Demonstrations Light from Excited Atoms

All the previous discussion in this chapter has been concerned with absorption or emission of a single photon. However, it is possible for an atom or molecule to absorb two or more photons simultaneously from a light beam to produce an excited state whose energy is the sum of the energies of the photons absorbed. This can happen even when there is no intemrediate stationary state of the system at the energy of one of the photons. The possibility was first demonstrated theoretically by Maria Goppert-Mayer in 1931 [29], but experimental observations had to await the development of the laser. Multiphoton spectroscopy is now a iisefiil technique [30, 31]. 

If the isotopic shift of a spectral line in an atom or in a molecule is more Hi... the Doppler width, it is in principle possible to selectively excite a parti, id... isotopic species from isotopic mixtures by monochromatic light of w.u. length in coincidence with the absorption of the particular isotopic spe> < In a typical example, 2°2Hg atoms in natural Hg vapor containing 204. o 201, 200, 199, and 198 isotopes are preferentially excited by the 2b i V resonance line of 202Hg atoms. It has recently been demonstrated tlm 235U atoms are enriched in the photoionization processes of Mi. t... 

Of course, predictable differences between He(2 5) and He(23S) occur regarding the polarization of Penning ions and electrons caused by total spin conservation. The fact that a component of spin angular momentum is conserved in Pgl has been demonstrated by the observation of the transfer of spin polarization from optically pumped He(23S) atoms to the Penning ions of cadmium, zinc, and strontium.72 The polarization was detected by measuring the polarization of the light emitted from the excited 2Ds/2 ions in 2Ds/2- 2D2/2 transitions. If a component of spin angular momentum is conserved, we may write the Pgl process as... 

Our previous results on hypericin indicate that excited-state H-atom transfer occurs even when one of the carbonyls is prohibited from accepting a hydrogen. The presence of such a transfer is apparent under very acidic conditions in AOT reverse micelles and cannot be excluded upon chelation of Tb3+ [76]. There is thus no evidence for a concerted H-atom-transfer mechanism in hypericin. In the present study, contrary to our initial expectations, we are not even able to demonstrate that hypomycin B executes an excited-state H-atom transfer hence our investigation sheds no light on the general question of how many H atoms are transferred in the perylene quinones and whether the transfer is concerted or stepwise. On the other hand, if further investigation reveals that H-atom transfer does not occur in hypomycin B, the result would have considerable implications for an understanding of the reaction coordinate for the H-atom transfer. 

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