Search

We report results from a novel fully ab initio method for simulating the time-resolved photoelectron angular distributions around conical intersections in CS_2. The technique employs wavepacket densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of the calculated molecular-frame photoelectron angular distributions with measured values for CS_2 demonstrates that this approach can successfully illuminate, and disentangle, the underlying coupled nuclear and electronic dynamics around conical intersections in polyatomic molecules.

We describe a theoretical approach to molecular photoionization that includes first-order corrections to the dipole approximation. The theoretical formalism is presented and applied to photoionization of H_2 over the 20- to 180-eV photon energy range. The angle-integrated cross section σ, the electric dipole anisotropy parameter β_e, the molecular alignment anisotropy parameter β_m, and the first-order nondipole asymmetry parameters γ and δ were calculated within the single-channel, static-exchange approximation. The calculated parameters are compared with previous measurements of σ and β_m and the present measurements of β_e and γ+3δ. The dipole and nondipole angular distribution parameters were determined simultaneously using an efficient, multiangle measurement technique. Good overall agreement is observed between the magnitudes and spectral variations of the calculated and measured parameters. The nondipole asymmetries of He 1s and Ne 2p photoelectrons were also measured in the course of this work.

Measurements of photoelectron angular distributions for carbon K-shell ionization of fixed-in-space CO2 molecules with the molecular axis oriented along, perpendicular and at 45 degrees to the electric vector of the light are reported. The major features of these measured spectra are fairly well reproduced by calculations employing a relaxed-core Hartree-Fock approach. In contrast to the angular distribution for K-shell ionization of N-2, which exhibits a rich structure dominated by the f-wave (l = 3) at the shape resonance, the angular distribution for carbon K-shell photoionization of CO2 is quite unstructured over the entire observed range across the shape resonance.

The photoionizationdynamics of rotationally hot CO, photodissociated from OCS, have been studied using laser photoelectron spectroscopy via the intermediate B^1Σ^+Rydberg state leading to the X^2Σ^+ of the ion. The photodissociation of OCS near 230 nm produces rotationally hot, but vibrationally cold CO (X^1Σ^+,N″,v″=0,1) fragments along with S (^1D) atoms. These high rotational levels show photoelectron spectra with a very strong ΔN=0 transition and weaker ΔN=±1, ±2, and ±3 transitions. Agreement between measured and calculated spectra is good and suggests that there is significant angular momentum coupling in the photoelectron orbital. In the ionization step not only Δv=0, but also off-diagonal, non-Franck–Condon (Δv≠0) transitions are observed. The intensities of these transitions vary strongly within the region studied and can be explained by the excitation of superexcited Rydberg states with an A^2Π core.

We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame photoelectron angular distributions around conical intersections in ${\mathrm{CS}}_{2}$. The technique employs wave packet densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of these results with measured molecular-frame photoelectron angular distributions for ${\mathrm{CS}}_{2}$ demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.

We present experimental and theoretical results on rotational distributions of CO^+ (B^2Σ^+) photoions. Rotational distributions were determined for both the v^+=0 and v^+=1 vibrational levels following photoionization of cold (T_0≈9K) neutral CO target molecules. Data were generated using dispersed ionic fluorescence over a wide range of photoelectron kinetic energies, 0⩽E_k⩽120eV, which allows one to interrogate the ionization dynamics. This wide spectral coverage permits illustrative comparisons with theory, and calculated spectra are presented to interpret the data. In particular, the comparison between theory and experiment serves to identify the strong continuum resonant enhancement at hν_(exc)≈35eV in the l=3 partial wave of the 4σ→kσ ionization channel, as this feature has profound effects on the ion rotational distributions over a wide range of energy. Second, there are differences between the rotational substructure for the v^+=0 and v^+=1 vibrational levels. All of the experimentally observed features and trends are reproduced by theory, and the consequences of these comparisons are discussed.

