Pulsed Field Ionization-photoelectron Bands Research Articles

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 vacuum-ultraviolet laser pulsed field ionization-photoelectron study of sulfur monoxide (SO) and its cation (SO+)

Vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectroscopy has been applied to the study of the sulfur monoxide radical (SO) prepared by using a supersonically cooled radical beam source based on the 193 nm excimer laser photodissociation of SO(2). The vibronic VUV-PFI-PE bands for the photoionization transitions SO(+)(X(2)Π(1∕2); v(+) = 0) ← SO(X(3)Σ(-); v = 0); and SO(+)((2)Π(3∕2); v(+) = 0) ← SO(X(3)Σ(-); v = 0) have been recorded. On the basis of the semiempirical simulation of rotational branch contours observed in these PFI-PE bands, we have obtained highly precise ionization energies (IEs) of 83,034.2 ± 1.7 cm(-1) (10.2949 ± 0.0002 eV) and 83,400.4 ± 1.7 cm(-1) (10.3403 ± 0.0002 eV) for the formation of SO(+)(X(2)Π(1∕2); v(+) = 0) and SO(+)((2)Π(3∕2); v(+) = 0), respectively. The present VUV-PFI-PE measurement has enabled the direct determination of the spin-orbit coupling constant (A(0)) for SO(+)(X(2)Π(1∕2,3∕2)) to be 365.36 ± 0.12 cm(-1). We have also performed high-level ab initio quantum chemical calculations at the coupled-cluster level up to full quadruple excitations and complete basis set (CBS) extrapolation. The zero-point vibrational energy correction, the core-valence electronic correction, the spin-orbit coupling, and the high-level correction are included in the calculation. The IE[SO(+)(X(2)Π(1∕2,3∕2))] and A(0) predictions thus obtained are found to be in remarkable agreement with the experimental determinations.

Vacuum ultraviolet laser pulsed field ionization-photoelectron study of allyl radical CH2CHCH2

The pulsed field ionization-photoelectron (PFI-PE) spectrum of allyl radical CH2CHCH2 (C3H5) in the energy range of 65 200-66 600 cm-1 has been measured using vacuum ultraviolet laser. Based on the simulation of the rotational structures resolved in the vibrational PFI-PE bands of C3H5+(X 1A1;0(0+) and nu7+=1), the ionization energies (IEs) of C3H5(X 2A2;0(0)) to form C3H5+(X 1A1;0(0+) and nu7+=1) are determined to be 65 584.6+/-2.0 cm-1 (8.131 46+/-0.000 25 eV) and 66 020.9+/-2.0 cm-1 (8.185 56+/-0.000 25 eV), respectively, where nu7+(a1) is the symmetric C-C-C bending mode of C3H5+(X 1A1). These values are compared to IE(C3H5) values obtained in previous experimental and high-level ab initio quantum theoretical studies.

Vacuum ultraviolet laser pulsed field ionization-photoelectron study of cis-dichloroethene

The vacuum ultraviolet (VUV) laser pulsed field ionization photoelectron (PFI-PE) spectrum of cis-dichloroethene (cis-ClCH[Double Bond]CHCl) has been measured in the energy region of 77 600-79 500 cm(-1). On the basis of the semiempirical simulation of the origin PFI-PE band, we have obtained the IE(cis-ClCH[Double Bond]CHCl) to be 77 899.5+/-2.0 cm(-1) (9.658 39+/-0.000 25 eV). The assignment of the vibrational bands resolved in the VUV-PFI-PE spectrum are guided by high-level ab initio calculations of the vibrational frequencies for cis-ClCH[Double Bond]CHCl(+) and the Franck-Condon factors for the ionization transitions. Combining the results of the present VUV-PFI-PE measurement and the recent VUV-infrared-photoinduced Rydberg ionization study, the vibrational frequencies for eleven of the twelve vibrational modes of cis-ClCH[Double Bond]CHCl(+) have been experimentally determined: nu(1) (+)(a(1))=181 cm(-1), nu(2) (+)(a(2))=277 cm(-1), nu(3) (+)(b(2))=580 cm(-1), nu(4) (+)(b(1))=730 cm(-1), nu(5) (+)(a(1))=810 cm(-1), nu(6) (+)(a(2))=901 cm(-1), nu(8) (+)(a(1))=1196 cm(-1), nu(9) (+)(b(2))=1348 cm(-1), nu(10) (+)(a(1))=1429 cm(-1), nu(11) (+)(b(2))=3067 cm(-1), and nu(12) (+)(a(1))=3090 cm(-1)). These values are compared to theoretical anharmonic vibrational frequencies obtained at the MP2/6-311G(2df,p) and CCSD(T)/6-311G(2df,p) levels. The IE prediction for cis-ClCH[Double Bond]CHCl has also been calculated with the wave function based CCSD(T)/CBS method, which involves the approximation to the complete basis set (CBS) and the high-level correlation corrections. The theoretical IE(cis-ClCH[Double Bond]CHCl)=9.668 eV thus obtained is found to have a deviation of less than 10 meV with respect to the experimental IE value.

