  In this letter, we report the first observation of dramatic decrease in
nanosecond (ns) pulsed laser induced transient absorption (TA) in
a-GexAs35-xSe65 thin films by tuning the amorphous network from floppy to
rigid. Our results provide the direct experimental evidence of a self trapped
exciton mechanism, where trapping of the excitons occurs through bond
rearrangements. Taken together, a rigid amorphous network with more constraints
than degrees of freedom, are unable to undergo any such bond rearrangements and
results in weaker TA. However, we also demonstrate that excitation fluence can
be effectively utilized as a simple tool to lift up enough constraints to
introduce large TA even in rigid networks. Apart from this, we also show that
TA is tunable with network rigidity as it blueshift when the mean coordination
is increased from 2.35 to 2.6.
