Zhihong Luo

State‐to‐state Photoionization Dynamics of Vanadium Nitride by Two‐color Laser Photoionization and Photoelectron Methods

We have conducted a two‐color visible‐ultraviolet (VIS‐UV) resonance‐enhanced laser photoionization and pulsed field ionization‐photoelectron (PFI‐PE) study of gaseous vanadium mononitride (VN) in the total energy range of 56900–59020 cm−1. The VN molecules were selectively excited to single rotational levels of the intermediate VN(D3Π0, v′=0) state by using a VIS dye laser prior to photoionization by employing a UV laser. This two‐color scheme allows the measurements of rovibronically selected and resolved PFI‐PE spectra for the VN+(X2Δ; v+=0, 1, and 2) ion vibrational bands. By simulating the rotationally resolved PFI‐PE spectra, J+=3/2 is determined to be the lowest rotational level of the ground electronic state, indicating that the symmetry of the ground VN+ electronic state is 2Δ3/2. The analysis of the PFI‐PE spectra for VN+ also yields accurate values for the adiabatic ionization energy for the formation of VN+(X2Δ3/2), IE(VN)=56909.5±0.8 cm−1 (7.05588±0.00010 eV), the vibrational frequency ωe+=10...

Rotationally resolved state-to-state photoelectron study of niobium carbide radical

By employing the two-color visible (VIS)-ultraviolet (UV) laser photoexcitation scheme and the pulsed field ionization-photoelectron (PFI-PE) detection, we have obtained rovibronically selected and resolved photoelectron spectra for niobium carbide cation (NbC(+)). The fully rotationally resolved state-to-state VIS-UV-PFI-PE spectra thus obtained allow the unambiguous assignments of rotational photoionization transitions, indicating that the electronic configuration and term symmetry of NbC(+)(X̃) ground state are …10σ(2) 5π(4) 11σ(2) (X̃(1)Σ(+)). Furthermore, the rotational analysis of these spectra yields the ionization energy of NbC [IE(NbC)] to be 56,369.2 ± 0.8 cm(-1) (6.9889 ± 0.0001 eV) and the rotation constant B0 (+) = 0.5681 ± 0.0007 cm(-1). The latter value allows the determination of the bond distance r0 (+) = 1.671 ± 0.001 Å for NbC(+)(X̃(1)Σ(+)). Based on conservation of energy, the IE(NbC) determined in the present study along with the known IE(Nb) gives the difference of 0 K bond dissociation energies (D0s) for NbC(+) and NbC, D0(NbC(+)) - D0(NbC) = -1855.4 ± 0.9 cm(-1) (-0.2300 ± 0.0001 eV). The energetic values and the B0 (+) constant determined in this work are valuable for benchmarking state-of-the-art ab initio quantum calculations of 4d transition metal-containing molecules.

Rotationally Resolved State-to-State Photoelectron Study of Molybdenum Monoxide Cation (MoO+)

By employing the two-color visible–ultraviolet (vis–UV) laser pulsed field ionization–photoelectron (PFI–PE) measurement, we have obtained rotationally selected and resolved photoelectron spectra for the MoO+(X4Σ–; v+ = 0, 1, and 2) and MoO+(a2Δ3/2,5/2; v+ = 0 and 1) cationic states. The unambiguous rotational assignments have made possible the determination of highly precise values for the band origin v00+ = 60 147.9 ± 0.8 cm–1, rotation constant B0+ = 0.4546 ± 0.0006 cm–1, spin–spin coupling constant λ = 26.454 ± 0.017 cm–1, and bond length re+ = 1.642 ± 0.001 A for the MoO+(X4Σ–) ground state; v00+ = 60 556.4 ± 0.8 cm–1, B0+ = 0.4711 ± 0.0005 cm–1, and r0+ = 1.613 ± 0.001 A for the MoO+ (a2Δ3/2) excited state; and v00+ = 61 718.2 ± 0.8 cm–1, B0+ = 0.4695 ± 0.0006 cm–1, and r0+ = 1.616 ± 0.001 A for the MoO+ (a2Δ5/2) excited state. The ionization energy (IE) for MoO is determined to be IE(MoO) = 60 095.1 ± 0.8 cm–1 [7.4508 ± 0.0001 eV]. Furthermore, the vibrational constants are determined as ωe+ = 1000...

