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Recent measurements [R. Panajotovic, M. Jelisavcic, R. Kajita, T. Tanaka, M. Kitajima, H. Cho, H. Tanaka, and S. J. Buckman, J. Chem. Phys. 121, 4559 (2004)] and calculations [C. Trevisan, A. E. Orel, and T. N. Rescigno, Phys. Rev. A 70, 012704 (2004)] of the elastic electron cross section for C2F4 differ materially from our earlier calculations [C. Winstead and V. McKoy, J. Chem. Phys. 116, 1380 (2002)]. Some of the differences are readily attributed to approximations made in our computations, but an overall difference in cross section magnitude above ca. 10 eV was surprising. Here we report a reexamination of the electron-C2F4 elastic cross section. After eliminating or minimizing various possible sources of error, we continue to predict a substantially larger cross section at higher energies. [ABSTRACT FROM AUTHOR]

Recent measurements [R. Panajotovic, M. Jelisavcic, R. Kajita, T. Tanaka, M. Kitajima, H. Cho, H. Tanaka, and S. J. Buckman, J. Chem. Phys. 121, 4559 (2004)] and calculations [C. Trevisan, A. E. Orel, and T. N. Rescigno, Phys. Rev. A 70, 012704 (2004)] of the elastic electron cross section for C2F4 differ materially from our earlier calculations [C. Winstead and V. McKoy, J. Chem. Phys. 116, 1380 (2002)]. Some of the differences are readily attributed to approximations made in our computations, but an overall difference in cross section magnitude above ca. 10 eV was surprising. Here we report a reexamination of the electron-C2F4 elastic cross section. After eliminating or minimizing various possible sources of error, we continue to predict a substantially larger cross section at higher energies. [ABSTRACT FROM AUTHOR]

We report calculated cross sections for elastic and electronically inelastic collisions of low-energy electrons with sulfur hexafluoride, SF6. Elastic cross sections are computed within the fixed-nuclei approximation, with polarization effects incorporated. Inelastic cross sections for nine low-lying states are computed in a few-channel approximation. We compare our cross sections to previous experimental and computational results where possible. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report calculated cross sections for elastic and electronically inelastic collisions of low-energy electrons with sulfur hexafluoride, SF6. Elastic cross sections are computed within the fixed-nuclei approximation, with polarization effects incorporated. Inelastic cross sections for nine low-lying states are computed in a few-channel approximation. We compare our cross sections to previous experimental and computational results where possible. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report calculated elastic and inelastic cross sections for low-energy electron collisions with methylsilane, CH[SUB3]SiH[SUB3], obtained using the Schwinger multichannel method. The elastic cross sections, obtained within the static-exchange approximation, are compared with elastic results for C[SUB2]H[SUB6] and Si[SUB2]H[SUB6]. Electron-impact excitation cross sections were computed for sixteen electronic states arising from excitation out of the two highest-lying valence orbitals. The dissociation of the lowest few states was examined through limited electronic-structure calculations, which indicated that the 2[SUP1,3] A[SUB1] states dissociate to CH[SUB3]SiH + [SUB2] while the 1[SUP1,3]E states dissociate to CH[SUB3] + SiH[SUB3]. [ABSTRACT FROM AUTHOR]

