Absence of Triplets in Single-Photon Double Ionization of Methanol

Abstract

Despite the abundance of data concerning single-photon double ionization of methanol, the spin state of the emitted electron pair has never been determined. Here we present the first evidence that identifies the emitted electron pair spin as overwhelmingly singlet when the dication forms in low-energy configurations. The experimental data show that while the yield of the CH 2 O + + H 3 + Coulomb explosion channel is abundant, the metastable methanol dication is largely absent. According to high-level ab initio simulations, these facts indicate that photoionization promptly forms singlet dication states, where they quickly decompose through various channels, with significant H 3 + yields on the low-lying states. In contrast, if we assume that the initial dication is formed in one of the low-lying triplet states, the ab initio simulations exhibit a metastable dication, contradicting the experimental findings. Comparing the average simulated branching ratios with the experimental data suggests a >3 order of magnitude enhancement of the singlet:triplet ratio compared with their respective 1:3 multiplicities.

An “inverse” harpoon mechanism

“…Few H2+-forming trajectories were found to exhibit long-range inverse harpoon while on an excited state. However, similar to the simulated roaming H 2 dynamics reported by Gope et al ( 50 ), most of the initially excited systems undergo nonadiabatic surface hopping to the ground state before the separation of the neutral H 2 . On the one hand, as can be expected, higher initial excitation exhibits higher KER than trajectories initiated on the ground and first excited state, respectively indicated in Fig.…”

“…3B show the simulated KER distribution for the CHOH++H2+ channel. More than 200 trajectories are initiated on each of the seven lowest singlet states of the methanol dication, in accordance with the high energy of the EUV photons ( 34 , 50 ). The simulation is terminated once the system is significantly extended after the dissociation.…”

Two pathways and an isotope effect in H3+ formation following double ionization of methanol

“…Furthermore, Gope et al. recently confirmed that SPDI predominantly forms the singlet CH 3 OH 2+ states 42 . The fact that theoretical simulations for these systems agree so well with the SPDI experiments allows us to use them to explore questions that cannot be directly resolved by experiments.…”

“…In the interstellar medium, trihydrogen is believed to form primarily by the collisions 3 . Nevertheless, it has been ubiquitously identified as a surprising product of many types of organic molecule ionization processes, including electron impact, 25,26 fast‐ion bombardment, 27,28 multi‐photon double‐ionization (MPDI) by intense laser pulses, 29–37 as well as single photon double‐ionization (SPDI) 38–42 . In particular, formation from the simplest alcohol (methanol) received special attention due to its abundance in a wide range of interstellar environments 43 …”

Strong-field ionization of CH3Cl: proton migration and association

“…In the next step, the neutral H 2 roams on the PES in the vicinity of the precursor and extracts a proton to form H 3 + . [11][12][13][14][15][16][17] H 2 + , on the other hand, is formed by the long-range electron transfer from the neutral H 2 to the parent dication by a process known as ''inverse harpooning''. 11 The timescale of such intramolecular hydrogen migration has been reported to be B100 fs.…”