Pulsed Field Ionization Photoelectron Research Articles

Vacuum ultraviolet laser pulsed field ionization photoelectron studies of polyatomic species: Accurate ionization energies of CH3SH and CH3CH2SH

The vacuum ultraviolet (VUV) pulsed field ionization photoelectron (PFI-PE) spectra for CH3SH and CH3CH2SH have been obtained near their ionization thresholds. Using a semiempirical simulation scheme, we have obtained satisfactory fits to fine structures resolved in the VUV-PFI-PE spectra, yielding accurate ionization energies of 76 256.3±2.9 cm−1 (9.454 58±0.000 36 eV) and 74 948.7±2.9 cm−1 (9.292 46±0.000 36 eV) for CH3SH and CH3CH2SH, respectively.

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.

High-resolution zero-kinetic-energy pulsed field ionization photoelectron spectra of the Na(H2O) complex

Measured single-photon zero-kinetic-energy pulsed field ionization (ZEKE-PFI) photoelectron spectra of the sodium–water complex are presented and compared with the results of rotationally resolved ab initio calculations. The very nonatomiclike behavior of the photoionization of this Na(H2O) complex is essential in accounting for several significant features in these spectra. Agreement between the calculated and measured photoelectron spectra is encouraging. Furthermore, these results suggest that combined experimental and theoretical studies of ZEKE-PFI spectra can be very useful in elucidating the molecular structure and intermolecular force fields of small clusters.

Rotationally resolved photoelectron study of O2: Identification of the vibrational progressions for O2+(2 2Πu,2Σu−) at 19.6–21.0 eV

On the basis of rotationally resolved pulsed field ionization photoelectron measurements, we have shown that the weak vibrational progressions in the energy region of 19.6–21.0 eV observed in recent high resolution photoelectron spectroscopic studies of O2 are associated with transitions to the 2 2Πu 1/2 (2 2Πu 3/2) and 2Σu− states of O2+, with ionization energies of 19.6268±0.0005 eV (19.6329±0.0005 eV) and 20.3528±0.0005 eV, respectively.

A high resolution pulsed field ionization photoelectron study of O2 using third generation undulator synchrotron radiation

We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1–19.4 eV, yielding accurate ionization energies for the transitions O2+(b 4Σg−, v+=0–9, N+=1)←O2(X 3Σg−, v=0, N=1).

High resolution threshold and pulsed field ionization photoelectron spectroscopy using multi-bunch synchrotron radiation

We have demonstrated a resolution of 0.8 meV [full width at half-maximum (FWHM)] for threshold photoelectron measurements using a steradiancy-type zero kinetic energy photoelectron (ZEKE-PE) analyzer and the high resolution monochromatized vacuum ultraviolet (VUV) undulator synchrotron radiation of the chemical dynamics beamline at the advanced light source (ALS). Using this high resolution ZEKE-PE energy analyzer to filter prompt electrons and by employing a proper voltage pulsing scheme adapted to the timing structure of the ALS, we have achieved a resolution of 0.5 meV (FWHM) for pulsed field ionization photoelectron (PFI-PE) measurements with little contamination from prompt photoelectrons produced from direct photoionization and autoionizing processes. The experiment scheme presented here is generally applicable to PFI-PE studies using multi-bunch VUV synchrotron radiation at other synchrotron radiation facilities.

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.

Nonresonant two-photon pulsed field ionization photoelectron spectroscopic study of CH 3SH and CH 3CH 2SH

The threshold photoelectron spectra for CH 3SH and CH 3CH 2SH in the photon energy regions of 75 600–77 400 and 74 600–76400 cm −1, respectively, have been measured using the nonresonant two-photon pulsed field ionization photoelectron (N2P-PFI-PE) spectroscopic technique. The respective ionization energies (IEs) for CH 3SH and CH 3CH 2SH are determined to be 76 262 ± 5 cm −1 (9.4553 ± 0.0006 eV) and 74 951 ± 5 cm −1 (9.2927 ± 0.0006 eV). The theoretical IEs of 9.46 eV for CH 3SH and 9.30 eV for CH 3CH 2SH obtained by the Gaussian-2 (G2) procedure are in excellent agreement with the experimental values. Ab initio calculations for CH 3CH 2SH and CH 3CH 2SH + at the G2 level indicate that both CH 3CH 2SH and CH 3CH 2SH + exist as a gauche- or a trans-conformer with the gauche-conformer slightly more stable. Comparison of the experimental and theoretical results show that the main vibrational features resolved in the PFI-PE spectra can be assigned to excitation of vibrational modes involving the H-S torsional, C-S stretching, and/or H-S bending vibrations of CH 3SH + and gauche-CH 3CH 2SH +. A doublet observed for the H-S torsional mode is tentatively attributed to mixing of the harmonic vibrational levels due to the low potential barriers between the potential wells of the gauche- and trans- conformers of CH 3CH 2SH and CH 3CH 2SH +. The vibrational structures observed in the PFI-PE spectra for CH 3SH and CH 3CH 2SH are consistent with the expectation that ionization involves the ejection of a mostly nonbonding electron, associated with the S.

Two-photon ionization and dissociation of ethyl iodide

Both spin–orbit states of C2H5I+, the X̃1 2E1/2 and the X̃2 2E1/2 electronic states, are studied by one-color two-photon ionization of ethyl iodide, utilizing the zero kinetic energy pulsed field ionization (ZEKE-PFI) photoelectron technique. Compared with conventional photoelectron data the two-photon spectra exhibit a more extensive vibrational structure mainly involving the C–I stretching and the C–C–I bending modes. This additional vibrational excitation in the two-photon spectra is discussed in terms of mode selective resonance enhancement by the dissociative Ã-state continuum. Accurate values for the vibrational frequencies and the ionization energies are obtained. The appearance of nontotally symmetric modes in the two-color ZEKE spectra is reported.