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Notes: Absolute total cross sections for H2O+ and OH+ formed by the O+(4So)+H2O reaction have been measured in the center-of-mass collision energy (Ec.m.) range of ≈0.1–60 eV. The cross sections for H2O+ are greater than those reported previously. The structure observed in the cross section curve for OH+ suggests that the OH++OH channel is produced at Ec.m.≈0.1–5.0 eV via a long-lived complex mechanism, while the OH++O+H channel is formed at Ec.m.≈5–60 eV by a short range charge transfer-predissociation pathway. © 1995 American Institute of Physics.

Notes: 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. © 1999 American Institute of Physics.

Notes: Accurate spin–orbit splitting constants (Av+) for the vibrational levels v+=0–41 of CO+(A 2Π3/2,1/2) have been determined in a rotationally resolved pulsed field ionization photoelectron study. A change in slope is observed in the v+ dependence for Av+ at v+(approximate)19–20. This observation is attributed to perturbation of the CO+(A 2Π) potential by the CO+(B 2Σ+) state. Theoretical Av+ values for CO+(A 2Π3/2,1/2, v+=0–41) have also been obtained using a newly developed ab initio computational routine for spin–orbit coupling calculations. The theoretical Av+ predictions computed using this routine are found to be in agreement with the experimental Av+ values for CO+(A 2Π3/2,1/2, v+=0–41). Similar Av+ calculations obtained for O2+(X 2Π3/2,1/2g, v+=0–38) are also in accord with the recent experimental Av+ values reported by Song et al. [J. Chem. Phys. 111, 1905 (1999)]. © 1999 American Institute of Physics.

Notes: 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 (approximate)0.2–0.6 μs, which are significantly shorter than those of (approximate)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. © 1999 American Institute of Physics.

Notes: 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 (approximate)0.33 μs, which are significantly shorter than the values of (approximate)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. © 1998 American Institute of Physics.

Notes: The absolute total cross sections for CH3CH2+, C2H4+, C2H3+, CH3+, CH2SH+(CH3S+), CH2S+(HCSH+), CHS+(CSH+), and H2S+ produced by the collision-induced dissociation (CID) reaction of CH3CH2SH++Ar have been measured in the center-of-mass collision energy (Ec.m.) range of 1–42 eV. Using the charge transfer probing technique, we found that the mass 47 product ions have overwhelmingly the CH2SH+ structure. The onsets for CH3CH2+, C2H4+, C2H3+, CH2SH+, H2S+, and CH3+ are consistent with their corresponding thermochemical thresholds. The formation of the higher energy channels CH3CH2++SH and CH3+CH2SH+, which involve the C–S and C–C bond scissions, are found to dominate in the entire Ec.m. range. The lower energy channel corresponding to the formation of CH3CHSH++H is not found. The strong preference observed for the formation of the higher energy channels is in accord with the conclusion obtained in the recent CID study of CH3SH+, providing evidence that the CID of CH3CH2SH+ is also nonstatistical. The high yields of CH3CH2++SH and CH2SH++CH3 are attributed to the more efficient translational to vibrational energy transfer for the low frequencies C–S and C–C stretching modes than for the high frequencies C–H and S–H stretching modes, along with the weak couplings between these low and high frequencies vibrational modes of CH3CH2SH+. The relative abundances of product ions formed by the single-photon ionization of CH3CH2SH were also measured for comparison with the CID results. The CH3CHSH++H channel is observed in the photoionization of CH3CH2SH. Similar to the finding in the photoionization of CH3SH, the relative abundances of fragment ions formed in the photoionization of CH3CH2SH are in qualitative accord with statistical predictions. To rationalize the dissociation mechanisms of CH3CH2SH+, we have also performed ab initio calculations to locate the possible transition structures for the observed dissociation channels. © 1998 American Institute of Physics.

Notes: The photoionization efficiency (PIE) spectra for M(CO)n+ (n=0–6) from M(CO)6, M=Cr, Mo, and W, have been measured in the photon energy range of 650–1600 Å. Based on the ionization energies for M(CO)6 and appearance energies (AEs) for M(CO)n+ (n=0–5) determined here, we have obtained estimates for the sequential bond dissociation energies (D0) for CO–M(CO)n−1+ (n=1–6). The comparison between the D0 values for the Cr(CO)6+ system obtained here and in the recent collisional induced dissociation and theoretical studies suggests that D0 values for CO–M(CO)n−1+ (n=3–6) based on this PIE experiment are reliable. The PIE results reveal the general trend for individual D0 values that D0[CO–Cr(CO)n−1+]〈D0[CO–Mo(CO)n−1+]〈D0[CO–W(CO)n−1+] (n=3–6). The comparison of the first D0 values for M(CO)6+ obtained here and those for M(CO)6 reported previously provides strong support for the theoretical analysis that the importance of relativistic effects, which give rise to more efficient M to CO π-back-donation in M(CO)6, is in the order W(CO)6〉Mo(CO)6〉Cr(CO)6. © 1997 American Institute of Physics.

Notes: 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+(X˜ 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+(X˜ 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+(X˜ 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+(X˜ 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 (approximate)10 cm−1 below the IE of CS2, has a minor contribution to the observed profile for the X˜ 2Π3/2(000) PFI-PE band. © 1997 American Institute of Physics.