Subject: Re: CCL:Two-photon spectroscopy

From: "ghunter" <[email-masked]>
Date: Sun, 21 Feb 1999 22:52:12 -0500

Dear Brian:
                     This may not answer your specific need, but it may help
you with 2-photon spectroscopy.  Just 10 years ago I published a paper
(co-author R.L.P.Wadlinger, "Photons and Neutrinos as Electromagnetic
Solitons", Physics Essays, Vol. 2, pp.158-172, 1989) that (among other things)
predicts the threshold intensity for multi-photon (minimally 2-photon)
absorption to take place.  The expression for the threshold intensity is:

           Intensity =  4 pi h c^2/lambda^4

where h=Planck's constant, c=velocity of light, lambda=photon-wavelength.
At lambda=523 nanometres the Intensity=1 megawatt per cm^2, and (as shown
above) it scales like  1/lambda^4.

The theory behind this expression predicts that the photon is a soliton wave
whose EM field is contained within a circular ellipsoid of length lambda, and
cross-sectional circumference=lambda.  The expression (above) arises from the
condition for closest packing of these ellipsoidal solitons.
That is you need to have 2 photon-solitons in the same place at the same time
(and in phase of course) for the absorption of both of them to take place.  In
addition to the energy of both (=twice the energy of one - which is equal to
the energy difference between the initial and final states), the transition
absorbs 2 times h/2pi of angular momentum, so the selection rule differs from
the same-energy one-photon absorption.

    Since the intensity at the focus of a focused laser beam is not uniform in
practice, you may need to vary the focussing around the above predicted
intensity to optimise the experimental yield.  A number of different
experimental results (some of them intensity sensitive to optimise 2-photon
[rather than 3-or-more] absorption) have confirmed the validity of the above
expression for the threshold intensity.  Two of them are cited in our 1989
paper.

Geoffrey Hunter,
Department of Chemistry,
York University, Toronto,
Canada   M3J 1P3
Business Tel: 416-736-5306  Fax: 416-736-5936
Home      Tel: 905-880-5146  Fax: 905-880-1997
email: [email-masked]
-----Original Message-----
From: Brian Williams <[email-masked]>
To: [email-masked] <[email-masked]>
Date: Saturday, February 20, 1999 8:47 PM
Subject: CCL:Two-photon spectroscopy

>Dear all- Is there anyone on the CCL list who has any experience trying to
>use a computational approach to estimating what the two-photon spectra for an
organic might be?  I have some understanding of how to get at UV spectra using
CIS methods, but I am curious to know whether this approach or the information
derived from such a calcuation might be used to get a
>quick/dirty estimate of two-photon spectra.  I'm starting pretty much from
>scratch here, so any/all advice would be helpful. Thanks.
>
>Brian Williams, Chemistry
>
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