Femtosecond energy- and angle-resolved pump-probe photoelectron spectra of wavepackets in a double-minimum state of Na 2 are studied. These studies illustrate that a robust description of the photoionization dynamics significantly enhances the utility of time-resolved photoelectron spectroscopy as a probe of wavepackets and of the evolution of electronic structure, particularly in regions of avoided crossings, and that the dependence of these angular distributions on the relative polarizations of the pump and probe lasers can be exploited to monitor molecular alignment.

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.

Several recent studies have demonstrated how well-suited femtosecond time-resolved photoelectron spectra are for mapping wavepacket dynamics in molecular systems. Theoretical studies of femtosecond photoelectron spectra which incorporate a robust description of the underlying photoionization dynamics should enhance the utility of such spectra as a probe of wavepackets and of the evolution of electronic structure. This should be particularly true in regions of avoided crossings where the photoionization amplitudes and electronic structure may evolve rapidly with geometry. In this paper we present the results of studies of energy- and angle-resolved femtosecond photoelectron spectra for wavepackets in the diatomic systems, Na2 and NaI. Both cases involve motion through regions of avoided crossings. In Na2, however, wavepacket motion occurs on a single adiabatic potential with an inner and outer well and a barrier between them, while in NaI wavepackets move on the nonadiabatically coupled covalent (NaI) and ionic (Na+I−) potentials. Results of these studies will be used to illustrate the insight into wavepacket dynamics that time-resolved photoelectron spectra provide. For example, in the case of NaI these angle-resolved photoelectron spectra seem to offer some promise for probing real-time dynamics of intramolecular electron transfer occurring in the crossing region of the ionic and covalent states.

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.

Results of calculations of the rotationally resolved photoelectron spectra of the V_2^+ ground state obtained by two-color excitation via the A ^3Π_u ← X^3 Σ_g^- system are reported and compared with recent pulsed-field-ionization zero-kinetic-energy measurements of these spectra. Agreement between the measured and calculated spectra is very encouraging. Both illustrate that the spin-orbit interaction plays an important role in photoionization of the A ^3Π_u state of V_2 leading to the X^4Σ_g^− state of V_2^+. Striking differences are seen in the ion rotational distributions for ionization via the A ^3Π_(1u) and A ^3Π_(2u spin-orbit components.

We report results of experimental and theoretical studies of the vibrational branching ratios for CO 4sigma(-1) photoionization from 20 to 185 eV. Comparison with results for the 2sigma(u)(-1) channel of the isoelectronic N-2 molecule shows the branching ratios for these two systems to be qualitatively different due to the underlying scattering dynamics: CO has a shape resonance at low energy but lacks a Cooper minimum at higher energies whereas the situation is reversed for N-2.

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.

We present the first measurements of ion rotational distributions for photoionization over an extended range [0 ≤ E_K ≤ 200 eV for N_2 (2σ^(−1)_u ) and 3 ≤ E_K ≤ 125 eV for CO (4σ^(−1))]. The N_2 ion rotational distributions are seen to change dramatically over this energy range, indicating that characteristically molecular behavior of the photoelectron persists far from ionization threshold. In addition, the N_2 and CO results show a strikingly different dependence on energy. Although differences are expected due to the absence of a center of symmetry in CO, detailed calculations reveal that this behavior arises from the presence of Cooper minima in the 2σ_u →kσ_g continuum in the case of N_2 and from an f‐wave shape resonance in the 4σ→kσ channel in CO. Agreement between measured and calculated ion rotational distributions is excellent. The N_2 results are also compared with electron bombardment ionization data. This comparison demonstrates that previous interpretations of electron bombardment data are prone to errors.

Results of calculations of single-photon pulsed-field ionization of the 1π orbital of the X ^1Σ^+ (v" = 0) ground state of CO leading to the A Π (v+ = 0-2) excited ionic state are reported. Agreement between the calculated and measured spectra is reasonable except for the A ^2Π_(1/2) (v^+ = 0) level which shows strong autoionization at larger positive ΔJ transitions. Our studies show that the direct photoionization of the In orbital is very atomiclike.