Vacuum ultraviolet pulsed-field ionization-photoelectron study of H2S in the energy range of 10-17 eV.

Vacuum ultraviolet pulsed-field ionization-photoelectron (PFI-PE) spectra of H(2)S have been recorded at PFI-PE resolutions of 0.6-1.0 meV in the energy range of 10-17 eV using high-resolution synchrotron radiation. The PFI-PE spectrum, which covers the formation of the valence electronic states H(2)S(+) (X (2)B(1), A (2)A(1), and B (2)B(2)), is compared to the recent high-resolution He I photoelectron spectra of H(2)S obtained by Baltzer et al. [Chem. Phys. 195, 403 (1995)]. In addition to the overwhelmingly dominated origin vibrational band, the PFI-PE spectrum for H(2)S(+)(X (2)B(1)) is found to exhibit weak vibrational progressions due to excitation of the combination bands in the nu(1) (+) symmetric stretching and nu(2) (+) bending modes. While the ionization energy (IE) for H(2)S(+)(X (2)B(1)) obtained here is in accord with values determined in previously laser PFI-PE measurements, the observation of a new PFI-PE band at 12.642+/-0.001 eV suggests that the IE for H(2)S(+)(A (2)A(1)) may be 0.12 eV lower than that reported in the He I study. The simulation of rotational structures resolved in PFI-PE bands shows that the formation of H(2)S(+)(X (2)B(1)) and H(2)S(+)(A (2)A(1)) from photoionization of H(2)S(X (1)A(1)) is dominated by type-C and type-B transitions, respectively. This observation is consistent with predictions of the multichannel quantum defect theory. The small changes in rotational angular momentum observed are consistent with the dominant atomiclike character of the 2b(1) and 5a(1) molecular orbitals of H(2)S. The PFI-PE measurement has revealed perturbations of the (0, 6, 0) K(+)=3 and (0, 6, 0) K(+)=4 bands of H(2)S(+)(A (2)A(1)). Interpreting that these perturbations arise from Renner-Teller interactions at energies close to the common barriers to linearity of the H(2)S(+) (X (2)B(1) and A (2)A(1)) states, we have deduced a barrier of 23,209 cm(-1) for H(2)S(+)(X (2)B(1)) and 5668 cm(-1) for H(2)S(+)(A (2)A(1)). The barrier of 23 209 cm(-1) for H(2)S(+)(X (2)B(1)) is found to be in excellent agreement with the results of previous studies. The vibrational PFI-PE bands for H(2)S(+)(B (2)B(2)) are broad, indicative of the predissociative nature of this state.

Vacuum ultraviolet laser pulsed field ionization photoelectron study of trans-2-butene

