  Isotopic substitution in molecular systems can affect fundamental molecular
properties including the energy position and spacing of electronic, vibrational
and rotational levels, thus modifying the dynamics associated to their coherent
superposition. In extreme ultraviolet spectroscopy, the photoelectron leaving
the molecule after the absorption of a single photon can trigger an ultrafast
nuclear motion in the cation, which can lead, eventually, to molecular
fragmentation. This dynamics depends on the mass of the constituents of the
cation, thus showing, in general, a significant isotopic dependence. In
time-resolved attosecond photoelectron interferometry, the absorption of the
extreme ultraviolet photon is accompanied by the exchange of an additional
quantum of energy (typically in the infrared spectral range) with the
photoelectron-photoion system, offering the opportunity to investigate in time
the influence of isotopic substitution on the characteristics of the
photoionisation dynamics. Here we show that attosecond photoelectron
interferometry is sensitive to isotopic substitution by investigating the
two-color photoionisation spectra measured in a mixture of methane (CH$_4$) and
deuteromethane (CD$_4$). The isotopic dependence manifests itself in the
modification of the amplitude and contrast of the oscillations of the
photoelectron peaks generated in the two-color field with the two
isotopologues. The observed effects are interpreted considering the differences
in the time evolution of the nuclear autocorrelation functions in the two
molecules.
