Presentation Isotope-selective ionization utilizing molecular alignment and angular dependent ionization induced by intense laser fields

Akagi, Hiroshi  ,  Kasajima, Tatsuya  ,  Kumada, Takayuki  ,  Itakura, Ryuji  ,  Yokoyama, Atsushi  ,  Hasegawa, Hirokazu  ,  Ohshima, Yasuhiro

An intense short laser pulse can rotationally excite a simple molecule through rotational Raman excitation, and can induce rotational wave packet in the molecule, which exhibits transient alignment and anti-alignment distributions periodically with interval of its revival time. Utilizing the laser induced alignment, we are studying isotope-selective ionization of 14N2 and 15N2 isotopologues, as a new molecular laser isotope separation (MLIS) method. An intense short laser pulse with linear polarization (alignment pulse) creates rotational wave packets in these two isotopologues. At specific time delays after the alignment pulse irradiation, one isotopologue aligns and the other anti-aligns because of the different revival times Tirev (i=14 or 15). Another linearly-polarized intense laser pulse (ionization pulse) at one of the time delays ionizes the aligning one preferentially, because of angular dependence of the N2 ionization probability. Our simulation indicates that the isotopic selectivity is not significantly lowered for heavy isotopes, in contrast to the conventional MLIS methods. As the experimental demonstration, we measure time delay dependence of the ion yield ratio R [=I(15N2)/I(14N2)]. In a vacuum chamber, gas jet including 14N2 and 15N2 is intersected by the alignment pulse (λ~795 nm, Δτ~60 fs) and the ionization pulse (λ~795 nm, Δτ~60 fs), which are focused on the gas jet with a lens (f=+250 mm). Ions produced at the focus are mass-selected with a time-of-flight mass spectrometer and detected with an MCP. The ratio R is changed in the range of R=0.68~1.41, depending on the time delay t between the pulses. This result indicates that the present method works properly. Pulse train irradiation with the interval of Tirev can improve the degree of iN2 alignment. To obtain higher isotope selectivity, we use a train of four alignment pulses with the interval of 125.7 ps, instead of the single alignment pulse. The pulse interval corresponds to the fifteenth T14rev of 14N2 and the fourteenth T15rev of 15N2. In this case, the range of R is obtained to be 0.49-2.00, indicating that the four pulse irradiation improves the selectivity by ca. 1.4 times. Our simulation indicates that the selectivity further increases by 1.5 times (R=0.31~3.0) when the number of the alignment pulses is increased to 12. The high selectivity predicted for a train of twelve pulses raises possibility of the present approach as a practical isotope separation technique.
The EMN Meeting on Light-Matter Interactions 2016

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