We present results of experiment and theory for the alignment of CO^+(B^2Σ^+) and N_2^+(B^2Σ^+_u) photoions over an extended energy range (0 ≤ E_k ≤ 210 eV for CO and 0 ≤ E_k ≤ 250 eV for N_2). The polarization of CO^+(B^2Σ^+ → X^2Σ^+) and N_2^+(B^2Σ^+ → X^2Σ^+_g) fluorescence is used to interpret the oscillator strength distributions for normally unresolved degenerate ionization channels. The results show the influence of a CO 4σ → kσ shape resonance clearly and agreement between theory and experiment is excellent. However, agreement between the calculated and measured values is less satisfactory for N_2. This behavior is somewhat surprising, as previous rotationally resolved fluorescence experiments have shown excellent agreement between theory and experiment. This comparison helps to illustrate the complementarity of alignment studies relative to alternative probes of ionization. For both N_2 and CO, the data indicate that the photoions retain significant alignment even at high energies. The results demonstrate that even well above threshold the spectral dependence of the alignment (i.e., polarization) is very sensitive to the molecular environment for photoejection. Such behavior provides useful insight into fundamental scattering phenomena in chemical physics.

Results of theoretical studies of rotationally resolved photoelectron spectra for (2 + 1’) resonance enhanced multiphoton ionization (REMPI) of the C ^2Σ^-, D ^2Σ^-, E ^2Σ^-, and F ^2Σ^- Rydberg states of ClO are presented. Cooper minima are predicted to occur in the electronic continua for photoionization of the D and F states and lead to unusual behavior in the ion rotational distributions. Strong partial wave I mixing is also predicted in the continua for the C and E states due to the nonspherical molecular ion potentials. These Cooper minima and l mixing make the associated photoionization dynamics quite nonatomiclike.

Combined theoretical and experimental studies of rotationally resolved photoelectron spectra for single‐photon ionization of the outermost valence orbitals of H2O, H2S, H2CO, and CH3 are reported. Agreement between calculated and measured spectra is very encouraging. Both show that photoionization dynamics is very molecular in origin for H2O, H2S, and H2CO but quite atomiclike for CH3. Parity selection rules and the angular momentum composition of the photoelectron are used to illustrate the dynamical aspects of photoionization of polyatomics as molecular symmetry changes in a group of structurally related systems.

We report both measured and calculated rotationally resolved threshold photoelectron spectra for single-photon coherent VUV ionization of the X ^1A_1 ground state of jet-cooled H_2O and H_2S. Agreement between the calculated and measured spectra is very encouraging. Both the H_2O and H_2S spectra show type a transitions which arise from odd angular momentum components of the photoelectron matrix element and are quite molecular in origin. These type a transitions, however, are much weaker in H_2S due to its more atomiclike character.

We present the first measurements of rotational distributions for photoionization over extended energy ranges [0 ≤ E_k ≤ 200 eV for N_2 (2σ^(-1)_u) and of 3 ≤ E_k ≤ 125 eV for CO (4σ^(−1))]. The N_2 and CO results show a strikingly unusual and different energy dependence. Although differences are expected due to the absence of a center of symmetry in CO, detailed calculations reveal that this behavior arises from the presence of Cooper minima in the photoelectron continuum (kσ_g) in the case of N_2 and from an f-wave shape resonance for 4σ^(−1) photoionization in CO.

Results of theoretical studies of rotational branching ratios and photoelectron angular distributions for (2 + 1) resonance-enhanced multiphonon ionization (REMPI) of NO via the Q_(21)(13.5) branch of the C ^2Π(3pπ) Rydberg state are reported and compared with recent measurements. Both calculated and measured spectra reveal that the ΔN=±2 peaks (ee line), which are very weak for the laser polarization parallel to the direction of photoelectron detection, have about 1/4 the intensity of the main ΔN=0 peak when the laser polarizaiton is rotated by 90°. This dependence on laser polarization is due to strong inteference among partial wave components of the photoelectron matrix element. This behavior is very evident in the photoelectron angular distribution. In addition, a Cooper minimum is predicted in the d wave of the 3pπ→Kδ ionization channel.