The single-photon pulsed field ionization photoelectron (PFI-PE) spectrum of trans-2-butene (trans-CH3CH=CHCH3) in the energy range of 73 500–75 850 cm−1 has been measured using vacuum ultraviolet laser sources. The semi-empirical simulation of fine structures resolved in the original PFI-PE band yields a value of 73 624.7±2.0 cm−1 for the ionization energy (IE) of trans-2-butene. The vibrational bands for trans-CH3CH=CHCH3+ resolved in the PFI-PE spectrum are assigned based on ab initio calculations of the vibrational frequencies and Franck-Condon factors (FCFs) for ionization transitions. This assignment has provided reliable vibrational frequencies (ν1+=104 cm−1, ν2+=127 cm−1, ν3+=131 cm−1, ν5+=484 cm−1, ν8+=798 cm−1, ν13+=1164 cm−1, ν14+=1264 cm−1, ν16+=1307 cm−1, ν20+=1407 cm−1, and ν22+=1567 cm−1) for trans-CH3CH=CHCH3+. The PFI-PE spectrum is compared to the recently reported PFI-photoion (PFI-PI) spectrum for trans-2-butene. The major difference observed between the PFI-PE and PFI-PI spectra is that the intensities for excited vibrational bands were significantly suppressed or indiscernible in the PFI-PI spectrum, suggesting that the lifetimes for high-n Rydberg states associated with these excited vibrational bands were greatly reduced under the conditions used in the PFI-PI study. The experimental conditions used in the PFI-PI study also led to an IE value of about 20 cm−1 lower than that obtained in the PFI-PE measurement. We have also reassigned the vibrational bands resolved in the PFI-PE spectrum for cis-2-butene based on the FCF calculation and a more reliable set of theoretical vibrational frequencies.

Pulsed field ionization-photoelectron bands for CS2+ in the energy range of 13.2–17.6 eV: An experimental and theoretical study

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for CS2 have been obtained in the energy range of 13.2–17.6 eV, revealing complex vibronic structures for the CS2+(B̃ 2Σu+ and C̃ 2Σg+) states. The PFI-PE spectra for CS2+(B̃ 2Σu+ and C̃ 2Σg+) are dominated by the ν1+ (symmetric stretching) mode. However, PFI-PE bands due to the ν2+ (bending) and ν3+ (antisymmetric stretching) modes with both even and odd quanta are clearly resolved. The simulation of rotational contours resolved in the origin PFI-PE bands yields accurate ionization energies of 14.4742±0.0005 eV (116 742±4 cm−1) and 16.1883±0.0005 eV (130 567±4 cm−1) for the formation of CS2+(B̃ 2Σu+ and C̃ 2Σg+) states from CS2(X̃ 1Σg+), respectively. The PFI-PE bands for Σu+2(302) and Σg+2(303) at 14.805 and 14.965 eV, which are in near energy resonance with the 0 K dissociation thresholds for the formation of S+(4S)+CS(X 1Σ+; ν″=0 and 1) from CS2, respectively, are found to be enhanced. These enhancements are rationalized by the prompt dissociation of excited CS2 in high-n (n⩾100) Rydberg states prior to PFI. Three-dimensional potential energy functions (PEFs) for CS2+(2 2Πu, B̃ 2Σu+, and C̃ 2Σg+) states have been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. On the basis of these PEFs, the harmonic frequencies for CS2+(2 2Πu, B̃ 2Σu+, and C̃ 2Σg+) and vibronic energy levels for CS2+(B̃ 2Σu+) have been calculated variationally. These theoretical predictions have made possible a satisfactory assignment of the vibronic bands resolved in the PFI-PE spectra for CS2+(B̃ 2Σu+ and C̃ 2Σg+). Using the theoretical predictions obtained here, we have also assigned vibronic bands for CS2+(2 2Πu) observed in the HeI photoelectron spectrum [Baltzer et al., Chem. Phys. 202, 185 (1996)].

Unimolecular decay pathways of state-selected CO2+ in the internal energy range of 5.2–6.2 eV: An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO2 has been measured in the energy region of 19.0–20.0 eV. The PFI-PE vibrational bands resolved for CO2+(C 2Σg+) are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the ν1+ (symmetric stretching), ν2+ (bending), and ν3+ (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911±0.0005 eV for the ionization energy of CO2 to form CO2+(C 2Σg+). A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O+(4S)+CO(X 1Σ+) and CO+(X 2Σ+)+O(3P). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO2 in high-n (n⩾100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO2+ in the internal energy range of 5.2–6.2 eV using the PFI-PE-photoion coincidence scheme. The coincidence TOF data show unambiguously the formation of O+(4S)+CO(X 1Σ+;ν″=0,1) and CO+(X 2Σ+;ν+=0,1)+O(3P). Analysis of the kinetic energy releases of fragment ions suggests that the dissociation of excited CO2+ involved is nonstatistical and proceeds with an impulsive mechanism. Potential energy functions (PEFs) for the CO2+(C 2Σg+) state and the lowest quartet states of CO2+, together with their spin–orbit interactions, have been calculated using the complete active space self-consistent field and internal contracted multireference configuration interaction methods. Based on these PEFs, vibrational levels for CO2+(C 2Σg+) have been also calculated using a variational approach. With the aid of these theoretical calculations, vibrational bands resolved in the PFI-PE spectrum for CO2+(C 2Σg+) have been satisfactorily assigned, yielding a ν3+ value of 2997 cm−1. The theoretical calculation also provides a rationalization that the predissociation for CO2+(C 2Σg+) to form O+(4S)+CO(X 1Σ+) and CO+(X 2Σ+)+O(3P) most likely proceeds via the repulsive a 4Σg− and b 4Πu (or B14 in a bent geometry) states.

