RE: orbitals and reality

This is a copy of a note I sent to Sengen Sun.  I am posting it now because
 there does seem to have been some interest in the issue he raised on CCL -
 at the time there had been no response.  Dr. Sun has promised to respond to
 my comments in the future.
 ====================================
 Dr. Sun:
 I noted your request for further response to the "orbital debate" that
 has
 arisen from the Nature paper by Villeneuve et al.
 The orbital model is undeniably a key part of modern electronic structure
 theory, and it is therefore understandable that experimentalists would seek
 physical confirmation (if not proof) of its validity.  On the other hand, it
 is equally true that theoreticians have always maintained that orbitals are
 "merely" mathematical constructs, that any arbitrary set of orbitals
 does
 not represent a unique solution to the Schroedinger equation, and that
 orbitals are therefore non-physical and non-observable.
 So, to get to the argument over Villeneuve et al: Having read the paper I
 must confess that I do not fully understand the experiment that they did.
 They assert that the overtone spectra that they acquire encode symmetry
 information about the electronic state (which they call an orbital) from
 which the electron is ejected and to which it returns.  Since they cite
 literature support for this assertion I am prepared to accept it for the
 sake of argument until someone provides a persuasive counter argument.
 So, the question is, what did they actually observe and how should it be
 interpreted?  I suggest that denying their experiment on theoretical grounds
 is scientifically inappropriate.  Observation must have priority over
 theory, or we are talking about religion instead of science.  This doesn't
 mean that they actually observed an orbital - but rather, if you are going
 to criticise their interpretation of the data you should present an
 alternative explanation of the origin of their spectra that is consistent
 with your view of electronic theory as well as with their observations.
 While the fact that orbitals are "only a model" means we must not
 assume
 that they are "real", I suggest that it is inappropriate to take it as
 axiomatic that the orbital model cannot embody at least some aspect of
 physical "reality".  It is entirely possible that electronic structure
 behaves under certain specific probes as if orbitals were real, that is,
 that our artifical model fortuitously captures an observable aspect of
 electrons.
 I don't want to argue that Villeneuve et al are correct.  But please, if you
 really want to conduct a scientific debate, address their experimental
 observations directly and don't simply say that "because they are
 inconsistent with theory they must be false".
 Dr. Philip G. Hultin
 Associate Professor of Chemistry,
 University of Manitoba
 Winnipeg, MB
 R3T 2N2
 hultin..at..cc.umanitoba.ca
 http://umanitoba.ca/chemistry/people/hultin