On Thursday, March 13, 2014 8:07:36 AM UTC-4, Phil Hobbs wrote:   
   > On 3/13/2014 1:21 AM, Bret Cannon wrote:   
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   > > wrote in message   
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   > > news:67a52766-f7e3-413d-a2eb-40fa4cebfcca@googlegroups.com...   
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   > >> Are you working with these forks for photo-acoustic spectroscopy that   
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   > >> Frank   
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   > >> Tittel's group at Rice University has been doing for about 15 years?   
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   > >> Bret Cannon   
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   > > The PAS sounds interesting. An interference effect or doppler?   
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   > > Any references on?   
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   > > JB   
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   > > The PAS is generation of sound waves from modulated heating produced by   
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   > > absorption of modulated modulated light.  This effect was discovered by   
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   > > Alexander Graham Bell of telephone fame.  In the work of Tittel's group,   
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   > > quartz tuning forks are used as very frequency selective and sensitive   
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   > > microphones with a laser beam passing between the tines of the tuning   
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   > > fork, which generates pressure waves that couple to the correct   
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   > > vibrational mode of the tuning fork.   
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   > > A Google search on "QEPAS" (Quartz enhanced photoacoustic spectroscopy)   
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   > > turns up several references.   
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   > > Bret Cannon   
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   > Photoacoustics has been around forever--my lab mate at grad school, Rick   
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   > Stearns, was doing it at the thermal noise level in 1984.  (I think he   
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   > was the first to try using a piezo transducer instead of an   
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   > interferometer to detect the surface displacement.)   
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   > Cheers   
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   > Phil Hobbs   
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   >   
   > --   
   >   
   > Dr Philip C D Hobbs   
   >   
   > Principal Consultant   
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   You man the "study" of photoacoustics? It seems to persist, as systems are   
   being developed for laser-assisted PAS detection of nerve agents, etc. at ppB   
   level, they claim.   
      
      
   http://www.rdmag.com/news/2012/08/photoacoustic-technique-hears-   
   ound-dangerous-chemical-agents   
      
      
   The researchers note that "midIR" lasers would be needed to make th system   
   effective. This raises the question in my mind as to whether that is true,   
   since NIR spectroscopy, when coupled with pattern recognition ('chemometrics'   
   for the sober-minded), has been shown to be useful for a variety of food and   
   agricultural discriminant tasks.   
      
   The NIR spectrum usually does not have the features that endear the mid-IR   
   regions to organic chemists.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)