Rotationally resolved pulsed-field ionization photoelectron bands for O2+(A 2Πu,v+=0–12) in the energy range of 17.0–18.2 eV

We have obtained rotationally resolved pulsed-field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 17.05–18.13 eV, covering the ionization transitions O2+(A 2Πu,v+=0–12,N+)←O2(X 3Σg−,v″=0,N″). Although these O2+(A 2Πu,v+) PFI-PE bands have significant overlaps with vibrational bands for O2+(a 4Πu) and O2+(X 2Πg), we have identified all the O2+(A 2Πu,v+=0–12) bands by simulation of spectra obtained using supersonically cooled O2 samples with rotational temperatures ≈20 and 220 K. While these v+=0–12 PFI-PE bands represent the first rotationally resolved photoelectron data for O2+(A 2Πu), the PFI-PE bands for O2+(A 2Πu,v+=9 and 10) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O2+(A 2Πu,v+=0–12). The analysis of the PFI-PE spectra supports the conclusion of the previous emission study that the O2+(A 2Πu,v+=9 and 10) states are strongly perturbed by a nearby electronic state.

Rotationally resolved pulsed field ionization photoelectron bands for O2+(a 4Πu, v+=0–18) in the energy range of 16.0–18.0 eV

We have obtained rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra of O2 in the energy range of 16.0–18.0 eV, covering ionization transitions O2+(a 4Πu, v+=0–18,J+)←O2(X 3Σg., v″=0,N″). Although these vibrational PFI-PE bands for O2+(a 4Πu, v+) have significant overlaps with those for O2+(X 2Πg) and O2+(A 2Πu), we have identified all O2+(a 4Πu, v+=0–18) bands by simulation of spectra obtained using supersonically cooled O2 samples with rotational temperatures ≈20 and 220 K. While the v+=4–18PFI-PE bands represent the first rotationally resolved photoelectron data for O2+(a 4Πu), the PFI-PE bands for O2+(a 4Πu, v+=10–18) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O2+(a 2Πu, v+=0–18). The observed intensities of spin–orbit components for the majority of O2+(a 2Πu, v+) vibrational bands are in accordance with the forced spin–orbit/rotational autoionization mechanism.

Rotationally resolved pulsed field ionization photoelectron study of CO+(X 2Σ+,v+=0–42) in the energy range of 13.98–21.92 eV

We have obtained rotationally resolved pulsed field ionization–photoelectron (PFI-PE) spectra of CO in the energy range of 13.98–21.92 eV, covering the ionization transitions CO+(X 2Σ+,v+=0–42,N+)←CO(X 1Σ+,v″=0,N″). The PFI-PE bands for CO+ (X 2Σ+, v+=8–22, 24, and 28–39) obtained here represent the first rotationally resolved spectroscopic data for these states. The high-resolution features observed in the PFI-PE spectra allow the identification of vibrational bands for the CO+ (X 2Σ+, v+=10, 14, 15, 17, 18, 21, 24, 25, 29–31, 33, 35–37, and 39) states, which strongly overlap with prominent vibrational bands of the CO+(A 2Π3/2,1/2,B 2Σ+) states. The simulation using the Buckingham–Orr–Sichel model has provided accurate molecular constants for CO+(X 2Σ+,v+=0–42), including ionization energies, vibrational constants (ωe+=2218.8±3.5 cm−1, ωe+xe+=16.20±0.32 cm−1, ωe+ye+=0.074±0.011 cm−1, and ωe+ze+=−0.001 83±0.000 13 cm−1), and rotational constants [Be+=1.9797±0.0051 cm−1, αe+=0.0201±0.0011 cm−1, γe+=0.000 122±0.000 067 cm−1, ze+=−(5.2±1.1)×10−6 cm−1]. Enhancement of ΔN<0 rotational branches, attributable to field-induced rotational autoionization, was clearly discernible in PFI-PE bands for CO+ (X 2Σ+, v+=0–5, 11, and 12). Significant local enhancements due to near-resonance autoionization were observed for low v+ (<10) PFI-PE bands of CO+(X 2Σ+), where the density of interloper Rydberg states converging to higher ionic levels is high as manifested in the photoion spectrum. The observation of a long vibrational progression in the Franck–Condon gap region, where strong autoionization states are absent, is consistent with the suggestion that high-n Rydberg states converging to highly excited vibrational levels of CO+(X 2Σ+) are partially populated via direct excitation to a repulsive neutral state. The relatively minor band intensity variation observed for high v+ PFI-PE bands is also in accord with the direct excitation model. Since ΔN=0, ±1, ±2, and ±3 rotational branches are observed in the PFI-PE spectra, we conclude that the ejected photoelectrons are restricted to angular momentum continuum states l=0–4.

