Photoelectron multiple excitation

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... 

Zarowin (68) has made use of a multiple-sampling technique in the measurement of decay times. This method uses a periodically pulsed- or chopped-excitation source and a continuously operating photomultiplier detector. The fluorescent signal is displayed on an oscilloscope. The response of the photomultiplier tube must be fast enough to resolve individual photoelectron pulses, and the time density of pulses is then proportional to the light intensity. 

Fig. 15. Pictorial view of the scattering processes of the excited internal photoelectron determining the EXAFS oscillations (single-scattering regime) and the resonances in the XANES (multiple-scattering regime). From Bianconi (30).

To summarize, the scintillation detector works by (1) formation of a photoelectron in the Nal(Tl) crystal after an X-ray photon hits the crystal, (2) emission of visible light photons from an excited state in the crystal, (3) production of photoelectrons from the cathode in the photomultiplier, and (4) electron multiplication. 

As the laser intensity is increased, the probability that a molecule will absorb more than one photon increases rapidly. When the initial excitation is to a real intermediate state, further excitation is efficient and can lead directly to ionization, if the energy of the photons is sufficient this forms the basis of the simplest REMPI scheme (i.e. 1 + 1 REMPI) described in Chapter 9. Excitation processes of this type are referred to as multiple-photon excitation, as the two steps are sequential and essentially independent (i.e. the process is not coherent). If, in the second step, another tuneable laser is used to further excite the molecule, from the real intermediate state, threshold ionization processes can be studied, and this is exploited in the ZEKE technique, which provides high-resolution photoelectron spectra of molecules (see Section 18.3). 

We have studied the application of pulse trains to probe some important aspects of the electronic excitation/deexcitation dynamics coupled with vibrational dynamics, with the LiH system as an example. A train of very short pulses well separated in time including frequency components suited for transfer between multiple electronic states and for photoionization resulted in step-like population transfers that may be recorded in the transient photoelectron signal. 

---