Spectroscopic techniques excited state lifetimes

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

Precise measurements of the excited state lifetimes of the DNA constituents were not available till very recently, mainly due to the limited time resolution of conventional spectroscopic techniques. Studying the DNA nucleosides by transient absorption spectroscopy, Kohler and co-workers observed a very short-lived induced absorption in the visible which they assigned to the first excited state [5,6]. The lifetimes observed were all well below 1 picosecond. The first femtosecond fluorescence studies of DNA constituents were performed using the fluorescence upconversion technique. Peon and Zewail [7] reported that the excited state lifetimes of DNA/RNA nucleosides and nucleotides all fall in the subpicosecond time, thus corroborating the results obtained by transient absorption. 

All the methods used to evaporate metals for atom synthesis were developed originally for the deposition of thin metal films. The more important of these techniques are shown schematically in Fig. la-d. Most of the evaporation devices can be scaled to give amounts of metal ranging from a few milligrams per hour for spectroscopic studies to 1-50 gm/hour for preparative synthetic purposes. Evaporation of metals from heated crucibles, boats, or wires (Fig. la-c) generally gives metal atoms in their ground electronic state. Electronic excitation of atoms is possible when metals are vaporized from arcs, by electron bombardment, or with a laser beam (Fig. Id). The lifetime of the excited states of... 

Spectroscopic evidence for the transient formation of the trans-stilbene radical cation could be obtained when colloidal TiOj suspended in an acetonitrile solution containing trans-stilbene (a species which should also be exothermically oxidized by a TiO valence band hole) was excited with a laser pulse The observed transient was identical in spectroscopic features and in lifetime with an authentic sample of the stilbene cation radical generated in the same medium via pulse radiolytic techniques. That the surface influences the subsequent chemistry of this species can be seen in the distribution of products observed under steady state illumination, Eq. (4) 2 . ... 

An electronically excited molecule may, under some conditions, absorb another quantum and be raised to a higher excited state. Usually the population of excited species is so low that the probability of this occurrence is very slight. However, in recent years the technique of flash photolysis has been developed, which allows us to investigate the absorption properties of excited states. An extremely high intensity laser, which has approximately one million times the power of a conventional spectroscopic lamp, is turned on for a tiny fraction of a second, and a large population of excited species is produced. Immediately after this photolysis flash is turned off, a low-power spectroscopic flash may be turned on and the absorption spectrum of the already-excited system determined. By varying the delay between photolysis and spectroscopic flashes, much can be learned about the absorption and lifetime of singlet and triplet excited states. 

National Laboratory by several standard spectroscopic techniques, including the measurements of angular correlations by a fixed-four detector system, y-y and 8-y coincidence, and lifetimes of some excited states. 

The longer lifetime and enhanced biradical character of the triplet state produce a difference in the style of research and the influence of time resolved spectroscopic techniques on the subject is less marked than for excited singlet states. 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

The use of triplet excited states as probes to study the surface properties of many solids is particulary interesting, due to the fact that they usually exhibit long lifetimes which in most cases come closer to those obtained for rigid matrices. These long lifetimes increase in many cases the efficiency of several photochemical processes. Therefore spectroscopic and kinetic studies can be performed in a wide and interesting variety of situations. As we said before, the development of the diffuse reflectance laser flash photolysis technique [1,10] by Wilkinson et al. was crucial for the development of these studies on surfaces. 

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