Rotational-resolved pulsed field ionization-photoelectron study of NO+(A′ 1Σ−,v+=0–17) in the energy range of 17.70–20.10 eV

We have obtained partially rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra of NO in the energy range of 17.70–20.10 eV, covering ionization transitions of NO+(A′ 1Σ−,v+=0–17,J+)←NO(X 2Π3/2,1/2,v″=0,J″). The PFI-PE bands for NO+(A′ 1Σ−,v+=0–17) obtained here represent the first spectroscopic data that allow the direct determination of rotational constants for these states. As in previous studies, the PFI-PE bands are simulated using the Buckingham–Orr–Sichel model as an empirical scheme to derive reliable spectroscopic constants. The simulation, together with comparison with previous He I photoelectron data, has also provided information on intensity enhancements of PFI-PE bands. Accurate spectroscopic constants for NO+(A′ 1Σ−,v+=0–17) obtained in the present study include ionization energies, vibrational constants (ωe=1272.03±1.45 cm−1, ωeχe=11.924±0.188 cm−1, and ωeye=−0.059±0.007 cm−1), and rotational constants [Be=1.3562±0.0024 cm−1, αe=0.017 80±0.000 61 cm−1, and γe=−(1.574±0.328)×10−4 cm−1]. The simulation indicates that continuum photoelectron states can be formed with orbital angular momenta l up to 5, which corresponds to the maximum total angular momentum transfer of |ΔJ|=13/2. Since the dominant rotational branches observed are ΔJ=±1/2, ±3/2, ±5/2, ±7/2, and +9/2, we conclude that continuum photoelectron states are predominantly produced in l=0–3 in the threshold photoionization of NO(X 2Π3/2,1/2) to form NO+(A′ 1Σ−,v+=0–17).

Rotationally resolved pulsed field ionization photoelectron bands of O2+(X 2Π1/2,3/2g, v+=0–38) in the energy range of 12.05–18.15 eV

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 12.05–18.15 eV, covering ionization transitions O2+(X 2Π1/2,3/2g, v+=0–38,J+)←O2(X 3Σg−, v+=0,N″). While the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=3–5, 9, 11, 12, 22, and 25–38) reported here are the first rotational-resolved photoelectron measurements, the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=25–38) represent the first rotationally resolved spectroscopic data for these states. The simulation of spectra obtained at rotational temperatures of ≈20 and 220 K allows the unambiguous identification of O2+(X 2Π1/2,3/2g, v+⩾21) PFI-PE bands, the majority of which overlap with prominent PFI-PE bands for O2+(A 2Πu, v+=0–12) and O2+(a 4Πu, v+=0–18). Combined with spectroscopic data obtained in the previous emission study and the present PFI-PE experiment, we have obtained accurate Dunham-type expansion coefficients for ionization energies, vibrational constants, rotational constants, and spin–orbit splitting constants covering the O2+(X 2Π1/2,3/2g, v+=0–38) states. Significant local intensity enhancements due to near-resonant autoionization were observed in PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=0–14). The energy region of these states is known to manifest a high density of very strong autoionizing low-n-Rydberg states. The observation of a long PFI-PE vibrational progression with a relatively smooth band intensity profile is also in accord with the direct excitation model for the production of highly vibrationally excited O2+(X 2Π1/2,3/2g) states in the Franck–Condon gap region. Since this experiment was carried out under relatively high rotational temperatures for O2, the PFI-PE data reveal higher rotational transitions and numerous local intensity enhancements, which were not observed in previous vacuum ultraviolet laser studies using a cold O2 molecular beam. The rotational branches found here indicate that photoelectrons are formed predominantly in continuum states with orbital angular momenta l=1,3, and 5.

