Anomalous isotopic effect on electron-directed reactivity by a 3-μm midinfrared pulse

Abstract

We have theoretically studied the effect of nuclear mass on electron localization in dissociating H2+ and its isotopes subjected to a few-cycle 3-µm laser pulse. Compared to the isotopic trend in the near-infrared regime, our results reveal an inverse isotopic effect in which the degree of electrondirected reactivity is even higher for heavier isotopes. With the semi-classical analysis, we find, for the first time, the pronounced electron localization is established by the interferences through different channels of one-and, more importantly, higher-order photon coupling. Interestingly, due to the enhanced high-order above-threshold dissociation of heavier isotopes, the interference maxima gradually become in phase with growing mass and ultimately lead to the anomalous isotopic behavior of the electron localization. This indicates that the multi-photon coupling channels will play an important role in controlling the dissociation of larger molecules with midinfrared pulses. PACS numbers: 33.20. Xx, 33.80.Wz, 32.80.Qk, 42.50.Hz The electronic motion inside the molecule is of fundamental importance in determining the formation and fracture of chemical bonds. For more than two decades, many efforts have been done to study the electronic dynamics in laser-matter interactions [1][2][3][4], aiming at the control over ultrafast reactions. With the recent development of laser techniques and attosecond science [5][6][7], it has become feasible to steer the electron localization in dissociating molecules with the carrier-envelope phase (CEP) stabilized few-cycle laser pulses [8][9][10][11] or the sequential ultraviolet and near-infrared pulses [12][13][14]. The asymmetric electron localization in molecules can be understood as the quantum interference of the populations that are resonantly transferred among at least two electronics states of different parity [15,16].More recently, the control of electron-directed reactivity in midinfrared laser pulses has been explored to enhance the electron localization probability via the match between the duration of the few-cycle pulse and the dissociation time of the molecule [17][18][19]. To hold the control efficiency for further heavier nuclei, one may have to use the pulse with longer wavelength [18]. While this constitutes an important step towards the control of chemical reactions in larger molecules, we are wondering whether the physical mechanism responsible for the electron localization in midinfrared pulses remains the same as that in the near-infrared regime [9]. Meanwhile, for extending the control scheme to the larger molecules, the influence of nuclear mass on the reactions must be taken into account. However, it remains unclear how the nuclear mass affects the control efficiency when the midinfrared pulses are applied, though it has been shown that the control of electron localization is weakened by the growing mass in the few-cycle near-infrared field [9,20] To understand the influence of mass in reactions, the use of isotopes is one of the practice means. This is ...

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