Coulomb-explosion imaging of concurrent ${\mathbf{CH}}_2\mathbf{BrI}$ photodissociation dynamics

The dynamics following laser-induced molecular photodissociation of gas-phase ${\mathrm{CH}}_2\mathrm{BrI}$ at 271.6 nm were investigated by time-resolved Coulomb-explosion imaging using intense near-IR femtosecond laser pulses. The observed delay-dependent photofragment momenta reveal that ${\mathrm{CH}}_2\mathrm{BrI}$ undergoes C-I cleavage, depositing 65.6% of the available energy into internal product states, and that absorption of a second UV photon breaks the C-Br bond of ${\mathrm{CH}}_2\mathrm{Br}$. Simulations confirm that this mechanism is consistent with previous data recorded at 248 nm, demonstrating the sensitivity of Coulomb-explosion imaging as a real-time probe of chemical dynamics.

Figure 1

Centroided ${\mathrm{CH}}_2\mathrm{BrI}$ time-of-flight spectrum accumulated by PImMS during the 0–2.65 ps UV-IR delay scan. Each red circle represents a time stamp and an ion image.

Figure 2

${\mathrm{I}}^{+}$ and ${\mathrm{CH}}_2{\mathrm{Br}}^{+}$ PImMS images before and after pbasex Abel inversion acquired at pump-probe delays of $0.50\pm 0.10$ and $1.00\pm 0.10$ ps. The arrowed rings contract with increasing delay, and are consistent with a Coulomb explosion occurring after neutral photodissociation.

Figure 3

${\mathrm{I}}^{+}$ (left) and ${\mathrm{CH}}_2{\mathrm{Br}}^{+}$ (right) kinetic energy distributions, obtained from pbasex Abel inverted ion images, as a function of pump-probe delay. Projections of the 1.85 ps data are shown to the right of each figure (black lines) and compared with normalized UV (violet lines) and IR background (red lines), as well as the sum of the asymptotic experimental data from Ref. [28] and the electrostatic kinetic energy at 1.85 ps (blue lines). Simulations of the expected kinetic energy are overlaid on the figures and labeled as ${\mathrm{X}}^{+}+{\mathrm{Y}}^{+}$ for the plotted fragment X and co-fragment Y. The $\mathrm{I}({}^{2}P_{3/2}$) (I, white solid lines) and $\mathrm{I}({}^{2}P_{1/2}$) (${\mathrm{I}}^{*}$, white dotted lines) pathways were modeled by assuming that the neutral fragments instantly reach their final velocities following photodissociation. The black dotted curves model $\mathrm{I}\left({}^{2}P_{3/2}\right)+{\mathrm{CH}}_2\mathrm{Br}$ acceleration using a 240 fs lifetime.

Figure 4

Schematic of the photodissociation (left) and Coulomb explosion (right) of ${\mathrm{CH}}_2\mathrm{BrI}$. C, H, Br, and I are respectively colored gray, white, red, and purple. Absorption at 271.6 nm primarily breaks the C-I bond, creating fragments with conserved momenta along the C-I axis. This force rotationally excites ${\mathrm{CH}}_2\mathrm{Br}$ as its center of mass (white circle) lies near bromine. After a set delay $t$, strong-field ionization of the co-fragments at 815 nm results in electrostatic repulsion.

Figure 5

${\mathrm{CH}}_2{\mathrm{I}}^{+}$ (top), ${\mathrm{Br}}^{+}$ (middle), and ${{\mathrm{CH}}_2}^{+}$ (bottom) kinetic energy distributions, obtained from pbasex Abel inverted ion images, as a function of pump-probe delay. The 1.85 ps data are projected to the right of each figure (black lines) and compared with normalized UV (violet lines) and IR background (red lines), and the sum of the asymptotic experimental data from Ref. [28] and the electrostatic kinetic energy at 1.85 ps (blue lines). Simulated curves for the expected kinetic energy release are labeled as ${\mathrm{X}}^{+}+{\mathrm{Y}}^{+}+{\mathrm{Z}}^{+}$ for the plotted fragment X and cofragments Y and Z.

Figure 6

${\mathrm{I}}^{+}$ (left) and ${\mathrm{CH}}_2{\mathrm{Br}}^{+}$ (right) kinetic energy distributions, obtained from pbasex Abel inverted ion images, as a function of pump-probe delay. The observed kinetic energies of the delay-dependent curves (white circles) are fit to Eq. (5) with and without (dashed white and solid red lines) a $t_0$ constraint of $\pm 100$ fs to determine the parameters in Table 1.