From owner-chemistry@ccl.net Sat Oct 11 17:50:00 2008 From: "John McKelvey jmmckel###gmail.com" To: CCL Subject: CCL:G: fluorescence spectra Message-Id: <-37896-081011123059-12829-wB8gLMkBhBkRuc5OGX7INg[]server.ccl.net> X-Original-From: "John McKelvey" Content-Type: multipart/alternative; boundary="----=_Part_111281_15100817.1223742627605" Date: Sat, 11 Oct 2008 09:30:27 -0700 MIME-Version: 1.0 Sent to CCL by: "John McKelvey" [jmmckel%gmail.com] ------=_Part_111281_15100817.1223742627605 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline An overly simplistic response: Exactly the same way one does absorbtion spectra, except that one uses the excited state geometry instead of the ground state geometry. This involves computing typically the excited state geometry for the lowest excited state found using the ground state geometry. One uses the excited state geometry exactly the same as for the ground state for the TD- or CI- calculation. Hope this helps. John McKelvey On Thu, Oct 9, 2008 at 1:21 PM, Melissa melchem{:}gmail.com < owner-chemistry]![ccl.net> wrote: > Dear CCL readers, > > I would like to know how to calculate fluorescence spectra of organic > molecules using g03? Could anyone please help me. > > Thanks in advance, > Melissa > ------=_Part_111281_15100817.1223742627605 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline
 An overly simplistic response: Exactly the same way one does absorbtion spectra, except that one uses the excited state geometry instead of the ground state geometry.  This involves computing typically the excited state geometry for the lowest excited state found using the ground state geometry.  One uses the excited state geometry exactly the same as for the ground state for the TD- or CI- calculation.

Hope this helps.

John McKelvey

On Thu, Oct 9, 2008 at 1:21 PM, Melissa melchem{:}gmail.com <owner-chemistry]![ccl.net> wrote:
Dear CCL readers,
 
I would like to know how to calculate fluorescence spectra of organic molecules using g03? Could anyone please help me.
 
Thanks in advance,
Melissa

------=_Part_111281_15100817.1223742627605-- From owner-chemistry@ccl.net Sat Oct 11 20:35:00 2008 From: "David Gallagher gallagher.da.-*-.gmail.com" To: CCL Subject: CCL:G: fluorescence spectra Message-Id: <-37897-081011202857-9890-x0TUxWh/hTUqmZQJGMO1GQ-*-server.ccl.net> X-Original-From: David Gallagher Content-Type: multipart/alternative; boundary="=====================_18324015==.ALT" Date: Sat, 11 Oct 2008 17:19:24 -0700 Mime-Version: 1.0 Sent to CCL by: David Gallagher [gallagher.da+/-gmail.com] --=====================_18324015==.ALT Content-Type: text/plain; charset="us-ascii"; format=flowed Hi Melissa, Briefly, you need an accurate geometry of the excited singlet, then use a ZINDO C.I. to calculate the spectrum of this geometry. Fluorescence emission usually occurs from the first excited singlet state to the ground singlet state. After excitation, as the rate constant for fluorescence is typically much slower than the rate constant for vibrational relaxation, the molecule will have time for the geometry to relax to an excited-singlet geometry before fluorescing. However, the rate of photon emission is so fast that the excited-singlet geometry does not have time to relax during the fluorescence (Frank-Condon principle applies). As the probability of emission and absorption are the same, a ZINDO UV-visible absorption spectrum calculation (C.I.) on the excited-singlet geometry should provide a reasonable approximation to the fluorescence emission. However, just as with UV-visible absorption spectra, ZINDO estimates only pure S0 - S1 electronic transitions without any vibrational fine structure so vibrational fine structure is not reproduced. Some slides and exercises which illustrate the concepts and an approach to calculating fluorescence spectra are posted at: http://cacheresearch.com/presentations.html Hope this helps, David Gallagher CACheResearch.com At 01:21 PM 10/9/2008, Melissa melchem{:}gmail.com wrote: >Dear CCL readers, > >I would like to know how to calculate fluorescence spectra of >organic molecules using g03? Could anyone please help me. > >Thanks in advance, >Melissa --=====================_18324015==.ALT Content-Type: text/html; charset="us-ascii" Hi Melissa,

Briefly, you need an accurate geometry of the excited singlet, then use a ZINDO C.I. to calculate the spectrum of this geometry. 

Fluorescence emission usually occurs from the first excited singlet state to the ground singlet state. After excitation, as the rate constant for fluorescence is typically much slower than the rate constant for vibrational relaxation, the molecule will have time for the geometry to relax to an excited-singlet geometry before fluorescing. However, the rate of photon emission is so fast that the excited-singlet geometry does not have time to relax during the fluorescence (Frank-Condon principle applies). As the probability of emission and absorption are the same, a ZINDO UV-visible absorption spectrum calculation (C.I.) on the excited-singlet geometry should provide a reasonable approximation to the fluorescence emission. However, just as with UV-visible absorption spectra, ZINDO estimates only pure S0 - S1 electronic transitions without any vibrational fine structure so vibrational fine structure is not reproduced.

Some slides and exercises which illustrate the concepts and an approach to calculating fluorescence spectra are posted at:

http://cacheresearch.com/presentations.html

Hope this helps,

David Gallagher
CACheResearch.com


At 01:21 PM 10/9/2008, Melissa melchem{:}gmail.com wrote:
Dear CCL readers,
 
I would like to know how to calculate fluorescence spectra of organic molecules using g03? Could anyone please help me.
 
Thanks in advance,
Melissa
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