We report calculated cross sections for elastic and inelastic collisions of low-energy electrons with tetrafluoroethene, C[sub 2]F[sub 4]. The elastic cross section shows a number of resonance features, which we classify according to symmetry and analyze in relation to available experimental data. Electron-impact excitation cross sections are obtained for the 1 [sup 3]B[sub 1u] (T) and 1 [sup 1]B[sub 1u] (V) states arising from the π→π* transition, as well as for eight other low-lying excited states arising from excitations out of the highest occupied molecular orbital. As expected, the T and V states make the largest individual contributions to electron-impact excitation at low energies; however, the other states are shown to contribute significantly to the total excitation cross section at impact energies from 10 to 25 eV. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present experimental and theoretical results on rotational distributions of CO[sup +](B [sup 2]Σ[sup +]) photoions. Rotational distributions were determined for both the v[sup +]=0 and v[sup +]=1 vibrational levels following photoionization of cold (T[sub 0]approx. 9 K) neutral CO target molecules. Data were generated using dispersed ionic fluorescence over a wide range of photoelectron kinetic energies, 0≤Ek≤120 eV, 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[sub exc]approx. 35 eV 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[sup +]=0 and v[sup +]=1 vibrational levels. All of the experimentally observed features and trends are reproduced by theory, and the consequences of these comparisons are discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Results of calculations of the photoelectron spectra for single-photon pulsed-field ionization of the 1π orbital of the X 2Π (v″=0) ground state NO leading to the a 3Σ+ (v+=0–2) excited ionic state are reported. Agreement between these calculated and recently measured spectra is very encouraging. Comparison of these spectra for ionization of the 1π level of NO (X 2Π) with those for the 2π orbital reveal significantly different spectral profiles and underlying dynamics for these two cases. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An ab initio electronic structure technique has been developed to study highly excited states of molecules by combining Schwinger variational methods of collision theory with generalized quantum defect theory. The technique exploits methods of scattering theory to study the region of highly excited Rydberg levels below and across ionization thresholds for molecules. The reaction matrix K, which describes the interaction of the Rydberg electron with the ionic core, is found at arbitrary negative electron energies by employing an unbounded Coulomb Green’s function in the Lippmann–Schwinger equation for the electronic wave function. Quantal conditions are imposed to obtain discrete molecular energy levels, associated Rydberg wave functions, and quantum defect functions, all as a function of the internuclear distance. Results within the static-exchange approximation for the 1,3Σ+u(1σgnσu) and 1,3Πu(1σgnπu) Rydberg states of H2, for n=2–20 and R=1.2–5.0 a0, are presented and discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report the results of theoretical studies of the rotationally resolved photoelectron spectra of ground state O2 leading to the X 2Πg state of O+2 via the absorption of a single vacuum ultraviolet photon. These studies elaborate on a recent report [M. Braunstein et al., J. Chem. Phys. 93, 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. [ABSTRACT FROM AUTHOR]

Results of studies of rotational ion distributions in the X 2Π3/2 and X 2Π1/2 spin-orbit states of HCl+ resulting from (2+1’) resonance enhanced multiphoton ionization (REMPI) via the S(0) branch of the F 1Δ2 Rydberg state are reported and compared with measured threshold-field-ionization zero-kinetic-energy spectra reported recently [K. S. Haber, Y. Jiang, G. Bryant, H. Lefebvre-Brion, and E. R. Grant, Phys. Rev. A (in press)]. These results show comparable intensities for J+=3/2 of the X 2Π3/2 ion and J+=1/2 of the X 2Π1/2 ion. Both transitions require an angular momentum change of ΔN=-1 upon photoionization. To provide further insight into the near-threshold dynamics of this process, we also show rotationally resolved photoelectron angular distributions, alignment of the ion rotational levels, and rotational distributions for the parity components of the ion rotational levels. About 18% population is predicted to occur in the (+) parity component, which would arise from odd partial-wave contributions to the photoelectron matrix element. This behavior is similar to that in (2+1) REMPI via the S(2) branch of the F 1Δ2 state of HBr and was shown to arise from significant l mixing in the electronic continuum due to the nonspherical molecular ion potential. Rotational ion distributions resulting from (2+1) REMPI via the S(10) branch of the F 1Δ2 state are also shown. [ABSTRACT FROM AUTHOR]

Results of theoretical studies of rotational ion distributions in the X 2Π1/2 ground state of HBr+ resulting from (2+1) resonance enhanced multiphoton ionization (REMPI) via the S(2) branch of the F 1Δ2 Rydberg state are reported. These results show a strongly parity-favored ion distribution with about 80% population in the (-) component of the Λ doublet of J+ rotational levels. The 20% population in the other parity component of the Λ doublet can be seen to be due to odd partial wave contributions to the photoelectron matrix elements which arise primarily from non-atomic-like behavior of the electronic continuum. This, in turn, is due to angular momentum coupling in the photoelectron orbital brought about by the torques of the nonspherical molecular ion potential. We demonstrate that the effect of alignment on these ion distributions, although not large, is important. Photoelectron angular distributions and alignment of the J levels of the HBr+ ions are also presented. Rotational branching ratios and photoelectron angular distributions resulting from (2+1’) REMPI of HBr via several S branches of the F 1Δ2 state are also shown for near-threshold photoelectron energies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the photoionization dynamics of the D 2Σ-(1π23pσ) and 3 2Σ-(1π24sσ) Rydberg states of OH, emphasizing the critical role that Rydberg orbital evolution plays at intermediate to larger internuclear distances in determining vibrational and rotational molecular ion distributions. The orbital evolution process is discussed in terms of diabatic and adiabatic molecular states, united atom–separated atom correlation rules, and quantum defect functions. Vibrationally resolved photoelectron spectra and angular distributions for resonance enhanced multiphoton ionization (REMPI) of OH via the D 2Σ-(1π25σ) and 3 2Σ-(1π26σ) Rydberg states are considered as examples. The results and conclusions are relevant to vibrationally and rotationally resolved REMPI studies of all first-row molecular hydrides, due to the similarity of their electronic structure and correspondence to their associated united atom. [ABSTRACT FROM AUTHOR]

