Vibrational relaxation pump-probe absorption

The above theory is usually called the generalized linear response theory because the linear optical absorption initiates from the nonstationary states prepared by the pumping process [85-87]. This method is valid when pumping pulse and probing pulse do not overlap. When they overlap, third-order or X 3 (co) should be used. In other words, Eq. (6.4) should be solved perturbatively to the third-order approximation. From Eqs. (6.19)-(6.22) we can see that in the time-resolved spectra described by x"( ), the dynamics information of the system is contained in p(Af), which can be obtained by solving the reduced Liouville equations. Application of Eq. (6.19) to stimulated emission monitoring vibrational relaxation is given in Appendix III. 

The 8- and 4.5-nm particles are at most weakly emissive and the polarizations of spectral features assigned to these particles are determined by polarized bleach measurements. The bleach anisotropy was determined using femtosecond pulses, with the probe delayed a few picoseconds from the pump. This delay ensures electronic and vibrational relaxation as well as relaxation of optical Kerr effects induced in tire solvent. As a control, transient absorption experiments were performed with excitation at 475 nm and detection at 550 nm. This detection wavelengtli is to the red of the wavelengths at which a bleach would be observed and provides a measure of the transient absorption intensity in this general spectral... 

With a similar setup as used by Ippen et al. for pump-probe experiments, except for an intensity stabilizer in both beams, we performed experiments on the electronic origin at 6027 A and vibronic transitions at 5933 and 5767 A. The results of these experiments are shown in Fig. 22. Except for minor details, the transient on the purely electronic transition is in agreement with our expectation that the singlet excited state is long lived (19.5 ns) on a picosecond time scale. The transient on the 261 cm vibration confirms what was already known from the optical absorption spectrum, namely, that it is very short lived. From the near Lorentzian lineshape at low temperature we calculate a 3.3 ps relaxation time in... 

Oscillations of fluorescence, stimulated emission and excited-state absorption have been studied by pump-probe techniques and fluorescence upconversion, and have been seen in numerous small molecules in solution (Fig. 11.7A [120, 122-124]), and also in photosynthetic bacterial reaction centers [27, 125, 126]. They typically damp out over the course of several picoseconds as a result of vibrational relaxations and dephasing. Vibrational coherences generally decay more slowly than electronic coherences because the energies of vibrational states are not coupled as strongly to fluctuating interactions with the surroundings. Vibrational dephasing also tends to be less dependent on the temperature. 

Such experiments are usually based on pump-probe schemes where an intense picosecond IR pulse, resonant to a vibrational transition in an adsorbate, creates a nonequilibrium population of an excited state. The subsequent evolution of this population is probed with a second, time-delayed IR pulse. The first time-resolved measurements of adsorbate vibrational relaxation were carried out for hydroxyl groups boimd to colloidal Si02 (Heilweil et al. 1984). The use of colloidal particles of about 10 nm in diameter leads to an increase in the effective number of adsorbate monolayers up to 10. The frequency of the vibrational transition v = 0—rv=l of surface hydroxyl groups is much higher than the frequencies of the substrate phonon modes. This allows one to monitor the evolution of the excited vibrational state population in the transmission of IR radiation. Due to the anharmonicity of the vibrations, absorption of the pump IR light does not lead to transitions to higher vibrational levels with... 

The guanine CO-stretch vibration is excited at three different pump frequencies with a spectrally narrow laser pulse (FWHM 12 cm 1). Transient-absorption spectra are measured using a systematically delayed, spectrally broad, probe pulse. A negative transient-absorption signal indicates a bleaching, whereas a positive signal reflects excited-state absorption. The overall decay of the signal is due to T relaxation. 

Fig. 2.4. (a) The excitation process. The pump radiation (solid line) populates a high-lying vibronic level. Relaxation (dashed line) populates lower-lying states. The population is redistributed black circles correspond to occupied states and white circles to empty states, (b) Probing. Photobleaching (dotted lines), photoinduced absorption (PA) and stimulated emission (SE) (solid lines). Only one vibrational replica is shown (tilted arrow)... 

In this method, two pulsed lasers are used, both usually in the nanosecond regime. One (the burn laser) is operated at high power, and is scanned across the absorption spectrum. It excites molecules (or clusters) from the particular vibrational level (usually the i = 0 level) to an electronically excited state. The upper state relaxes (radiatively or otherwise) back to the ground state, but not necessarily to i = 0. Thus, depletion in the population of this species is achieved. A second, low-power laser (the probe laser) is fired after a suitable time delay (to allow complete decay of the emission induced by the pump laser). It is tuned to one of the excitation spectrum vibronic bands of the system, and the fluorescence induced by it (the signal ) is continuously monitored. Whenever the frequency of the bum laser corresponds to excitation of the species giving rise to the absorption of the probe laser, the signal is reduced. This reduction appears as a hole that is burned in the spectrum—hence the name of the method. If a different species is excited (another molecule or a different vibrational level) no change in fluorescence intensity is incurred. 

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