Communication: A vibrational study of titanium dioxide cation using the vacuum ultraviolet laser pulsed field ionization-photoelectron method.

We have successfully measured the vacuum ultraviolet (VUV) laser photoionization efficiency and pulsed field ionization-photoelectron (PFI-PE) spectra of cold titanium dioxide (TiO(2)) prepared by a supersonically cooled laser ablation source. The VUV-PFI-PE spectrum thus obtained exhibits long progressions of the v(2)(+)(a(1)) bending and the combination of v(1)(+)(a(1)) stretching and v(2)(+)(a(1)) bending vibrational modes of the TiO(2)(+)(X(2)B(2)) ion. The pattern of Franck-Condon factors observed indicates that the O-Ti-O bond angle of the TiO(2)(+)(X(2)B(2)) ion is significantly different from that of the TiO(2)(X(1)A(1)) neutral, whereas the change of the Ti-O bond distance is very minor upon the photoionization transition. The analysis of the PFI-PE bands has made possible the determination of the adiabatic ionization energy for TiO(2), IE(TiO(2)) = 77215.9 ± 1.2 cm(-1) (9.57355 ± 0.00015 eV), the harmonic vibrational frequencies, ω(1)(+) = 829.1 ± 2.0 cm(-1) and ω(2)(+) = 248.7 ± 0.6 cm(-1), and the anharmonic coefficients, χ(11)(+) = 5.57 ± 0.65 cm(-1), χ(22)(+) = 0.08 ± 0.06 cm(-1), and χ(12)(+) = -4.51 ± 0.30 cm(-1) for the TiO(2)(+)(X(2)B(2)) ground state.

Rotationally resolved state-to-state photoelectron study of zirconium monoxide cation (ZrO+)

Using two-colour visible (Vis)–ultraviolet (UV) photoionisation and pulsed field ionisation–photoelectron (PFI–PE) methods, we have obtained cleanly rotationally resolved photoelectron spectra for ZrO+(X 2Δ3/2,5/2; v+ = 0, 1, and 2). The rotation assignment of these state-to-state Vis–UV–PFI–PE spectra has allowed the unambiguous determination of the ground state term symmetry for ZrO+(X) to be 2Δ3/2, and the adiabatic ionisation energy of 90Zr16O, IE(90Zr16O) = 54,948.3(8) cm−1 [6.81272(10) eV]. The symmetry of the ionic ZrO+(X 2Δ3/2) ground state determined here disagrees with that reported in previous experiments. The rotational and vibrational constants determined in this experiment for the ionic 90Zr16O+(X 2Δ3/2) ground state are: Be+ = 0.4343(8) cm−1 and αe+ = 0.0019(5) cm−1, and ωe+ = 991.2(8) cm−1 and ωe+xe+ = 3.5(8) cm−1; and those for the ionic 90Zr16O+(X 2Δ5/2) excited spin-orbit state are: Be+ = 0.4357(6) cm−1 and αe+ = 0.0022(4) cm−1, and ωe+ = 991.9(8) cm−1 and ωe+xe+ = 3.6(8) cm−1, respectively. Based on the latter Be+ value, the equilibrium bond distances are determined to be re+ = 1.691(2) Å for 90Zr16O+(X 2Δ3/2) and re+ = 1.688(1) Å for 90Zr16O+(X 2Δ5/2). The IE(ZrO) along with the spectroscopic constants obtained here are valuable for benchmarking the ab initio quantum chemical calculations for energetic and structural predictions of ZrO/ZrO+.

