Dissociative Ionization and Coulomb Explosion of Molecular Bromocyclopropane in an Intense Femtosecond Laser Field

Dissociative Ionization and Coulomb Explosion of CHBrCl2 in Intense Near-Infrared Femtosecond Laser Fields

We experimentally demonstrate the dissociative photoionization of CHBrCl2 molecules in a femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. The results suggest that the low kinetic energy components are from the dissociative ionization process of single-charged molecular ions. The angular distribution of fragment Cl+ ions can be attributed to the features of dissociative state and molecular configuration, and that of Br+ ions results from the electronic wave-packet evolution and combination of the multi-dissociation processes. The high kinetic energy components are from the Coulomb explosion of multi-charged molecular ions, and the error of the C-Br distance involved in the Coulomb explosion can be explained by the movement of the effective charge center of the polyatomic molecule.

Dissociative Ionization of Molecular CF2Br2 under 800 and 400 nm Intense Femtosecond Laser Fields

The interaction between the CF2Br2 molecule and 800/400 nm intense femtosecond laser fields is investigated by direct current (dc) sliced velocity mapping imaging implementation. By analyzing the kinetic energy release distribution and angular distribution of fragment ions, the dissociation channels along C-Br bond cleavage have been determined. The isotropic structure of the angular distribution for CF2Br+ ions is attributed to the coupling between the excited states. Additionally, a unique elimination channel of CF2Br2+ → CF2 + Br2+ has been observed and identified in the case of 400 nm laser field, in which the two C-Br bonds break asynchronously.

Direct Production of CH(A2Δ) Radical from Intense Femtosecond Near-IR Laser Pulses

CH(A²Δ) radical formation was observed in bromoform and methanol vapor in argon plasma with near-infrared femtosecond laser pulses (43 fs, 1030 nm, 100 kHz, 250 μJ/pulse). The beam was focused with an achromatic lens, creating very high intensity in the plasma that caused Coulomb explosion (calculated intensity was ∼1.1 × 10¹⁶ W/cm² in the focal point). The emitted fluorescence light was measured with high spectral (1-10 cm^-1) and temporal resolution (5 ns) with an FT-Vis spectrometer. The step-scan technique allowed the reconstruction of the time-resolved fluorescence spectra from CH(A-X) emission. The emission from atomic lines such as H, Br, C, and O was observed and also from C⁺ cations and CH and C₂ radicals. This indicates that in a significant portion of these organic molecules, all chemical bonds were cleaved in the Coulomb explosion. For both organics, the peak maximum of the CH(A) emission occurred at about 10 ns after excitation by the femtosecond pulse. After the maximum, a rapid emission decay was observed in the case of bromoform (monoexponential decay, t = 10 ns). The fluorescence decay was biexponential when methanol was used as the source for CH(A) generation. It can be assumed that CH(A) generation involved a fast and a slower path with some secondary reactions via the stepwise loss of hydrogen atoms from the CH₃ group. The time constants were t₁ = 7.8-8.3 ns and t₂ = 78-82 ns for the fast and slow components, respectively, and very similar values were obtained at 10 and 25 mbar total pressures. However, in the case of bromoform, the C-Br bonds are significantly weaker; therefore, these atoms can be removed even in a single step via multiphoton absorption. The rotational temperature of CH(A) radicals generated from methanol decreased rapidly in the 30-55 ns time period from 2770 ± 80 to 1530 ± 50 K. The vibrational temperature increased from 3530 ± 450 to 9810 ± 760 K in the 30-80 ns time period and then started to decrease (the average temperatures were T_rot = 910 ± 20 K and T_vib = 7490 ± 340 K at 100 ns). This initial increase of T_vib is thought to be the result of electron collision with the CH radicals. The high temperatures of the fragment may indicate the roaming reaction associated with the Coulomb explosion of the parent molecule. We demonstrated that CH(A) radicals can be produced from both organic compounds, and the step-scan technique is ideal for the characterization of their time-resolved spectra using the 100 kHz high repetition rate near-infrared femtosecond laser pulses. The FT/UV-vis step-scan technique can detect neutral species directly with high spectral and time resolution, thus it is a complementary technique to the experiments utilizing ion detection schemes, such as velocity map imaging.