Impulsive Raman excitation

The electric field envelope of the femtosecond pump pulse which is short compared to the period of the oscillations in Fig. 15.3 (b) covers a frequency range much broader than the energy spacing of individual levels of the low-frequency mode. In other words, the pump spectrum overlaps with several lines of the vibrational progression depicted in Fig. 15.1 (b). As a result, impulsive dipole excitation from the Vqd = 0 to 1 state creates a nonstationary superposition of the wavefunc-tions of low-frequency levels in the Vqd = 1 tate with a well-defined mutual phase. This quantum-coherent wavepacket oscillates in the Vqd = 1 state with the frequency Q of the low-frequency mode and leads to a modulation of O-H stretching absorption which is measured by the probe pulses. In addition to the wavepacket in the Vqd = 1 state, impulsive Raman excitation within the spectral envelope of... 

We employ the time-dependent adiabatic state approach [24-26] to examine the dynamical behavior of Ceo vibrationally excited by near-IR laser pulses (impulsive Raman excitation [36]). The time-dependent adiabatic potentials E R, t) and time-dependent adiabatic electronic states ir R, f) used in the approach are functions... 

Vibrational Impulsive Raman Excitation by a Single Pulse... 

To qualitatively understand the mechanism of impulsive Raman excitation, we apply the above-mentioned cycle average condition to the harmonic oscillator model for a Raman active mode I of which the reduced mass and vibrational frequency are ft/ and an, respectively. The applied pulse is assumed to be rectangular, and the duration is Tp./(f) is constant, that is,/(f) = / for the pulse duration. From Eqs. (7.7) and (7.8), we can derive the classical equation of motion for Qi. The field-induced... 

In this study, at first, we further examine the efficiency of impulsive Raman excitation for two models the sequential ionization model and the neutral model where Ceo remains neutral without ionization. MD simulations were performed by using the time-dependent adiabatic state approach with the B3LYP/3-21G method of DFT. Figures 7.1 and 7.2 show snapshots of the dynamics in the sequential... 

The peaks of hg(l) and ag(l) in Fig. 7.3 do not overlap with each other that is, in the initial excitation stage, Ceo can be treated as an ensemble of independent oscillators, though all the anharmonic mode couplings are taken into account in the MD simulations. It is shown in the neutral model that the peak of the ag(l) mode increases when Tpg decreases from 70 to 30 fs. As expected, the Raman active mode is efficiently excited if 7 p 7 vib/2. Although the peaks of hg(l) and ag(l) in the sequential ionization model shift to the lower frequency sides of the neutral ones (of which the frequencies correspond to those of Cjo" "), the two spectra for Tpg = 30 fs indicate that impulsive Raman excitation in the neutral model qualitatively agrees with that in the sequential ionization model. 

As already mentioned, electronically resonant, two-pulse impulsive Raman scattering (RISRS) has recently been perfonned on a number of dyes [124]. The main difference between resonant and nom-esonant ISRS is that the beats occur in the absorption of tlie probe rather than the spectral redistribution of the probe pulse energy [124]. These beats are out of phase with respect to the beats that occur in nonresonant ISRS (cosinelike rather tlian sinelike). RISRS has also been shown to have the phase of oscillation depend on the detuning from electronic resonance and it has been shown to be sensitive to the vibrational dynamics in both the ground and excited electronic states [122. 124]. 

The mechanism of control with multipulse excitation is likely due to dynamics of the carotenoid donor. The presumably incoherent EET process [1] would not support the observed dependence on the carrier phase via the parameter c. Furthermore, the control effect does not suffer from annihilation at higher excitation intensities [2], as would be characteristic for the delocalised excitons in the B850 ring [1], However, it is well known that femtosecond pulses populate higher ground state vibrational levels by impulsive Raman scattering (IRS) [4], and that the periodic phase modulation (Eq. 1) makes IRS selective for specific vibrations... 

Figure 6.12 shows the vibrational spectra for e+ and e excitations. For e+ excitation, the maximum value of S u>) appears at i> 1,400 cm and another peak is found at v 2,500 cm while for e+ excitation, the spectrum reaches its maximum at 2,500 cm and also exhibits a couple of strong peaks at v> 3,000 cm The wave numbers of 1,400, 2,500, and 3,000 cm are very close to those of the lowest three vibrational states of G) owing to the similarity between G) and L) in the PES around its minimum. The spectral features emerging in Fig. 6.12 confirm that at f > fnV L (Q, 0 consists of different vibrational quantum states between e+ and e excitations. The vibrational structure changes of aromatic molecules can be measured experimentally with femtosecond optical spectroscopic methods, for example, transient impulsive Raman spectroscopy [38]. 

