Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime

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 PAH2+* 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 PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Covariance-Map Imaging: A Powerful Tool for Chemical Dynamics Studies

“…However, calculations show that the relevant dissociation energies are all energetically accessible following double ionization by a 30.3 nm photon. 59 On closer inspection of the array of covariance images, we note an interesting pattern in the data. In working down the first column of images, for example, we see the expected covariance between the C 2 + and C 11 + fragment ions.…”

“…This is perhaps surprising given the extensive molecular rearrangement and multiple bond cleavages needed to access some of these pathways. However, calculations show that the relevant dissociation energies are all energetically accessible following double ionization by a 30.3 nm photon …”

“…Having demonstrated that recoil-frame covariancemap imaging can be useful in distinguishing between competing dissociation mechanisms in a small molecule, one might wonder whether such an approach can be extended to much larger molecular systems. We have recently investigated the dissociation pathways followed by multiply charged polycyclic aromatic hydrocarbons (PAHs) generated by XUV photoionization 59 and have found covariance mapping to be an extremely powerful tool for unravelling the complex dissociation dynamics of these molecules.…”

The kinetic energy of PAH dication and trication dissociation determined by recoil-frame covariance map imaging

“…Firstly, the signal level is significantly lower than from dication dissociation owing to the fact that the 40.9 eV photons are only a few eV above energy required to triply ionize these molecules. 8,29 Secondly, two-body dissociation of the parent trication will result in a monocation and a dication. As mentioned in Section 3.1, dications containing an even number of carbons overlap with monocations in flight time due to having an equiva- lent value of m/z, for example, C 6 H 2+ x has the same arrival time, and is therefore recorded in the same ion image as C 3 H + y .…”

Fragmentation Dynamics of Fluorene Explored Using Ultrafast XUV-Vis Pump-Probe Spectroscopy

“…Despite the remarkable stability of PAHs, they can undergo ionization, isomerization, fragmentation, and dissociative ionization when exposed to high-energy radiation [16][17][18][19][20][21]. Understanding these processes is challenging for both computational and experimental methods because a large number of nuclear and electronic degrees of freedom are strongly coupled when the system is highly excited and typically enters the non-adiabatic regime, in which the Born-Oppenheimer approximation breaks down [22][23][24][25].…”