The results of studies of ionic rotational branching ratios and photoelectron angular distributions resulting from (2+1’) resonance enhanced multiphoton ionization of NO via various high J (≊21.5) rotational branches of the E2∑+(4s,3d) and H2∑+(3d,4s) Rydberg states are presented. The rotational branching ratios show the expected ΔN=even rotational propensity rule with very small ΔN=odd signals. The branching ratios for the E2∑+ state are seen to be independent of photoelectron energy with the ΔN=+2 signals strongest and no appreciable higher rotational transfer peaks (|ΔN|≥3). The higher rotational transfer signal for ionization of the H2∑+ state are also negligible but the rotational branching ratios are strongly energy dependent due to a Cooper minimum in the l=3 partial wave of the kσ- and kπ-continua at a photoelectron kinetic energy of 2.6 eV and 2.9 eV, respectively. This leads to a strong rotational selectivity that can be exploited to produce ions in a specific rotational level. These consequences of Cooper minima close to threshold are quite general and their influence on rotational distributions should be readily observable in other molecular systems. The photoelectron angular distributions via both states show a strong energy dependence with a rapid change in the angular distributions around the Cooper minimum associated with the H2∑+ state. [ABSTRACT FROM AUTHOR]

We report photoionization cross sections and photoelectron angular distributions for the 5σg, 2πu and 2πg orbitals of Cl2 from threshold to ∼20 eV photoelectron energy obtained using Hartree–Fock photoelectron orbitals. These studies were motivated by recent (2+1) resonance enhanced multiphoton ionization (REMPI) experiments via the 2 1Πg(2π3g4sσg) state which show strong non-Franck–Condon effects in the ion vibrational distribution suggesting that shape and/or autoionizing resonances may play a role near threshold. Previous single-photon experiments on the valence orbitals of Cl2 do not give a consistent picture of these cross sections at low energies. Our results show that there is a shape resonance in the kπu continuum. However, preliminary studies show that these πu cross sections have almost no dependence on internuclear distance and could not be the cause of non-Franck–Condon effects observed in the REMPI experiments. [ABSTRACT FROM AUTHOR]

We report calculated ionic rotational branching ratios and associated photoelectron angular distributions for (1+1’) resonance enhanced multiphoton ionization (REMPI) via the R21(20.5), P21+Q11(25.5), and P11(22.5) branches of the A 2 Σ+(3sσ) state of NO. The branching ratios are dominated by even angular momentum transfer peaks, in agreement with the ΔN+l=odd (ΔN≡N+-Ni ) selection rule. Whereas the calculated photoelectron angular distributions are very branch dependent alignment, the ionic branching ratios are found to be less so. The present calculated results agree well with the experimental results of Allendorf et al. [ABSTRACT FROM AUTHOR]

We report multiplet-specific vibrationally resolved photoionization cross sections and photoelectron angular distributions for the 3σg orbital of O2 leading to the v+=0–3 levels of the b 4Σ-g and B 2Σ-g states of O+2. These studies were motivated by recent work which shows significant nonstatistical behavior in the vibrationally unresolved spectrum at low photoelectron energies arising from the sensitivity of the kσu shape resonance to the multiplet-specific exchange potentials. In addition to the anticipated non-Franck–Condon vibrational distributions arising from the kσu shape resonance, we also find substantial nonstatistical effects in our vibrationally resolved cross sections and particularly in our photoelectron angular distributions over a broad energy range. Extensive electronic autoionization due to Rydberg levels leading to the c 4Σ-u (2σ-1u) ion makes it difficult to assess these effects in the available experimental data. [ABSTRACT FROM AUTHOR]

We report multiplet-specific photoionization cross sections and photoelectron angular distributions for the 3σg orbital of O2 leading to the b 4Σ-g and B 2Σ-g ion states obtained using Hartree–Fock photoelectron orbitals. These cross sections show significant nonstatistical behavior at low photoelectron energies which arises from the sensitivity of the kσu shape resonance to the exchange potentials of these two molecular ions. Specifically, the oscillator strength associated with the shape resonance in the B 2Σ-g cross section is shifted to lower energy compared to that of the b 4Σ-g cross section. This shift gives rise to a quartet to doublet cross section ratio of more than 15:1 near threshold. These effects are difficult to assess in the measured cross sections due to the presence of strong autoionization features. Significant multiplet dependence is also seen in the calculated photoelectron angular distributions. [ABSTRACT FROM AUTHOR]

