Yih-Chung Chang

Rovibronically selected and resolved two-color laser photoionization and photoelectron study of the iron carbide cation.

By using a two-color laser excitation-photoionization scheme, we have obtained rovibronically selected and resolved state-to-state pulsed field ionization-photoelectron (PFI-PE) bands for FeC+(X2delta5/2; v+=0-2, J+), allowing unambiguous rotational assignments for the photoionization transitions. The finding of the J+ = 5/2 level as the lowest rotational state confirms that the ground FeC+ ion state is of 2delta5/2 symmetry. The observed changes in total angular momentum upon photoionization of FeC are |deltaJ+| = |J+ - J| </= 3.5, indicating that the photoelectron orbital angular momentum is limited to l </= 3. This observation is also consistent with the conclusion that the photoionization involves the removal of an electron from the highest occupied molecular orbital of the pi-type. The ionization energy, IE = 61243.1 +/- 0.5 cm(-1) (7.59318 +/- 0.00006 eV), for the formation of FeC+ (X2delta5/2, v+=0; J+=5/2) from FeC (X3delta3, v=0; J=3), the rotational constants, Be+ = 0.7015 +/- 0.0006 cm(-1) and alphae+ = 0.00665 +/- 0.00036 cm(-1), and the vibrational constants, omegae+ = 927.14 +/- 0.04 cm(-1) and omegae+chie+ = 6.35 +/- 0.04 cm(-1), for FeC+(X2delta5/2) determined in the present study are compared to the recent state-of-the-art ab initio quantum chemical calculation at the C-MRCI+Q level of theory. The large deviation (0.49 eV) observed between the present experimental IE value and the C-MRCI+Q theoretical IE prediction highlights the great need for the further development of ab initio quantum theoretical procedures for more accurate energetic predictions of transition metal-containing molecules.

Rotationally resolved state-to-state photoionization and photoelectron study of titanium carbide and its cation (TiC/TiC⁺).

Titanium carbide and its cation (TiC/TiC(+)) have been investigated by the two-color visible (VIS)-ultraviolet (UV) resonance-enhanced photoionization and pulsed field ionization-photoelectron (PFI-PE) methods. Two visible excitation bands for neutral TiC are observed at 16,446 and 16,930 cm(-1). Based on rotational analyses, these bands are assigned as the respective TiC((3)Π1) ← TiC(X(3)Σ(+)) and TiC((3)Σ(+)) ← TiC(X(3)Σ(+)) transition bands. This assignment supports that the electronic configuration and term symmetry for the neutral TiC ground state are …7σ(2)8σ(1)9σ(1)3π(4) (X(3)Σ(+)). The rotational constant and the corresponding bond distance of TiC(X(3)Σ(+); v″ = 0) are determined to be B0″ = 0.6112(10) cm(-1) and r0″ = 1.695(2) Å, respectively. The rotational analyses of the VIS-UV-PFI-PE spectra for the TiC(+)(X; v(+) = 0 and 1) vibrational bands show that the electronic configuration and term symmetry for the ionic TiC(+) ground state are …7σ(2)8σ(1)3π(4) (X(2)Σ(+)) with the v(+) = 0 → 1 vibrational spacing of 870.0(8) cm(-1) and the rotational constants of B(e)(+) = 0.6322(28) cm(-1), and α(e)(+) = 0.0085(28) cm(-1). The latter rotational constants yield the equilibrium bond distance of r(e)(+) = 1.667(4) Å for TiC(+)(X(2)Σ(+)). The cleanly rotationally resolved VIS-UV-PFI-PE spectra have also provided a highly precise value of 53u2009200.2(8) cm(-1) [6.5960(1) eV] for the adiabatic ionization energy (IE) of TiC. This IE(TiC) value along with the known IE(Ti) has made possible the determination of the difference between the 0 K bond dissociation energy (D0) of TiC(+)(X(2)Σ(+)) and that of TiC(X(3)Σ(+)) to be D0(Ti(+)-C) - D0(Ti-C) = 0.2322(2) eV. Similar to previous experimental observations, the present state-to-state PFI-PE study of the photoionization transitions, TiC(+)(X(2)Σ(+); v(+) = 0 and 1, N(+)) ← TiC((3)Π1; v, J), reveals a strong decreasing trend for the photoionization cross section as |ΔN(+)| = |N(+) - J| is increased. The maximum |ΔN(+)| change of 7 observed here is also consistent with the previous experimental results for the 3d transition-metal carbides, oxides, and nitrides. However, the VIS-UV-PFI-PE spectra for TiC(+)(X(2)Σ(+); v(+) = 0 and 1, N(+)) are found to display only the negative ΔN(+) (N(+)-J≤ 0) transitions, indicating that the cross sections for the formation of positive ΔN(+) (N(+)-J > 0) transitions by both the channel coupling mechanism and direct photoionization are negligibly small.

Branching ratio measurements for vacuum ultraviolet photodissociation of 12C16O.

