Destructive Interference During HHG in Mixed Gases

In this section, we present the first experimental evidence of the destructive interference (DI) and the constructive interference (Cl) in a mixed gas of He and Ne, which prove the validity of the method. The observed interference modulation is, as discussed in Sect. 4.2, attributed to the difference between the phases of the intrinsically chirped harmonic pulses from He and Ne, which leads to the novel method for broadband measurement of the harmonic phases and for observing the underlying attosecond electron dynamics. 

One of the important consequences of these results can be revealed when one regards the observed interference as an inverse problem. In fact, from (4.3), observing the relative phase corresponds to the broadband measurement of the excursion time T and thus the estimation of the individual harmonic phase [24]. Measuring attosecond excursion times offer a crucial basis 

In Sect. 4.3, we discussed the role of ionization potential in harmonic phase Ip [8,11], As the geometric structures of the atomic or molecular valence orbitals should give additional effects to the harmonic phases [26,55], by measuring the harmonic phases as well as intensities from a pair of atoms and/or molecules with the same Jp such as H2-D2 [12], N2-Ar, and CO2-KX [12,30], the net effects due to the structures of them and their dynamics can be extracted [11] (Fig. 4.5). The main advantage of the present method, which we call heterodyne interferometry, is that it makes use of the interference of harmonics, and faint signals from the dynamics in the atoms and molecules can be observed. 

In this section, by applying the heterodyne interferometry to a mixed gas of H2 and D2 molecules, we probe attosecond dynamics of nuclear wavepackets in the molecules. We find that not only the single molecule responses but also the propagation effects of harmonics differ between the two isotopes and that to discuss dynamics of molecules in the single molecule responses, the propagation effects need to be excluded from the raw harmonic signals. The measured relative phase as well as intensity ratio are found to be monotonic functions of the harmonic order and are successfully reproduced by applying 

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