We discuss results of theoretical studies of rotationally resolved photoelectron spectra of the CH, NH, and OH radicals, including predictions of zero-kinetic-energy (ZEKE) spectra for molecular ions produced via resonance enhanced multiphoton ionization. The influence of dynamical features such as Cooper minima, orbital evolution, and 1 mixing are examined for photoionization of the 3σ Rydberg orbital in these radicals. We also make specific predictions for the photoelectron spectra for each molecule within the context of current experimental capabilities.

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Recent advances in experimental techniques now make it possible to readily achieve rotational resolution in molecular photoelectron spectra. Here we discuss results of our recent theoretical studies of rotationally resolved photoelectron spectra at low and very near-threshold energies for molecules such as HBr, NH, OH, NO, CO, H2_O, and H_2S. These studies serve to reveal the rich dynamics of quantum-state specific studies of molecular photoionization and, where possible, provide a robust description of key spectral features of interest in related experimental studies.

Combined theoretical and experimental studies of He I photoelectron spectra for photoionization of the X^1Σ^+ (v″ = 0) ground state HF leading to the X^2Π (v^+ = 0–2) and A ^2Σ^+ (v^+ = 0–3) ionic states, including detailed analyses of the rotational bandshapes, are reported. Agreement between the measured bandshapes and the calculated spectra, convoluted with the experimental resolution function (22 meV FWHM), is excellent. Ionization potentials of 16.046(1) eV and 19.116(2) eV are obtained for formation of the X and A ionic states, respectively. The vibrational spacings, deduced from the measured data by careful bandshape analysis, are in very good agreement with spectroscopic values.

Single‐photon threshold photoionization spectra for jet‐cooled NO have been measured for the v^+=0 and 1 vibrational levels of the X ^1Σ^+ ground state of NO^+. The NO^+ rotational state distribution for the v^+=0 level is shown to be perturbed by nearby autoionizing levels, whereas the v^+=1 level exhibits a cation rotational distribution which is in near quantitative agreement with calculated spectra near threshold. Only small changes in total angular momentum are observed (‖ΔJ‖=‖J^+ − J‘‖≤5/2) even though a wide range of photoelectron angular momenta (l=0–3) are predicted to contribute to the near‐threshold photoelectron continua. The present results are also discussed in light of recently published two‐photon threshold photoionization spectra of NO which exhibit nearly identical NO^+ rotational state distributions.

Results of theoretical studies of rotationally resolved photoelectron spectra for (2 + 1’) resonance enhanced multiphoton ionization (REMPI) of the C ^2Σ^-, D ^2Σ^-, E ^2Σ^-, and F ^2Σ^- Rydberg states of ClO are presented. Cooper minima are predicted to occur in the electronic continua for photoionization of the D and F states and lead to unusual behavior in the ion rotational distributions. Strong partial wave I mixing is also predicted in the continua for the C and E states due to the nonspherical molecular ion potentials. These Cooper minima and l mixing make the associated photoionization dynamics quite nonatomiclike.

We report the results of theoretical studies of the time-resolved femtosecond photoelectron spectroscopy of quantum wavepackets through the conical intersection between the first two (2)A' states of NO(2). The Hamiltonian explicitly includes the pump-pulse interaction, the nonadiabatic coupling due to the conical intersection between the neutral states, and the probe interaction between the neutral states and discretized photoelectron continua. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Photoelectron angular distributions are seen to provide a clearer picture of the ionization channels and underlying wavepacket dynamics around the conical intersection than energy-resolved spectra. Time-resolved photoelectron velocity map images are also presented.

