Photoionization, pump-probe

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... 

We have investigated the vibrational relaxation of Na3F by time-resolved photoionization at the threshold. Among the two isomers of Na3F, we have studied the excited electronic states of the C2v one. The pump-probe signal clearly shows damped oscillations, the period of which is fitted to 390 8 fs, close to twice the previously measured bending mode of Na2F,[l] while the relaxation time is 1275 50 fs. 

For pump-probe photoionization (PPI, Fig.l) the first laser pulse is tuned into resonance with the (vibrationless) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (1+1 ) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. 

Fig, 1. Pump-probe photoionization (PPI) and time-resolved degenrate four-wave mixing (TRDFWM) schemes for rotational coherence spectroscopy (RCS). 

Fig. 2. RCS of cyclohexylbenzene. Experimental data a) from pump-probe photoionization in a molecular beam (T 10 K) [2], b) from time-resolved degenerate four-wave mixing in a gas cell (T 305 K).

A further possibility is that the signals arise from hydrated electrons or base radical ions produced by monophotonic ionization of the polymers. However, the quantum yield for photoionization of adenosine is reported to be approximately the same as that of poly(A) and poly(dA) [25], It is unlikely that photoionization of the polymers can account for the signals seen here since there is no detectable signal contribution from the photoionization of single bases [4], The most compelling argument that our pump-probe experiments monitor excited-state absorption by singlet states is the fact that ps and ns decay components have been observed in previous time-resolved emission experiments on adenine multimers [23,26-28]. 

Figure 5. A femtosecond pump-probe photoionization scheme for studying excited-state dynamics in DT. The molecule is excited to its S> electronic origin with a pump pulse at 287 nm (4.32 eV). Due to nonadiabatic coupling, DT undergoes rapid internal conversion to the lower lying Si state (3.6eV). The excited-state evolution is monitored via single-photon ionization. As the ionization potential is 7.29 eV, all probe wavelengths <417 nm permit single-photon ionization of the excited state.

Pump-probe experiments using single photon (/.piimp =267 nm) coupled with non-resonant photoionization probing ( probe =800 nm) produced ions of all fragments Fe(CO) + (n =4-0) along with the parent ion [63], In these experiments the pump pulse intensity was 10-2 to 10-4 times those used in the multiphoton experiments described in Sect. 3.2 (109 compared to 10u-1013 W cm-2). 

This operation correlates the ground and excited states on both surfaces. The two-level charge-induced interchange of the conformers can occur on a timescale of a few picoseconds, which is typical for the resonant photoionization process [35], The dynamics of such a process, 0)° -> 1)+1 and 1)° -> 0)+1, is monitored in real time by the change in the anchoring A-N stretch, equal to Av(Au-N) = 145, 165 (due to the appearance of the A-N stretch doublet), and by the disappearance of the vibrational mode -v(N-H - N) (see Table 3) using, e.g. time-resolved picosecond UV/IR pump-probe ionization depletion spectroscopy [35]. 

Fig. 1. (a) The two isomers of NsbF. (b) Photoionization efficiency curve of NasF. Ab initio vertical ionization energies of the isomers are indicated by vertical lines. The UV probe photon energy used in the photodepletion experiment, as well as the femtosecond pump+probe total energy, is indicated by down arrows, (c) Photoabsorption spectrum of NasF determined by photodepletion spectroscopy. The calculated oscillator strengths for vertical excitations are indicated for both the Cjy and Csv structures by straight and dotted lines respectiveIy.[S] The spectra of the femtosecond pulses are also indicated. 

Fig. 2. Experimental setup of the pump-probe experiment showing the molecular beam irradiated by laser pulse sequences coming from a titanium sapphire fs-laser system. The resulting photoions are detected by a quadrupole mass spectrometer.

In femtosecond experiments, as shown in Fig. 4.1, the pump-probe methods are most commonly used to study the dynamic processes in chemical compounds or materials. It should be noted that for probing, one can use the optical excitation, photoionization up-conversion, and stimulated emission [18]. From the uncertainty principle, AEAt w /2, we can see that AE depends on the pumping-pulse duration At. For short At, both population and coherence (or phase) can be created. In other words, in this case, both population and coherence dynamics have to be... 

Figure 10.3 illustrates an application of a pump-probe system in a study of the excited states dynamics of DNA and RNA bases cytosine, guanine, thymine, and uracil [8], The experiment was conducted using a TOF mass analyzer equipped with a photoionization ion source. The samples were heated and desorbed using an oven. The vapor was carried into the ionization region using a supersonic argon jet. The pump and probe lasers... 

An example where insight into the detailed mechanism has been achieved is seen in the work by Woeste s group (Daniel et al., 2003). They combined femtosecond pump-probe experiments, ab initio quantum calculations and wave-packet dynamics simulations in order to decipher the reaction dynamics that underlie the optimal laser fields for producing the parent molecular ion and minimizing fragmentation when CpMn(CO)3 is photoionized (Cp = cyclopentadienyl) ... 

To describe photoionization in a pump-probe context, the total wavefunc-tion of our system, r,R,t), may be decomposed into several neutral electronic states n(r R), whose dynamics we wish to probe, and several ionized states ( i R) use for probing,... 

We have used the formulation, potential curves (see Fig. 3.5), and photoionization matrix elements outlined in the previous sections to study the pump-probe photoelectron spectra in Na2. Important parameters in these studies include. 

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