dependence of the coherent vibrational amplitudes obtained from the Fourier transform (FT) on the probe wavelength detected was found to be given by the sum of the ground-state absorption spectrum and its first and second derivatives. This indicates that the change of the transition probability caused by a wave packet motion can be explained as induced by both the non-Condon effect (non-Condon (NC) mechanism) and the time-dependent Franck–Condon factor (Frank-Condon (FC) mechanism). The FC mechanism can contribute to the first and second derivatives' dependence. On the other hand, the NC mechanism is dominant in the zeroth-order derivative. This result proves that the 11Bu exciton is strongly coupled with the excited 1Ag state, which is known to be essential in third-order optical nonlinearity. The amounts of shift of the absorption peaks and changes in the bandwidth due to the wave packet motions were determined for the four most prominent modes in the FT power spectra. The shift due to the FC mechanism was about 1.3–1.1% of the peak transition energy and the broadening of the vibronic transition due to the NC mechanism was about 8.0–7.6% of the bandwidth for the four modes. A novel ultrafast optical switch utilizing the modulation of electronic transition probability by molecular vibration through vibronic coupling and the interference of the wave packets is proposed.
Highlights
Vibrational real-time spectra of poly-[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) were measured in a 5 fs pump–probe experiment simultaneously at 128 probe wavelengths with a multichannel detection system
We report the results of real-time vibrational spectroscopy for MEH-PPV excited by a laser pulse of 5.7 fs duration, which is short enough to impulsively excite several vibrations of the molecule, and we show that the non-Condon effect is substantially large in the vibronic coupling mechanism
The amplitudes of all four modes have very large values in the spectral range of 2.18–2.28 eV, which indicates that the transition probability change due to the deformation induced by molecular vibration is large in this spectral range
Summary
A noncollinear optical parametric amplifier (NOPA) [26]–[29] was used as a light source for the pump–probe experiment as described in our previous papers [30]–[32]. Pulse duration, repetition rate and average output power were 790 nm, 50 fs, 5 kHz and 800 mW, respectively. The output pulse from the NOPA was compressed with an optical pulse compressor composed of a pair of Brewster-cut prisms and a pair of chirped mirrors. The pulse broadening due to the transmission through the air and some optical components was controlled by fine-tuning of the distance between the above-mentioned prism pair. The modulation frequency was set at 2.5 kHz. Absorption and fluorescence spectra were measured with an absorption spectrometer (model UV-3101PC; Shimadzu) and a fluorophotometer (model F-4500; Hitachi), respectively. All the experiments were performed at room temperature (293 ± 1 K)
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