Results of calculations of rotationally resolved photoelectron spectra for resonance enhanced multiphoton ionization (REMPI) of SiF via the B ^2Σ^+ (4sσ), C" ^2Σ^+ (4pσ), and C’ ^2Π (4pπ) Rydberg states are reported. In addition to the expected ΔN=even peaks, unusually strong ΔN=±1 transitions are predicted for photoionization of the B ^2Σ^+ state. These unusual transitions are due to even angular momentum components of the photoelectron matrix element and arise from the formation of Cooper minima in the ionization channels and strong l mixing in the electronic continuum induced by the nonspherical molecular ion potential. Unexpected ΔN=0,±2 transitions, due to odd wave contributions to the photoelectron matrix element, are also predicted for photoionization of the C" ^2Σ^+ state. Asymmetrical ion distributions with respect to ΔN=0 are also predicted for the C’ ^2Π state. Cooper minima are predicted to occur in the l=2 wave of the kπ photoelectron channel for the B state and in the l=4 wave of the kσ and kπ channels for the C‘ state. Photoelectron angular distributions provide further insight into the photoionization dynamics.

Ion rotational distributions for single‐photon VUV photoionization of the 1b_1 orbital of the X̃ ^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.

We report the results of theoretical studies of the rotationally resolved photoelectron spectra of ground state O_2 leading to the X ^2Π_g state of O^+ 2 via the absorption of a single vacuum ultraviolet photon. These studies elaborate on a recent report [M. Braunstein e t a l., J. Chem. Phys. 9 3, 5345 (1990)] where we showed that a shape resonance near threshold creates a significant dependence of the rotational branching ratios on the ion vibrational level. We also showed that analysis of the rotational branches yields detailed information on the angular momentum composition of the shape resonance. We continue this analysis giving a comprehensive derivation of the rotationally resolved cross sections and photoelectron angular distributions. We discuss the selection rules implied by these expressions and present very high resolution cross sections (J→J^+) obtained using static‐exchange photoelectron orbitals and explicitly taking into account the internuclear distance dependence of the electronic transition moment. These cross sections illustrate the selection rules and show more explicitly the angular momentum composition of the shape resonance. We also present rotationally resolved photoelectron angular distributions which would be expected at low energy.

Cooper minima are predicted to occur in the 3pσ→kσ(l=2) and 3pσ→kπ(l=2) channels in the resonance enhanced multiphoton ionization of NO via the D ^2Σ^+(3pσ) Rydberg state. The low energy kσ(l=2) Cooper minimum leads to the observed ΔN = N^+ − N’=0 photoelectron peak, in addition to the ΔN = ±1, ±3 peaks seen in the rotational spectra. The Cooper minima are accompanied by significant l mixing in the continuum due to the nonspherical molecular potential and result in a strong dependence of rotational branching ratios and angular distributions on photoelectron kinetic energy. A Cooper minimum is also predicted in the 3pπ→kδ(l=2) channel for photoionization of the C ^2Π(3pπ) Rydberg state. The effect of this Cooper minimum on photoelectron spectra and photoelectron angular distributions is also investigated.

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.

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay.

Strong non‐Franck–Condon behavior is predicted to occur in the vibrationally resolved photoionization spectra of NH for (3+1) resonance enhanced multiphoton ionization processes via the 3 ^3Π Rydberg state. The non‐Franck–Condon effects are interpreted on the basis of rapid orbital evolution, Cooper minima, and internuclear distance dependence of the dipole transition moment and cross sections. A Cooper minimum occurs in the 5σ→kπ channel at small internuclear distances, where NH resembles its united atom, oxygen. The iterative Schwinger variational method and multiplet‐specific ion potentials are employed in the calculation of the photoelectron continuum wave functions. Cross sections and asymmetry parameters for photoionization of the NH ground state leading to the X ^2Π, a ^4Σ^−, and A ^2Σ^− ions are also reported.

