Kwanghsi Wang

Rotationally resolved threshold photoelectron spectra of OH and OD

The results of combined experimental and theoretical studies of the rotationally resolved photoelectron spectra of OH and OD following single‐photon ionization are presented. The measured zero‐kinetic‐energy (ZEKE) spectra were obtained using pulsed field ionization in conjunction with a vacuum ultraviolet laser source. The OH^+ and OD^+ (X ^3Σ^−, v^+=0) rotational distributions were studied over the range 95.0–95.4 nm. Agreement between the observed and calculated spectra is very encouraging. Improved values for the ionization potentials of OH and OD (104 989 and 105 085 ± 2 cm^(−1), respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed.

Femtosecond energy- and angle-resolved photoelectron spectroscopy

We present a formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets and results of its application to the ^1Σ^+_u double-minimum state of aligned Na_2. The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump–probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.

Rotationally resolved photoelectron spectroscopy of the 2Σ- Rydberg states of OH: the role of Cooper minima

We have measured rotationally resolved photoelectron spectra of the OH radical using (2+1) resonance enhanced multiphoton ionizationspectroscopy via the D  ^2Σ^−(3pσ) and 3 ^2Σ^−(4sσ) Rydberg states. For the D  ^2Σ^−(3pσ) state, we observe primarily ΔN=even distributions of ionic rotational states, in contrast to the ΔN=odd distribution expected for ionization of a 3pσ Rydberg electron. The observations are described quantitatively by ab initio calculations which predict a Cooper minimum in the 3pσ→kπ(l=2) channel, whose occurrence determines the ΔN=even ion rotational distribution. In contrast, the 3 ^2Σ^−(4sσ) photoelectron spectra reveal a broad distribution in rotational levels, arising from greater l mixing in the higher Rydberg orbital and much weaker Cooper minima in the continuum.

FEMTOSECOND ENERGY- AND ANGLE-RESOLVED PHOTOELECTRON SPECTRA

Abstract We present energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wavepackets in the 1 Σ u + double-minimum state for aligned Na2. These results illustrate that a robust description of the underlying photoionization amplitudes can significantly enhance the utility of photoelectron spectroscopy as a probe of wavepacket motion and of the evolution of electronic structure, particularly in cases of avoided crossings and motion over large distances. The angular dependence of these photoelectron spectra provide insightful fingerprints of vibrational wavepacket dynamics and can be a useful real-time probe of molecular rotation.

Rotational branching ratios and photoelectron angular distributions in resonance enhanced multiphoton ionization of diatomic molecules

In this paper we extend a previous formulation of molecular resonance enhanced multiphoton ionization (REMPI) photoelectron spectra to explicitly include multiplet‐specific final state wave functions and intermediate coupling schemes. The results of this formulation should be well suited and helpful in quantitative theoretical studies of rotationally resolved REMPI spectra in many diatomic molecules of interest. As an example, we use this formulation to study the rotational branching ratios and photoelectron angular distributions for (3+1) REMPI of NH via the 3 3Π Rydberg resonant state. The predicted anomalous rotational distributions are interpreted as arising from a Cooper minimum in the l=2 component of the kπ photoionization channel. A number of other results are obtained and discussed.

Studies of electron transfer in NaI with pump–probe femtosecond photoelectron spectroscopy

We discuss an extension of our formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets to nonadiabatically coupled states and present results of its applications to wave packet motion on the ionic (Na^+I^−) and covalent (NaI) states of sodium iodide. The results of these studies suggest that the energy and angular distributions of these photoelectron spectra provide a useful mapping of the bifurcation of the wave packets through the crossing region and a valuable window on the intramolecular electron transfer occurring between the covalent and ionic states (NaI→Na^+I^−).

High-Resolution Photoelectron Spectroscopy of Molecules

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Here, we discuss and compare results of recent theoretical studies of rotationally resolved photoelectron spectra with measurements for molecules such as HBr, OH, NO, N2, CO, H20, H2CO, and CH3. These studies reveal the rich dynamics of quantum-state-specific studies of molecular photoionization and provide a robust description of key spectral features resulting from Cooper minima, autoionization, alignment, partial-wave mixing, and interference in related experimental studies.

Ion rotational distributions for near‐threshold photoionization of H2O

Ion rotational distributions for single‐photon VUV photoionization of the 1b_1 orbital of the X ^1A_1 ground state of the jet‐cooled water are reported. These spectra reveal significant type a transitions which are seen to arise from odd angular momentum components of the photoelectron matrix element. The resulting photoionization dynamics are quite nonatomic‐like.

Pulsed‐field ionization threshold photoelectron spectroscopy with coherent extreme ultraviolet radiation: A comparison of CO and N2

Single‐photon zero‐kinetic‐energy pulsed‐field‐ionization spectra have been measured for the v^+=0 and 1 levels of CO^+ (X  ^2Σ^+) and the v^+=0 level of N_2^+ (X  ^2Σ_g^+) by coherent XUV radiation. In spite of similarities in the electronic structure of CO and N_2, the measured ion spectra show dramatically different intensities for the Q branches. These threshold spectra are interpreted on the basis of ab initio calculations of the ion rotational distributions. Agreement between the calculated and measured spectra is very encouraging. Improved values for the ionization potentials of CO (113 025.6 and 115 211.2±1.5 cm^(−1) for v^+=0 and 1, respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed. The CO spectra show quite different behavior for the ΔN<0 transitions for v^+=0 and v^+=1 bands, which is interpreted in terms of the relative importance of rotational autoionization in the two bands.

Zero kinetic energy‐pulsed field ionization and resonance enhanced multiphoton ionization photoelectron spectroscopy: Ionization dynamics of Rydberg states in HBr

The results of rotationally resolved resonance enhanced multiphoton ionization photoelectron spectroscopy and zero kinetic energy‐pulsed field ionization studies on HBr via various rotational levels of the F^ 1Δ_2 and f^ 3Δ_2 Rydberg states are reported. These studies lead to an accurate determination of the lowest ionization threshold as 94 098.9±1 cm^(−1). Observed rotational and spin–orbit branching ratios are compared to the results of ab initio calculations. The differences between theory and experiment highlight the dominant role of rotational and spin–orbit interactions for the dynamic properties of the high‐n Rydberg states involved in the pulsed field ionization process.