Laser and Q-switching dyes.
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From: Salmon Egg on 21 Nov 2009 19:51 In article <9a501d30-b057-461b-8703-ade22846a59b(a)a21g2000yqc.googlegroups.com>, Darwin123 <drosen0000(a)yahoo.com> wrote: > I have looked into all sorts of dyes. There is a lot of information > on the spectroscopy of dyes provided by chemical companies that make > laser dyes. Once, I was really interested in infrared dyes. I wanted > to study fluorescence at infrared wavelengths. I noticed something > really interesting that I have no model for. > Dyes with short emission wavelengths usually have a large Stokes > shift. That is, the peak of their exicitation spectrum lies at a much > shorter wavelength than the peak of their emission spectrum. However, > IR dyes have almost no Stokes shift. Their emission spectrum and > excitation spectrum have almost total overlap. They peak at the same > wavelength. > Stokes shift is caused by phonons (i.e., vibrations). When an > electron in an electronic excited state relaxes, goes into the > electronic ground state. However, it doesn't necessarily go into the > vibrational ground state. Some of the energy in the excited electron > state goes into making the molecule vibrate. That cause the emission > photon to generally be lower in energy than the excitation photon. > Somehow, the vibration is being suppressed in dye molecules > which have a small difference in electronic energies. I am not sure > why this is so. > I would love to have a model that explains that, even a rough one. > It sounds like we may be talking about the same phenomenon. I am not a professional chemist. I also change the subject line to start a new thread. The first Q-switching dyes were low soluble pthalocyanines and silver halide photo-sensitizers. My thought was that laser Q-switch operation would lead to multiple excitation of the dye until the the molecule disintegrated. Such a cascade would be easier to excite for an infrared dye compared to short wavelength dye. Then, some IBM researchers came up with a nickel complex dye abbreviated BDN bis-(4-dimethylaminodithiobenzil)-nickel for 1.06 microns. It was much more photostable than other Q-switching dyes of the day. The US Army was sponsoring development of longer wavelength dyes. This was more than 20 years ago and I lost track of the developments. If this is of interest to you, we can have more discussion. Bill -- As the years go by, dying just before having to fill out a tax return has merit. |