Plenary 7 7. P M Champion et al, e-mail address champ neu.edu (TRRRS). Femtosecond impulsive preparation and timing of ground and excited state Raman coherences in heme proteins. Discovery of coherence transfer along a de-ligation coordinate. See above for fiirther connnent. 

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...

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. 

Ifourth(fd, 2 Q) was multiplied with a window function and then converted to a frequency-domain spectrum via Fourier transformation. The window function determined the wavenumber resolution of the transformed spectrum. Figure 6.3c presents the spectrum transformed with a resolution of 6cm as the fwhm. Negative, symmetrically shaped bands are present at 534, 558, 594, 620, and 683 cm in the real part, together with dispersive shaped bands in the imaginary part at the corresponding wavenumbers. The band shapes indicate the phase of the fourth-order field c() to be n. Cosine-like coherence was generated in the five vibrational modes by an impulsive stimulated Raman transition resonant to an electronic excitation. 

The wavenumbers of the observed bands are identical with those of the spontaneous Raman spectrum of the solution and oxazine solid [27]. The impulsive stimulated Raman transition may initiate coherent vibrations in the electronic excited state. However, there was no sign of the excited-state vibrations superimposed on the ground-state bands in the spectrum of Figure 6.3. 

Fig. 2.2. Two generation models of coherent optical phonons, (a), (c), (e) impulsive stimulated Raman scattering (ISRS). (b), (d), (f) displacive excitation of coherent phonons (DECP). Graphs (e) and (f) display the time evolution of the driving force (grey areas) and that of the displacement (solid, curves) for ISRS and DECP, respectively...

We used short broadband pump pulses (spectral width 200 cm 1, pulse duration 130 fs FWHM) to excite impulsively the section of the NH absorption spectrum which includes the ffec-exciton peak and the first three satellite peaks [4], The transient absorbance change signal shows pronounced oscillations that persist up to about 15ps and contain two distinct frequency components whose temperature dependence and frequencies match perfectly with two phonon bands in the non-resonant electronic Raman spectrum of ACN [3] (Fig. 2a, b). Therefore the oscillations are assigned to the excitation of phonon wavepackets in the ground state. The corresponding excitation process is only possible if the phonon modes are coupled to the NH mode. Self trapping theory says that these are the phonon modes, which induce the self localization. 

D. M. Neumark We are interested in generating coherent vibrational motion in negative ions, which typically do not have bound excited electronic states. Does your Impulsive Stimulated Raman Scattering (ISRS) scheme work if the excited state is not bound ... 

The implementation of time-resolved CARS for microspectroscopy and its application for vibrational imaging based on RFID was first demonstrated by Volkmer et al. [64] using three incident pulses that are much shorter than the relevant material time scale. Here, a pair of temporally overlapped pump and Stokes femtosecond pulses was used to impulsively polarize the molecular vibrations in the sample. Impulsive excitation with a single ultrashort pulse is also possible provided that the spectral bandwidth of the pulse exceeds the Raman shift of the molecular vibration of interest [152]. The relaxation of the induced third-order nonlinear polarization is then probed by scattering of another pulse at a certain delay time, r. A measurement of the RFID consists of the CARS signal collected at a series of delay times. 

In impulsive multidimensional (1VD) Raman spectroscopy a sample is excited by a train of N pairs of optical pulses, which prepare a wavepacket of quantum states. This wavepacket is probed by the scattering of the probe pulse. The electronically off-resonant pulses interact with the electronic polarizability, which depends parametrically on the vibrational coordinates (19), and the signal is related to the 2N + I order nonlinear response (18). Seventh-order three-dimensional (3D) coherent Raman scattering, technique has been proposed by Loring and Mukamel (20) and reported in Refs. 12 and 21. Fifth-order two-dimensional (2D) Raman spectroscopy, proposed later by Tanimura and Mukamel (22), had triggered extensive experimental (23-28) and theoretical (13,25,29-38) activity. Raman techniques have been reviewed recently (12,13) and will not be discussed here. 

The experimental setup for the broadband CARS is rather simple because only two pulses are needed for three-color CARS emission, as shown in Fig. 5.4a a broadband first pulse impulsively promotes molecules to vibrationally excited states through a two-photon Raman process, and a delayed narrowband second pulse induces anti-Stokes Raman emission from coherent superpositions to the ground state [29]. By changing the delay time for the second pulse, therefore, one can expect to probe dynamical behaviors of multiple RS-active modes. Such a two-dimensional observation in the time-frequency domains should be effective for detailed analysis of nanomaterials. 

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