The branching ratios for the spin-forbidden photodissociation channels of (12)C(16)O in the vacuum ultraviolet (VUV) photon energy region from 102,500 (12.709 eV) to 106,300 cm(-1) (13.180 eV) have been investigated using the VUV laser time-slice velocity-map imaging photoion technique. The excitations to three (1)Σ(+) and six (1)Π Rydberg-type states, including the progression of W(3sσ) (1)Π(v = 0, 1, and 2) vibrational levels of CO, have been identified and investigated. The branching ratios for the product channels C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) of these predissociative states are found to depend on the electronic, vibrational, and rotational states of CO being excited. Rotation and e/f-symmetry dependences of the branching ratios into the spin-forbidden channels have been confirmed for several of the (1)Π states, which can be explained using the heterogeneous interaction with the repulsive D(1)Σ(+) state. The percentage of the photodissociation into the spin-forbidden channels is found to increase with increasing the rotational quantum number for the K(4pσ) (1)Σ(+) (v = 0) state. This has been rationalized using a (1)Σ(+) to (1)Π to (3)Π coupling scheme, where the final (3)Π state is a repulsive valence state correlating to the spin-forbidden channel.

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+ = 60u202f147.9 ± 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+ = 60u202f556.4 ± 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+ = 61u202f718.2 ± 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) = 60u202f095.1 ± 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.

A HIGH-RESOLUTION PHOTOIONIZATION STUDY OF 56Fe USING A VACUUM ULTRAVIOLET LASER

The photoionization efficiency (PIE) spectrum for 56 Fe formed by laser ablation has been measured using tunable vacuum ultraviolet (VUV) laser radiation in the energy range of 63000–74700 cm −1 , covering the threshold regions for the photoionization transitions of Fe + (3d 6 4s 6 D) ← Fe(3d 6 4s 25 D) and Fe + (3d 6 4s 4 D) ← Fe(3d 6 4s 25 D). The fact that no step-like structures in the PIE spectrum are observed at these ionization thresholds indicates that direct photoionization plays a very minor role in the photoionization of Fe in this VUV energy range. Comparing the VUVPIE and the VUV-absorption spectra of Fe shows that all prominent absorption bands of Fe correspond to strong autoionizing structures in the PIE spectrum of Fe. Due to the significantly narrower VUV laser optical bandwidth of 0.12 cm −1 used in the present study, complex autoionizing resonances are resolved in the PIE spectrum. Two autoionizing Rydberg series (3d 74 F7/2)np (n = 9–27) and (3d 74 F9/2)np (n = 10–32) formed by two-electron excitations from the (3d 6 4s 25 D4) ground state are identified to converge to the respective Fe + (3d 74 F7/2) and Fe + (3d 74 F9/2) ion levels. The photoionization cross sections and well resolved autoionizing Rydberg resonances observed in this study are relevant to astrophysics for understanding the Fe contribution to the VUV opacity in the solar atmosphere, and for benchmarking theoretical calculations under the Opacity Project and the IRON Project.

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).

A HIGH-RESOLUTION PHOTOIONIZATION AND PHOTOELECTRON STUDY OF 58Ni USING A VACUUM ULTRAVIOLET LASER

In order to provide high-resolution spectroscopic data of nickel (58Ni) and its cation (58Ni+) for the assignment of vacuum ultraviolet (VUV) stellar spectra, we have obtained the photoionization efficiency (PIE) spectra of 58Ni by using a supersonically cooled laser ablation transition-metal beam source and a broadly tunable VUV laser in the range of 61,100-73,600xa0cm–1, covering the photoionization transitions: Ni+ (3d 9 2 D) ← Ni (3d 84s 2 3 D), Ni+(3d 9 2 D) ← Ni(3d 84s 2 3 F), and Ni+ (3d 84s 4 F) ← Ni(3d 84s 2 3 F). We have also measured the VUV laser pulsed-field-ionization-photoelectron (PFI-PE) spectra of 58Ni in these regions. The VUV-PFI-PE measurement has allowed the determination of a precise value of 61,619.89 ± 0.8xa0cm–1 (7.6399 ± 0.0001xa0eV) for the ionization energy (IE) of 58Ni. Due to the narrow VUV laser optical bandwidth of 0.4xa0cm–1 used in the present study, many complex autoionizing resonances exhibiting Fano line shape profiles are resolved in the PIE spectra. Four autoionizing Rydberg series originating from two-electron and one-electron excitations from the Ni(3d 84s 2 3 F 4) ground state to converge to the respective Ni+(2 D 3/2) and Ni+(4 FJ ) (J = 9/2, 7/2, and 5/2) ion states are identified. The Rydberg analysis, along with VUV-PFI-PE measurements, has yielded highly precise IE values for the formation of these excited ionic states from the Ni(3d 84s 2 3 F 4) ground state. The IE values, relative photoionization cross sections, and autoionizing Rydberg resonances observed in the present study are relevant to astrophysics by enhancing the atomic database of iron group transition metal atoms and for understanding the Ni and Ni+ contribution to the VUV opacity in the solar atmosphere.

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 (58u202f380.0 ± 0.7 cm(-1) or 7.238u202f20 ± 0.000u202f09 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).