Pump-probe measurements

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. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

The methods diseussed so far, fluoreseenee upeonversion, the various pump-probe speetroseopies, and the polarized variations for the measurement of anisotropy, are essentially eonventional speetroseopies adapted to the femtoseeond regime. At the simplest level of interpretation, the infonnation eontent of these eonventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to ehemieal kineties, for most slow reaetions (on the ten pieoseeond and longer time seales), populations adequately speeify the position of the reaetion eoordinate intemiediates and produets show up as time-delayed speetral entities, and assignment of the transient speetra to ehemieal stnietures follows, in most oases, the same prinoiples used in speotrosoopio experiments perfomied with oontinuous wave or nanoseoond pulsed lasers. 

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

Figure C3.5.10. Frequency-dependent vibronic relaxation data for pentacene (PTC) in naphthalene (N) crystals at 1.5 K. (a) Vibrational echoes are used to measure VER lifetimes (from [99]). The lifetimes are shorter in regime I, longer in regime II, and become shorter again in regime III. (b) Two-colour pump-probe experiments are used to measure vibrational cooling (return to the ground state) from [1021.

Sitz G. O., Farrow R. L. Pump-probe measurements of state-to-state rotational energy transfer rates in N2 (v = 1), J. Chem. Phys. 93, 7883-93 (1990). 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

Two redox states of one complex, (168) and (169), exhibit very similar respective values of ca. 0.6 J cm-2 and 0.7 J cm-2 with 32 ps pulses at 532 nm (in benzene).452 A 532 nm OL study of the two neutral complexes (170) and (171) using ns and ps pulses has also been reported.453-455 Low values of ca. 0.3 Jem-2 are observed with ps pulses in benzene, and both ps time-resolved pump-probe and Z-scan measurements reveal that RSA and nonlinear refraction are responsible for the OL behavior.453-455 Because (170) and (171) are transparent in the region 400-900 nm, their OL responses should cover a wider range than those of fullerenes and MPcs.453-455 Dai et al. have applied ps 532 nm DFWM to the tetrahedral Zn11 or Cd11 complexes (172) and (173), the modest 7 responses of which are resonance enhanced by the n- 7r transition at 512nm.456 The dimeric square pyramidal Zn11 complex (174) exhibits a broad n —> tt absorption with Amax = 497 nm in DMF and is shown by 532 nm Z-scan to exhibit SF behavior.457... 

We consider a model for the pump-probe stimulated emission measurement in which a pumping laser pulse excites molecules in a ground vibronic manifold g to an excited vibronic manifold 11 and a probing pulse applied to the system after the excitation. The probing laser induces stimulated emission in which transitions from the manifold 11 to the ground-state manifold m take place. We assume that there is no overlap between the two optical processes and that they are separated by a time interval x. On the basis of the perturbative density operator method, we can derive an expression for the time-resolved profiles, which are associated with the imaginary part of the transient linear susceptibility, that is,... 

The absorption spectrum measured in the typical pump-probe experiment is the difference between the spectrum of the remaining irradiated precursor and the created intermediate(s). Assignment of the transient absorption spectrum typically is done by reference to the low-temperature spectra described above, and (sometimes more certainly) by analysing the chemical behavior of the intermediate. For example, many carbenes are known to react with alcohols to give ethers (see below). If the detected intermediate can be observed to react with an alcohol, then this is taken as additional evidence for its assignment as a carbene. 

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