Multiorbital effects in strong-field ionization and dissociation of aligned polar molecules ${\mathrm{CH}}_3\mathrm{I}$ and ${\mathrm{CH}}_3\mathrm{Br}$

Controlling the molecular axis offers additional ways to study molecular ionization and dissociation in strong laser fields. We measure the ionization and dissociation yields of aligned polar ${\mathrm{CH}}_{3}X$ ($X=\mathrm{I}$, Br) molecules in a linearly polarized femtosecond laser field. The current data show that maximum ionization occurs when the laser polarization is perpendicular to the molecular $\mathrm{C}-X$ axis, and dissociation prefers to occur at the laser polarization parallel to the $\mathrm{C}-X$ axis. The observed angular distributions suggest that the parent ions are generated by ionization from the HOMO. The angular distribution of fragment ions indicates that dissociation occurs mainly from an ionic excited state produced by ionization from the HOMO-1.

Figure 1

(a) Pump-probe scheme for measuring the alignment-dependent ionization of ${\mathrm{CH}}_{3}X$ ($X=\mathrm{I}$, Br) molecules. (b) HOMO and HOMO-1 of the ${\mathrm{CH}}_3\mathrm{I}$ molecule. (c) Schematic of ionization and fragmentation pathways and relevant energy levels of ${\mathrm{CH}}_{3}X$ ($X=\mathrm{I}$, Br) molecules; colored arrows indicate directly tunneling ionization from different orbitals.

Figure 2

Measured parent ion yields of ${\mathrm{CH}}_3{\mathrm{I}}^{+}$ and ${\mathrm{CH}}_3{\mathrm{Br}}^{+}$ as a function of the pump-probe delay. Black circles represent the alignment-dependent yield; blue lines, the time-dependent alignment evolution $\langle {\cos }^2\theta \rangle$ calculated by the time-dependent Schrödinger equation.

Figure 3

Time-of-flight mass spectra of (a) ${\mathrm{CH}}_3\mathrm{I}$ and (b) ${\mathrm{CH}}_3\mathrm{Br}$ irradiated with 800-nm, 50-fs laser pulses at intensities of $5.4\times {10}^{13}$ and $6.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 4

Angular distribution of parent and fragment ions measured from the well-aligned molecular targets ${\mathrm{CH}}_3\mathrm{I}$ and ${\mathrm{CH}}_3\mathrm{Br}$ at the pump-probe delay time of 68.1 and 53.1 ps, respectively. Angular distributions for (a) ${\mathrm{CH}}_3{\mathrm{I}}^{+}$, (b) ${\mathrm{I}}^{+}$, and (c) ${\mathrm{CH}}_3{}^{+}$ and for (d) ${\mathrm{CH}}_3{\mathrm{Br}}^{+}$, (e) ${\mathrm{Br}}^{+}$, and (f) ${\mathrm{CH}}_3^{+}$. Error bars are estimated from the different experimental runs and blue lines are the fitted curves using Legendre polynomials $I=1+\sum {\beta }_{2i}{P}_{2i}$.

Figure 5

Experimentally deconvolved angular-dependent ionization and dissociation probability $\mathrm{S}(\theta$) of ${\mathrm{CH}}_{3}X$ molecules, together with the MO-SFA and TDDFT calculation results. (a, b) Angular distributions of parent and fragment ions from ${\mathrm{CH}}_3\mathrm{I}$ and ${\mathrm{CH}}_3\mathrm{Br}$ (c, d) Angular distributions of HOMO (solid and dashed black lines) and HOMO-1 (solid and dashed red lines) orbitals from MO-SFA and TDDFT calculations for ${\mathrm{CH}}_3\mathrm{I}$ and ${\mathrm{CH}}_3\mathrm{Br}$. Angular distributions are normalized at the maximum intensities.

Figure 6

Normalized angular distributions of parent ions for (a) ${\mathrm{CH}}_3\mathrm{I}$ and (b) ${\mathrm{CH}}_3\mathrm{Br}$ at different laser intensities.