In this paper we discuss the single‐photon ionization dynamics of the d ^1Π_g Rydberg state of O_2. Comparison is made with vibrationally resolved measurements of photoelectron spectra which employ (2+1) resonance enhanced multiphoton ionization (REMPI) through the d ^1Π_g state. A σ_u shape resonance near the ionization threshold leads to non‐Franck–Condon vibrational branching ratios and a substantial dependence of photoelectron angular distributions on the vibrational state of the X ^2Π_g ion. Significant differences exist between our one‐electron predictions and experiment. These are mainly attributed to electronic autoionization of repulsive ^1∑^−_u , ^1∑^+_u , and ^1Δ_u states associated with the 1π^3_u 1π^3_g configuration. A proposed singlet "K" ^1Π_u Rydberg state converging to the A ^2Π_u ion probably also contributes to autoionization in the d ^1Π_g state spectrum. We also show that autoionizing H and J ^3Π_u Rydberg states of O_2 converging to the a ^4Π_u and A ^2Π_u ionic thresholds, respectively, may play a previously unsuspected role in the C ^3Π_g state one‐color REMPI spectra. We discuss multiplet‐specific (spin‐dependent) effects via comparison of these results with recent experimental and theoretical studies of O_2 C ^3Π_g photoionization.

We review and discuss some recent resonance enhanced multiphoton ionization (REMPI) studies of small molecules, emphasizing the role that shape resonances and Rydberg orbital evolution play in determining vibrational and rotational ion distributions. Strong non-Franck-Condon effects observed in ion vibrational distributions for REMPI of O_2 via the C ^3Π_g and d ^1Π_g Rydberg states are partially attributed to a σ_u shape resonance, previously observed in ground state photoionization of diatomic molecules. Autoionization of repulsive valence states also induces non-Franck-Condon effects in these REMPI spectra. Significant non-Franck-Condon effects in molecular REMPI spectra also arise from a mechanism associated with rapid evolution of the resonantly populated Rydberg orbital with changing inter-nuclear distance. These effects should be most pronounced in diatomic hydrides, and are illustrated by theoretical predictions of vibrational and rotational ion distributions for REMPI of OH and CH via the D ^2Σ^− and E' ^2Σ^+ Rydberg states, respectively.

The iterative Schwinger variational method is used to obtain cross sections and photoelectron asymmetry parameters for photoionization of the three outermost valence orbitals (1b1, 3a1, and 1b2) of H2O for photon energies from near threshold to 50 eV. A comparison of these calculated results with available experimental data is encouraging.

Observations are reported for the first time of significant nondipole effects in the photoionization of the outer-valence orbitals of diatomic molecules. Measured nondipole angular-distribution parameters for the 3sigma(g), 1pi(u), and 2sigma(u) shells of N2 exhibit spectral variations with incident photon energies from thresholds to approximately 200 eV which are attributed via concomitant calculations to particular final-state symmetry waves arising from (E1)multiply sign in circle(M1,E2) radiation-matter interactions first-order in photon momentum. Comparisons with previously reported K-edge studies in N2 verify linear scaling with photon momentum, accounting in part for the significantly enhanced nondipole behavior observed in inner-shell ionization at correspondingly higher momentum values in this molecule.

We have measured photoelectron angular distributions in the molecular frame (MF-PADs) for O 1s photoemission from CO2, using photoelectron-O+–CO+ coincidence momentum imaging. Results for the molecular axis at 0, 45 and 90° to the electric vector of the light are reported. The major features of the MF-PADs are fairly well reproduced by calculations employing a relaxed-core Hartree–Fock approach. Weak asymmetric features are seen through a plane perpendicular to the molecular axis and attributed to symmetry lowering by anti-symmetric stretching motion.

The application of femtosecond pump-probe photoelectron spectroscopy to directly observe vibrational wave packets passing through an avoided crossing is investigated using quantum wave packet dynamics calculations. Transfer of the vibrational wave packet between diabatic electronic surfaces, bifurcation of the wave packet, and wave packet construction via nonadiabatic mixing are shown to be observable as time-dependent splittings of peaks in the photoelectron spectra.

We present results of calculations of the angular distributions of K-shell photoelectrons from fixed-in-space CO and N_2 molecules emitted by circularly polarized light. These calculated distributions account well for the strong circular dichroism recently reported in measurements of photoelectron distributions in these two systems. The calculations also serve to identify regions of photoelectron energy where pronounced circular dichroism can be expected in these spectra.

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.

