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We measure high order harmonics from the molecule SF6 over a large range of phase matching conditions and observe several features in the harmonics that are largely independent of such macroscopic conditions. The experimental data are then compared to the quantitative rescattering theory for the generation of harmonics from three orbitals. With this comparison, we are able to assign spectroscopic features in the harmonics to contributions from 1t1g (HOMO) and 5t1u (HOMO-1) orbitals. [ABSTRACT FROM AUTHOR]

High-resolution photoelectron spectra of the gas phase pyrimidine-type nucleobases, thymine, uracil, and cytosine, were collected using synchrotron radiation over the photon energy range 17 = hv = 150 eV. These data provide the highest resolution photoelectron spectra of thymine, uracil, and cytosine published to date. By comparing integrated regions of the energy dependent photoelectron spectra of thymine, the ionization potentials of the first four ionic states of thymine were estimated to be 8.8, 9.8, 10.3, and 10.8 eV. The thymine data also show evidence for low energy shape resonances in three of the outermost valence electronic states. Comparing the uracil spectrum with the thymine spectrum, the four outermost valence electronic states of uracil likely begin at binding energies 9.3, 9.9, 10.5, and 11.0 eV. High-resolution spectra indicate only one tautomeric form of cytosine contributes significantly to the spectrum with the four outermost valence electronic states beginning at binding energies 8.9, 9.9, 10.4, and 10.85 eV. [ABSTRACT FROM AUTHOR]

The vibrational branching ratios in the photoionization of acrolein for ionization leading to the ... ²A' ion state were studied. Computed logarithmic derivatives of the cross section and the corresponding experimental data derived from measured vibrational branching ratios for several normal modes (v9, v10, v11, and v12) were found to be in relatively good agreement, particularly for the lower half of the 11-100 eV photon energy range considered. Two shape resonances have been found near photon energies of 15.5 and 23 eV in the photoionization cross section and have been demonstrated to originate from the partial cross section of the A' scattering symmetry. The wave functions computed at the resonance complex energies are delocalized over the whole molecule. By looking at the dependence of the cross section on the different normal mode displacements together with the wave function at the resonant energy, a qualitative explanation is given for the change of the cross sections with respect to changing geometry. [ABSTRACT FROM AUTHOR]

The vibrational branching ratios in the photoionization of C6F6 leading to the C 2B2u state of C6F6+ are considered. Computational and experimental data are compared for the excitation of two totally symmetric modes. Resonant features at photon energies near 19 and 21 eV are found. A detailed analysis of the computed results shows that the two resonance states have different responses to changes in the C–C and C–F bond lengths. We find that the energies of both of the resonant states decrease with increasing bond lengths. In contrast to the energy positions, however, the resonant widths and the integrated oscillator strength of the resonances can either increase or decrease with increasing bond length depending on the nature and location of the resonant state and the location of the bond under consideration. With increasing C–F bond length, we find that the energy of the antibonding σ resonance localized on the ring has a decreasing resonance energy and also a decreasing lifetime. This behavior is in contrast to the usual behavior of shape resonance energies where increasing a bond length leads to decreasing resonance energies and increasing resonance lifetimes. Finally, for the first time, we examine the effect of simultaneously occurring multiple vibrations on the resonance profile for valence photoionization, and we find that the inclusion of more than a single vibrational mode substantially attenuates the strength of resonance. [ABSTRACT FROM AUTHOR]

