Photodetachment and Tunneling Dissociation of Cryogenic Double-Rydberg Anions NH4.

The Rydberg radical NH4 and the double Rydberg anion (DRA) NH4- have long aroused researchers' interests due to their potential for exploring the reaction dynamics of the H + NH3 → H2 + NH2 reaction, a prototypical penta-atomic system. In this study, we present high-resolution photodetachment spectroscopy of DRA NH4- and ion-molecule complex H-(NH3). We observed multiple new photodetachment channels of DRA NH4-. The energy level of the excited state (3p 2T2) of the Rydberg radical NH4 was determined to be 15052(94) cm-1, in excellent agreement with the principal Schüler band (15061.61 cm-1). Additionally, we observed the tunneling dissociation of NH4- in a cryogenic ion trap with its dissociation lifetime determined to be 19(2) ms.

Probing the activated complex of the F + NH3 reaction via a dipole-bound state.

Experimental characterization of the transition state poses a significant challenge due to its fleeting nature. Negative ion photodetachment offers a unique tool for probing transition states and their vicinity. However, this approach is usually limited to Franck-Condon regions. For example, high-lying Feshbach resonances with an excited HF stretching mode (vHF = 2-4) were recently identified in the transition-state region of the F + NH3 → HF + NH2 reaction through photo-detaching FNH3- anions, but the direct photodetachment failed to observe the lower-lying vHF = 0,1 resonances and bound states due apparently to negligible Franck-Condon factors. Indeed, these weak transitions can be resonantly enhanced via a dipole-bound state (DBS) formed between an electron and the polar FNH3 species. In this study, we unveil a series of Feshbach resonances and bound states along the F + NH3 reaction path via a DBS by combining high-resolution photoelectron spectroscopy with high-level quantum dynamical computations. This study presents an approach for probing the activated complex in a reaction by negative ion photodetachment through a DBS.

Spectroscopic observation of Feshbach resonances in the tellurium dimer anion.

We report on the high-resolution photodetachment spectroscopy of the cryogenically cooled anionic tellurium dimer (Te2-). The high-resolution resonant photoelectron spectrum yields an accurate electron affinity of 16 689.7(92) cm-1 or 2.0693(11) eV for Te2. Two resonant states of Te2- anions have been identified, positioned at 1092(17) cm-1 below and 250(11) cm-1 above the photodetachment threshold, respectively. The spectra of resonant two-photon detachment (R2PD) and autodetachment from a specific vibrational level through a Feshbach resonance exhibit notable non-Franck-Condon behaviors. Using the spectroscopic data from the current experiment, the equilibrium bond distances and spectroscopic constants of the ground state and two electronically excited states of Te2- were determined.

The unusual quadruple bonding of nitrogen in ThN.

Nitrogen has five valence electrons and can form a maximum of three shared electron-pair bonds to complete its octet, which suggests that its maximum bond order is three. With a joint anion photoelectron spectroscopy and quantum chemistry investigation, we report herein that nitrogen presents a quadruple bonding interaction with thorium in ThN. The quadruple Th≣N bond consists of two electron-sharing Th-N π bonds formed between the Th-6dxz/6dyz and N 2px/2py orbitals, one dative Th←N σ bond and one weak Th←N σ bonding interaction formed between Th-6dz2 and N 2s/2pz orbitals. The ThC molecule has also been investigated and proven to have a similar bonding pattern as ThN. Nonetheless, due to one singly occupied σ-bond, ThC is assigned a bond order of 3.5. Moreover, ThC has a longer bond length as well as a lower vibrational frequency in comparison with ThN.

Electron affinity of atomic scandium and yttrium and excited states of their negative ions.

The latest experimental electron affinity (EA) values of atomic scandium and yttrium were 0.189(20) and 0.308(12) eV as reported by Feigerle et al. in 1981. The measurement accuracy of these was far lower than that of other transition elements, and no conclusive result had been made on the excited states of their negative ions. In the current work, we report more accurate EA values of Sc and Y and the electronic structure of their negative ions using the slow-electron velocity-map imaging method. The EA values of Sc and Y are determined to be 0.179 378(22) and 0.311 29(22) eV, respectively. The ground state of Sc- is identified as 3d4s24p 1D2, and the ground state is 4d5s25p 1D2 for Y-. Furthermore, several excited states of Sc- and Y- are observed: Sc- (3D1) and Y- (3D1, 3D2, 3D3, 3F2, and 3F3), and their energy levels are determined to be 1131.8(28), 1210.0(13), 1362.3(30), 1467.7(26), 1747(16), and 1987(33) cm-1, respectively.

Probing Isomerization Dynamics via a Dipole-Bound State.

