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Boundary Condition Effect On Photoacoustic Response Of Absorbing Liquid To Modulation Of Laser Intensity

The  photoacoustic  (PA)  pressure  response  of  the absorbing  liquid  (water, ethanol)  with  free  and confined  surface  to  laser  pulse  with  modulated intensity is studied.  Modulation  amplitude  of  the PA  signal  demonstrates  non-monotonous behavior during laser pulse action which can be explained as interference  between thermo-acoustic  and vaporization  mechanisms  of  pressure  generation. The phase behavior of modulated part of PA signal also  gives  important  information  on processes  in the  irradiated  matter.  Doppler  photoacoustic monitoring  (DPM)  is used  to  determine  surface movement  during  laser  irradiation  (wavelength  – 2.94 um,  pulse  duration  ~  200-300  ns,  fluence  ~ 0.3-1.6  J/cm2).  In  the  case  of free  surface  this movement  is  possible  into  the  upward  and downward  directions while  for  the  constrained irradiated surface only one movement is supposed. This supposition  is  in  agreement  with  the experiment.  However  the  total  behavior  of PA response  is  more  complicated  due  to  probably vapor  cavity  formation between liquid  and  solid surfaces.  In  particular,  despite  the  imposed constraint PA response behavior resembles the free surface case.