Vibrationally resolved photoelectron spectra of OCS+(C 2Σ+) are used to probe coupling between photoelectron motion and molecular vibration for a simple asymmetric system. Spectra are reported over the photon energy range of 21≤hν≤55 eV. Vibrational branching ratios for all of the normal modes are determined and the results exhibit mode-specific deviations from Franck–Condon behavior. Schwinger variational calculations indicate the presence of four shape resonances, two kσ resonances and two kπ resonances. All of the resonances play a role in the observed vibrationally resolved behavior. Two results are striking; first, the resonances are more sensitive to the C–O stretch than to the C–S stretch, particularly for photon energies above 30 eV. This relative insensitivity of the resonance to geometry changes involving a third-row element is similar to other systems studied. Second, theoretical results lead to the counterintuitive conclusion that bending the molecule suppresses the high energy resonance, even though there is an enhancement in the vibrational branching ratio curve for the single quantum bending excitation. The agreement between the theoretical and experimental branching ratio curves is good. Finally, the results unambiguously demonstrate that the forbidden bending excitation is caused by photoelectron-mediated vibronic coupling, i.e., the variation in the electronic transition matrix element with geometry, rather than the traditional explanation of interchannel vibronic coupling with intensity borrowing between ionic states. [ABSTRACT FROM AUTHOR]

Vibrationally resolved photoelectron spectroscopy of the CF4+ (D 2A1) state is studied for the first time over an extended energy range, 26.5≤=hν≤=50 eV. It is found that the energy dependence of the totally symmetric stretching vibration is qualitatively different from all of the other vibrational modes. Moreover, the vibrational branching ratio curves for all of the symmetry forbidden vibrations are nearly identical. Qualitative arguments are used to show that it is likely that at least two shape resonances are present in the continuum, and that their characteristics, such as energy dependence and spatial localization, are distinctly different [ABSTRACT FROM AUTHOR]

The authors report a fully vibrationally resolved photoelectron spectroscopy investigation of a nonplanar molecule studied over a range of excitation energies. Experimental results for all four fundamental vibrational modes are presented. In each case significant non-Franck-Condon effects are seen. The vibrational branching ratio for the totally symmetric mode ν1+ is found to be strongly affected by resonant excitation in the SiF4+ (D 2A1) photoionization channel. This is shown to be the result of two distinct shape resonances, which for the first time have been both confirmed by theoretical calculations. Vibrationally resolved Schwinger photoionization calculations are used to understand the vibronic coupling for the photoelectrons, both using ab initio and harmonic vibrational wave functions. [ABSTRACT FROM AUTHOR]

Evidence is presented demonstrating that an electron launched into the continuum is trapped in an unprecedented quasibound state, namely, one that extends through the backbone of the six-member carbon ring of C6F6. The mode specificity of the vibrational sensitivity to the electron trapping provides an experimental signature for this phenomenon, while adiabatic static model-exchange scattering calculations are used to map the wave function, which corroborate the interpretation. [ABSTRACT FROM AUTHOR]

Evidence is presented demonstrating that an electron launched into the continuum is trapped in an unprecedented quasibound state, namely, one that extends through the backbone of the six-member carbon ring of C6F6. The mode specificity of the vibrational sensitivity to the electron trapping provides an experimental signature for this phenomenon, while adiabatic static model-exchange scattering calculations are used to map the wave function, which corroborate the interpretation. [ABSTRACT FROM AUTHOR]