The observation of molecular isomerization dynamics is a long-standing goal in physical chemistry. The loosely bound electron in a dipole-bound state (DBS) can be a messenger for probing the isomerization of the neutral core. Here we study the isomerization dynamics of the salt dimer (NaCl)2 from linear to rhombic via a DBS using cryogenic photoelectron spectroscopy in combination with ab initio calculations. Although the energy level of the DBS is below the electron affinity of the linear (NaCl)2, (NaCl)2- in its DBS can autodetach due to the linear-to-rhombic isomerization. (NaCl)2- in the ground DBS has a relatively long lifetime of a few nanoseconds due to the quantum tunneling through a potential barrier during the transformation from linear to rhombic. In contrast, the vibrationally excited DBS has a much shorter lifetime on the order of picoseconds. The energy distribution of autodetachment electrons has an unexpected characteristic of the thermionic emission.

Electron affinity of tantalum and excited states of its anion.

The tantalum anion has the most complicated photoelectron spectrum among all atomic anions of transition elements, which was the main obstacle to accurately measure its electron affinity via the generic method. The latest experimental value of the electron affinity of Ta was 0.323(12) eV, reported by Feigerle et al. [J. Chem. Phys. 74, 1580 (1981)]. In the present work, we report the high-resolution photoelectron spectroscopy of Ta- via the slow-electron velocity-map imaging method combined with a cryogenic ion trap. The electron affinity of Ta was measured to be 2652.38(17) cm-1 or 0.328 859(23) eV. Three excited states 5D1, 3P0, and 5D2 of Ta- were observed, and their energy levels were determined to be 1169.64(17) cm-1 for 5D1, 1735.9(10) cm-1 for 3P0, and 2320.1(20) cm-1 for 5D2 above the ground state 5D0, respectively.

Structural Versatility and Energy Difference of Salt-Water Complex NaCl(H2O) Encoded in Cryogenic Photoelectron Spectroscopy.

A weakly bound complex usually has multiple structural isomers with small energy differences. The sophisticated ab initio calculations are the main workhorse for providing theoretical results of different isomers. In contrast, the experimental determination of the energy difference is very rare. We report the energy-difference measurement of a model complex: salt-water complex NaCl(H2O). We measured the energy difference among the structural isomers of the negatively charged NaCl(H2O) complex and the neutral counterpart using cryogenic photoelectron spectroscopy. The temperature-dependent photoelectron spectra (15-300 K) revealed that the negatively charged NaCl(H2O) and the neutral counterpart both have three isomers. The two higher-lying isomers are 186(22) and 481(48) cm-1, respectively, above the most stable isomer for the negatively charged and 123(10) and 1821(24) cm-1 for the neutral. These results provide a benchmark for the development of theoretic methods of weakly bound complexes. The experimental technique demonstrated here can be employed to investigate other weakly bound complexes with multiple isomers.

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.

Dipole-bound and valence excited states of AuF anions via resonant photoelectron spectroscopy.

Gold fluoride is a very unique species. In this work, we reported the resonant photodetachment spectra of cryogenically cooled AuF- via the slow-electron velocity-map imaging method. We determined the electron affinity of AuF to be 17 976(8) cm-1 or 2.2287(10) eV. We observed a dipole-bound state with a binding energy of 24(8) cm-1, a valence excited state with a binding energy of 1222(11) cm-1, and a resonant state with an energy of 814(12) cm-1 above the photodetachment threshold. An unusual vibrational transition with Δn = -3 was observed in the autodetachment from the dipole-bound state. Moreover, two excited states of neutral AuF were recognized for the first time, located at 13 720(78) cm-1 and 16 188(44) cm-1 above the AuF ground state.

High-Resolution Photoelectron Imaging and Photodetachment Spectroscopy of Cryogenically Cooled IO.

We report a high-resolution photoelectron imaging and photodetachment spectroscopy study of cryogenically cooled IO-. The high-resolution photoelectron spectra yield a more accurate electron affinity (EA) of 2.3805(5) eV for IO as well as a more accurate spin-orbit splitting energy between the 2Π3/2 and 2Π1/2 states of IO as 2093(5) cm-1. Photodetachment spectroscopy confirmed several excited states for the IO- anion predicted by theoretical calculations, including two valence-type excited states, the repulsive 3Π state, and a shallow bound 1Π state. More interestingly, we have observed two vibrational resonances which are proposed to be due to a dipole-induced resonant state, about 230 cm-1 above the detachment threshold of IO-.

Accurate electron affinity of Ga and fine structures of its anions.

We report the high-resolution photoelectron spectra of negative gallium anions obtained via the slow-electron velocity-map imaging method. The electron affinity of Ga is determined to be 2429.07(12) cm-1 or 0.301 166(14) eV. The fine structures of Ga are well resolved: 187.31(22) cm-1 or 23.223(27) meV for 3P1 and 502.70(28) cm-1 or 62.327(35) meV for 3P2 above the ground state 3P0, respectively. The photoelectron angular distribution for photodetachment from Ga-(4s24p2 3P0) to Ga(4s25s 2S1/2) is measured. An unexpected perpendicular distribution instead of an isotropic distribution is observed, which is due to a resonance near 3.3780 eV.

