Communication: Evidence for dipole-bound excited states in gas-phase I− ⋅ MI (M = Na, K, Cs) anionic salt microclusters

Harvey, Andrew J.A.; Yoshikawa, Naruo; Wang, Jinguo; Dessent, Caroline E. H.

doi:10.1063/1.4930919

Cited by 7 publications

(10 citation statements)

References 44 publications

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“…and is also consistent with the fact that dipole-bound excited states can be observed via photodepletion spectroscopy in our instrument [50,51]. Our calculations indicate that the I -…”

Section: Assignment Of the I -•G Excited Statessupporting

confidence: 90%

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

2019

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Laser photodissociation spectroscopy of the I -•guanine complex has been conducted for the first time across the regions above the electron detachment threshold to explore the excited states and whether vertical ionization occurs from the iodide or the nucleobase.The photofragment spectra reveal a prominent dipole-bound excited state (I) close to the calculated vertical electron detachment energy (~4.0 eV) and a second excited (II) centred around 4.8 eV, which we assign to -* nucleobase-localized transitions. The ionic photofragments are identified as Iand I -•[G-H], with the later fragment being produced significantly more strongly than the former. Both photofragments are observed across the two excited states, with production of the iodide being attributed to internal conversion to the ground state followed by evaporation. We trace the formation of the I -•[G-H] photofragment to initial vertical ionization of guanine, followed by ejection of a proton. This two-step process is important as it follows known steps in radiation-induced damage to DNA, namely initial formation of a guanine radical cation which then forms a free radical [G-H] moiety through deprotonation. Production of the I -•[G-H] photofragment is pronounced through II indicating that its formation is enhanced by coupling of the -* transitions to the electron detachment continuum.

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“…and is also consistent with the fact that dipole-bound excited states can be observed via photodepletion spectroscopy in our instrument [50,51]. Our calculations indicate that the I -…”

Section: Assignment Of the I -•G Excited Statessupporting

confidence: 90%

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

2019

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“…Band I in the photodepletion spectrum of I -•A (Figure 4a) being accessed in this region. We have previously shown that dipole-bound excited states can be observed in our instrument following near-threshold anion excitation [27,38,39]. The identity of the shoulder peak at 4.17 eV resembles similar structures present in the nearthreshold photodepletion spectra of anionic alkali halide clusters (e.g.…”

Section: Assignment Of the Excited States Of The X -•A Clusterssupporting

confidence: 66%

“…The identity of the shoulder peak at 4.17 eV resembles similar structures present in the nearthreshold photodepletion spectra of anionic alkali halide clusters (e.g. I -NaI) [38], which were assigned to excitation of the vibrational envelope in the dipole-bound excited state of the cluster.…”

Section: Assignment Of the Excited States Of The X -•A Clusterssupporting

confidence: 62%

Near-threshold electron transfer in anion-nucleobase clusters: does the identity of the anion matter?

2019

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Laser photofragmentation spectroscopy of the I-•adenine (I-•A) and H2PO3-•adenine (H2PO3-•A) clusters has been utilized for the first time across the electron detachment thresholds to explore how the anion identity affects intra-cluster electron transfer. Ionic photofragmentation is weak for both clusters, despite strong photodepletion, revealing that both clusters decay predominantly by electron detachment. The spectra of I-•A display a prominent dipole-bound excited state in the region of the vertical detachment energy, which relaxes to produce deprotonated adenine. In contrast, photoexcitation of H2PO3-•A in the near-threshold region does not access a dipole-bound state, but instead displays photofragmentation properties associated with ultrafast decay of an adenine-localized -* transition. Notably, the experimental electron detachment onset of H2PO3-•A is around 4.7 eV. This value is substantially reduced compared to the VDE expected for detachment of a simple anion-dipole complex. The lower VDE of H2PO3-•A can be traced to initial ionization of the adenine, which is followed by significant rearrangement of a hydrogen atom on the neutral surface. We conclude that these dynamics quench access to a dipole-bound excited state for H2PO3-•A. Despite the fact that the excess negative charge is located on H2PO3in the ground-state cluster, the anion in this cluster does not act as a free electron source to initiate free electron attachment or dissociation in the nucleobase. However, the H2PO3-•A cluster represents an important new example of an anionic cluster where ionization occurs from the initial neutral moiety of the cluster, and where photodetachment initiates intra-molecular hydrogen atom transfer.

