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.

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.

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.

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-.

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.

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.

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.

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.

Precision measurement of electron affinity of Zr and fine structures of its negative ions.

The high-resolution photoelectron spectra of Zr- were obtained via the slow-electron velocity-map imaging method. The electron affinity of Zr was measured to be 3494.67(72) cm-1 or 0.433 283(89) eV. The accuracy has been improved by a factor of 160 compared with the previous result. The fine structures of Zr- were also well resolved: 251.0(37) (4F5/2), 579.6(8) (4F7/2), and 971.7(12) cm-1 (4F9/2) above the ground state 4F3/2.

Perovskite CH3NH3PbI3(Cl) Single Crystals: Rapid Solution Growth, Unparalleled Crystalline Quality, and Low Trap Density toward 10(8) cm(-3).

Single crystal reflects the intrinsic physical properties of a material, and single crystals with high-crystalline quality are highly desired for the acquisition of high-performance devices. We found that large single crystals of perovskite CH3NH3PbI3(Cl) could be grown rapidly from chlorine-containing solutions. Within 5 days, CH3NH3PbI3(Cl) single crystal as large as 20 mm × 18 mm × 6 mm was harvested. As a most important index to evaluate the crystalline quality, the full width at half-maximum (fwhm) in the high-resolution X-ray rocking curve (HR-XRC) of as-grown CH3NH3PbI3(Cl) single crystal was measured as 20 arcsec, which is far superior to so far reported CH3NH3PbI3 single crystals (∼1338 arcsec). The unparalleled crystalline quality delivered a low trap-state density of down to 7.6 × 10(8) cm(-3), high carrier mobility of 167 ± 35 cm(2) V(-1) s(-1), and long transient photovoltaic carrier lifetime of 449 ± 76 µs. The improvement in the crystalline quality, together with the rapid growth rate and excellent carrier transport property, provides state-of-the-art single crystalline hybrid perovskite materials for high-performance optoelectronic devices.

Accurate electron affinity of Pb and isotope shifts of binding energies of Pb(.).

Lead (Pb) was the last element of the group IVA whose electron affinity had a low accuracy around 10 meV before the present work. This was due to the generic threshold photodetachment measurement that cannot extent well below 0.5 eV due to the light source limitation. In the present work, the electron affinity of Pb was determined to be 2877.33(13) cm(-1) or 0.356 743(16) eV for the isotope m = 208. The accuracy was improved by a factor of 500 with respect to the previous laser photodetachment electron spectroscopy. Moreover, remarkable isotope shifts of the binding energy of Pb(-) 6p(3) (4)S3/2 - Pb 6p(2) (3)P2 were observed for m = 206, 207, and 208.

Accurate electron affinity of V and fine-structure splittings of V- via slow-electron velocity-map imaging.

We report the high-resolution photoelectron spectra of negative vanadium ions obtained via the slow-electron velocity-map imaging method. The electron affinity of V was determined to be 4255.9(18) cm-1 or 0.527 66(20) eV. The accuracy was improved by a factor of 60 with regard to the previous measurement. The fine structure of V- was well resolved: 35.9(11) (5D1), 103.8(12) (5D2), 204.17(74) (5D3), and 330.58(40) cm-1 (5D4) above the ground state 5D0, respectively.

Activation of Methane Promoted by Adsorption of CO on Mo2 C2 (-) Cluster Anions.

Atomic clusters are being actively studied for activation of methane, the most stable alkane molecule. While many cluster cations are very reactive with methane, the cluster anions are usually not very reactive, particularly for noble metal free anions. This study reports that the reactivity of molybdenum carbide cluster anions with methane can be much enhanced by adsorption of CO. The Mo2 C2 (-) is inert with CH4 while the CO addition product Mo2 C3 O(-) brings about dehydrogenation of CH4 under thermal collision conditions. The cluster structures and reactions are characterized by mass spectrometry, photoelectron spectroscopy, and quantum chemistry calculations, which demonstrate that the Mo2 C3 O(-) isomer with dissociated CO is reactive but the one with non-dissociated CO is unreactive. The enhancement of cluster reactivity promoted by CO adsorption in this study is compared with those of reported systems of a few carbonyl complexes.

C-H Bond Activation by Early Transition Metal Carbide Cluster Anion MoC3 (-).

