Search for: All records

Investigating the behavior of polyatomic molecules in strong laser fields allows the examination of coupled nuclear and electronic dynamics. In this article} we examine multibody fragmentation of formic acid dications, produced by ultrashort intense laser pulses, using coincidence momentum imaging and deuterium tagging to distinguish the hydrogen sites. Our measurements indicate that double hydrogen elimination leads to 3- and 4-body breakup, specifically H + D + CO$$_2^{2+}$$ and H + D + O$^+$ + CO$^+$, respectively. We find that the latter breakup channel also proceeds via the CO$$_2^{2+}$$ metastable dication, but with shorter lifetimes than the CO$$_2^{2+}$$ dications detected intact. This intriguing sequential breakup is identified using native frames analysis and simulations to confirm the signature of this process \new{in spite of} the inferior momentum resolution associated with breakup channels for which not all fragments are detected. In addition to the fragmentation channel above, we found no evidence of sequential breakup for single hydrogen elimination, H + CO$^+$ + OD$^+$. The elimination of three atoms, H + D + O + C$^+$ + O$^+$, shows evidence of both concerted fragmentation and sequential breakup via metastable intermediate CO$$^{2+}$$ ions. Measuring the dissociation dynamics of these highly excited molecular ions yields information that can be used to test theoretical approaches to these challenging few-body systems. 

We explore the formation of metastable CO$$_2^{2+}$$ from linear and ``bent" carbon dioxide by intense ultrashort laser pulses. In particular, we evaluate the fraction of these dications that survive to the detector and determine the lifetime of those that dissociate in flight through our apparatus. Curiously, the fraction of those dissociating in flight changes significantly when cooling the linear CO$$_2$$ target using a He carrier gas, while the lifetimes change only slightly. To determine these lifetimes we employ an improved method that takes into account the dissociation velocity of the O$^+$$\,+\,CO$$^+$ fragments in a self-consistent manner. We also determine the dissociation-in-flight lifetime of an initially bent CO$$_2^{2+}$$ fragment of HCOOD$$^{2+}$$ following rapid double hydrogen elimination. 

Site-selective probing of iodine 4d orbitals at 13.1 nm was used to characterize the photolysis of CH2I2 and CH2BrI initiated at 202.5 nm. Time-dependent fragment ion momenta were recorded using Coulomb explosion imaging mass spectrometry and used to determine the structural dynamics of the dissociating molecules. Correlations between these fragment momenta, as well as the onset times of electron transfer reactions between them, indicate that each molecule can undergo neutral three-body photolysis. For CH2I2, the structural evolution of the neutral molecule was simultaneously characterized along the C–I and I–C–I coordinates, demonstrating the sensitivity of these measurements to nuclear motion along multiple degrees of freedom. 

We report on the use of extreme ultraviolet (XUV, 30.3 nm) radiation from the Free-electron LASer in Hamburg (FLASH) and visible (Vis, 405 nm) photons from an optical laser to investigate the relaxation and fragmentation dynamics of fluorene ions. The ultrashort laser pulses allow to resolve the molecular processes occurring on the femtosecond timescales. Fluorene is a prototypical small polycyclic aromatic hydrocarbon (PAH). Through their infrared emission signature, PAHs have been shown to be ubiquitous in the universe, and they are assumed to play an important role in the chemistry of the interstellar medium. Our experiments track the ionization and dissociative ionization products of fluorene through time-of-flight mass spectrometry and velocity-map imaging. Multiple processes involved in the formation of each of the fragment ions are disentangled through analysis of the ion images. The relaxation lifetimes of the excited fluorene monocation and dication obtained through the fragment formation channels are reported to be in the range of a few tens of femtoseconds to a few picoseconds. 

Abstract Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH * , PAH +* and PAH 2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH 2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.