  The report focuses on the introduction and validation of photoacoustics
measurements in the presence of two simultaneous probing beams. The
experiments, performed for three model systems: a perfect absorber - carbon
black membrane, a multilayer system - acrylic paint-covered copper plate, and
luminescent sample - ruby, were aimed for comparative analyses of frequency
domain photoacoustic responses to a single probing beam (classical approach)
and two probing beams (of different wavelengths). A modulation frequency shift
criterion, allowing for the simultaneous acquisition of two independent
signals, was recognized. Any synergistic or cross effects of sample dual beam
excitation on the photoacoustic signal generation (impacts of the beams
intensities or modulation frequency effects) were not observed. The multibeam
photoacoustics data appears to be (at least) equivalent to classically acquired
results for stationary systems. It is considered, that the multibeam approach
will broaden the applicability of the photothermal methods to the
time-dependent systems, where the evolutions of distinct absorption bands is of
special interest.
