Second harmonic generation Subject

In conclusion, one should mention that other techniques are available for examination of surface states, in particular 2PPE, which has yielded a wealth of information on the dynamics of electrons in surface states recently [12]. As a chapter in this book is devoted specially to the subject, it is not followed up here. Also, optical techniques such as second harmonic generation are sensitive to surface states, but they yield more indirect information and are not discussed here. 

The literature is now abimdant in this field. However, the mechanisms of the extraction processes are still the subject of speculation and controversy. Nevertheless, this situation is probably begiiming to change as new experimental techniques are now able to study liquid interfaces on a molecular scale optical second harmonic and simi frequency generation (SHG and SFG) are described in chapter 2 of this book. In particular, these techniques may have the potential to obtain dynamical information in the future, as has been demonstrated already for the water/air interface [7]. 

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