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“…so that the 4.0 eV band for the I -•U and I -•T clusters can be confidently assigned to excitation of a dipole-bound excited state. Indeed, we have found clear evidence for the formation of dipole-bound excited states within our laser-interfaced mass spectrometry instrument following near-threshold excitation of anionic salt clusters (I -•MI where M = Na, K, Cs),44 and are therefore confident that these states can be observed using our experimental method. The absorption spectrum of the I -•C cluster in the vicinity of band I strongly resembles the absorption spectra of the I -•U and I -•T clusters, displaying a prominent absorption band in the spectral region where the VDE of the cluster is expected to occur.…”

supporting

confidence: 53%

“…We note that these excited states are resolved in the photofragment action spectra but not in the photodepletion spectra since the photodepletion spectra include spin-orbit state. 44 However, the [KI]photofragment spectrum that accompanies the photodepletion spectrum of this cluster revealed that no ionic photofragments were produced across the region of the upper spin-orbit state of the cluster. We concluded that the lack of photofragments resulted from rapid spin-orbit relaxation of the upper iodine 2 P½ state accompanied by electron detachment occurring on a timescale that is faster than decay to ionic photofragments.…”

Section: B Photodepletion Of the I -•M (M = U T C) Clustersmentioning

confidence: 93%

Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold: Intracluster electron transfer versus nucleobase-centred excitations

Matthews

Cercola

Mensa-Bonsu

et al. 2018

The Journal of Chemical Physics

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Laser photodissociation spectroscopy of the I·thymine (I·T) and I·cytosine (I·C) nucleobase clusters has been conducted for the first time across the regions above the electron detachment thresholds to explore the excited states and photodissociation channels. Although photodepletion is strong, only weak ionic photofragment signals are observed, indicating that the clusters decay predominantly by electron detachment. The photodepletion spectra of the I·T and I·C clusters display a prominent dipole-bound excited state (I) in the vicinity of the vertical detachment energy (∼4.0 eV). Like the previously studied I·uracil (I·U) cluster [W. L. Li et al., J. Chem. Phys. 145, 044319 (2016)], the I·T cluster also displays a second excited state (II) centred at 4.8 eV, which we similarly assign to a π-π* nucleobase-localized transition. However, no distinct higher-energy absorption bands are evident in the spectra of the I·C. Time-dependent density functional theory (TDDFT) calculations are presented, showing that while each of the I·T and I·U clusters displays a single dominant π-π* nucleobase-localized transition, the corresponding π-π* nucleobase transitions for I·C are split across three separate weaker electronic excitations. I and deprotonated nucleobase anion photofragments are observed upon photoexcitation of both I·U and I·T, with the action spectra showing bands (at 4.0 and 4.8 eV) for both the I and deprotonated nucleobase anion production. The photofragmentation behaviour of the I·C cluster is distinctive as its I photofragment displays a relatively flat profile above the expected vertical detachment energy. We discuss the observed photofragmentation profiles of the I·pyrimidine clusters, in the context of the previous time-resolved measurements, and conclude that the observed photoexcitations are primarily consistent with intracluster electron transfer dominating in the near-threshold region, while nucleobase-centred excitations dominate close to 4.8 eV. TDDFT calculations suggest that charge-transfer transitions [Iodide n (5p) → Uracil σ*] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed.

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Communication: Evidence for dipole-bound excited states in gas-phase I− ⋅ MI (M = Na, K, Cs) anionic salt microclusters

Cited by 7 publications

References 44 publications

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

Near-threshold electron transfer in anion-nucleobase clusters: does the identity of the anion matter?

Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold: Intracluster electron transfer versus nucleobase-centred excitations

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