Using orthogonally polarized 800 nm and 400 nm laser pulses, we have generated high harmonics in ethane (C2H6). We observe that the intensity of each harmonic order modulates with the attosecond delay between the two laser fields. The modulation period of the low even harmonics is twice that of the period of modulation of the other harmonics. By comparing with theoretical calculation, we show that the double periodicity is a result of the electron wave packet motion in the valence shell of C2H 6 on the attosecond time-scale. Our method is a general approach to measuring internal electron dynamics which does not require molecular alignment, making it applicable to more complex molecules than previous approaches.
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