Excited states of polar negative ions

Investigating Possible Dipole-Bound States of Cyanopolyynes: the Case for the C5 N- Anion Detected in Interstellar Space

We present results of quantum structure calculations aimed at demonstrating the possible existence of dipole-bound states (DBS) for the anionC 5 N - ${{\rm{C}}_5 {\rm{N}}^ - }$ , a species already detected in the Interstellar medium (ISM). The positive demonstration of DBS existence using ab initio studies is an important step toward elucidating possible pathways for the formation of the more tightly bound valence bound states (VBS) in environments where free electrons from starlight ionization processes are known to be available to interact with the radical partner of the title molecule. Our current calculations show that such excited DBS states can exist inC 5 N - ${{\rm{C}}_5 {\rm{N}}^ - }$ , in agreement with what we had previously found for the smallercyanopolyyne in the series: theC 3 N - ${{\rm{C}}_3 {\rm{N}}^ - }$ anion. This system has a very weakly bound anion with binding energies of about 3 and 9 cm-1 for the1 Σ + ${^1 \Sigma ^ + }$ and3 Σ + ${^3 \Sigma ^ + }$ DBS, respectively.

Observation of an Excited Dipole-Bound State in a Diatomic Anion

We report the observation of σ-type and π-type excited dipole-bound states (DBSs) in cryogenically cooled potassium iodide (KI) anions for the first time. Two DBSs were observed 39.7(10) meV and 5.0(12) meV below the photodetachment threshold via the resonant two-photon detachment. The different photoelectron angular distributions and binding energies suggest that the two DBSs are of different types. The existence of one σ-type and one π-type DBS in the KI anion was also supported by the high-level ab initio theoretical calculations. The excellent agreement between experimental and theoretical results confirms the prediction that a dipolar molecule with a large enough dipole moment can have an excited DBS.

Investigating the mapping of chromophore excitations onto the electron detachment spectrum: photodissociation spectroscopy of iodide ion-thiouracil clusters

Laser photodissociation spectroscopy (3.1-5.7 eV) has been applied to iodide complexes of the non-native nucleobases, 2-thiouracil (2-TU), 4-thiouracil (4-TU) and 2,4-thiouracil (2,4-TU), to probe the excited states and intracluster electron transfer as a function of sulphur atom substitution. Photodepletion is strong for all clusters (I-·2-TU, I-·4-TU and I-·2,4-TU) and is dominated by electron detachment processes. For I-·4-TU and I-·2,4-TU, photodecay is accompanied by formation of the respective molecular anions, 4-TU- and 2,4-TU-, behaviour that is not found for other nucleobases. Notably, the I-·2TU complex does not fragment with formation of its molecular anion. We attribute the novel formation of 4-TU- and 2,4-TU- to the fact that these valence anions are significantly more stable than 2-TU-. We observe further similar behaviour for I-·4-TU and I-·2,4-TU relating to the general profile of their photodepletion spectra, since both strongly resemble the intrinsic absorption spectra of the respective uncomplexed thiouracil molecule. This indicates that the nucleobase chromophore excitations are determining the clusters' spectral profile. In contrast, the I-·2-TU photodepletion spectrum is dominated by the electron detachment profile, with the near-threshold dipole-bound excited state being the only distinct spectral feature. We discuss these observations in the context of differences in the dipole moments of the thionucleobases, and their impact on the coupling of nucleobase-centred transitions onto the electron detachment spectrum.

Threshold photodetachment spectroscopy of the astrochemical anion CN. J Chem Phys 2020;153:184309. [PMID: 33187436 DOI: 10.1063/5.0029841]

Threshold photodetachment spectroscopy has been performed on the molecular anion CN^- at both 16(1) K and 295(2) K in a 22-pole ion trap and at 295(2) K from a pulsed ion beam. The spectra show a typical energy dependence of the detachment cross section yielding a determination of the electron affinity of CN to greater precision than has previously been known at 31 163(16) cm^-1 [3.864(2) eV]. Allowed s-wave detachment is observed for CN^-, but the dependence of the photodetachment cross section near the threshold is perturbed by the long-range interaction between the permanent dipole moment of CN and the outgoing electron. Furthermore, we observe a temperature dependence of the cross section near the threshold, which we attribute to a reduction of the effective permanent dipole due to higher rotational excitation at higher temperatures.

