Isotope-selective ionization utilizing field-free alignment of isotopologues with a train of femtosecond laser pulses

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We propose a strategy of isotope-selective ionization for a binary mixture of isotopologues of homonuclear diatomic molecules, utilizing field-free alignment with a train of femtosecond laser pulses. Field-free alignment can be achieved simultaneously for two isotopologues consisting of two atoms with the same atomic mass number \ensuremath{\alpha} or \ensuremath{\beta} $({}^{\ensuremath{\alpha}}{X}_{2}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\beta}}{X}_{2})$, utilizing a pulse train with their time interval of ${T}_{\mathrm{com}}^{\mathrm{rev}}=\ensuremath{\beta}{T}_{\ensuremath{\alpha}}^{\mathrm{rev}}=\ensuremath{\alpha}{T}_{\ensuremath{\beta}}^{\mathrm{rev}}$, where ${T}_{\ensuremath{\alpha}}^{\mathrm{rev}}$ and ${T}_{\ensuremath{\beta}}^{\mathrm{rev}}$ are the rotational revival times of the isotopologues. We demonstrate experimentally that a train of four alignment pulses with their interval of ${T}_{\mathrm{com}}^{\mathrm{rev}}(\ensuremath{\alpha}=14,\phantom{\rule{0.16em}{0ex}}\ensuremath{\beta}=15)$ creates transiently aligned ${}^{14}{\mathrm{N}}_{2}$ and antialigned ${}^{15}{\mathrm{N}}_{2}$ just before ${T}_{\mathrm{com}}^{\mathrm{rev}}/2$ after the last alignment pulse and vice versa just after ${T}_{\mathrm{com}}^{\mathrm{rev}}/2$. Highly isotope-selective ${\mathrm{N}}_{2}$ ionization is achieved at these timings with another femtosecond laser pulse, which induces the nonresonant multiphoton ionization with the cross section remarkably depending on the angle between the molecular axis and the laser-electric-field direction. The ion yield ratio $I({}^{15}{N}_{2}^{+})/I({}^{14}{N}_{2}^{+})$ ranges from 0.49 to 2.00, which is wider than the range obtained with a single alignment pulse [H. Akagi et al., Appl. Phys. B 109, 75 (2012)].