Communication: State-to-state photoionization and photoelectron study of vanadium methylidyne radical (VCH)

By employing the infrared (IR)-ultraviolet (UV) laser excitation scheme, we have obtained rotationally selected and resolved pulsed field ionization-photoelectron (PFI-PE) spectra for vanadium methylidyne cation (VCH(+)). This study supports that the ground state electronic configuration for VCH(+) is …7σ(2)8σ(2)3π(4)9σ(1) (X(2)Σ(+)), and is different from that of …7σ(2)8σ(2)3π(4)1δ(1) (X(2)Δ) for the isoelectronic TiO(+) and VN(+) ions. This observation suggests that the addition of an H atom to vanadium carbide (VC) to form VCH has the effect of stabilizing the 9σ orbital relative to the 1δ orbital. The analysis of the state-to-state IR-UV-PFI-PE spectra has provided precise values for the ionization energy of VCH, IE(VCH) = 54,641.9 ± 0.8 cm(-1) (6.7747 ± 0.0001 eV), the rotational constant B(+) = 0.462 ± 0.002 cm(-1), and the v2(+) bending (626 ± 1 cm(-1)) and v3(+) V-CH stretching (852 ± 1 cm(-1)) vibrational frequencies for VCH(+)(X(2)Σ(+)). The IE(VCH) determined here, along with the known IE(V) and IE(VC), allows a direct measure of the change in dissociation energy for the V-CH as well as the VC-H bond upon removal of the 1δ electron of VCH(X(3)Δ1). The formation of VCH(+)(X(2)Σ(+)) from VCH(X(3)Δ1) by photoionization is shown to strengthen the VC-H bond by 0.3559 eV, while the strength of the V-CH bond remains nearly unchanged. This measured change of bond dissociation energies reveals that the highest occupied 1δ orbital is nonbonding for the V-CH bond; but has anti-bonding or destabilizing character for the VC-H bond of VCH(X(3)Δ1).

Rotationally Selected and Resolved State-to-State Photoelectron Study of Vanadium Monoxide Cation VO+(X3Σ–; v+ = 0–3)

Vanadium monoxide cation VO(+)(X(3)Σ(-)) has been investigated by two-color visible (VIS)-ultraviolet (UV) pulsed field ionization-photoelectron (PFI-PE) methods. The unambiguous rotational assignment of rotationally selected and resolved VIS-UV-PFI-PE spectra thus obtained confirms the ground state term symmetry of VO(+) to be X(3)Σ(-). The rotational analysis also yields the rotational constants Be(+) = 0.5716 ± 0.0012 cm(-1) and αe(+) = 0.0027 ± 0.0005 cm(-1) for VO(+)(X(3)Σ(-)), from which the equilibrium bond distance of VO(+)(X(3)Σ(-)) is determined to be re(+) = 1.557 ± 0.002 Å. This PFI-PE study covers the vibrational bands, VO(+)(X(3)Σ(-); v(+) = 0, 1, 2, and 3) ← VO(X(4)Σ(-); v″ = 0), which has made possible the determination of the vibrational constants for VO(+)(X(3)Σ(-)) to be ωe(+) = 1068.0 ± 0.7 cm(-1) and ωe(+)xe(+) = 5.5 ± 0.7 cm(-1). The present state-to-state measurement also yields a more precise value (58 380.0 ± 0.7 cm(-1) or 7.238 20 ± 0.000 09 eV) for the ionization energy of VO [IE(VO)]. This value along with the known IE(V) has allowed the determination of the difference between the 0 K bond dissociation energy (D0) of VO(+)(X(3)Σ(-)) and that of VO(X(4)Σ(-)) to be D0(V(+)-O) - D0(V-O) = IE(V) - IE(VO) = -3967 ± 1 cm(-1).

High-Level ab Initio Predictions for the Ionization Energies, Bond Dissociation Energies, and Heats of Formation of Titanium Oxides and Their Cations (TiOn/TiOn+, n = 1 and 2)