Observation of a π-Type Dipole-Bound State in Molecular Anions

We report the observation of a π-type dipole-bound state (π-DBS) in cryogenically cooled deprotonated 9-anthrol molecular anions (9AT^{-}) by resonant two-photon photoelectron imaging. A DBS is observed 191  cm^{-1} (0.0237 eV) below the detachment threshold, and the existence of the π-DBS is revealed by a distinct (s+d)-wave photoelectron angular distribution. The π-DBS is stabilized by the large anisotropic in-plane polarizability of 9AT. The population of the dipole-forbidden π-DBS is proposed to be via a nonadiabatic coupling with the dipole-allowed σ-type DBS mediated by molecular rotations.

High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions

Valence-bound anions with polar neutral cores (μ > ∼2.5 D) can support dipole-bound excited states below the detachment threshold. These dipole-bound states (DBSs) are highly diffuse and the weakly bound electron in the DBS can be readily autodetached via vibronic coupling. Excited DBSs can be observed in photodetachment spectroscopy using a tunable laser. Tuning the detachment laser to above-threshold vibrational resonances yields vibrationally enhanced resonant photoelectron spectra, which are highly non-Franck-Condon with much richer vibrational information. This perspective describes recent advances in the studies of excited DBSs of cryogenically cooled anions using high-resolution photoelectron imaging (PEI) and resonant photoelectron spectroscopy (rPES). The basic features of dipole-bound excited states and highly non-Franck-Condon resonant photoelectron spectra will be discussed. The power of rPES to yield rich vibrational information beyond conventional PES will be highlighted, especially for low-frequency and Franck-Condon-inactive vibrational modes, which are otherwise not accessible from non-resonant conventional PES. Mode-selectivity and intra-molecular rescattering have been observed during the vibrationally induced autodetachment. Conformer-specific rPES is possible due to the different dipole-bound excited states of molecular conformers with polar neutral cores. For molecules with μ ≪ 2.5 D or without dipole moments, but large quadrupole moments, excited quadrupole-bound states can exist, which can also be used to conduct rPES.

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

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 (5p⁶) → Uracil σ*] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed.

Bound and continuum-embedded states of cyanopolyyne anions

Equation-of-motion coupled-cluster calculations reveal systematic trends across bound and continuum-embedded excited states in cyanopolyyne anions.

Critical binding and electron scattering by symmetric-top polar molecules

Quantum treatments of electron interactions with polar symmetric-top rotor molecules show features not present in the treatment of the linear-polar-rotor model. For symmetric tops possessing non-zero angular momentum about the symmetry axis, a new critical dipole can be defined that guarantees an infinite set of dipole-bound states independent of the values of the components of the inertial tensor. Additionally, for this same class, the scattering cross section diverges for all nonzero values of dipole moments and inertial moments, similar to solutions for the fixed linear dipole. Additional predictions are presented for electron affinities and rotational resonances of these systems.

Critical conditions for stable dipole-bound dianions

We present finite size scaling calculations of the critical parameters for binding two electrons to a finite linear dipole field. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. A complete ground state stability diagram for the dipole-bound dianion is obtained using accurate variational and finite size scaling calculations. We also study the near threshold behavior of the ground state energy by calculating its critical exponent.

Resonant electron capture by some amino acids esters

Resonant electron capture by Gly, Ala and Phe esters have shown that the most efficient negative ion (NI) fragmentations are associated with the C-termini. A new mechanism for the negative ion-forming processes at energies lower than those associated with the pi*(OO) shape resonance involves coupling between dipole-bound and valence negative ion states of the same symmetry for amino acid conformers with high permanent dipoles. The interaction avoids crossing of the NI states and instead leads to formation of two adiabatic potential energy surfaces. Underivatized amino acids most effectively fragment from the bottom adiabatic surface via generation of [M-H](-) carboxylate anions by hydrogen-atom tunneling through the barrier; fragmentation of the their esters with formation of analogues [M-X](-) NIs occurs through the upper adiabatic state without penetration of the barrier in which the energy of the valence sigma*OX resonance exceeds the bond dissociation energy of the neutral molecule. Low and high temperature resonant electron capture experiments point to the importance of conformational preferences of the amino acids for optimum dissociation of the parent NIs in the gas phase.

