The autodetachment dynamics of vibrational Feshbach resonances of the quadrupole-bound state (QBS) for the first time has been investigated in real time for the first excited state of the 4-cyanophenoxide (4-CP) anion. Individual vibrational resonances of the cryogenically cooled 4-CP QBS have been unambiguously identified, and their autodetachment rates state-specifically measured using the picosecond time-resolved pump–probe technique employing the photoelectron velocity-map imaging method. The autodetachment lifetime (τ) is found to be strongly dependent on mode, giving τ values of ∼56, ∼27, and ≤2.8 ps for the 12′1 (E vib = 406 cm–1), 12′2 (E vib = 806 cm–1), and 21′1 (E vib = 220 cm–1) modes, respectively. The striking mode-specific behavior of the QBS lifetime has been invoked by the physical model in which the loosely bound electron falls off by the dynamic wobbling of the three-dimensional quadrupole moment ellipsoid associated with the corresponding vibrational motion in the autodetachment process.

Real-time autodetachment dynamics of the loosely-bound excess electron from the vibrational Feshbach resonances of the dipole-bound states (DBS) of 4-bromophonoxide (4-BrPhO-) and 4-chlorophenoxide (4-ClPhO-) anions have been thoroughly investigated. The...

Anion chemical dynamics of autodetachment and fragmentation mediated by the dipole-bound state (DBS) have been thoroughly investigated in a state-specific way for the first time by employing the picosecond time-resolved or the nanosecond frequency-resolved spectroscopy combined with the cryogenically cooled ion trap and velocity-map imaging techniques. For the ortho-, meta-, or para-iodophenoxide anion (o-, m-, or p-IPhO -), the C-I bond rupture giving the anionic iodide (I -) fragment occurs via the nonadiabatic transition from the DBS to the nearby valence-bound states (VBS) of the anion where the vibronic coupling into the S1 (πσ*) state (which is repulsive along the C-I bond extension coordinate) should be largely responsible. The dynamic details are governed by the isomer-specific nature of the potential energy surfaces in the vicinity of the DBS-VBS curve crossings, as manifested in the huge different chemical reactivity of o-, m-, or p-IPhO -. It is confirmed here that the C-I bond dissociation is mediated by DBS resonances, providing the foremost evidence that the metastable DBS plays the essential role as the doorway into the anion chemistry especially of the dissociative electron attachment (DEA). The fragmentation channel is dominant when it is mediated by the DBS resonances located below the electron-affinity (EA) threshold, whereas it is kinetically adjusted by the competitive autodetachment process when the DBS resonances lying above EA convey the electron to the valence orbitals. The product yield of the C-I bond cleavage is strongly mode-dependent as the rate of the concomitant autodetachment is much influenced by the characteristics of the individual vibrational modes, paving a new way of the reaction control of the anion chemistry.

Conspectus Nonvalence bound states (NBS) are anionic states where the excess electron is extremely loosely bound to the neutral core through long-range potentials. In contrast to the valence orbitals of which the electron occupancy determines the molecular structure, as well as the chemical reactivity, the nonvalence orbital is quite diffuse and located far from the neutral core. The NBS can be classified into the dipole-bound state (DBS), quadruple-bound state (QBS), or correlation-bound state (CBS) according to the nature of the electron-neutral interaction, although their interaction potentials may cooperatively contribute. The NBS is ubiquitous in nature and has the strong implications in atmospheric, interstellar, or biological chemistry. Accordingly, NBS has long been conceived to play the role of the doorway into the formation of a stable anion or dissociative electron attachment (DEA). Despite intensive and extensive studies, however, the quantum-mechanical nature of NBS is still far from being thorough understanding. Herein, we describe a new aspect of state-specific NBS-mediated chemical dynamics, which has been revealed through a series of recent studies by our group. We have employed picosecond time–resolved pump–probe spectroscopy combined with cryogenically cooled ion trap and velocity-map imaging techniques to study closed-shell anions generated by electrospray ionization. DBS vibrational Feshbach resonances are prepared by the optical excitation of phenoxide, for instance, and their individual lifetimes have been precisely measured in a state-specific manner to reveal the strong mode-dependency of the autodetachment rate. Fermi’s golden rule turns out to be extremely useful for a rational explanation of the experiment, although the more sophisticated theoretical model is desirable for the more quantitative analysis. For the DBS of para-chlorophenoxide or para-bromophenoxide where the polarizability of neutral core is substantial, the Fermi’s golden rule based on the charge-dipole potential needs to be significantly modified to include the correlation effects to explain the exceptionally slow autodetachment rates. For the QBS of 4-cyanophenoxide, the mode-specific behavior of the quadrupole ellipsoid tensor explains the strong mode-dependent autodetachment rate. Meanwhile, the nonadiabatic transition of the excess electron into the valence orbital can result in stable anion formation or immediate chemical bond rupture. In the DBS of ortho-, meta-, or para-iodophenoxide, the transformation of the loosely bound excess electron into the πσ* antibonding orbital occurs to give I– as a final fragment. The fragmentation mediated by DBS occurs competitively with the concomitant autodetachment, paving a new way of the reaction control by tuning the quantum-mechanical nature of the DBS Feshbach resonance. This experimental observation provides the foremost evidence for the dynamic role of the DBS as a doorway into anion chemistry, such as DEA. The ponderomotive force on the electron in the nonvalence orbital...

We investigated vibrational Feshbach resonances of multipole-bound states in the time-domain for the first time. State-specific autodetachment rate measurement in phenoxide (PhO-) dipole-bound state (DBS) [1,2] and 4-cyanophenoxide (4-CP-) quadrupole-bound state (QBS) [3] was performed in the cryogenically-cooled ion trap combined with the time-resolved photoelectron velocity map imaging technique. For the phenoxide DBS, the most prominent 11 ′1 (+519 cm −1) peak showed τ ~ 33.5 ps. The lifetime of the each vibrational peak is highly mode-dependent to give ~5 ps for the 18 ′1 (+632 cm −1) , and the multi-quanta excitation of the ν 11 mode gives ~ 11.9 ps for the 11 ′2 (+1036 cm −1) and ~9.2 ps for the 11 ′3 (+1556 cm −1). These mode-specific autodetachment rates are well-described by the Fermi's golden rule. Interestingly, the 11 ′1 18 ′1 combination mode showed bifurcation dynamics to the 11 0 18 1 and 11 1 18 0 neutral product with much faster autodetachment rate of τ ~ 1.4 ps. This result implies that the each bifurcated channel showed accelerated autodetachment dynamics compared to the detachment from the single-mode respectively. For the 4-CP-QBS, the lifetime of the most prominent 12 ′1 peak gives τ ~ 56.4 ps lifetime, which is ~1.68-fold slower than the similar 11 ′1 mode of phenoxide DBS. This may originate from the weaker interaction of the charge-quadrupole potential in long-range region compared to that of the charge-dipole interaction. The mode-dependency is also confirmed by measuring the lifetime on the other vibrational modes of QBS, which could be described by the wobbling of the quadrupole moment elliposoid and Fermi's golden rule. This research provides unprecedental insights into the interaction between the electronic and nuclear dynamics of the non-valence bound states.