Rotational-resolved pulsed field ionization photoelectron study of NO+(a 3Σ+,v+=0–16) in the energy range of 15.6–18.2 eV

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of NO in the energy range of 15.6–18.2 eV, covering ionization transitions of NO+(a 3Σ+,v+=0–16,J+)←NO(X 2Π3/2,1/2,v″=0,J″). The PFI-PE bands for NO+(a 3Σ+,v+=1–5,7–10,12–14,16) obtained in this experiment represent the first rotationally resolved spectroscopic data for these states. The simulation of these PFI-PE bands provides accurate molecular constants for NO+(a 3Σ+,v+=0–5,7–10,12–14,16), including ionization energies, vibrational constants (ωe+=1295.2±0.1 cm−1, ωe+χe+=15.198±0.002 cm−1), and rotational constants (Be+=1.3501±0.0070 cm−1, αe+=0.0206±0.0001 cm−1). As observed in the PFI-PE study of NO+(X 1Σ+), this experiment reveals a generally increasing trend for the maximum ΔJ value and intensities of higher ΔJ branches as v+ (or bond distance) for NO+(a 3Σ+) is increased. This observation can be taken as strong support of the electron-molecular-ion-core scattering model for angular momentum and energy exchanges in the threshold photoionization of NO.

High resolution pulsed field ionization photoelectron spectroscopy using multibunch synchrotron radiation: Time-of-flight selection scheme

We have developed an efficient electron time-of-flight (TOF) selection scheme for high resolution pulsed field ionization (PFI) photoelectron (PFI-PE) measurements using monochromatized multibunch undulator synchrotron radiation at the Advanced Light Source. By employing a simple electron TOF spectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. A near complete suppression of prompt electrons was achieved in PFI-PE measurements by gating the PFI-PE TOF peak, as indicated by monitoring background electron counts at the Ar(11s′) autoionizing Rydberg peak, which is adjacent to the Ar+(2P3/2) PFI-PE band. The rotational-resolved PFI-PE band for H2+ (X 2Σg+,v+=0) measured using this electron TOF selection scheme is nearly free from residues of nearby autoionizing features, which were observed in the previous measurement by employing an electron spectrometer equipped with a hemispherical energy analyzer. This comparison indicates that the TOF PFI-PE scheme is significantly more effective in suppressing the hot-electron background. In addition to attaining a high PFI-PE transmission, a major advantage of the electron TOF scheme is that it allows the use of a smaller pulsed electric field and thus results in a higher instrumental PFI-PE resolution. We have demonstrated instrumental resolutions of 1.0 cm−1 full width at half maximum (FWHM) and 1.9 cm−1 FWHM in the PFI-PE bands for Xe+(2P3/2) and Ar+(2P3/2) at 12.123 and 15.760 eV, respectively. These resolutions are more than a factor 2 better than those achieved in previous synchrotron based PFI-PE studies.

Rotationally resolved pulsed field ionization photoelectron study of O2+(B 2Σg−,2Σu−; v+=0–7) at 20.2–21.3 eV

