Velocity Map Imaging of Dissociative Ionization and Coulomb Explosion of CH₃I Induced by a Femtosecond Laser

Abstract

The dissociative ionization and the Coulomb explosion of CH3I irradiated by a 35 fs 800 nm laser with a laser intensity of 4 x 10(13) to 6 x 10(14) W/cm2 was studied. In a relatively weak laser field (about 10(13) W/cm2), the dissociative ionization of CH3I took place. The speed distributions of the CH3+ and I+ fragments were measured and fitted using multiple Gaussian functions. Different product channels were found for CH3+ and I+, respectively. In a strong laser field (about 10(14) W/cm2), the multiply ionized fragment ions of Iq+ (q </= 3), which experienced a Coulomb explosion, were observed. The angular and speed distributions of the I+, I2+, and I3+ fragments were obtained. All of these fragment angular distributions are anisotropic and peaked along the laser polarization direction. The Iq+ ion angular distributions all were of similar widths, which would imply that the geometric alignment dominated the process, as opposed to a dynamic alignment mechanism.

Dissociative ionization and Coulomb explosion of CH3I in intense femto second laser fields

“…More recently, Corrales et al [43] applied a short (50 fs) and non-resonant (804 nm) laser pulse to study the CE and DI of CH 3 I in the high density region of a molecular beam. This is in contrast to the other experiments described in this paper and in previous studies [39][40][41][42]. Employing, 1 to 6 × 10 13 W/cm 2 laser field intensities, much lower than those applied in previous studies [25,[39][40][41][42], the recoil energy distribution of the I q+ and CH p+ 3 CE ion pair fragments were found to exhibit a double peak structure at recoil energies much less than those observed in previous studies.…”

“…The KER's agreed reasonable well with those found by Liu et al [41] earlier at the same laser intensity but with a longer 180 fs laser pulse duration [42]. More recently, Corrales et al [43] applied a short (50 fs) and non-resonant (804 nm) laser pulse to study the CE and DI of CH 3 I in the high density region of a molecular beam.…”

“…The white circle radii in Figure 2 denote the peak intensity position of the kinetic energy released for each of the distinct DI and CE channels. Table 1 lists both the E peak and W values of the CE channels obtained in our experiment and those of previous experiments [41][42][43]. Typically, for most CE channels the KER's as observed by Liu et al [41] and also those obtained by Wang et al [42] are somewhat larger than the ones obtained in our experiment.…”

“…R C ≈ 3.7Å resulted as a typical value for CH 3 I independent on p and q [42]. The KER's obtained in this study refer to a geometric alignment, where the laser polarization vector pointed along the TOF axis.…”

“…The KER's obtained in this study refer to a geometric alignment, where the laser polarization vector pointed along the TOF axis. In a subsequent study Wang et al [42] applied the Velocity Mapped Ion imaging (VMI) method to explore the recoil of the ionic fragments resulting from the exposure of CH 3 I to a 35 fs, 800 nm laser pulse. At their lowest intensity 5 × 10 13 W/cm 2 , the observed recoil velocity distributions for of CH + 3 and I + were found to originate from 3 different DI channels.…”