Coherent Control of Photofragment Separation in the Dissociative Ionization of IBr

We have investigated coherent control of the dissociative ionization of IBr using phase-controlled two-color $\omega +2\omega$ laser pulses with an intensity of $1.0\times {10}^{12}\mathrm{W}/\mathrm{cm}$ and a pulse duration of 130 fs. The directional asymmetries of the photofragment angular distributions showed oscillation behavior dependent on the relative phase difference between the $\omega$ and $2\omega$ pulses. The phase dependencies of the directional asymmetries observed for iodine ions and bromine ions were out of phase with each other. This result shows that a phase-controlled $\omega +2\omega$ optical field can produce molecular orientation in which the optical field discriminates between parallel and antiparallel configuration of molecules that have a permanent dipole.

Figure 1

(a) TOF spectra of ${\mathrm{Br}}^{+}$, ${\mathrm{I}}^{+}$, and ${\mathrm{IBr}}^{+}$ produced by the dissociative ionization of IBr molecules at 800 nm. The scales in the figure have the kinetic energy of the photofragments calibrated by means of nanosecond-pulse excitation. (b) The TOF spectrum of spin-orbit excited iodine and bromine atoms produced by one-photon dissociation of IBr molecules at 280 nm for the calibration of kinetic energy under the conditions used in the experiment with femtosecond-pulse excitation.

Figure 2

TOF spectra of ${\mathrm{I}}^{+}$ and ${\mathrm{Br}}^{+}$ ions produced by dissociative ionization of IBr when they were irradiated with both the 800 and the 400 nm pulses at relative phase difference (a) $\varphi =0$ and (b) $\varphi =\pi$.

Figure 3

The forward/backward yield ratio (${\mathrm{I}}_f/{\mathrm{I}}_b$) as a function of relative phase difference $\varphi$: (open circles) iodine ion; (solid circles) bromine ion. The inset shows the forward/backward yield ratio as a function of the angle ($\theta$) of the phase-shifting plate.