  TetraPhenyl Butadiene is the wavelength shifter most widely used in
combination with liquid Argon. The latter emits scintillation photons with a
wavelength of 127 nm that need to be downshifted to be detected by
photomultipliers with glass or quartz windows. TetraPhenyl Butadiene has been
demonstrated to have an extremely high conversion efficiency, possibly higher
than 100 % for 127 nm photons, while there is no precise information about the
time dependence of its emission. It is usually assumed to be exponentially
decaying with a characteristic time of the order of one ns, as an extrapolation
from measurements with exciting radiation in the near UV. This work shows that
TetraPhenyl Butadiene, when excited by 127 nm photons, reemits photons not only
with a very short decay time, but also with slower ones due to triplet states
de-excitations. This fact can strongly contribute to clarify the anomalies in
liquid Argon scintillation light reported in literature since seventies, namely
the inconsistency in the measured values of the long decay time constant and
the appearance of an intermediate component. Similar effects should be also
expected when the TPB is used in combination with Helium and Neon, that emit
scintillation photons with wavelengths shorter than 127 nm.