We report calculated rotational branching ratios for very low energy (50 meV) photoelectrons resulting from (1+1’) resonant enhanced multiphoton ionization (REMPI) via the Ji =1/2, 3/2, 5/2, and 7/2 levels of the P11 branch of the A 2Σ+ (3sσ) state of NO. Even angular momentum transfer (ΔN≡N+-Ni) peaks are dominant in these rotational distributions, in agreement with the selection rule ΔN+l=odd. Angular momentum coupling in the photoelectron wave function arising from the molecular ion potential leads to smaller but appreciable ΔN=odd peaks. The calculated ΔN=0 to ΔN=+2 peak ratios show the same strong decrease when Ji increases from 1/2 to 3/2 as seen in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra [Sander et al., Phys. Rev. A 36, 4543 (1987)], but do not show the rapid die-off of the ΔN≠0 peaks for higher Ji observed experimentally. The calculated trend in the ΔN=+2 vs ΔN=0 peaks could be understood on the basis of simple angular momentum transfer arguments. These same arguments indicate that this trend in the ΔN=0 and +2 peaks with increasing angular momentum is not generally expected in other branches. Spectra via the R21 ( J) branch are presented to support this assertion. We also present photoelectron angular distributions which show a strong dependence on ΔN reflecting the changing composition of the photoelectron wave function. [ABSTRACT FROM AUTHOR]

We report calculations of vibrationally resolved cross sections and photoelectron angular distributions for photoionization of O2 leading to the X 2 Πg (ν+ =0–4) states of O+2 using Hartree–Fock continuum photoelectron orbitals. These studies were motivated by recent results which show that a σu shape resonance plays a dominant role in producing non-Franck–Condon vibrational distributions in resonant multiphoton ionization of O2 via the C 3Πg (1πg3sσg) Rydberg state. In the present study, we investigate how this shape resonance influences photoionization dynamics in single-photon ionization. Below 21 eV photon energy, we find significant non-Franck–Condon effects in the vibrational branching ratios as well as in the vibrationally resolved photoelectron angular distributions. Substantial autoionization hinders a direct comparison between theory and experiment. [ABSTRACT FROM AUTHOR]

Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N2O(7σ-1) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong non-Franck–Condon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a σ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, N–N or N–O. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from Franck–Condon values observed in the symmetric than in the antisymmetric mode. The σ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes. [ABSTRACT FROM AUTHOR]

We present cross sections and asymmetry parameters for photoionization of the 1t2 orbital of CH4 using static-exchange continuum orbitals of CH+4 to represent the photoelectron wave function. The calculations are done in the fixed-nuclei approximation at a single internuclear geometry. To approximate the near-threshold behavior of these cross sections, we assumed that the photoelectron spectrum is a composite of three electronic bands associated with the Jahn–Teller components of the distorted ion. The resulting cross sections reproduce the sharp rise seen at threshold in the experimental data and are in good agreement with experiment at higher energy. The agreement between the calculated and measured photoelectron asymmetry parameters is, however, less satisfactory. [ABSTRACT FROM AUTHOR]

We report the results of studies of the photoionization cross sections and asymmetry parameters for the 3σ and 1π levels of OH, corresponding to the production of the A 3Π, c 1Π, a 1Δ, b 1Σ+, and X 3Σ- molecular ions. The calculations employed multiplet-specific Hartree–Fock potentials and numerical photoelectron continuum orbitals, obtained using the iterative Schwinger variational method. Noticeable nonstatistical behavior of the cross sections is seen, mainly for the 1π level, although deviations are not as pronounced as in other open-shell systems. Comparison with fragmentary experimental data is encouraging, although synchrotron radiation studies are needed to fully assess the accuracy of the calculated cross sections. [ABSTRACT FROM AUTHOR]

Two distinct (n+1) REMPI techniques for obtaining the alignment of gas phase atoms and molecules from photoelectron angular distributions are presented. In both methods, the alignment is extracted from the angular distributions independently of the photoionization dynamics. The first method, which takes advantage of circular dichroism in the angular distributions (CDAD) has already been established experimentally as a useful probe of state alignment. The theory outlined in previous work is expanded here. The second method involves photoionization with light linearly polarized along the photoelectron collection direction and is presented here for the first time. [ABSTRACT FROM AUTHOR]

