Excited-State Molecular Vibration Observed for a Probe Pulse Preceding the Pump Pulse by Real-Time Optical Spectroscopy (original) (raw)
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Spectrally resolved two-colour three-pulse photon echoes (PE) have been investigated on a femtosecond time scale for Rhodamine B in Methanol. The time evolution of the spectra of the PE signals and their dependence on the wavelength of the excitation pulses are analysed. New spectral features are observed which differ from the spectral profile of the probe pulse and which decay with a range of time constants. The influence of vibrational-electronic coupling on the PE spectra is discussed. The vibrational relaxation times (80-200 fs), which depend on the levels selected in the vibrational manifold, and the optical dephasing time (B400 fs) can be determined directly from the spectrally resolved PE measurements. r
The theory of ultrafast vibrational spectroscopy
Chemical physics, 1995
This paper discusses the use of ultrafast time-resolved IR spectroscopy to probe vibrational modes that are coupled to photochemically induced reactions. A detailed theory is derived that describes the IR probe signals in three different schemes: pulsed-pump, pulsed-probe spectroscopy in which the total power of the transmitted probe pulse is measured, pulsed-pump, pulsed-probe spectroscopy in which the probe pulse is spectrally dispersed after the sample and pulsed-pump, cw-probe with gating after the sample. It is found that the three schemes lead to subtly different signals which may greatly affect the interpretation of experiments. Also the coherent excitation of vibrational modes that are both IR and Franck-Condon active is considered. 0301-0104/95/$09.50
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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. 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 1 1 B u exciton is strongly coupled with the excited 1 A g 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
Slow Intramolecular Vibrational Relaxation Leads to Long-Lived Excited-State Wavepackets
The Journal of Physical Chemistry A, 2016
Broadband optical pump and compressed white light continuum probe was used to measure the transient excited state absorption, ground state bleach, and stimulated emission signals of cresyl violet solution in methanol. Amplitude oscillations caused by wavepacket motion in the ground and excited electronic states are analyzed. It is found that vibrational coherences in the excited state persist for more than the experimental waiting time window of 6 ps and the strongest mode has a dephasing time constant of 2.4 ps. We hypothesize the dephasing of the wavepacket in the excited state is predominantly caused by intramolecular vibrational relaxation (IVR). Slow IVR indicates weak mode-mode coupling and therefore weak anharmonicity of the potential of this vibration. Thus the initially prepared vibrational wavepacket in the excited state is not significantly perturbed by non-adiabatic coupling to other electronic states and hence the diabatic and adiabatic representation of the system are essentially identical within the Born-Oppenheimer approximation. The wavepacket therefore evolves with time in an almost harmonic potential, slowly dephased by IVR and the pure vibrational decoherence. The consistence in the position of node (phase change in the wavepacket) in the excited state absorption and stimulated emission signals without undergoing any frequency shift till the wavepacket is completely dephased, conforms to the absence of any reactive internal conversion.
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Real-time traces of vibrational amplitude detected by the intensity modulation of electronic transition under high-density excitation with sub-5-fs pulse revealed that a combination tone of the C u C and the C v C stretching modes is generated by the exciton-exciton interaction. They were found to be generated in high quantum number vibrational levels in the ground electronic state after internal conversion from higher excited electronic state created by the Auger process. This indicates that the Auger induced highly excited state results in the fusion of vibrational quantum states. At the same time, parametric difference frequency generation of the vibrational modes takes place in the same process.
Journal of Optics B: Quantum and Semiclassical Optics, 2002
A pair of coherent femtosecond pulse excitations applied to a molecule with strong electron-phonon coupling creates a coherent superposition of a low momentum and a high momentum wavepacket in the vibrational states of both the excited state and the ground state of the coherent transition. As the excited state is accelerated further, interference between the high momentum ground state contribution and the low momentum excited state contribution causes a photon echo. This photon echo is a direct consequence of quantum interference between separate vibrational trajectories and can therefore provide experimental evidence of the non-classical properties of molecular vibrations.
New Journal of Physics, 2008
Time-resolved spectrum after ultrashort pulse excitation revealed fine structure of instantaneous vibronic absorption spectra in a thiophene derivative. The probe photon energy-dependent amplitudes of molecular vibration coupled to the induced absorption were composed of several peaks. An absorbance-change peak-tracking method revealed four vibronic transitions buried in the time-integrated spectra over several vibrational periods of typical molecular vibration. Four vibronic transitions located at 2.024, 1.921, 1.818 and 1.731 eV were found to be correlated among themselves with respect to the photon energies and intensities of the peaks in the difference absorbance change spectra. From the size and sign of the correlation strengths the mechanism of the vibronic coupling was related to non-Condon mechanism and Herzberg-Teller vibronic coupling.
The Journal of Physical Chemistry A, 2007
Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and bandintegrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zerothorder derivatives due to quasi-pure non-Condon type and first-and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.
Chemical Physics, 2013
Ultrafast two-dimensional infrared (2DIR) and IR pump-probe (PP) spectroscopy was used to study the intermolecular vibrational energy transfer process from the excited state of asymmetric stretching vibration of HN 3 to the overtone band of CO stretching vibration of solvent methanol. A series of timeresolved 2DIR spectra indicate an intermolecular vibrational excitation transfer between the two modes, since the corresponding cross peaks appear at longer waiting times (>20 ps). However, detailed analyses of temperature-dependent FTIR, dispersed IR PP, and 2DIR spectra showed that the vibrational relaxation of the azido stretch mode and its energy transfer to solvent methanol CO stretch overtone mode involve not only heat dissipation directly to the solvent bath modes but also production of transient intermediate states. The present experimental work demonstrates that ultrafast nonlinear IR spectroscopy is quite useful to shed light into the complicated vibrational relaxation dynamics of H-bonded solute-solvent systems.