Photon time-delay spectrum

The photomultipliers feed the coincidence-counting electronics, that includes a time-to-amplitude converter and a multichannel analyzer, yielding the time-delay spectrum of the two-photon detections (Fig. 11). This spectrum involves a flat background due to accidental coincidences (i.e. between photons emitted by different atoms). True coincidences yield a peak around the null-delay, with an exponential decrease (time constant t ). 

Fig. 11. Time-delay spectrum. Number of detected pairs as a function of the delay between the detections of two photons.

Figure 7-19 shows theps transient PM spectra lor time delays f=0 and /= 1 ns, where —A77T is ploLLed versus the probe photon energy. The PM spectrum at... 

A different perspective of the vibrational structure of the Sj electronic state is illustrated in Figure 2.13b. This is an OODR that was obtained by sequentially exciting CI2CS with two photons of different colors. In this experiment, a photon from the first laser (the pump photon) induces a Si <— So vibronic transition that is followed after a short time delay by a second S2 Si, probe photon that carries the excitation to the S2 state. The pump laser is advanced to the blue and interrogates the bands of the S2 <— So system while the probe laser is scanned at the same rate to the red such that the total energy matches a selected vibrational level of the S2 state. In this way, an excitation spectrum of the vibrational band structure of the S2 state is constructed by monitoring the fluorescence that originates from the S2 state. 

What is not too surprising is that the one-photon LIF spectrum (Fig. 2.13a) and two-photon OODR spectra (Fig. 2.13b) are similar, since these spectra sample the same Si level structure. The major differences between these panels lies in the intensity relationships of the bands within the progressions. These differences can be understood by recognizing that the OODR is a sequential process where a substantial time delay is introduced between the pump and the probe photons. Thus the Franck-Condon factors for the S2<—Si <— So process is a... 

Fig. 2. Doppler-free spectra of the 15 — 2S two-photon transition (F = 1 —> F = 1) in atomic hydrogen, a) Spectra for three different nozzle temperatures and no delay time, b) Time resolved spectrum (nozzle temperature 6.5 K). This plot gives the 2S count rate as a function of the absolute optical frequency for different delay times. The inset shows the spectra with longer delay times on a magnified scale...

Consider a time-resolved, electronically nonresonant CARS spectrum from a molecular liquid. In the CARS process, the laser pump pulses create a linear combination (that is the inteimolecular rovibrational coherence) of Raman active rovibrational transitions between molecules at position rr and r in the mixture. This stimulated Raman scattering process is carried out by two-coincident laser pulsesfl, II) with central frequenciesfwave vectors) C0i(k ) and (Oiiikii). By applying the third pulse with C0 (kni) to the liquid after time delay t, the time dependence of the inteimolecular rovibrational coherence is detected through the measurement of the intensity of the scattered photon with kj... 

This is a third-order nonlinear spectroscopic method that does not involve time delay. It consists of sending two coherent beams on a sample simultaneously one in the visible-UV region and the other one in the IR region and observing the photon that is emitted at the sum of their frequencies and is concomitant with the absorption of two photons, one in each of the two incident beams (72). The spectroscopic regions of the two incident beams are regions of transparency of the sample. The emitted photon requires absence of a centre of symmetry at molecular level to appear. It means that it practically does not appear in the bulk of a liquid, for instance, which is isotrope and consequently displays a centre of inversion in the average, but may appear on its surface, or at the interface between this liquid and another medium, where this centre of inversion disappears. It will consequently be most useful in the study of surfaces and interfaces, particularly the structures of the molecules thereon that can be deduced from the spectrum of these surfaces or interfaces (73). In many situations, it may be the unique tool to study liquid surfaces and interfaces and we shall see this in Ch. 9, which is devoted to liquid water-related examples. 

Figure 18. A typical time correlation spectrum for the experiment of Perrie, Duncan, Beyer, and Kleinpoppen after subtraction of the spectrum obtained with the metastable component of the beam quenched. Polarizer plates removed. Time delay per channel 0.8 nsec. Total collection time 21.5 h. Singles rate with metastables present (quenched) about 1.15 x lO sec" (0.85 X 10 sec" ). True two-photon coincidence rate 490 h" .

Fig. 3. Phase Locked IR Pulses Time domain interferometry. (A) Output IR pulses from two tunable OPA-DFGs in the 4-pm frequency regime. (B) Three examples of interferograms generated by these IR pulses. (C) Linear IR absorption spectrum of acetic acid overlapped with the output of two OPAs. (D) Photon echo signal from acetic acid upon t-scan. The x-axis is the delay of the translation stage and the insert is a blow-up of a small region.

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