Time-Resolved Laser Spectroscopy

While the previous chapter emphasized the high speotral resolution achievable with different sub-Doppler techniques, this chapter concentrates on some methods which allow high time resolution. The generation of extremely short and intense laser pulses has opened the way for the study of fast transient phenomena, such as molecular relaxation processes in gases or liquids due to spontaneous or collision-induced transitions. A new field of laser spectroscopy is the time-resolved detection of coherence and interference effects such as quantum beats or coherent transients monitored with pulse Fowoiev transform spectroscopy. 

Recent progress in the generation of picosecond and sub-picosecond light pulses makes investigations of ultrafast processes occurring during excitation and deactivation of molecular states in solvents possible. The population decay times and the phase decay times of coherently excited molecules can be studied with extremely high time resolution. 

The spectral resolution Av of most time-resolved techniques is in principle limited by the Fourier limit Av = a/AT where aT is the duration of the short light pulse and the factor a depends on the profile I(t) of the pulse. Generally the spectral bandwidth Av of these pulses is still much narrower than that of light pulses from incoherent light sources, such as flashlamps or sparks. Some time-resolved methods based on regular trains of short pulses even circumvent the Fourier limit Av of a single pulse and simultaneously reach extremely high spectral and time resolutions (see Sect.11.4). 

We discuss first some techniques of generating short laser pulses and then illustrate different applications. Methods for lifetime measurements, the quantum beat technique, pulse Fourier transform spectroscopy, and multiple coherent interactions are some of the recently developed methods which demonstrate the capabilities of pulsed lasers for high time-resolution 

The investigation of fast processes, such as radiative or collision-induced decays of excited levels, isomerization of excited molecules, or the relaxation of an optically pumped system toward thermal equilibrium, opens the way to study in detail the dynamic properties of excited atoms and molecules. A thorough knowledge of dynamical processes is of fundamental importance for many branches of physics, chemistry, or biology. Examples are predissociation rates of excited molecules, femtosecond chemistry, or the understanding of the visual process and its different steps from the photoexcitation of rhodopsin molecules in the retina cells to the arrival of electrical nerve pulses in the brain. 

In order to study these processes experimentally, one needs a sufficiently good time resolution, which means that the resolvable minimum time interval At must still be shorter than the time scale T of the process under investigation. While the previous chapters emphasized the high spectral resolution, this chapter concentrates on experimental techniques that allow high time resolution. 

The development of ultrashort laser pulses and of new detection techniques that allow a very high time resolution has brought about impressive progress in the study of fast processes. The achievable time resolution has been pushed recently into the femtosecond range (1 fs = 10 s). Spectroscopists can now quantitatively follow up ultrafast processes, which could not be resolved ten years ago. 

We will at first discuss techniques for the generation and detection of short laser pulses before their importance for different applications is demonstrated by some examples. Methods for measuring lifetimes of excited atoms or molecules and of fast relaxation phenomena are presented. These applications illustrate the relevance of pico- and femtosecond molecular physics and chemistry for our understanding of fundamental dynamical processes in molecules. 

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Viappiani, C. Rudzki-Small, J. In Time-resolved Laser Spectroscopy in Biochemistry 111, J.R. Lakowicz, Ed. SPIE Proc. 1640 Bellingham, WA, 1992. 

J. R. Lakowicz, ed., Time-Resolved Laser Spectroscopy in Biochemistry III, SPIE (The Society of Photo-Optical Instrumentation Engineers) 1640, Billingham, Washington (1992). 

D. J. S. Birch, K. Suhling, A. S. Holmes, A. D. Dutch and R. E. Imhof, Array fluorometry the theory of the statistical multiplexing of single-photon timing, in Time-Resolved Laser Spectroscopy in Biochemistry II, (J. R. Lakowicz, ed.), Proc. SPIE 1204, 26-34 (1990). 

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

Electron transfer from I- into the oxidized Ru photosensitizer (cation), or regeneration of the Ru photosensitizer, is one of the primary processes needed to achieve effective charge separation. The kinetics of this reaction has also been investigated by time-resolved laser spectroscopy [48,51]. The electron-transfer rate from I into the Ru(III) cation of the N3 dye was estimated to be 100 nsec... 

Beechem, J.M. Gratton, E. In Time-Resolved Laser Spectroscopy in... 

The first valence isomer to be investigated was the bicyclic (hexamethyl-, HM-) Dewar benzene. However, y irradiation of this substrate produced the electronic spectrum of HM-128 [360]. Similarly, nsec time-resolved laser spectroscopy failed to reveal evidence for the bicyclic radical cation [362]. The first indication for the existence of such a species as a discrete entity was provided by a CIDNP study [361]. These results are best discussed in connection with ab initio calculations carried out for the parent C6H6 system. At the 6-31 G level these calculations support the existence of two cationic states with the unpaired spin density either... 

Kochi et al. [111-113] have utilized time-resolved laser spectroscopy to examine the excited complexes. For the indene (IN)/TCNE system, excitation of the CT complex (532 nm) afforded not only absorption bands assigned to TCNE anion radical and IN cation radical, but also an absorption band due to an unidentified intermediate. Kochi proposed a 1,4-diradical or 1,4-zwitterionic intermediate, and postulated bond-formation as follows [111] ... 

In this chapter, we will focus on photosensitive systems that are used in free radical photopolymerization reactions. We will give the most exhaustive presentation of the commercially used or potentially interesting systems developed on a laboratory scale together with the characteristics of their excited-state properties. We will also show how modem time resolved laser spectroscopy techniques and quantum mechanical calculations allow to probe the photophysical/photochemical properties as well as the chemical reactivity of a given photoinitiating system. 

Time resolved laser spectroscopy is likely the best method to get kinetic information on transient states. More information can currently be gained from quantum mechanical calculations at relatively high level of theory using the Gaussian Suite of programs (ab initio and density functional theory). 

Figure 1. Experimental set-up for time-resolved laser spectroscopy (From Ref.

Knutson J R 1988 Time-resolved laser spectroscopy In biochemistry SPIE 909 51-60... 

T. M. Nordlund, in Time-Resolved Laser Spectroscopy in Biochemistry (J. R. Lakow-icz, ed.), p. 35. SPIE, Bellingham, Washington, 1988. 

Time-Resolved Laser Spectroscopy of Synergistic Processes in Photoinitiators of Polymerization... 

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