Time-Resolved Atomic and Molecular Spectroscopy

If the primary excitation is performed, e.g., with tt light (Fig. 4.9) the magnetic sublevels of the D j2 state will be non-uniformly populated. The fluorescence in the different decay Imes will then also be linearly polarized. Radio-frequency transitions, that are induced in the primary state or in cascade states, will give rise to a depolarization and redistribution of the light that can be detected. Thus the properties of many P, D and F states can be investigated. Examples of ODR signals in excited S states, populated by stepwise excitation, were given in Fig. 7.11. 

In Chap. 7 we have discussed how hyperfine structure can be determined by level-crossing spectroscopy. Clearly, alkali atom states can readily be studied using this technique after stepwise excitation. We will here instead choose an example illustrating fine-structure measurements. In Fig. 9.17 the example of the inverted sodium 4d 5/2,3/2 state is given. From the measured level-crossing positions the fine-structiue splitting can be calculated using the Breit-Rabi formula for the fine structure, (2.31). 

In this section we study time-resolved laser spectroscopy and generally discuss radiative properties of atoms and molecules and methods of studying these properties. Since very short laser pulses with a power density sufficient to saturate optical transitions can be obtained, a large fraction of the irradiated ground-state atoms can be transferred to the excited state. Using stepwise excitation with synchronized lasers a large number of atoms can be excited into very higlfly excited states. When the laser pulse ceases, the exponential decay of the excited state can be monitored. Note, that primarily the population number N t) decays exponentially, i.e., 

This decay can be monitored by observing the decay of the fluorescence light in an arbitrary spectral line orighiating in the state. For the light intensity I[t) we have 

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Early experiments in this new field of femtosecond chemistry took the form of time-resolved spectroscopy since the probing involved absorption or emission spectroscopy. Theoretical interpretation of the spectroscopic data is clearly required in order to obtained the desired information, i.e., snapshots of the time-dependent distribution of atomic positions. To that end, extensive quantum chemical calculations of energies of excited electronic states are needed, which even today can be cumbersome for larger molecular systems. Soon after the first successful experiments using time-resolved spectroscopy, there was, therefore, efforts to use alternative probing techniques like diffraction. The advantage is that a simpler and more direct connection between the diffraction signals and molecular structure is available. 

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