Vibrational coherence

Wang Q, Schoenlein R W, Peteanu L A, Mathies R A and Shank C V 1994 Vibrationally coherent photochemistry in the femtosecond primary event of vision Science 266 422... 

Figure Bl.3.5. Four WMEL diagrams for fiilly resonant Raman scattering (RRS). Diagrams (a) and (b) both have doorway stage rr(A.j2 ) (Figure B 1.3.4(a)), in which a vibrational coherence is created in the ground electronic state, g. For the window event in (a), field 1 promotes the bra from the ground electronic state, g, to...

This is followed by two field actions which again create a vibrational coherence but, now, with opposite phase to the first coherence. Hence one obtains a partial rephasing, or echo, of the macroscopic polarization. The final field action creates the seventh order optical polarization which launches the signal field (the eighth field). Just as for the spin echo in NMR or the electronic echo in 4WM, the degree of rephasing (tlie... 

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...

An alternative fifth order Raman quasi-echo experiment can also be perfomied [130. 131. 132. 133 and 134]. Unlike the true Raman echo which involves only two vibrational levels, this process requires the presence of tliree very nearly evenly spaced levels. A WMEL diagram for the Raman quasi-echo process is shown in figure Bl.3.8. Here again the first two field actions create a vibrational coherence which is allowed to dephase. This is followed by a second pair of... 

Zhu I, Wdom A and Champion P M 1997 A multidimensional Landau-Zener description of chemical reaction dynamics and vibrational coherence J. Chem. Phys. 107 2859-71... 

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... 

Lotshaw WT, McMorrow D, Thantu N, Melinger J S and Kitchenbaum R 1995 Intermolecular vibrational coherence in molecular liquids J. Raman Spectrosc. 26 571-83... 

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

Vos M H, Jones M R, Hunter C N, Breton J, Lambry J C and Martin J L 1996 Femtosecond spectroscopy and vibrational coherence of membrane-bound RCs of Rhodobacfe/ sp/raero/des genetically modified at positions M210 and LI 81 The Reaction Center of Photosynthetic Bacteria—Structure and Dynamics ed M E Michel-Beyerle (Berlin Springer) pp 271-80... 

Hayashi M, Yang T-S, Yu J, Mebel A, Chang R, Lin S H, Rubtsov I V and Yoshihara K 1998 Vibronic and vibrational coherence and relaxation dynamics in the TCNE-HMB complex J. Phys. Chem. A 102 4256-65... 

The use of the rotational coherent state is then analogous to the use of the vibrational coherent state and can be used to study rotational state resolved properties. We note that the resolution of the identity applies here as well, that is. 

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. 

When the full width at half maximum (fwhm) of a Gaussian pulse is 20 fs, its frequency width is 740 cm as the fwhm. Frequency components Ql and fis are present in the pulse and are used to generate the vibrational coherence, where Ql — iis is equal to the vibration frequency ox... 

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. 

To ensure interface-selective detection of the Raman-pumped vibrational coherence, one more incident electric field is required. A fourth-order optical response is thereby generated. The requirement is fulfilled by observing the second harmonic (SH) light generated at the interface, instead of the transmitted fundamental light. 

The fourth-order coherent Raman spectrum of a liquid surface was observed by Fujiyoshi et al. [28]. The same authors later reported a spectrum with an improved signal-to-noise ratio and different angle of incidence [27]. A water solution of oxazine 170 dye was placed in air and irradiated with light pulses. The SH generation at the oxazine solution was extensively studied by Steinhurst and Owrutsky [24]. The pump and probe wavelength was tuned at 630 nm to be resonant with the one-photon electronic transition of the dye. The probability of the Raman transition to generate the vibrational coherence is enhanced by the resonance. The efficiency of SH generation is also enhanced. 

With the resonance to the electronic transition, the ground-state population is partially depleted by the pump irradiation and restored with the time delay. The raw intensity of SH light was accordingly damped at fa = 0 and recovered in picoseconds, as seen in Figure 6.3a. Intensity modulation due to the vibrational coherence was superimposed on the non-modulated evolution as expected from Eq. (6.3). The coherence continued for picoseconds on this solution surface. The non-modulated component Isecond(fd> 2 ii) was fitted with a multiexponential... 

A 0.2-mm thick hexadecane layer was placed on the oxazine solution. The vibrational coherence at the hexadecane/solution interface was pump-probed in a similar manner [27]. The light pulses traveled in the hexadecane layer and experienced group velocity dispersion before arriving at the interface. This undesired dispersion... 

Figure 6.4 Vibrational coherence at a liquid/liquid interface.

A number of solid compounds have been examined with this time-domain method since the first report of coherent phonons in GaAs [10]. Coherent phonons were created at the metal/semiconductor interface of a GaP photodiode [29] and stacked GaInP/GaAs/GalnP layers [30]. Cesium-deposited [31-33] and potassium-deposited [34] Pt surfaces were extensively studied. Manipulation of vibrational coherence was further demonstrated on Cs/Pt using pump pulse trains [35-37]. Magnetic properties were studied on Gd films [38, 39]. 

Figure 6.5 Vibrational coherence at a Ti02(l 10) surface covered with TMA monolayer, (a) The raw SH intensity, (b) the modulated component, and (c) the Fourier-transformed spectrum. The TMA-covered surface was irradiated in air with p-polarized pump (14mjcm ) and p-polarized probe (6mjcm ) pulses.

In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of Ti02 at 826 cm. Direct observation of a frequency-domain spectrum is free from the high-wavenum-ber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses. 

Successful applications of fourth-order coherent Raman scattering are presented. Interface-selective detection of Raman-active vibrations is now definitely possible at buried interfaces. It can be recognized as a Raman spectroscopy with interface selectivity. Vibrational sum-frequency spectroscopy provides an interface-selective IR spectroscopy in which the vibrational coherence is created in the IR resonant transition. The two interface-selective methods are complementary, as has been experienced with Raman and IR spectroscopy in the bulk. 

Note that the usage of 10-fs laser pulse leads to rich oscillatory components as well as these rapid kinetics in their pump-probe time-resolved profiles. Obviously in this timescale, the temperature T will have no meaning except for the initial condition before the pumping process. In addition, such oscillatory components may be due not only to vibrational coherence but also to electronic coherence. A challenging theoretical question may arise, for such a case, as to how one can describe these ultrafast processes theoretically. 

In the secular approximation and with the ladder model, the time evolution of the vibrational coherence, pw(x), is determined by... 

Group State Vibrational Coherence Induced by Impulsive Absorption in Csl. A Computer Simulation. 

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. 

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