The ejection of triatomic molecular hydrogen ions H3+ produced by the interaction of benzene molecules with ultrafast laser pulses

Strong-field ionization of CH3Cl: proton migration and association

Strong-field ionization of CH₃Cl using femtosecond laser pulses, and the subsequent two-body dissociation of CH₃Cl²⁺ along H_n⁺ (n = 1-3) and HCl⁺ forming pathways, have been experimentally studied in a home-built COLTRIMS (cold target recoil ion momentum spectrometer) setup. The single ionization rate of CH₃Cl was obtained experimentally by varying the laser intensity from 1.6 × 10¹³ W cm^-2 to 2.4 × 10¹⁴ W cm^-2 and fitted with the rate obtained using the MO-ADK model. Additionally, the yield of H_n⁺ ions resulting from the dissociation of all charge states of CH₃Cl was determined as a function of intensity and pulse duration (and chirp). Next, we identified four two-body breakup pathways of CH₃Cl²⁺, which are H⁺ + CH₂Cl⁺, H₂⁺ + CHCl⁺, H₃⁺ + CCl⁺, and CH₂⁺ + HCl⁺, using photoion-photoion coincidence. The yields of the four pathways were found to decrease on increasing the intensity from I = 4.2 × 10¹³ W cm^-2 to 2I = 8.5 × 10¹³ W cm^-2, which was attributed to enhanced ionization of the dication before it can dissociate. As a function of pulse duration (and chirp), the H_n⁺ forming pathways were suppressed, while the HCl⁺ forming pathway was enhanced. To understand the excited state dynamics of the CH₃Cl dication, which controls the outcome of dissociation, we obtained the total kinetic energy release distributions of the pathways and the two-dimensional coincidence momentum images and angular distributions of the fragments. We inferred that the H_n⁺ forming pathways originate from the dissociation of CH₃Cl dications from weakly attractive metastable excited states having a long dissociation time, while for the HCl⁺ forming pathway, the dication dissociates from repulsive states and therefore, undergoes rapid dissociation. Finally, quantum chemical calculations have been performed to understand the intramolecular proton migration and dissociation of the CH₃Cl dication along the pathways mentioned above. Our study explains the mechanism of H_n⁺ and HCl⁺ formation and confirms that intensity and pulse duration can serve as parameters to influence the excited state dynamics and hence, the outcome of the two-body dissociation of CH₃Cl²⁺.

Identifying the Multielectron Effect on Chemical Bond Rearrangement of CH3Cl Molecules in Strong Laser Fields

Strong field double ionization that triggers the chemical bond rearrangement of CH₃Cl is investigated by impulsive control of the alignment of molecules. The alignment and laser intensity dependent H₂⁺ and H₃⁺ yields in linearly polarized femtosecond laser have been measured, and the obtained data show that the maximum signal of H₂⁺ appears at the laser polarization parallel to the C-Cl axis of molecules and H₃⁺ species are more likely to eject at the laser polarization parallel to the C-Cl axis at low laser intensity while the H₃⁺ signal peaks at laser polarization perpendicular to the C-Cl axis at high laser intensity. The measurements indicate that electrons from HOMO - 1 and HOMO - 2 orbitals have been ionized for the generation of bond rearrangement at different laser intensity. Our results demonstrate the importance of multielectron effects and also provide an effective control method in the process of chemical bond rearrangement of the molecules in strong laser fields.

Tailoring Ion Charge State Distribution in Tetramethyltin Clusters under Influence of Moderate Intensity Picosecond Laser Pulse: Role of Laser Wavelength and Rate of Energy Deposition

Systematic manipulation of ionic-outcome in laser-cluster interaction process has been realized for studies carried out on tetramethyltin (TMT) clusters under picosecond laser conditions, determined by choice of laser wavelength and intensity. As a function of laser intensity, TMT clusters exhibit gradual enhancement in overall ionization of its cluster constituents, up to a saturation level of ionization, which was distinct for different wavelengths (266, 355, and 532 nm). Simultaneously, systematic appearance of higher multiply charged atomic ions and shift in relative abundance of multiply charged atomic ions towards higher charge state was observed, using time-of-flight mass spectrometer. At saturation level, multiply charged atomic ions up to (C²⁺, Sn²⁺) at 266 nm, (C⁴⁺, Sn⁴⁺) at 355 nm, and (C⁴⁺, Sn⁶⁺) at 532 nm were detected. In addition, at 355 nm intra-cluster ion chemistry within the ionized cluster leads to generation of molecular hydrogen ion (H₂⁺) and triatomic molecular hydrogen ion (H₃⁺). Generation of multiply charged atomic ions is ascribed to efficient coupling of laser pulse with the cluster media, facilitated by inner-ionized electrons produced within the cluster, at the leading edge of laser pulse. Role of inner-ionized electrons is authenticated by measuring kinetic energy distribution of electrons liberated upon disintegration of excessively ionized cluster, under the influence of picosecond laser pulse. Graphical Abstract ᅟ.

Ultrafast proton migration and Coulomb explosion of methyl chloride in intense laser fields

We investigated the ultrafast proton migration and the Coulomb explosion (CE) dynamics of methyl chloride (CH₃Cl) in intense femtosecond laser fields at the wavelengths of 800 nm (5.5 × 10¹⁴ W/cm²) and 400 nm (4 × 10¹⁴ W/cm²), respectively. Various fragment channels from molecular dication and trication were observed by coincidence momentum imaging through the measurement of their kinetic energy releases (KERs). The proton migration from different charged parent ions was analyzed from the obtained KER distributions. For the direct CE channel of CH₃⁺ + Cl⁺ and CH₃⁺ + Cl²⁺, the contribution of multiply excited electronic states and multicharged states is identified. In addition, the measurements of relative yields of the fragmentation channel at different laser wavelengths provide a selective control of proton migration for CH₃Cl molecules in intense laser fields.

Sub-cycle steering of the deprotonation of acetylene by intense few-cycle mid-infrared laser fields

Directional breaking of the C-H/C-D molecular bond is manipulated in acetylene (C₂H₂) and deuterated acetylene (C₂D₂) by waveform controlled few-cycle mid-infrared laser pulses with a central wavelength around 1.6 μm at an intensity of about 8 × 10¹³ W/cm². The directionality of the deprotonation of acetylene is controlled by changing the carrier-envelope phase (CEP). The CEP-control can be attributed to the laser-induced superposition of vibrational modes, which is sensitive to the sub-cycle evolution of the laser waveform. Our experiments and simulations indicate that near-resonant, intense mid-infrared pulses permit a higher degree of control of the directionality of the reaction compared to those obtained in near-infrared fields, in particular for the deuterated species.