Pump-probe experiment

A main feature of ultrafast processes under consideration takes place in the time scale shorter than picoseconds. Thus, it is necessary to employ the laser with pulse-duration 10 fsec to study these ultrafast processes. From the uncertainty principle AE At h/2 it can be seen that using this pulse-duration, numerous vibronic states can be coherently pumped (or excited) and thus the probing signal in a pump-probe experiment will contain the information of the dynamics of both population and coherence (or phase). In other words, in order to obtain the information of ultrafast dynamics it is 

It has been shown that for the case in which the pumping and probing lasers do not overlap, one can use the GLRT. In this section, it shall be shown how the GLRT can be applied to calculate the ultrafast time-resolved spectra. For this purpose, start from the stochastic Liouville equation to describe the EOM for the density matrix r(t) of the system embedded in a heat bath 

Applying the perturbation method, the first-order solution is given by 

the polarization P(t) can be calculated. Consider the polarization due to the transitions between the two electronic manifolds ( and m, where m is the upper manifold to the first order, one obtains 

for example, a mk = a)mk — iymk and ymk is the dephasing constant. Since the system is initially prepared in the manifold, the first term in the bracket involving can be removed. In other words, 

In Section 1.3 the shift of the single molecule excitation line under the influence of a static electric field, the DC Stark effect, is discussed. The interaction of molecular electronic energy levels with a strong optical field is also expected to lead to level shifts and splittings and additionally to a change of relaxation rates. The shift of energy levels under optical excitation is called light shift or AC Stark effect where 

An alternative description of the observed phenomena can be obtained by the dressed atom model [80]. In this approach the eigenvalues of the global system atom + field, the dressed states, are sought. The strong pump beam dresses the atom with laser photons which leads to energy levels split by the Rabi frequency. The resonances discussed in Fig. 20 then arise naturally as transitions between the (infinite) ladder of dressed states. The dressed atom model delivers a physically comprehensible description of atomic energy levels in strong fields and the reader is referred to Ref. 80 for a detailed presentation. 

The strong inhomogeneous broadening of optical transitions in solids at low temperature prevented the observation of the AC Stark effect within an ensemble of molecules. In this case also line narrowing techniques did not provide a solution to realize the experiment. With the advent of single molecule spectroscopy the proper means to perform experiments of the piunp-probe type on absorbers in solids without excessive interfering backgrounds were available. Due to the favourable photo- 

Figore 21. Variations of the light shift with inverse of pump detuning for a single terry-lene molecule in / -terphenyl. The pump intensities are as given in the figure. The two plots relate to two different molecules measured with different setups, under different illumination conditions (from Ref. 81). 

The light shift of the resonance line of a single terrylene molecule was measured with a near-resonant pump (A F) by either varying the pump intensity at a iked detuning or vice versa. As can be derived from a perturbative approach as well as from the dressed atom model, the light shift d for a near-resonant ptimp can be approximated (A Qi) by the simple expression, 

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Femtosecond pump-probe experiments have burgeoned in the last ten years, and this field is now connnonly referred to as laser femtochemistry [26, 27, 28 and 22],... 

Pollard W T, Lee S-Y and Mathies R A 1990 Wavepacket theory of dynamic absorption spectra in femtosecond pump-probe experiments J. Chem. Phys. 92 4012... 

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. 

Balk M W and Fleming G R 1985 Dependence of the coherence spike on the material dephasIng time In pump-probe experiments J. Chem. Phys. 83 4300-7... 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Tamarat P, Lounis B, Bernard J, Orrit M, Kummer S, Kettner R, Mais S and Basche T 1995 Pump-probe experiments with a single molecule ac-Stark effect and nonlinear optical response Phys. Rev. Lett. 75 1514-17... 

Figure C3.4.5. Typical scheme of a single-colour pump-probe experiment utilizing lock-in-amplifier detection.

Another important breaktlirough occurred with the 1974 development by Laubereau et al [24] of tunable ultrafast IR pulse generation. IR excitation is more selective and reliable than SRS, and IR can be used in pump-probe experiments or combined with anti-Stokes Raman probing (IR-Raman method) [16] Ultrashort IR pulses have been used to study simple liquids and solids, complex liquids, glasses, polymers and even biological systems. 

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.

Hill J R ef a/1994 Vibrational dynamios of oarbon monoxide at the aotive site of myoglobin piooseoond infrared free-eleotron laser pump-probe experiments J. Phys. Chem. 98 11213-19... 

We have studied the temporal dynamics of CPG in m-LPPP by performing field-assisted pump-probe experiments on LED structures, as described in Section 8.3.2. The narrow line-width PA assigned to polarons (see Section 8.5.2) is a fingerprint of charge generation in m-LPPP. Monitoring the dynamics of these PA band enables us, for the first time, to directly observe the CPG dynamics in a conjugated polymer with sub-picosecond time resolution [40],... 

Figure 8-3. Layout of the experimental set-x / up for conventional pump-probe experi-...

Figure 3. Transient response of 1,2 C-vinyl acetate (MW=88) obtained from a typical pump-probe experiment with the palladium-based catalysts and 1,2 C-ethylene.

The extension of Eqs. (12)—(16) to a standard pump-probe experiment, where the final state is a (dissociation or ionization) continuum is straightforward. It requires only that we replace the pulse that spans two vibrational eigenstates by a shorter one that spans several eigenstates. 

Figure 7. Several of the ionization schemes employed in various pump-probe experiments of ammonia clusters the energy levels correspond to those of the ammonia monomer. The upper hatched region denotes the ionization limit. Taken with permission from ref. 68.

The results for the A state show that a different mechanism is operative. A series of femtosecond pump-probe experiments were performed at wavelengths corresponding to the Rydberg states A (v = 0,1,2) of ammonia molecules.64-66,68,69 The wavelengths used to access these vibrational levels were 214 nm, 211 nm, and 208 nm for the pump laser and 321 nm, 316.5 nm, and 312 nm for the probe laser, respectively. 

As discussed in Sect. 1.2.3, it is usually not possible to distinguish ESA from 2PA with Z-scan experiments if they are performed with only one excitation pulsewidth. However, since ESA is not an instantaneous process as is 2PA, the pump-probe technique can be successfully used to verify the origin of the nonlinearity for the spectral regions close to the main absorption band. Figure 7 illustrates how the influence of the ESA can be distinguished from the 2PA with pump-probe experiments. The curve labeled (1) shows an instantaneous 2PA response without ESA and the long-lived components of the transmittance change seen in (2) and (3) are due to ESA. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

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. 

Fig. 1.1. TOF data for I2 as a function of time delay in a pump-probe experiment showing a variety of vibrational signals in the I+ signal. The horizontal band at 5,145 ns corresponds to metastable t] 1. The pair of bands at 5,020 and 5,270 ns corresponds to the I+ + I+ dissociation channel, while the signal in between these bands corresponds to I+ + I. Pump and probe are both 25 fs, 800 nm pulses...

Finally, it has been observed through pump-probe experiments [9] that the I2+ fragment following asymmetric dissociation of can itself be in an excited state. This also represents a very high level of electronic excitation. 

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