<p>An ultrafast structural, Jahn-Teller (JT) driven, electronic
coherence mediated quantum dynamics in the CH<sub>4</sub><sup>+</sup> and CD<sub>4</sub><sup>+
</sup>cations that follows a sudden ionization by an XUV attopulse, exhibits a
strong isotope effect. The JT effect makes the methane cation unstable in the T<sub>d</sub>
geometry of the neutral. Upon the sudden ionization the cation is produced in a
coherent superposition of three electronic states that are strongly coupled. On
the ground state of the cation the few femtosecond structural rearrangement leads
first to a geometrically less distorted D<sub>2d</sub> minimum followed by a reorganization
to a shallow C<sub>2v</sub> minimum. The dynamics is computed for an ensemble
of 8000 ions randomly oriented with respect to the polarization of the XUV
pulse. The ratio, about 3, of the CD<sub>4</sub><sup>+</sup> and CH<sub>4</sub><sup>+</sup><sub>
</sub>autocorrelation functions, is in
agreement with experimental measurements of the high harmonic spectra. The high
value of the ratio is attributed to the faster electronic coherence dynamics in
CH<sub>4</sub><sup>+</sup>. </p>