Vibration coherently excited

Dephasing of the Vibrational Coherence Excitation Fluence Dependence [33, 35]... 

Figure Bl.3.7. A WMEL diagram for the seventh order Raman echo. The first two field actions create the usual Raman vibrational coherence which dephases and, to the extent that inliomogeneity is present, also weakens as the coherence from different cliromophores walks oflP. Then such dephasing is stopped when a second pair of field actions converts this coherence into a population of the excited vibrational state / This is followed by yet another pair of field actions which reconvert the population into a vibrational coherence, but now one with phase opposite to the first. Now, with time, the walked-oflP component of the original coherence can reassemble into a polarization peak that produces the Raman echo at frequency oi = 2(o - (O2...

DIffey W M, Homoelle B J, Edington M D and Beck W F 1998 Excited-state vibrational coherence and anisotropy decay In the bacterlochlorophyll a dimer protein B820 J. Phys. Chem. B 102 2776-86... 

Chudoba C, Riedle E, Pfeiffer M and Elsaesser T 1996 Vibrational coherence in ultrafast excited-state proton transfer Cham. Phys. Lett. 263 622-8... 

Figure 6.1 Nonlinear optical responses, (a) Second-order SF generation, the transition probability is enhanced when the IR light is resonant to the transition from the ground state g to a vibrational excited state V. CO is the angular frequency of the vibration, (b) Third-order coherent Raman scheme, the vibrational coherence is generated via impulsive stimulated...

Raman excitation. and I2s are the high-frequency and low-frequency components of the pump light pulse. A probe pulse of frequency 12 interacts with the coherence to present the optical response of the fundamental frequency 12 + C0fsl2. (c) Fourth-order coherent Raman scattering, the optical response of the second harmonic frequency 212 + co 2I2 is modulated by the vibrational coherence. 

In the current chapter, the principles of Raman excitation and interface-selective detection of vibrational coherence are described, including applications to air/liquid, liquid/liquid, air/solid interfaces, and an organic submonolayer. 

A light pulse of a center frequency Q impinges on an interface. Raman-active modes of nuclear motion are coherently excited via impulsive stimulated Raman scattering, when the time width of the pulse is shorter than the period of the vibration. The ultrashort light pulse has a finite frequency width related to the Fourier transformation of the time width, according to the energy-time uncertainty relation. 

Another light pulse of frequency comes at a time delay ta and interacts with the vibrationally excited molecules. The intensity of the probe light transmitted through the interface is modulated as a function of the delay. The modulation is Fourier-transformed to provide the frequency and phase of the vibrational coherence. 

CARS signal originates from a coherent excitation of vibrational level nsing a pair of optical pulses, tUj ( pump ) and (o ( Stokes ), separated by a freqnency of this vibrational level, Q i.e.. 

The third pulse at frequency (o ( probe ) is scattered off the coherently excited vibration to generate the signal at the CARS freqnency, (Fignre 6.13) ... 

The excited dibromide executes this vibrational mode and the vibrational coherence persists as some trajectories find the C Br bond-cleavage exit channel, a reaction favored when the dihedral angle is close to 60°. The proper phasing of two vibrations, the BrCCC dihedral-angle-modifying torsional mode and the C—Br stretching vibration, leads to the cleavage of the C Br bond. 

The theoretical model developed to explain these experiments is based on inelastic tunneling of electrons from the tip into the 2ir adsorbate resonance that induces vibrational excitation in a manner similar to that of the DIMET model (Figure 3.44(b)). Of course, in this case, the chemistry is induced by specific and variable energy hot electrons rather than a thermal distribution at Te. Another significant difference is that STM induced currents are low so that vibrational excitation rates are smaller than vibrational de-excitation rates via e-h pair damping. Therefore, coherent vibrational ladder climbing dominates over incoherent ladder climbing,... 

One of the most interesting applications of Femtochemistry is the stroboscopic measuring of observables related to molecular motion, for instance the vibrational periods or the breaking of a bond [1], Because femtosecond laser fields are broadband, a wave packet is created by the coherent excitation of many vibrational states, which subsequently evolves in the electronic potential following mostly a classical trajectory. This behavior is to be contrasted to narrow band selective excitation, where perhaps only two (the initial and the final) states participate in the superposition, following typically a very non-classical evolution. In this case one usually is not interested in the evolution of other observables than the populations. 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

The 196 cm"1 mode is antisymmetric and thereby optically inactive and does not appear in the Raman spectrum. Since a direct optical excitation of the mode is excluded by symmetry selection rules we conclude that it is solely excited by the single proton transfer which breaks the symmetry. This demonstrates for the first time that the coherent excitation of a vibrational mode results exclusively from an ultrafast reactive process. 

The protons of the hydroxy groups were deuterated by dissolving BP(OH)2 in cyclohexane and shaking the solution with deuterated water for several hours. After precipitation pump-probe measurements of BP(OD)2 in cyclohexane were recorded and are compared to BP(OH)2 in Fig. 4. Both samples were excited at 350 nm and probed at 505 nm. The delay of the emission rise of about 50 fs is equal in both cases and the coherent excitation of the vibrations is identical with respect to frequencies, phases and amplitudes. The ESIPT dynamics is obviously not altered by the deuteration and the mass of the proton has no influence on the transfer speed. This excludes that tunneling of the proton determines the speed of the transfer and the measurements provide the first proof for the passive behavior of the proton in the ESIPT. 

We observe the coherent excitation of an optically inactive mode proving that the reactive process itself and not only the optical excitation drives the observed vibrational motions. Further we demonstrate that during the ESIPT the proton is adiabatically shifted from one site to the other and tunneling of the proton is not rate determining. The dynamics is entirely controlled by the skeletal modes. Interestingly, this is quite similar to ground state proton transfer of HC1, where the fluctuations of the water environment enable the adiabatic process [8]. 

Figure 3. Field-matter interactions for a pair of electronic states. The zero and first excited vibrational levels are shown for each state (A). The fields are resonant with the electronic transitions. A horizontal bar represents an eigenstate, and a solid (dashed) vertical arrow represents a single field-matter interaction on a ket (bra) state. (See Refs. 1 and 54 for more details.) A single field-matter interaction creates an electronic superposition (coherence) state (B) that decays by electronic dephasing. Two interactions with positive and negative frequencies create electronic populations (C) or vibrational coherences either in the excited (D) or in the ground ( ) electronic states. In the latter cases (D and E) the evolution of coherence is decoupled from electronic dephasing, and the coherences decay by the vibrational dephasing process.

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