The ionization energies (IEs) of TiO and TiO2 and the 0 K bond dissociation energies (D0) and the heats of formation at 0 K (ΔH°f0) and 298 K (ΔH°f298) for TiO/TiO+ and TiO2/TiO2+ are predicted by the wave-function-based CCSDTQ/CBS approach. The CCSDTQ/CBS calculations involve the approximation to the complete basis set (CBS) limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy (ZPVE), high-order correlation (HOC), core-valence (CV) electronic, spin-orbit (SO) coupling, and scalar relativistic (SR) effect corrections. The present calculations yield IE(TiO) = 6.815 eV and are in good agreement with the experimental IE value of 6.819 80 ± 0.000 10 eV determined in a two-color laser-pulsed field ionization-photoelectron (PFI-PE) study. The CCSDT and MRCI+Q methods give the best predictions to the harmonic frequencies: ωe (ωe+) = 1013 (1069) and 1027 (1059) cm-1 and the bond lengths re (re+) = 1.625 (1.587) and 1.621 (1.588) Å, for TiO (TiO+) compared with the experimental values. Two nearly degenerate, stable structures are found for TiO2 cation: TiO2+(C2v) structure has two equivalent TiO bonds, while the TiO2+(Cs) structure features a long and a short TiO bond. The IEs for the TiO2+(C2v)←TiO2 and TiO2+(Cs)←TiO2 ionization transitions are calculated to be 9.515 and 9.525 eV, respectively, giving the theoretical adiabatic IE value in good agreement with the experiment IE(TiO2) = 9.573 55 ± 0.000 15 eV obtained in the previous vacuum ultraviolet (VUV)-PFI-PE study of TiO2. The potential energy surface of TiO2+ along the normal vibrational coordinates of asymmetric stretching mode (ω3+) is nearly flat and exhibits a double-well potential with the well of TiO2+ (Cs) situated around the central well of TiO2+(C2v). This makes the theoretical calculation of ω3+ infeasible. For the symmetric stretching (ω1+), the current theoretical predictions overestimate the experimental value of 829.1 ± 2.0 cm-1 by more than 100 cm-1. This work together with the previous experimental and theoretical investigations supports the conclusion that the CCSDTQ/CBS approach is capable of providing reliable IE and D0 predictions for TiO/TiO+ and TiO2/TiO2+ with error limits less than or equal to 60 meV. The CCSDTQ/CBS calculations give the predictions of D0(Ti+-O) - D0(Ti-O) = 0.004 eV and D0(O-TiO) - D0(O-TiO+) = 2.699 eV, which are also consistent with the respective experimental determination of 0.008 32 ± 0.000 10 and 2.753 75 ± 0.000 18 eV.

A HIGH-RESOLUTION VACUUM ULTRAVIOLET LASER PHOTOIONIZATION AND PHOTOELECTRON STUDY OF THE CO ATOM

We have measured the vacuum ultraviolet–photoionization efficiency (VUV–PIE) spectrum of Co in the energy range of 63,500–67,000 cm−1, which covers the photoionization transitions of Co(3d74s2 4F9/2) Co+(3d8 3F4), Co(3d74s2 4F7/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F3), Co(3d74s2 4F9/2) Co+(3d8 3F2), and Co(3d74s2 4F9/2) Co+(3d74s1 5F5). We have also recorded the pulsed field ionization photoelectron spectrum of Co in the same energy range, allowing accurate determinations of ionization energies (IEs) for the photoionization transitions from the Co(3d74s2 4F9/2) ground neutral state to the Co+(3F J ) (J = 4 and 3) and Co+(5F5) ionic states, as well as from the Co(3d74s2 4F7/2) excited neural state to the Co+(3d8 3F3) ionic state. The high-resolution nature of the VUV laser used has allowed the observation of many well-resolved autoionizing resonances in the VUV–PIE spectrum, among which an autoionizing Rydberg series, 3d74s1(5F5)np (n = 19–38), converging to the Co+(3d74s1 5F5) ionic state from the Co(3d74s2 4F9/2) ground neutral state is identified. The fact that no discernible step-like structures are present at these ionization thresholds in the VUV–PIE spectrum indicates that direct photoionization of Co is minor compared to autoionization in this energy range. The IE values, the autoionizing Rydberg series, and the photoionization cross sections obtained in this experiment are valuable for understanding the VUV opacity and abundance measurement of the Co atom in stars and solar atmospheres, as well as for benchmarking the theoretical results calculated in the Opacity Project and the IRON Project, and thus are of relevance to astrophysics.