The probe pulse

Block 4 a high-voltage generator of the probe pulse, the output value of the amplitude is from 100 to 400V, the duration of the pulse is from 15 to 300 ns. 

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 pump-probe concept can be extended, of course, to other methods for detection. Zewail and co-workers [16,18, 19 and 2Q, 93] have used the probe pulse to drive population from a reactive state to a state that emits fluorescence [94, 95, 96, 97 and 98] or photodissociates, the latter situation allowing the use of mass spectrometry as a sensitive and selective detection method [99, 100]. 

The main cost of this enlianced time resolution compared to fluorescence upconversion, however, is the aforementioned problem of time ordering of the photons that arrive from the pump and probe pulses. Wlien the probe pulse either precedes or trails the arrival of the pump pulse by a time interval that is significantly longer than the pulse duration, the action of the probe and pump pulses on the populations resident in the various resonant states is nnambiguous. When the pump and probe pulses temporally overlap in tlie sample, however, all possible time orderings of field-molecule interactions contribute to the response and complicate the interpretation. Double-sided Feymuan diagrams, which provide a pictorial view of the density matrix s time evolution under the action of the laser pulses, can be used to detenuine the various contributions to the sample response [125]. 

Figure B2.5.10. LIF signal of free Na atoms produced in the photodissociation of Nal. t - q is the delay between the photolysis pulse (at L) and the probe pulse. Adapted from [111].

Optical detectors can routinely measure only intensities (proportional to the square of the electric field), whether of optical pulses, CW beams or quasi-CW beams the latter signifying conditions where the pulse train has an interval between pulses which is much shorter than the response time of the detector. It is clear that experiments must be designed in such a way that pump-induced changes in the sample cause changes in the intensify of the probe pulse or beam. It may happen, for example, that the absorjDtion coefficient of the sample is affected by the pump pulse. In other words, due to the pump pulse the transparency of the sample becomes larger or smaller compared with the unperturbed sample. Let us stress that even when the optical density (OD) of the sample is large, let us say OD 1, and the pump-induced change is relatively weak, say 10 , it is the latter that carries positive infonnation. 

Figure 10-2. Experimental setup for pump and probe measurements. Two femtosecond pulses are focused onto the same spot of the sample. The pump pulse-induced changes A7/T0 of the normalized transmission of the probe pulse are measured as a function of the time delay between the two pulses.

Figure 10-10. (a) Semilogarillnnic plol of ihc stimulated emission transients for various excitation pulse energies measured for LPPP on glass. The excitation pulses have a duration of 150 fs and are centered at 400 nm. The probe pulse were spectrally filtered (Ao=500nin, Aa=l0nm). (b) Emission spectra recorded for the same excitation conditions. The spectra are normalized at the purely electronic emission baud (according lo Ref. [181). 

Figure 10-7. (a) Absorption spectrum of 3 LPPP. The arrow indicates the spectral po-.oj, silion of the excitation pulse in the time-re- i solved measurements, (b) PL spectrum for LPPP for low excitation pulse energies, (c) Differential transmission spectrum observed in LPPP after photoexcitation with a femtosecond pulse having a pulse energy of 80 uJ at a wavelength of 400 nm. The arrow indicates the spectral position of the probe pulses used for a more detailed investigation of the gain dynamics. 

Fig. 2.1. Double-pump and probe reflectivity signal of coherent LO phonons of GaAs (left) and coherent A g phonons of Bi (right). The horizontal axis gives the time delay of the probe pulse with respect to the first pump pulse in both panels. Ati2 (At) in the left (right) panel is the time delay between the two pump pulses in units of the LO phonon period i/ [q=114 fs (A g phonon period T = 341 fs). From [5] and [6]...

A broad band emission for the probe pulse is obtained by focussing part of the monochromatic laser pulse into a cell containing a suitable liquid, such as water or tetrachloromethane. The liquid converts the laser beam into a polychromatic pulse covering a wide range of wavelengths through much of the UV, visible and IR regions. 

Figure 10.15 shows that the fluorescence intensity increases as the delay between the pump and the probe pulses increases, until a constant level of fluorescence intensity is produced (where all the excited molecules have undergone photodissociation). The curve fitting the experimental data has the form ...

The intensity of the sum frequency light /sum at a given delay time r between the probe pulse and the fluorescence beam co( is proportional to the correlation function of the fluorescence intensity with the intensity of the probe pulse co ... 

The concentration of the iron porphyrins was adjusted to be between 0.2 and 0.3 OD for 2 mm cell at 530 nm. All relaxation times were calculated from the first order kinetic curves of excited state decay or ground state reappearance. This procedure eliminates error in delay times between the excitation and different wavelength probe pulses ("chirp") since constant delay times are subtracted out of the kinetic curves. There may, however, be some error introduced in the shorter decay times because of the excitation pulse and the probe pulse may overlap at the earliest points of the kinetic curve calculations. 

Both in linear and nonlinear methods, the minimum time delay accessible to the experimenter is the time resolution, and it is determined by either the duration of the pump or the probe pulse, whichever is longer. Two linear methods are discussed in section II, while a nonlinear method is presented in section IV. Typical timescales for protein catalyzed reactions range in the nanosecond (ns) to millisecond (ms) time range and the time resolution must be much better in order to sample the time range sufficiently. However, there are processes in proteins that are much faster, often occurring at femtosecond (fs) timescales (Franzen et al. 1995 Lim et al. 1993 Jackson et al. 1994 Armstrong et al. 2003 Nagy et al. 2005). To observe these processes. 

By making the excitation pulses overlap with the minimum of the probe pulse preceding its main maximum, the nonresonant background is further suppressed (Pestov et al. 2007). The same idea can be exploited with a single pulse excitation (Dudovich et al. 2003), when both pump pulses at frequencies i, 2 are derived from a single ultra-broadband pulse. 

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