Vibrationally resolved photoelectron spectroscopy of the N2O+(A 2Σ+) state is used to compare the dependence of the photoelectron dynamics on molecular geometry for two shape resonances in the same ionization channel. Spectra are acquired over the photon energy range of 18≤hν≤55 eV. There are three single-channel resonances in this range, two in the 7σ→kσ channel and one in the 7σ→kπ channel. Vibrational branching ratio curves are determined by measuring vibrationally resolved photoelectron spectra as a function of photon energy, and theoretical branching ratio curves are generated via Schwinger variational scattering calculations. In the region 30≤hν≤40 eV, there are two shape resonances (kσ and kπ). The kσ ionization resonance is clearly visible in vibrationally resolved measurements at hν=35 eV, even though the total cross section in this channel is dwarfed by the cross section in the degenerate, more slowly varying 7σ→kπ channel. This kσ resonance is manifested in non-Franck–Condon behavior in the approximately antisymmetric ν3 stretching mode, but it is not visible in the branching ratio curve for the approximately symmetric ν1 stretch. The behavior of the 35-eV kσ resonance is compared to a previously studied N2O 7σ→kσ shape resonance at lower energy. The mode sensitivity of the 35-eV kσ resonance is the opposite of what was observed for the lower-energy resonance. The contrasting mode-specific behavior observed for the high- and low-energy 7σ→kσ resonances can be explained on the basis of the “approximate” symmetry of the quasibound photoelectron resonant wave function, and the contrasting behavior reflects differences in the continuum electron trapping. An examination of the geometry dependence of the photoelectron dipole matrix elements shows that the kσ resonances have qualitatively different dependences on the individual bond lengths. The low-energy resonance is influenced only by changes in the end-to-end length of the molecule, whereas the higher-energy resonance depends on the individual N–N and N–O bond lengths. Branching ratios are determined for several vibrational levels, including the symmetry-forbidden bending mode, and all of the observed behavior is explained in the context of an independent particle, Born–Oppenheimer framework. [ABSTRACT FROM AUTHOR]

Vibrationally resolved photoelectron spectroscopy of the CS2+(B 2Σu+) state is used to show how nontotally symmetric vibrations “activate” a forbidden electronic transition in the photoionization continuum, specifically, a 5σu→kσu shape resonance, that would be inaccessible in the absence of a symmetry breaking vibration. This electronic channel is forbidden owing to inversion symmetry selection rules, but it can be accessed when a nonsymmetric vibration is excited, such as bending or antisymmetric stretching. Photoelectron spectra are acquired for photon energies 17≤hν≤72 eV, and it is observed that the forbidden vibrational transitions are selectively enhanced in the region of a symmetry-forbidden continuum shape resonance centered at hν≈42 eV. Schwinger variational calculations are performed to analyze the data, and the theoretical analysis demonstrates that the observed forbidden transitions are due to photoelectron-mediated vibronic coupling, rather than interchannel Herzberg–Teller mixing. We observe and explain the counterintuitive result that some vibrational branching ratios vary strongly with energy in the region of the resonance, even though the resonance position and width are not appreciably influenced by geometry changes that correspond to the affected vibrations. In addition, we find that another resonant channel, 5σu→kπg, influences the symmetric stretch branching ratio. All of the observed effects can be understood within the framework of the Chase adiabatic approximation, i.e., the Born–Oppenheimer approximation applied to photoionization. [ABSTRACT FROM AUTHOR]

Using high-resolution photoelectron spectroscopy, we have determined the energy dependent vibrational branching ratios for the symmetric stretch [v[sup +]=(100)], bend [v[sup +]=(010)], and antisymmetric stretch [v[sup +]=(001)], as well as several overtones and combination bands in the 4σ[sub g][sup -1] photoionization of CO[sub 2]. Data were acquired over the range from 20–110 eV, and this wide spectral coverage highlighted that alternative vibrational modes exhibit contrasting behavior, even over a range usually considered to be dominated by atomic effects. Alternative vibrational modes exhibit qualitatively distinct energy dependences, and this contrasting mode-specific behavior underscores the point that vibrationally resolved measurements reflect the sensitivity of the electron scattering dynamics to well-defined changes in molecular geometry. In particular, such energy-dependent studies help to elucidate the mechanism(s) responsible for populating the symmetry forbidden vibrational levels [i.e., v[sup +]=(010), (001), (030), and (110)]. This is the first study in which vibrationally resolved data have been acquired as a function of energy for all of the vibrational modes of a polyatomic system. Theoretical Schwinger variational calculations are used to interpret the experimental data, and they indicate that a 4σ[sub g]→kσ[sub u] shape resonance is responsible for most of the excursions observed for the vibrational branching ratios. Generally, the energy dependent trends are reproduced well by theory, but a notable exception is the symmetric stretch vibrational branching ratio. The calculated results display a strong peak in the vibrational branching ratio while the experimental data show a pronounced minimum. This suggests an interference mechanism that is not accounted for in the single-channel adiabatic-nuclei calculations. Electronic branching ratios were also measured and compared to the vibrational branching ratios to assess the relative contributions of interchannel (i.e., Herzberg–Teller) versus intrachannel (i.e., photoelectron-mediated) coupling. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