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

We present the results of applications of the iterative Schwinger variational method to obtain photoionization cross sections and phoelectron angular distributions for ionization out of the four outermost valence orbitals 1b_(3u), 1b_(3g), 3a_g and 1b_(2u) of ethylene for photon energies ranging from near threshold to 30 eV. The observed resonance-like maxima in the cross sections for ionization of the 1b_(3g) and 3a_g orbitals are reproduced in our calculations. The disagreement between our cross sections and the experimental data for the 1b_(3u) orbital below 17 e V is attributed to autoionization, which is not accounted for in our calculations.

Nondipole photoelectron parameters ζ have been obtained experimentally for the outer-valence 3σ_g, 1π_u and 2σ_u shells in molecular nitrogen from threshold to ∼ 200 eV photon energy. Significant nondipole effects are observed even in the immediate threshold regions of these valence-shell distributions. The results of preliminary calculations for the 3σ_gand 2σ_u clarify the origins of the observed features in terms of contributing molecular symmetry channels. Theory and experiment are in excellent accord, suggesting that the large nondipole effects previously observed in atoms and the K-shells of molecules can also appear at low photon energies in the outer-valence shells of molecules.

We determine the rotationally state‐resolved 2σ_u^(−1) photoionization of N_2 into alternative vibrational channels as a function of energy over a 200 eV range. Experiment and theory reveal that Cooper minima highlight the coupling between electronic, vibrational, and rotational degrees of freedom over this very wide range.

Ionization of K-shell or, more generally, of deep inner-shell electrons in atoms and molecules is accompanied by a considerable rearrangement of the valence (outer-shell) electrons in response to the reduced shielding of the nuclear attraction.(1) This adjustment of the valence electrons, referred to as electronic relaxation, leads to a significant energy lowering of the final ionic state relative to a state where the valence electron distribution of the initial state is maintained (“frozen”). The magnitude of this relaxation energy scales with the number of valence electrons.(2,3) In the case of the K-shell ionization of second-row atoms (Z = 3−10), for example, the relaxation energies (in eV) are approximately given by E R(Z) = 3.1 (Z − 2.2). In a molecular environment the corresponding relaxation energies are typically 2−3 eV larger than the values for the free atom. Relaxation not only plays a role in the ionic core but also affects the motion of the outgoing photoelectron. The relaxation of the valence electrons, being essentially a contraction of the valence charge distribution, quite effectively screens the inner-shell hole potential experienced by the photoelectron. This means that the potential of the relaxed ionic core is less attractive than its unrelaxed (frozen) counterpart. As a consequence, resonances in the photoionization cross section will appear at higher energy for a relaxed core than for an unrelaxed (frozen) core. Concomitantly with the shift to higher energy, the resonance peaks will be lowered and broadened as a result of relaxation.

Results of calculated and measuredphotoelectron spectra via the [a ^1Δ]3dπ ^2Φ state of SH are reported. These spectra show a strong asymmetry away from classical expectations and only transitions up to ΔN=±2 are observed, in contrast to ΔN=±4 expected for photoionization of a 3dπ orbital. Agreement between the calculated and measured spectra is excellent.

We explore the application of a quantum vibrational wavepacket dynamics formulation of femtosecond energy- and angle-resolved photoelectron spectroscopy to the study of nuclear and electron dynamics. The formulation incorporates geometry- and energy-dependent photoionization matrix elements and is well suited for studies of nonadiabatic dynamics where nuclear and electronic degrees of freedom are coupled. In this paper we explore two aspects of angle-resolved pump–probe photoelectron spectroscopy, using NO_2 and NaI as examples. The first is a refinement of femtosecond photoelectron spectroscopy in which the time derivative of the velocity map image is seen to more readily characterize the nuclear wavepacket dynamics in non-adiabatic regions. The other is an analysis to extract the effects of electronic-state fluctuation from sub-femtosecond scale photoelectron dynamics. Within the framework of the Born–Huang (Oppenheimer) expansion where the total wavefunction is expressed in products of stationary-state electronic wavefunctions and time-dependent nuclear wavepackets, we seek to identify how the effect of electron dynamics might be manifested in principle.