Laser-induced Coulomb explosion of 1,4–diiodobenzene molecules: Studies of isolated molecules and molecules in helium nanodroplets

Coulomb explosion of 1,4–diiodobenzene molecules, isolated or embedded in helium nanodroplets, is induced by irradiation with an intense femtosecond laser pulse. The recoiling ion fragments are probed by time-of-flight measurements and two-dimensional velocity map imaging. Correlation analysis of the emission directions of ${\mathrm{I}}^{+}$ ions recoiling from each end of the molecules reveals significant deviation from axial recoil, i.e., where the ${\mathrm{I}}^{+}$ ions leave strictly along the I-I symmetry axis. For isolated molecules, the relative angular distribution of the ${\mathrm{I}}^{+}$ ions is centered at ${180}^{\circ }$, corresponding to perfect axial recoil, but with a full width at half maximum of ${30}^{\circ }$. For molecules inside He droplets, the width of the distribution increases to ${45}^{\circ }$. These results provide a direct measure of the accuracy of Coulomb explosion as a probe of the spatial orientation of molecules, which is particularly relevant in connection with laser-induced molecular alignment and orientation. In addition, our studies show how it is possible to identify fragmentation pathways of the Coulomb explosion for the isolated 1,4–diiodobenzene molecules. Finally, for the 1,4–diiodobenzene molecules in He droplets, it is shown that the angular correlation between fragments from the Coulomb explosion is preserved after they have interacted with the He environment.

Figure 1

(a) Sketch of the molecular structure of 1,4–diiodobenzene (DIB). (b) TOF mass spectrum of isolated DIB molecules recorded with the probe pulse polarized along the TOF axis. The intensity is $3.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The predominant fragments are labeled. The inset shows a zoom of the region between $3.5$ and $7\mu \mathrm{s}$ covering fragments from $m/q=57$ to $m/q=210\mathrm{u}/e$.

Figure 2

(a) TOF covariance map for isolated DIB molecules. (b),(c) Average TOF spectrum. The panels (I–VI) show the covariance map for specific ion correlations with the color scale adjusted to enhance contrast. The insets are indicated on the main covariance maps by white circles.

Figure 3

(a1)–(a3) 2D ion images from isolated DIB molecules and (b1)–(b3) slices of the 3D ion distribution. Column (1): ${\mathrm{I}}^{+}$ ions; column (2): ${\mathrm{I}}^{2+}$ ions; column (3): ${\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{I}}^{+}$ ions. The probe pulse polarization is indicated in (a1). Each image is scaled to its maximum value and is 551 by $551\mathrm{pixels}$, which corresponds to 36 by $36\mathrm{mm}$ on the detector. See text for details.

Figure 4

(a) Radial distributions and (b) speed distributions of ${\mathrm{I}}^{+}$ ions (solid red), ${\mathrm{I}}^{2+}$ ions (dashed blue), and ${\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{I}}^{+}$ ions (dotted black) obtained from the ion images in Figs. 3(a1)–3(a3) and 3(b1)–3(b3), respectively.

Figure 5

The kinetic-energy distribution of ${\mathrm{I}}^{+}$ ions resulting from ionization of the DIB molecules with a probe pulse intensity of $4.1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 6

(a) Radial covariance map of ${\mathrm{I}}^{+}$ ions on the top half of the detector with the ${\mathrm{I}}^{+}$ ions on the bottom half. (b),(c) Average radial distribution from 2D ion images. The dashed lines are guides to help distinguish regions for the two separate channels.

Figure 7

Angular covariance map for ${\mathrm{I}}^{+}$ ions detected in (a) the inner channel and (b) the outer channel as a function of emission angle ${\theta }_1$ and ${\theta }_2$. The A-diagonal line has been put to zero.

Figure 8

(a) Angular covariance map for ${\mathrm{I}}^{+}$ ions detected in the outer channel as a function of emission angle ${\theta }_1$ and difference angle ${\theta }_2-{\theta }_1$. (b) Integrated covariance as a function of the difference angle ${\theta }_2-{\theta }_1$; see text for details.

Figure 9

(a) Radial covariance map for ${\mathrm{I}}^{+}$ and ${\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{I}}^{+}$ ions on the top half of the detector with those on the bottom half. (b),(c) Average radial distribution including both ion species. The white circle highlights the main difference compared to Fig. 6.

Figure 10

Angular covariance map for a radial range covering (a) the inner channel and (b) the outer channel for ${\mathrm{I}}^{+}$ ions. The white circle highlights the difference compared to Fig. 7.

Figure 11

TOF spectrum of DIB molecules inside helium droplets. The inset shows a zoom of the region between 4 and $8.2\mu \mathrm{s}$ covering fragments from $m/q=46$ to $m/q=210\mathrm{u}/e$. The main fragments are labeled. The probe pulse intensity was $4.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and the polarization was along the TOF direction.

Figure 12

(a) Time-of-flight covariance map for randomly aligned DIB molecules in helium droplets with the average TOF spectrum shown below in (c). (b) Time-of-flight covariance map for aligned DIB molecules in helium droplets with the average TOF spectrum shown below in (d). The intensity of the probe pulse was $2.2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and the polarization was along the TOF axis.

Figure 13

(a1)–(a3) 2D ion images from DIB molecules in helium droplets and (b1)–(b3) slices of the 3D ion distribution. Column (1): ${\mathrm{I}}^{+}$ ions recorded at a peak pulse intensity of $1.9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$; column (2): ${\mathrm{I}}^{+}$ ions recorded at peak intensity of $3.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$; column (3): ${\mathrm{IHe}}^{+}$ ions recorded at a peak intensity of $1.9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The laser polarization is indicated in (a1). Each image is $551\mathrm{pixels}$ by $551\mathrm{pixels}$, which corresponds to $36\mathrm{mm}$ by $36\mathrm{mm}$ on the detector.

Figure 14

(a) Radial distributions and (b) speed distributions of ions obtained from the images in Figs. 13(a1)–13(a3) and 13(b1)–13(b3), respectively. The distribution for ${\mathrm{I}}^{+}$ ions recorded at an intensity of $I=3.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ is shown in dashed blue. The distributions for ${\mathrm{I}}^{+}$ and ${\mathrm{IHe}}^{+}$ ions at an intensity of $I=1.9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ is shown in solid red and dotted black, respectively.

Figure 15

Speed distribution of ${\mathrm{I}}^{+}$ ions from isolated molecules for an intensity of $4.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (dotted red) and for molecules in helium droplets for an intensity of $3.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (solid blue). The dashed and dotted dashed line is the average speed for each distribution. See text for details.

Figure 16

(a) Radial covariance map of ${\mathrm{I}}^{+}$ ions on the top part of the detector with ${\mathrm{I}}^{+}$ ions on the bottom half recorded from molecules inside helium droplets. (b),(c) Average radial distribution from the 2D ion image.

Figure 17

(a) Angular covariance map for ${\mathrm{I}}^{+}$ ions from molecules inside helium droplets as a function of emission angle ${\theta }_1$ and difference angle ${\theta }_2-{\theta }_1$. (b) Integrated covariance as a function of the difference angle ${\theta }_2-{\theta }_1$.