Measurement of electron affinity of iridium atom and photoelectron angular distributions of iridium anion.

The latest electron affinity value of an iridium atom is 1.564 36(15) eV, determined via a method based on the Wigner threshold law by Bilodeau and co-workers. However, they observed a significant deviation from the Wigner threshold law in the threshold photodetachment experiment. To address this dilemma, we conducted high-resolution photoelectron spectroscopy of Ir- via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of Ir was measured to be 12 614.97(9) cm-1 or 1.564 057(11) eV. We find that the Wigner threshold law is still valid for the threshold photodetachment of Ir- through a p-wave fitting of the photodetachment channel Ir-5d86s23F4→Ir5d86sb4F9/2. The photoelectron angular distributions of photodetachment channels Ir-5d86s23F4→Ir5d76s2a4F9/2 and Ir-5d86s23F4→Ir5d86sb4F9/2 were also investigated. The behavior of anisotropy parameter ß indicates a strong interaction between the two channels. Moreover, the energy level 3P2 of Ir-, which was not observed in the previous works, was experimentally determined to be 4163.24(16) cm-1 above the ground state.

Candidate for Laser Cooling of a Negative Ion: High-Resolution Photoelectron Imaging of Th

Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La^{-} as the most promising candidate. We report on experimental and theoretical studies supporting Th^{-} as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35(48) cm^{-1} and 4832 cm^{-1}, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of Th^{-} is determined to be 6d^{3}7s^{2} ^{4}F_{3/2}^{e} rather than 6d^{2}7s^{2}7p ^{4}G_{5/2}^{o}. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations 6d^{3}7s^{2} and 6d^{2}7s^{2}7p in Th^{-}. The potential laser-cooling transition is ^{2}S_{1/2}^{o}↔^{4}F_{3/2}^{e} with a wavelength of 2.6 µm. The zero nuclear spin and hence lack of hyperfine structure in Th^{-} reduces the potential complications in laser cooling as encountered in La^{-}, making Th^{-} a new and exciting candidate for laser cooling.

Double- and multi-slit interference in photodetachment from nanometer organic molecular anions.

We present the predictions of double-slit and multislit interference of photoelectrons from a nanometer-size molecular negative ion. The interference clearly appears in both photoelectron angular distributions and photodetachment cross sections. In contrast to the diatomic photoelectron interference via the X-ray photon, the interference in the nanometer-size negative ions can be readily observed via a visible or extreme ultraviolet laser. Therefore, the phenomenon can be realized on a table-top setup, instead of a large accelerator.

Triple differential cross sections for electron-impact ionization of methane at intermediate energy.

We report an experimental and theoretical investigation of electron-impact single ionization of the highest occupied molecular orbital 1t2 and the next highest occupied molecular orbital 2a1 states of CH4 at an incident electron energy of 250 eV. Triple differential cross sections measured in two different laboratories were compared with results calculated within the molecular 3-body distorted wave and generalized Sturmian function theoretical models. For ionization of the 1t2 state, the binary peak was observed to have a single maximum near the momentum transfer direction that evolved into a double peak for increasing projectile scattering angles, as has been seen for ionization of atomic p-states. A detailed investigation of this evolution was performed. As expected because of its s-type character, for ionization of the 2a1 state, only a single binary peak was observed. Overall, good agreement was found between experiment and theory.

Ground-State Pd Anions React with H2 Much Faster than the Excited Pd Anions.

Recent advances in experimental techniques have made it relatively easy to prepare reactant cations in well-defined states of electronic excitation. Extensive studies on the role of excited states in the cation-neutral reactions have contributed significantly to our understanding of reaction kinetics and dynamics. The excited states are often more reactive than the ground state. However, the reactions involving the excited atomic anion are very rare because the negative ions usually have no bound excited states. In the present work, we report the state-specific reaction of Pd anions with H2. Surprisingly, we observed that the ground-state Pd anions react with H2 10 times faster than the excited Pd anions. The high-level calculations show that the difference is due to the reaction barrier.

Accurate electron affinity of Ti and fine structures of its anions.

The high-resolution photoelectron energy spectra of atomic titanium and its hydride anions were obtained on a slow-electron velocity-map imaging spectrometer equipped with a cold ion trap. The cold ion trap employed in the present measurement was found to be very helpful for reducing the interference from the titanium hydride anions. The electron affinity of Ti was determined to be 609.29(34) cm-1 or 75.54(4) meV. The accuracy was improved by a factor of 350 compared with the previous result. The fine structures of Ti- were clearly resolved: 70.0(12)(4F5/2), 165.2(15)(4F7/2), and 285.2(15) cm-1 (4F9/2) above its ground state 4F3/2. Moreover, the measured electron affinity and vibrational frequency of TiH can be reproduced well using the high level calculations.