Time-resolved spectroscopy
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص503-504
2025-12-09
23
Time-resolved spectroscopy
The ability of lasers to produce pulses of very short duration is particularly useful in chemistry when we want to monitor processes in time. Q-switched lasers produce nanosecond pulses, which are generally fast enough to study reactions with rates controlled by the speed with which reactants can move through a fluid medium. How ever, when we want to study the rates at which energy is converted from one mode to another within a molecule, we need femtosecond and picosecond pulses. These timescales are available from mode-locked lasers. In time-resolved spectroscopy, laser pulses are used to obtain the absorption, emission, or Raman spectrum of reactants, intermediates, products, and even transition states of reactions. It is also possible to study energy transfer, molecular rotations, vibrations, and conversion from one mode of motion to another. We shall see some of the information obtained from time-resolved spectroscopy in Chapters 22 to 24. Here, we describe some of the experimental techniques that employ pulsed lasers. The arrangement shown in Fig. 14.39 is often used to study ultrafast chemical reactions that can be initiated by light, such as the initial events of vision (Impact I14.1). A strong and short laser pulse, the pump, promotes a molecule A to an excited electronic state A* that can either emit a photon (as fluorescence or phosphorescence) or react with another species B to yield a product C:
A+ hν →A* (absorption)
A*→A (emission)
A*+B→[AB]→C (reaction)
Here [AB] denotes either an intermediate or an activated complex. The rates of appearance and disappearance of the various species are determined by observing time dependent changes in the absorption spectrum of the sample during the course of the reaction. This monitoring is done by passing a weak pulse of white light, the probe, through the sample at different times after the laser pulse. Pulsed ‘white’ light can be generated directly from the laser pulse by the phenomenon of continuum generation, in which focusing an ultrafast laser pulse on a vessel containing a liquid such as water, carbon tetrachloride, CaF, or sapphire results in an outgoing beam with a wide distribution of frequencies. A time delay between the strong laser pulse and the ‘white’ light pulse can be introduced by allowing one of the beams to travel a longer distance before reaching the sample. For example, a difference in travel distance of ∆d = 3 mm corresponds to a time delay ∆t =∆d/c ≈10 ps between two beams, where c is the speed of light. The relative distances travelled by the two beams in Fig. 14.39 are controlled by directing the ‘white’ light beam to a motorized stage carrying a pair of mirrors. Variations of the arrangement in Fig. 14.39 allow for the observation of fluorescence decay kinetics of A* and time-resolved Raman spectra during the course of the reaction. The fluorescence lifetime of A* can be determined by exciting A as before and measuring the decay of the fluorescence intensity after the pulse with a fast photo detector system. In this case, continuum generation is not necessary. Time-resolved resonance Raman spectra of A, A*, B, [AB], or C can be obtained by initiating the reaction with a strong laser pulse of a certain wavelength and then, sometime later, irradiating the sample with another laser pulse that can excite the resonance Raman spectrum of the desired species. Also in this case continuum generation is not necessary. Instead, the Raman excitation beam may be generated in a dye laser (see Further information 14.1) or by stimulated Raman scattering of the laser pulse in a medium such as H2(g) or CH4(g).
Fig. 14.39 A configuration used for time resolved absorption spectroscopy, in which the same pulsed laser is used to generate a monochromatic pump pulse and, after continuum generation in a suitable liquid, a ‘white’ light probe pulse. The time delay between the pump and probe pulses may be varied by moving the motorized stage in the direction shown by the double arrow.
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