Subject: SOME MORE REPLIES re ADIABATIC/VERICAL IONIZATION...

From: "E. Lewars" <[email-masked]>
Date: Wed, 21 Jul 1999 22:57:48 -0400 (EDT)

1999 July 20

MORE ANSWERS TO ADIABATIC AND VERTCAL IONIZATION ENERGIES

I have two more replies to my question on adiabatic & vertical IEs, and one
"reply" from me.
  Thanks again to everyone who responded.

   E. Lewars
=================

#1a

Subject: vertical vs. adiabatic ie's
Date: Mon, 19 Jul 1999 14:09:52 -0400

Dear Dr. Lewars,

You can find vertical and adiabatic ionization energies in many
experimental articles on UV photoelectron spectroscopy. Experimentally,
the vertical IE corresponds to the maximum of the band envelope, which
can easily be measured. You can generally find this kind of IE in PES
articles, unless otherwise stated. In contrast, the adiabatic IE (which
is due to the v=0 - v=0 transition) is often associated with the
photoelectron band onset. This, of course, is only an entirely valid
assumption if the band is resolved and the vibrational assignment has
been done. It is usually fairly easy to find out from the experimental
article whether the published value indeed refers to the experimental
band maximum, i.e. to the vertical value. However, you can generally get
a better estimate for the adiabatic IE of a molecule from photoionzation
mass spectroscopy. In a similar manner, EA values can be obtained from
papers on photodetachment experiments. 
I would like to note that there are a number of computational articles
dealing with the issue of ionization energies and electron affinities.
The vertical ionization energy is calculated as the energy difference
between the neutral molecule and the cation at the geometry of the
neutral. In contrast, the adiabatic ionization energy is usually taken
as the energy difference between the two at their respective equilibrium
geometries. The vertical and adiabatic electron affinities are
calculated analogously, as the energy difference between the anion and
the neutral. The IE value obtained using Koopmans' theorem will be close
to the vertical IE. However, this is only true if your molecule is
closed-shell (otherwise Koopmans' theorem will not hold anyway) and the
calculated value will neglect the so-called correlation and
reorganization energy contributions. On the other hand, if you calculate
the IE or EA as an energy difference, the reorganization contribution is
taken care of by definition and you can deal with the correlation energy
contribution by using an appropriate level of theory. You will also find
papers on diatomic molecules (and a few triatomics) where the complete
Franck-Condon envelope has been calculated, not only those two
characteristic energies.

I hope this helps. Best regards,
Miklos Feher

************************************************
Dr. Miklos Feher
senior scientist, computational chemistry
Nanodesign Inc., Suite 300, Research Park Centre
150 Research Lane, Guelph, ON, N1G 4T2, Canada
Tel: 519-823-9088    Fax: 519-823-9401  
e/mail:[email-masked] 
************************************************
===========

#2a

1999 July 21
>From: M. Feher
REFERENCES

Hello,
Thanks for your e/mail. There are a number of books on uv photoelectron
spectroscopy, which discuss the measurement of vertical and adiabatic
ionization energies. The couple of best general ones:
J.H.D. Eland: Photoelectron Spectroscopy, Butterworth, London, 1984.
J.H.M. Hollas: High Resolution Spectroscopy, Butterworth, London, 1982.
A more detailed discussion of the ionization process and a comparison of
IE values from different methods can be found in:
J. Berkowitz, Electron Spectroscopy: Theory, Techniques and
Applications, Academic Press, London, 1976.
You can find a lot of interpreted spectra for molecules of the size that
are of interest to you in the book of the inventor of PES:
D.W. Turner: Molecular photoelectron spectroscopy, Wiley, London, 1970
and a detailed compilation of spectra and assignments in:
K. Kimura: Handbook of HeI photoelectron spectra of fundamental organic
molecules : ionization energies, ab initio assignments, and valence
electronic structure for 200 molecules, Japan Scientific Societies
Press, New York, 1981.
Luckily copies of all of these books are available in the UoT Library.
In terms of vertical and adiabatic electron affinities, I am not aware
of any books. Nonetheless, you can find many articles and reviews if you
search the literature for negative ion photoelectron spectroscopy, UVPES
of anions or, as it is sometimes called, photodetachment spectroscopy. 
As you will see in the books and articles, my recipe for determining
vertical IE's of EA's holds in most cases, with a few exotic exceptions
(such as when the molecule autoionizes or pre-dissociation occurs,
etc.). The adiabatic value is a bit more uncertain, unless the
vibrational structure is resolved - there are plenty of examples where
this is the case. Your source for appearance potentials is probably more
reliable for the adiabatic values, though. One must be very careful with
Koopmans' theorem IE's: it breaks down more often than many people
realize, the simplest case of course being the nitrogen molecule. Just a
comment about the table of values: the relative magnitude of the
experimental vertical and adiabatic values for acetone seems odd. 

I hope this helps. You can e/mail me if you have further questions. Good
luck with the calculations!

Best regards,
Miklos Feher

************************************************
Dr. Miklos Feher
senior scientist, computational chemistry
Nanodesign Inc., Suite 300, Research Park Centre
150 Research Lane, Guelph, ON, N1G 4T2, Canada
Tel: 519-823-9088    Fax: 519-823-9401  
e/mail:[email-masked] 
************************************************
===========

#3a
(E. Lewars)
I have found that the compilation
R. D. Levin and S. G. Lias, "Ionization Potential and Appearance Potential
Measurements, 1971-1981", National Bureau of Standards, 1982
puts a V besides those IEs that are vertical, allowing one to conveniently
compare vertical and adiabatic energies.

This may be of some interest--here are some ionization energies I calculated:
-----------------------------------------------------------------------
 IEs are in eV
 The basis set for the calculations is 6-31G*
 The exp values are from R. D. Levin and S. G. Lias, "Ioniation Potential
and Appearance Measurements, 1971-1981", National Bureau of Standards, 1982,
except for the CH3SH value, which is quoted in  L. A. Curtis, R. H. Nobes, J. A.
Pople and L. Radom, J Chem Phys, 1992, 97, 6766.

=========================================================================
                    IE from deltaE          IE from Koopmans   some exp values
                      HF  MP2(FC)             HF    MP2(FC)
CH3OH adiabatic      9.38  10.57             ----     ----       10.90
CH3OH vertical       9.66  10.82             12.06   12.12       10.95

CH3SH adiabatic      8.34   8.97              ----    ----        9.44
CH3SH vertical       8.38   9.03              9.69    9.69 (sic)  ---

acetone adiabatic    8.19   9.63              ----    ----        9.71, 9.74
acetone vertical     8.37   9.78             11.07    11.19       9.5,  9.72

===========================================================================
 G2 calculations give accurate IEs and EAs, but are practical only for small
molecules, and can't be conveniently (i.e. using the automated prceedure
implemented in Gaussian) used for vertical energies (since the geometry of the
ion would be altered from that of the neutral precursor). G2: see e.g. Curtis et
al above, and L. A. Curtis, D. J. Lucas, and J. A. Pople, J Chem Phys, 1995,
102, 3292, and refs therein. Also J. B. Foresman and A. E. Frisch,
"Exploring Chemistry with Electronic Structure Methods", 2ns ed., Gaussian
Inc., Pittsburgh, PA, 1996, Chapter 7.
===============