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of O2 in the energy range of 20.2–21.3 eV, covering the ionization transitions of O2+(B 2Σg−, v+=0–7, N+)←O2(X 3Σg−, v″=0, N″). Only the ΔN=−2, 0, and +2 (or O, Q, and S) rotational branches are observed in the PFI-PE bands for O2+(B 2Σg−, v+=0–7), indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the ionization transitions. This experiment allows the determination of accurate spectroscopic constants, such as ionization energy (20.29825±0.0005 eV) for the formation of O2+[B 2Σg−, v+=0, N+=1 (F2)] from O2(X 3Σg−, v″=0, N″=1), vibrational constants (ωe+=1152.91 cm−1, ωe+χe+=20.97 cm−1_, and rotational constants (Be+=1.255±0.0015 cm−1, αe+=0.0241±0.00037 cm−1_ for O2+(B 2Σg−, v+). The (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are measured to be ≈0.2–0.6 μs, which are significantly shorter than those of ≈1.9 μs observed for O2+(b 4Σg−, v+=0–5). The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are attributed to the higher kinetic energy releases (or velocities) of O++O fragments resulting from predissociation of the O2+(B 2Σg−, v+=0–6) ion cores. Rotationally resolved PFI-PE measurements also make possible the identification of the weak vibrational progression with the origin at 20.35 eV as associated with transitions to O2+(2Σu−, v+=0–7). The analysis of the rotationally resolved PFI-PE bands for O2+(2Σu−, v+=0 and 1) has yielded accurate rotational constants and IE values for these states. The rotational structures resolved in the O2+(2Σu−, v+=0 and 1) PFI-PE bands are contributed overwhelmingly by the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches, showing that the angular momenta for the outgoing photoelectron are restricted to l=0, 2, and 4. Based on simulation of the observed rotational structures, we also obtain the predissociative lifetimes for O2+(B 2Σg−, v+=0–7) and O2+(2Σu−, v+=0–1) to be in the range of 0.45–2 ps.

High-resolution pulsed field ionization photoelectron study of O2: Predissociation lifetimes and high-n Rydberg lifetimes converging to O2+(c 4Σu−, v+=0,1)

We have measured the pulsed field ionization photoelectron (PFI-PE) spectrum of O2 in the energy range of 24.53–25.0 eV at a PFI-PE resolution of 11 cm−1 (full width at half maximum, FWHM). The PFI-PE bands for O2+(c 4Σu−, v+=0 and 1) obtained at O2 rotational temperatures of 35 and 298 K have been simulated using the Buckingham–Orr–Sichel model. Only the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches are observed, indicating that the outgoing electron continuum channels with angular momenta l=0, 2, and 4 dominate in the threshold ionization transitions O2+(c 4Σu−, v+=0 to 1, N+)←O2(X 3Σg−, v″=0, N″). The simulation yields natural rotational linewidths of 19.6±2.0 and 77±8 cm−1 (FWHM) for the respective v+=0 and 1 PFI-PE bands of the O2+(c 4Σu−) state. These linewidths make possible the determination of the predissociation lifetimes for the v+=0 and 1 levels of O2+(c 4Σu−) to be (2.7±0.3)×10−13 and (6.9±0.7)×10−14 s, respectively. This experiment also provides accurate ionization energies of 24.56227±0.0005 and 24.75445±0.0005 eV for transitions to O2+(c 4Σu−, v+=0, N+=0) and O2+(c 4Σu−, v+=1, N+=0) from O2(X 3Σg−, v″, N″=1), respectively. The rotational constants of 1.58±0.02 and 1.54±0.04 cm−1 obtained here for the O2+(c 4Σu−, v+=0) and O2+(c 4Σu−, v+=1) states allow the calculation of their corresponding equilibrium bond distances to be 1.155±0.011 and 1.170±0.015 Å. The (nominal) effective lifetimes for high-n Rydberg states converging to the O2+(c 4Σu−, v+=0 and 1) states are measured to be ≈0.33 μs, which are significantly shorter than the values of ≈1.9 μs measured for the O2+(b 4Σg−, v+=0–5) states. The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O2+(c 4Σg−, v+=0 and 1) observed are attributed to the higher kinetic energy releases (or velocities) of O+ fragments resulting from predissociation of the O2+ ion cores.

Rotational-resolved pulsed field ionization photoelectron bands for H2+(X 2Σg+, v+=0, 2, 9 and 11)

We present here the assignment and simulation of rotational transitions for the H2+(X 2Σg+, v+=0, 2, 9 and 11) vibronic bands using the Buckingham–Orr–Sichel (BOS) model. The simulation shows that perturbation of PFI-PE rotational line intensities due to near-resonance autoionization decreases as v+ increases. Experimental rotational constants for H2+(X 2Σg+, v+=0, 2, 9 and 11) are determined with higher accuracy than those obtained in previous HeI and NeI photoelectron studies. In agreement with previous experimental and theoretical investigations, only the ΔN=0 and ±2 rotational branches are observed in the PFI-PE spectrum for H2.

