Modulation pump probe

Figure C3.5.3. Schematic diagram of apparatus used for (a) IR pump-probe or vibrational echo spectroscopy by Payer and co-workers [50] and (b) IR-Raman spectroscopy by Dlott and co-workers [39]. Key OPA = optical parametric amplifier PEL = free-electron laser MOD = high speed optical modulator PMT = photomultiplier OMA = optical multichannel analyser.

An investigation of the diffraction signatures of specific vibronic states with the vibrations of the same symmetry reveals that even those vibrations have characteristic diffraction signatures. While the overall appearances of these diffraction patterns are similar, they show characteristic spacings and modulation amplitudes. We therefore conclude that different vibrational states have characteristic diffraction signatures, which can be observed using pump-probe electron diffraction. 

Fig. 5. Spectrogram of periodically oscillating components of pump-probe signals of polyacetylene probed at 750 nm shown in Fig. 4 and calculated using a Gaussian window function with a HWHM At = 96 fs. S and D denote the stretching modes of single and double bonds respectively. Short-lived satellite-bands (S , S and D , D associated with S and D modes, respectively) indicate the modulation induced by the breather state.

It is theoretically predicted that the formation of the breather is accompanied by the collective oscillation of the bond-length, which can be detected in the pump-probe experiment as modulation of the instantaneous vibrational frequencies. The simulation of a frequency distribution of the vibrational frequencies and a spectrogram was made with a modulation period of 44-fs and a modulation duration time of 50-fs. The evidence of the modulation appears in the spectrogram in the shape of satellite-bands S , S and D , D on both sides of the main vibrational modes S and D, respectively with the same separation. These sidebands do not appear in cis-rich samples. These results clearly suggests that the unidentified... 

Figure 8. Frequency-filtered Na2+ pump-probe signal in comparison to the averaged signal of Fig, 4. The filtered signal measures by how much the Na2+ signal is modulated with the laser frequency. Such modulations occur when there is interference between excitation by the probe pulse and the wavepackets formed by the pump laser pulse. This interference effect causes both the A EJ and the 2 1 Ilg state wavepacket motion to be observable in the signal.

On the other hand, additional spectroscopic information can be obtained by making use of this technique The Fourier transform of the frequency-filtered transient (inset in Fig. 8) shows that the time-dependent modulations occur with the vibrational frequencies of the A E and the 2 IIg state. In the averaged Na2+ transient there was only a vanishingly small contribution from the 2 IIg state, because in the absence of interference at the inner turning point ionization out of the 2 IIg state is independent of intemuclear distance, and this wavepacket motion was more difficult to detect. In addition, by filtering the Na2+ signal obtained for a slowly varying pump-probe delay with different multiples of the laser frequency, excitation processes of different order may be resolved. This application is, however, outside the scope of this contribution and will be published elsewhere. 

An ultraviolet-laser based thermo-optical absorbance detector for micrometer capillaries was used by Qi et al. [76] to monitor the separation of a mixture of 13 phenylthiohydantoin-amino acids. A modulated pump laser beam periodically illuminated the capillary at a point near its end. Complex deflection and diffraction effects occur at the capillary-solution interface. Perturbation of the refractive index at this interface changes the intensity of the probe beam that is measured using a small photodiode. 

Figure 11.24. Experimental arrangement used by Ernst and Kindt [44] in their pump/probe microwave/optical double resonance study of a rotational transition (18.2 GHz) in the ground state of CaCl. The photomultiplier tubes which monitor fluorescence are situated on the axis perpendicular to both the laser beam and the molecular beam. The C region, where the molecular beam is exposed to microwave radiation, is magnetically shielded to minimise stray Zeeman effects. The microwave power was amplitude modulated at 160 Hz and the modulated fluorescence detected by photomultiplier B. 

In a PA experiment, the excited-state absorption spectrum is measured. The sample was excited by an amplitude- modulated pump beam and changes in the transmission of a probe beam are measured by lockin amplification of a photoreceiver. The 363-nm line of an argon ion laser at a power density of 30 mW/cm2 was used as the pump beam and a monochromated tungsten white-light source was used as the probe. The pump beam was modulated at 120 Hz by an acousto-optical modulator and the spectra were measured under... 

The second major section (Section III), comprising the bulk of the chapter, pertains to the studies of IVR from this laboratory, studies utilizing either time- and frequency-resolved fluorescence or picosecond pump-probe methods. Specifically, the interest is to review (1) the theoretical picture of IVR as a quantum coherence effect that can be manifest in time-resolved fluorescence as quantum beat modulated decays, (2) the principal picosecond-beam experimental results on IVR and how they fit (or do not fit) the theoretical picture, (3) conclusions that emerge from the experimental results pertaining to the characteristics of IVR (e.g., time scales, coupling matrix elements, coupling selectivity), in a number of systems, and (4) experimental and theoretical work on the influence of molecular rotations in time-resolved studies of IVR. Finally, in Section IV we provide some concluding remarks. 

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

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