CW Photoacoustic Facility
Pulsed PA Facility
PA Spectrometer

Laser Dyes

 
 
  
 
 
 
 
 
 
 
 
 

 

 

 

 
 
 
 

 C70 Structure

 
 
 
 
 
 
 
 

 

 
 

 

 
 

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Pulsed Photoacoustic Facility

In the pulsed photoacoustic set-up, the excitation source is either a second harmonic output beam from a Q-switched Nd:YAG laser or the laser beam from an optical parametric oscillator, depending on the sample of study. The signal from the photoacoustic cell is analysed using a digital storage oscilloscope. Pulsed photoacoustic method is highly sensitive and hence it can be used for probing very weak nonradiative processes. The information thus obtained is complementary to that from fluorescence measurements.         

Pulsed PA Set-up

 

We are mainly using the pulsed photoacoustic set-up to study liquid samples. Some of the systems that we are interested in is given given below.

 

Laser dyes
These are organic compounds which form the active media of several kind of dye lasers. Most of these compounds have strong absorption generally in the visible and UV region of the spectrum, with fluorescence emission from the first excited singlet state at Stokes-shifted wavelengths. These molecules exhibit large optical nonlinerities due to the broad delocalization of pi electrons over the molecular chain. An interesting consequence is that, in a typical dye laser system, several nonlinear effects including two-photon absorption can occour in the pumped region of the dye jet. Another possibility is that of the excited singlet state absorption from the lasing level, which reduces the effective stimulated emission cross section of laser dye significantly in some cases. The identification and study of two photon absorption is usually based on observation of radiative transitions called anti-Stokes fluorescence. In organic dyes anti-Stokes fluorescence is weak since the excited singlet states relax non-radiatively. However these non-radiative relaxations release large amount of thermal energy into the medium and these phenomena can be easily detected using photoacoustic technique. We can also use the photoacoustic technique to measure the absolute value of fluorescence quantum efficiency of laser dyes and effect of processes like dimerization, aggregation, complex formation, etc.
 
Fullerence
There has been considerable interest in practical applications for fullerenes (buckyballs) since Wolfgang Kratschmer and Donald R. Huffman first demonstrated a way to produce these molecules in quantity (Nature, Vol. 347, 354-358). Fullerenes are large carbon-cage molecules with a diameter of about 7-15 angstroms in diameter. Chemically, they are quite stable; breaking the balls requires temperatures of over 1000 degrees C. Fullerence exhibit optical nonlinerities leading to second-harmonic generation and optical limiting and self-defocusing of laser beams due to high values of third-order susceptibility. Because of the very high rate of intersystem crossingto the excited triplet state at room temperature, the fluorescence emission spectra of these molecules are very weak with an extremely low fluoresccent quantum yield. The discovery of optical limiting in fullerence evoked considerable attention because of its comparatively lower threshold limiting fluence. We are using photoacoustic technique to study the nonlinear proparties of fullerence molecules. 
 

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