Generation of Short Optical Pulses

Because of the special properties of the exponential function the light decays with the same time constant r as the population decay. The light decay can be followed by a fast detector connected to fast, time-resolving electronics. If the excited state has a substructure, e.g. because of the Zeeman effect or hyperfine structure, and an abrupt, coherent excitation is made, oscillations (quantum beats) in the light intensity will be recorded. The oscillation frequencies correspond to the energy level separations and can be used for structure determinations. We will first discuss the generation of short optical pulses and measurement techniques for fast optical transients. 

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In Table 6.1 the different techniques for the generation of short laser pulses and their typical parameters are compared. It shows that pulse widths below I ps can be achieved with the CPM technique and with Kerr lens mode-locking. In the next section we will discuss how short laser pulses can be further compressed by nonlinear effects in optical fibres. 

The importance of this technique to chemistry and biology has been far less widely accepted than it deserves. The essential technical problem involves the generation of short pulses of ionizing radiation followed generally by optical detection of transient species (Swallow, 1973 von Sonntag, 1987 Kiefer, 1990). 

Fig. 9.26 Temporal characteristics of photo-induced absorptions associated with charge carriers generated by short laser pulses, (a) IR absorption in MEH-PPV (reprinted from Moses et ah, 2001 copyright 2001, with permission from Elsevier) and (b) optical absorption in a poly(indenofluor-ene) (reprinted from Silva et ah, 2001 copyright 2001, with permission from Elsevier). See text for details.

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

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