Dissociative photodetachment dynamics of the iodide-aniline cluster

The photodetachment dynamics of the iodide-aniline cluster, I-(C6H5NH2), were investigated using photoelectron-photofragment coincidence spectroscopy at several photon energies between 3.60 and 4.82 eV in concert with density functional theory calculations. Direct photodetachment from the solvated I- chromophore and a wavelength-independent autodetachment process were observed. Autodetachment is attributed to a charge-transfer-to-solvent reaction in which incipient continuum electrons photodetached from I- are temporarily captured by the nascent neutral iodine-aniline cluster configured in the anion geometry. Subsequent dissociation of the neutral cluster removes the stabilization, leading to autodetachment of the excess electron. The dependence of the dissociative photodetachment (DPD) and autodetachment dynamics on the final spin-orbit electronic state of the iodine fragment is characterized. The dissociation dynamics of the neutral fragments correlated with autodetached electrons were found to be identical to the DPD dynamics of the I atom product spin-orbit state closest to threshold at a given photon energy, lending support to the proposed sequential mechanism.

Finite-size scaling for critical conditions for stable quadrupole-bound anions

We present finite-size scaling calculations of the critical parameters for binding an electron to a finite linear quadrupole field. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. The model Hamiltonian consists of a charge Q located at the origin of the coordinates and k charges -Q/k located at distances R(i), i=1, em leader,k. After proper scaling of distances and energies, the rescaled Hamiltonian depends only on one free parameter q=QR. Two different linear charge configurations with q>0 and q<0 are studied using basis sets in both spherical and prolate spheroidal coordinates. For the case with q>0, the finite size scaling calculations give an extrapolated critical value of q(c)=1.469 70+/-0.000 05 a.u. by using a basis set with prolate spheroidal coordinates. For the quadrupole case with q<0, we obtained an extrapolated critical value of mid R:q(c)mid R:=3.982 51+/-0.000 01 a.u. for stable quadrupole bound anions. The corresponding critical exponent for the ground state energy alpha=1.9964+/-0.0005, with E approximately (q-q(c))(alpha).

Dipole-bound anions of highly polar molecules: Ethylene carbonate and vinylene carbonate

Results of experimental and theoretical studies of dipole-bound negative ions of the highly polar molecules ethylene carbonate (EC, C3H4O3, mu=5.35 D) and vinylene carbonate (VC, C3H2O3, mu=4.55 D) are presented. These negative ions are prepared in Rydberg electron transfer (RET) reactions in which rubidium (Rb) atoms, excited to ns or nd Rydberg states, collide with EC or VC molecules to produce EC- or VC- ions. In both cases ions are produced only when the Rb atoms are excited to states described by a relatively narrow range of effective principal quantum numbers, n*; the greatest yields of EC- and VC- are obtained for n*(max)=9.0+/-0.5 and 11.6+/-0.5, respectively. Charge transfer from low-lying Rydberg states of Rb is characteristic of a large excess electron binding energy (Eb) of the neutral parent; employing the previously derived empirical relationship Eb=23/n*(max)(2.8) eV, the electron binding energies are estimated to be 49+/-8 meV for EC and 24+/-3 meV for VC. Electron photodetachment studies of EC- show that the excess electron is bound by 49+/-5 meV, in excellent agreement with the RET results, lending credibility to the empirical relationship between Eb and n*(max). Vertical electron affinities for EC and VC are computed employing aug-cc-pVDZ atom-centered basis sets supplemented with a (5s5p) set of diffuse Gaussian primitives to support the dipole-bound electron; at the CCSD(T) level of theory the computed electron affinities are 40.9 and 20.1 meV for EC and VC, respectively.

Theory of dipole-bound anions

The problem of the binding of an excess electron to polar molecules and their clusters has long fascinated researchers. Although excess electrons bound to such species tend to be very extended spatially and to have little spatial overlap with the valence electrons of the neutral molecules, inclusion of electron correlation effects is essential for quantitatively describing the electron binding. The major electron correlation contribution may be viewed as a dispersion interaction between the excess electron and the electrons of the molecule or cluster. Recent work using a one-electron Drude model to describe excess electrons interacting with polar molecules is reviewed.

Electronic excitation and thermal effects in alkali-halide cluster anions

We have observed electronically excited states in alkali-halide cluster anions with one excess electron. Using photoelectron spectroscopy, we have found two narrow states in (KI)-2, (NaI)-2, and (NaCl)-2, consistent with a dipole-bound electron, while larger cluster anions exhibit a single broad excited state. In the larger systems, electronic excitation is often accompanied by vibrational excitation and thus a change in cluster temperature. Such temperature changes affect cluster structure and in some cases lead to rapid thermal isomerization.