High resolution vacuum ultraviolet pulsed field ionization photoelectron band for OCS+(X 2Π): An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization photoelectron (PFI-PE) band for OCS+(X 2Π) in the energy region of 11.09–11.87 eV has been measured using high resolution monochromatized synchrotron radiation. The ionization energies (IEs) for the formation of the (0,0,0) X 2Π3/2 and (0,0,0) 2Π1/2 states of OCS+ are determined to be 11.1831±0.0005 and 11.2286±0.0005 eV, respectively, yielding a value of 367±1.2 cm−1 for the spin–orbit splitting. Using the internally contracted multireference configuration interaction approach, three-dimensional potential energy functions (PEFs) for the OCS+(X 2Π) state have been generated and used in the variational Renner–Teller calculations of the vibronic states. The energies of all vibronic states (J=P) for J=1/2, 3/2, 5/2, and 7/2 have been computed in the energy range of ≈4000 cm−1 above the IE[OCS+(X 2Π3/2)] for the assignment of the experimental spectrum. By a minor modification of the ab initio PEFs, good correlations are found between the experimental and theoretical Renner–Teller structures. Similar to the PFI-PE bands for CO2+(X 2Πg) and CS2+(X 2Πg), weak transitions have been detected in the PFI-PE band for OCS+(X 2Π), which are forbidden in the Franck–Condon approximation. The nonvanishing single-photon ionization cross sections involving the excitation of the bending vibrational modes of OCS+, CO2+, and CS2+, in their ground electronic states are attributed to the symmetries of the geometry-dependent electronic transition dipole operator components.

A high-resolution vacuum ultraviolet photoionization, photoelectron, and pulsed field ionization study of CS2 near the CS2+(X 2Π3/2,1/2) thresholds

High-resolution photoionization efficiency (PIE) and pulsed field ionization photoelectron (PFI-PE) spectra for CS2 have been measured using coherent vacuum ultraviolet (VUV) laser radiation in the energy range of 81 050–82 100 cm−1. The PIE and threshold photoelectron (TPE) spectra for CS2 in the energy range of 80 850–82 750 cm−1 have also been obtained using synchrotron radiation for comparison with results of the VUV laser study. The analysis of the PIE spectra reveals three Rydberg series converging to the excited CS2+(2Π1/2) spin–orbit state. These series, with quantum defects of 1.430, 1.616, and 0.053, are associated with the [2Π1/2]npσu, [2Π1/2]npπu, and [2Π1/2]nfu configurations, respectively. The Stark shift effect on the ionization threshold of CS2 has been examined as a function of dc electric fields (F) in the range of 0.65–1071 V/cm. The observed F dependence of the Stark shift for the ionization onset of CS2 is consistent with the prediction by the classical adiabatic field ionization formula. The extrapolation of the ionization onset to zero F yields accurate values for IE[CS2+(X̃ 2Π3/2)]. This study shows that in order to determine accurate IEs and to probe autoionizing structures for molecular species by PIE measurements, it is necessary to minimize the electric field used for ion extraction. The assignment of Renner–Teller structures resolved in the VUV PFI-PE spectrum is guided by the recent nonresonant two-photon (N2P) PFI-PE and theoretical studies. The analysis of the PFI-PE spectrum also yields accurate values for IE[CS2+(X̃ 2Π3/2,1/2)]. Taking average of the IE values determined by VUV-PFI-PE, N2P-PFI-PE, and Stark field extrapolation methods, we obtain a value of 81 285.7±2.8 cm−1 for IE[CS2+(X̃ 2Π3/2)]. For IE[CS2+(2Π1/2)], we recommend a value of 81 727.1±0.5 cm−1 determined by the Rydberg series analysis. A theoretical simulation of the 2Π3/2(000) and 2Π1/2(000) VUV-PFI-PE band profiles reproduces the observed branching ratio of 1.9±0.3 for CS2+(X̃ 2Π3/2)/CS2+(2Π1/2). The relative intensities of vibronic structures observed in the VUV PFI-PE and TPE spectra are in agreement. Evidence is found, indicating that the strongly (Stark field induced) autoionizing Rydberg state, 17pσu, which is ≈10 cm−1 below the IE of CS2, has a minor contribution to the observed profile for the X̃ 2Π3/2(000) PFI-PE band.