Ultrafast ionization techniques

Once the alcohol or at least the cluster contains a soft ionization or fluorescence chromophore, a wide range of experimental tools opens up. Experimental methods for hydrogen-bonded aromatic clusters have been reviewed before [3, 19, 175]. Fluorescence can sometimes behave erratically with cluster size [176], and short lifetimes may require ultrafast detection techniques [177]. However, the techniques are very powerful and versatile in the study of alcohol clusters. Aromatic homologs of ethanol and propanol have been studied in this way [35, 120, 121, 178, 179]. By comparison to the corresponding nonaromatic systems [69], the O—H - n interaction can be unraveled and contrasted to that of O—H F contacts [30]. Attachment of nonfunctional aromatic molecules to nonaromatic alcohols and their clusters can induce characteristic switches in hydrogen bond topology [180], like aromatic side chains [36]. Nevertheless, it is a powerful tool for the size-selected study of alcohol clusters. 

Besides typical ESIPT compounds, other tautomeric systems have also been investigated by ultrafast spectroscopy. However, the number of studies is less. Cytosine was investigated by femtosecond ionization techniques to learn about the relevance of tautomerism for the photochemistry and photostabiHty of DNA [57, 58]. It was found that the isolated cytosine molecule exhibits a rather fast IC. It... 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

The purpose of this chapter is to describe the technology that enables the investigation of radiation chemical phenomena at picosecond and femtosecond timescales. Several research groups have used femtosecond laser photoionization techniques to examine electron solvation dynamics and other processes relevant to early radiation chemistry events [1,2]. This chapter will focus on ultrafast studies using ionizing radiation, primarily electron beams, as the excitation source. 

A rather different technique was recently demonstrated to transfer PA-produced Cs2 molecules to the lowest vibrational level of theX E+ state [37]. Ultrafast (100 fs) pulses were used for broadband pumping of the initial w = 1-7 molecules back to the excited state. These excited molecules would decay back into X E+. The specttum of this pumping light was shaped to eliminate any components tihat could excite from w = 0. As a result, if a molecule decayed into w = 0, it would stay there. After a sequence of many of these shaped broadband pulses, a large fraction, 70%, of the initial V = 1-7 population would thus accumulate in w = 0. The various X vibrational level populations were monitored by vibrationally state-selective pulsed ionization. 

In this chapter, we will focus on the discussion of experiments that directly monitor the time-evolution of the electronic excited-state dynamics. In particular, we shall consider transient transmittance, time-resolved fluorescence, and time-resolved ionization spectroscopy. This is because these techniques have the potential to directly observe the ultrafast photochemical excited-state processes triggered by conical intersections. 

To analyze the ultrafast dynamics of small molecules and clusters, the method of multiphoton ionization (MPI) spectroscopy combined with the pump probe technique was chosen. Adiabatic expansion or sputtering was used to produce the molecules and clusters of interest. Quadrupole mass spectrometers enabled a mass-selective detection of the ionized species. 

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