Pump-probe method

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

K., Wada, A., Kano, S. S. and Hirose, C. (1999) SFG study of unstable surface species by picosecond pump-probe method. Surf. Sci, 427-428, 349-357. 

More recently the application of sub-picosecond, time-resolved pump-probe methods revealed the timescale for vibrational relaxation of a diatomic molecule at a metal surface directly. See for example Refs. 19-21. In comparison to vibrational relaxation on NaCl salts,22 which occurs on the millisecond timescale, another relaxation mechanism is clearly at play. Theory of vibrational relaxation based on excitation of electron-hole pairs gave agreement with observed ps timescales for CO on copper.23... 

The pump-probe method provides the solution to this nanosecond barrier. Here, two light pulses are generated one to excite the sample (prepare the excited state) and one to probe the system at a given time after excitation (Figure 10.8). 

The outlook is good for applications of these picosecond methods to an increasing number of studies on reactive intermediates because of the limitations imposed by the time resolution of nanosecond methods and the generally greater challenges of the use of a femtosecond spectrometer. The pump-probe technique will be augmented in more widespread applications of the preparation-pump-probe method that permits the photophysics and photochemistry of reactive intermediates to be studied. 

Comprehensive experimental investigations of 2PA processes in fluorene derivatives were performed by Hales et al. [53,56-59] with open aperture Z-scan [26], two-photon induced fluorescence [60] and femtosecond white-light continuum pump-probe methods [61]. For degenerate two-photon excitation, the experimental 2PA spectra of symmetrical and asymmetrical fiuorenes are presented in Figs. 15 and 16. These spectra were obtained with the combination of open aperture Z-scan and two-photon fluorescence methods [57]. For centrosymmetric molecules, two-photon transitions from... 

Since the development of ultra-fast pump-probe methods [1], in which one detects the temporal evolution of molecular absorption [1, 2], or emission... 

Figure 6 shows an experimental setup to measure nonlinear transmittance by the pump-probe method. The pump is used to excite the sample and the transmission of the probe is measured. The probe beam may be delayed relative to the pump beam in order to perform time delay experiments. A continuum of frequencies may be used as the probe, thereby allowing... 

In summary, the presented results demonstrate the capacity of combining IR-pump-probe methods with calculations on microsolvated base pairs to reveal information on hidden vibrational absorption bands. The simulation of real condensed phase dynamics of HBs, however, requires to take into account all intra- and intermolecular interactions mentioned in the Introduction. As far as DNA is concerned, Cho and coworkers have given an impressive account on the dynamics of the CO fingerprint modes [22-25]. Promising results for a single AU pair in deuterochloroform [21] have been reported recently using a QM/MM scheme [65]. 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

Figure 3-5. Schematic of ultrafast pump-probe method applied to photoinitiated reactions in binary complexes.

It must be emphasized that such phenomena are to be expected for a statistical system only in the regime of low level densities. Theories like RRKM and phase space theory (PST) (Pechukas and Light 1965) are applicable when such quantum fluctuations are absent for example, due to a large density of states and/or averaging over experimental parameter such as parent rotational levels in the case of incomplete expansion-cooling and/or the laser linewidth in ultrafast experiments. However, in the present case, it is unlikely that such phenomena can be invoked to explain why different rates are obtained when using ultrafast pump-probe methods that differ only in experimental detail. 

Walther, Th., Bitto, H. and Huber, J.R. (1993). High-resolution quantum beat spectroscopy in the electronic ground state of a polyatomic molecule by IR-UV pump-probe method, Chem. Phys. 

These early lifetime results, obtained from bulb conditions, (room temperature) were compared to similar data recorded under jet-cooled conditions. Analyses of the decay characteristics of the 41 level fluorescence at Ev = 0.42 cm-1 yielded a lifetime of 1.5 ps and was found to be internally consistent with the value derived from the OODR pump-probe method with beam displacement... 

Fig. 2. Schematic description of the pulsed laser photolysis/laser-induced fluorescence (LP/LIF) pump-probe method for H-I-O2 chemical dynamics studies. TVanslationally energetic H atoms are generated by pulsed laser photolysis of appropriate precursor molecules HX and the nascent 0( Pj 2 i o) atoms produced in the reaction H -I- O2 are detected under collision free conditions via LIF.

In this experiment type, the detector of the transmitted probe light behaves as a photon integrator only and the time resolution derives from the instrument s capability for measuring the difference in the time of arrival of the pump and probe pulses. This capability provides an intrinsic limitation to the pump-probe method, which depends critically on the width of the pulses being employed. As indicated earlier, exceedingly short duration pulses are commonplace these days. 

In the first reported measurements made with picosecond pulses, an optical beam splitter was used to pick off a portion of the pulse train and a variable optical delay path was introduced between the two beams [7]. The main beam was used to excite (pump) a dye sample, and the weak (probe) beam was used to monitor the recovery of dye transmission as a function of delay. Over the past two decades, this pump-probe method has been extended to a variety of measurement geometries and used to measure electronic polarization dephasing times as well as population lifetimes. 

Secondly, many of the results from our laboratory will be summarized, in which binary complexes are formed in supersonic nozzle expansions, and standard laser pump-probe methods are used to initiate reactions and monitor products [10, 25-33]. This work commenced in 1985 with the report of nascent OH(X Il) deriving from the 193 nm excitation of C02-HBr complexes [10]. In the beginning, experiments mainly recorded OH state... 

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. 

To utilise the pump-probe method for such experiments, one can use a laser of ns, ps or fs pulse width, any of which is commercially available, with high-speed electronics such that the required time-gated synchronisation between X-ray and laser pulses for the experiment is achieved. 

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