Selective Ionization of Oriented Nonpolar Molecules with Asymmetric Structure by Phase-Controlled Two-Color Laser Fields

We report on the selective ionization of oriented nonpolar molecules with asymmetric structure by using phase-controlled two-color $\omega +2\omega$ laser pulses with an intensity of $1.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ (tunneling ionization regime) and a pulse duration of 130 fs. The orientation of 1-bromo-2-chloroethane was monitored by the directional asymmetries of the forward-backward emission in dissociative ionization. The observed direction of orientation clearly confirms that molecular orientation is induced not by dynamic orientation but by selective ionization of oriented molecules, which reflects the structure of the highest occupied molecular orbital. This method can be applied for the vast majority of molecules.

Figure 1

Waveform of phase-controlled two-color $\omega +2\omega$ laser fields at relative phase differences (a) $\varphi =\pi$ and (b) $\varphi =0$, with molecular structures and isocontours of the HOMO of 1-bromo-1-chloroethane. The different shadings indicate different signs of the wave functions. We define the relative phase difference as $\varphi =0$ when the electric field maxima point toward the detector (“forward” direction) and as $\varphi =\pi$ when the electric field maxima point away from the detector (“backward” direction).

Figure 2

(a) TOF mass spectra of ions produced by dissociative ionization of 1-bromo-2-chloroethane molecules irradiated only with 800-nm pulses. The parent molecules are denoted by “P.” The solid lines indicate the pair of forward and backward peaks for ${\mathrm{Cl}}^{+}$ and ${\mathrm{Br}}^{+}$.

Figure 3

Forward or backward yield ratio ($I_{\mathrm{f}}/I_{\mathrm{b}}$) as a function of relative phase difference, $\varphi$: (open circles) halogen ion; (solid circles) countercation.