We report the results of studies of the cross sections and photoelectron asymmetry parameters for photoionization of the 7σ level of N2O using Hartree–Fock photoelectron continuum orbitals. These studies were motivated by recent measurements which showed significant non-Franck–Condon vibrational distributions at low photoelectron energies where previously only autoionizing resonances, but no shape resonance, had been identified. Our results establish that there are two σ shape resonances in the 7σ ionization continuum, a pronounced resonance at low photoelectron energies, and another at higher energy which is essentially obscured in the vibrationally unresolved cross sections. The shape resonant structure that emerges from these studies differs significantly from the predictions of previous model studies. Studies in progress reveal a rich and unusual dependence of these resonances on changes in internuclear distances. [ABSTRACT FROM AUTHOR]

We report photoionization cross sections and asymmetry parameters for the 5σ and 4σ levels of NO obtained using numerical photoelectron continuum functions. For the 5σ orbital multiplet-specific potentials lead to shape-resonant cross sections which show significant nonstatistical multiplet behavior. This behavior, which reflects the sensitivity of shape resonances to the exchange component of the molecular ion potential, is most prominently characterized by resonance peaks at substantially different photoelectron kinetic energies in the two multiplet channels. Comparison of the calculated cross sections with available experimental data for NO shows somewhat poorer agreement than has been seen in the photoionization of closed-shell systems such as N2 and CO. This difference reflects the more complex nature of photoionization processes in open-shell molecules. [ABSTRACT FROM AUTHOR]

In this paper we formulate a theory for the analysis of resonant enhanced multiphoton ionization processes in molecules. Our approach consists of viewing the (n+m) photon ionization process from an isotropic initial state as m-photon ionization out of an oriented, excited state. The orientation in this resonant state, which is reached by n-photon excitation from the initial state, is nonisotropic, and is characteristic of this absorption process. The ionization simply probes this anisotropic population. The calculation of the REMPI process thus consists of determining the anisotropy created in the resonant state and then coupling this anisotropic population to ionization out of it. While the former is accomplished by the solution of appropriate density matrix equations, the latter is done by coupling these density matrix elements to angle-resolved ionization rates out of this state. An attractive feature of this approach is that the influence of saturation effects, and other interactions, such as collisions, on the photoelectron properties is easily understood and incorporated. General expressions are derived for photoelectron angular distributions. Based on these, several properties of the angular distributions that follow purely on symmetry considerations are discussed. One of the new features that emerge out of this work is the saturation induced anisotropy in REMPI. In this effect higher order contributions to the angular distributions appear since saturation influences different ionization channels differently thereby creating an additional anisotropy in the excited state. [ABSTRACT FROM AUTHOR]

Presents experimental and theoretical results on rotationally resolved photoelectron spectra of Rydberg states of the mercapto radical (SH). Ionization via a large number of members of different rotational branches; Asymmetries in rotational ionic distributions; Suppression of transitions involving a large change in angular momentum.

We determine the rotationally state-resolved 2σu-1 photoionization of N2 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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 1Δ2 and f 3Δ2 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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present the first measurements of ion rotational distributions for photoionization over an extended range [0≤EK≤200 eV for N2 (2σ-1u) and 3≤EK≤125 eV for CO (4σ-1)]. The N2 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 N2 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 2σu→kσg continuum in the case of N2 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 N2 results are also compared with electron bombardment ionization data. This comparison demonstrates that previous interpretations of electron bombardment data are prone to errors. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present results of measurements and calculations of the alignment for CO+(B 2Σ+) photoions over an extended energy range (0≤Ek≤210 eV). The polarization of CO+(B 2Σ+→X 2Σ+) fluorescence indicates that the photoions retain significant alignment even at high energies. Agreement between the measured and calculated polarization of the fluorescence is excellent. [ABSTRACT FROM AUTHOR]

State-selected delayed pulsed-field threshold photoionization spectra of HCl and DCl are recorded in double-resonant transitions through the F 1Δ, E 1Σ+, and g 3Σ- states of the 4pπ Rydberg configuration. Comparison of observed rotational line strengths with calculated spectra, as well as with available time-of-flight photoelectron spectra, provides useful insight on the influence of spin–orbit and rotational autoionization on delayed pulsed-field threshold photoionization of HCl. Spin–orbit and rotational autoionization are seen to dramatically reduce the ion rotational intensity associated with the upper spin–orbit level of the ion. [ABSTRACT FROM AUTHOR]

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 N2+ (X 2Σg+) by coherent XUV radiation. In spite of similarities in the electronic structure of CO and N2, 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]