Although early transition metal (ETM) carbides can activate CH bonds in condensed-phase systems, the electronic-level mechanism is unclear. Atomic clusters are ideal model systems for understanding the mechanisms of bond activation. For the first time, CH activation of a simple alkane (ethane) by an ETM carbide cluster anion (MoC3 (-) ) under thermal-collision conditions has been identified by using high-resolution mass spectrometry, photoelectron imaging spectroscopy, and high-level quantum chemical calculations. Dehydrogenation and ethene elimination were observed in the reaction of MoC3 (-) with C2 H6 . The CH activation follows a mechanism of oxidative addition that is much more favorable in the carbon-stabilized low-spin ground electronic state than in the high-spin excited state. The reaction efficiency between the MoC3 (-) anion and C2 H6 is low (0.23±0.05) %. A comparison between the anionic and a highly efficient cationic reaction system (Pt(+) +C2 H6 ) was made. It turned out that the potential-energy surfaces for the entrance channels of the anionic and cationic reaction systems can be very different.

Calculation of photodetachment cross sections and photoelectron angular distributions of negative ions using density functional theory.

Recently, the development of photoelectron velocity map imaging makes it much easier to obtain the photoelectron angular distributions (PADs) experimentally. However, explanations of PADs are only qualitative in most cases, and very limited works have been reported on how to calculate PAD of anions. In the present work, we report a method using the density-functional-theory Kohn-Sham orbitals to calculate the photodetachment cross sections and the anisotropy parameter ß. The spherical average over all random molecular orientation is calculated analytically. A program which can handle both the Gaussian type orbital and the Slater type orbital has been coded. The testing calculations on Li(-), C(-), O(-), F(-), CH(-), OH(-), NH2 (-), O2 (-), and S2 (-) show that our method is an efficient way to calculate the photodetachment cross section and anisotropy parameter ß for anions, thus promising for large systems.

Vibrational state-selective autodetachment photoelectron spectroscopy from dipole-bound states of cold 2-hydroxyphenoxide: o-HO(C6H4)O(-).

We report a photodetachment and high-resolution photoelectron imaging study of cold 2-hydroxyphenoxide anion, o - HO(C6H4)O(-), cooled in a cryogenic ion trap. Photodetachment spectroscopy revealed a dipole-bound state (DBS) of the anion, 25 ± 5 cm(-1), below the detachment threshold of 18 784 ± 5 cm−1 (2.3289 ± 0.0006 eV), i.e., the electron affinity of the 2-hydroxyphenoxy radical o - HO(C6H4)O(⋅). Twenty-two vibrational levels of the DBS are observed as resonances in the photodetachment spectrum. By tuning the detachment laser to these DBS vibrational levels, we obtain 22 high-resolution resonant photoelectron spectra, which are highly non-Franck-Condon due to mode-selective autodetachment and the Δv = - 1 propensity rule. Numerous Franck-Condon inactive vibrational modes are observed in the resonant photoelectron spectra, significantly expanding the vibrational information that is available in traditional high-resolution photoelectron spectroscopy. A total of 15 fundamental vibrational frequencies are obtained for the o - HO(C6H4)O(⋅) radical from both the photodetachment spectrum and the resonant photoelectron spectra, including six symmetry-forbidden out-of-plane modes as a result of resonant enhancement.

Photoelectron imaging spectroscopy of MoC(-) and NbN(-) diatomic anions: A comparative study.

The isoeletronic diatomic MoC(-) and NbN(-) anions have been prepared by laser ablation and studied by photoelectron imaging spectroscopy combined with quantum chemistry calculations. The photoelectron spectra of NbN(-) can be very well assigned on the basis of literature reported optical spectroscopy of NbN. In contrast, the photoelectron spectra of MoC(-) are rather complex and the assignments suffered from the presence of many electronically hot bands and limited information from the reported optical spectroscopy of MoC. The electron affinities of NbN and MoC have been determined to be 1.450 ± 0.003 eV and 1.360 ± 0.003 eV, respectively. The good resolution of the imaging spectroscopy provided a chance to resolve the Ω splittings of the X(3)Σ(-) (Ω = 0 and 1) state of MoC and the X(4)Σ(-) (Ω = 1/2 and 3/2) state of MoC(-) for the first time. The spin-orbit splittings of the X(2)Δ state of NbN(-) and the a(2)Δ state of MoC(-) were also determined. The similarities and differences between the electronic structures of the NbN and MoC systems were discussed.