X-ray absorption spectra are reported for two classes of ring systems, cyclic polymethylene sulfides and organochromium ring compounds with phosphorus-complexing bidentate ligands. For the cyclic polymethylene sulfides, spectra were acquired in the region of the sulfur K-edge. For the organochromium complexes, spectra were acquired at both the chromium K-edge and at the phosphorus K-edge. These systems allow one to interrogate how the spectra evolve as the bond angle about the absorbing atom is varied. Systematic trends are observed as a function of ring size/bond angle. For the cyclic sulfides, the peaks in the continuum above the sulfur K-edge shift to higher energy and broaden with increasing ring size. These continuum features are assigned to shape resonances. In the chromium K-edge measurements, changes in the P–Cr–C bond angle do not influence the spectra appreciably, indicating that the four carbonyl groups dominate the photoelectron scattering processes. However, the phosphorus K-edge spectra are sensitive to changes in the endocyclic Cr–P–C bond angle. The trends observed in the phosphorus K-edge region for the organometallic continuum features are consistent with those observed for the cyclic polymethylene sulfides. © 2003 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]

The energy dependence of the vibrational branching ratio for exciting one quantum of bending is determined for CO[sub 2] 4σ[sub g][sup -1] photoionization. This nominally forbidden transition becomes allowed for a photoionization transition as a result of instantaneous symmetry breaking due to zero point motion, and is strongly enhanced by a continuum shape resonance. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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]

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 have measured vibrational branching ratios for 2σ-1u photoionization of N2 in an effort to elucidate fundamental aspects of continuum channel coupling. Calculations have shown that photoejection of a 2σu electron from N2 should be influenced by a shape resonance in the 3σg →εσu photoionization channel and that this continuum channel coupling can result in deviations from Franck–Condon behavior for the resulting N+2(B 2Σ+u) ion. In the present study, the N2 molecules are ionized by monochromatic synchrotron radiation (25

We have measured vibrationally resolved fluorescence from SiF+4(D 2A1) photoions to determine the vibrational branching ratio σ[v=(1,0,0,0)]/σ[v=(0,0,0,0)] in the excitation energy range 22

We investigate the dynamics of molecular core–hole decay using dispersed fluorescence from an ionic valence–hole state in N2. A core–hole excited state is created via the N2(1s→1 π*g) transition, and this state electronically autoionizes to the N+2(B 2Σ+u) state. The vibrational branching ratios for the N+2(B 2Σ+u) state are then determined from N+2(B 2Σ+u→X 2Σ+g) fluorescence. Fundamental aspects of core–hole state decay emerge clearly from these measurements. In particular, interference effects due to lifetime broadening of vibrational levels of the core–hole state can be investigated quantitatively using this method. As a result, dispersed fluorescence detection serves as a powerful tool for investigating R-dependent aspects of molecular core–hole resonances. The experimental results do not agree with predictions based on previous theoretical developments, and possible causes for the discrepancy are discussed. [ABSTRACT FROM AUTHOR]

The polarization of the CS+2(A 2Π→X 2Π) transition was measured following photoionization of CS2 with synchrotron radiation excitation in the range 875 Å<λexc <967 Å. Autoionization features are prominent in the fluorescence polarization spectrum and were investigated in detail. The spectral assignments of the absorption spectrum by Ogawa and Chang [Can. J. Phys. 48, 2455 (1970)] are supported by the current measurements. Although fluorescence excitation and fluorescence polarization profiles normally align precisely, exceptions have been found for many resonances (σu →ndπ, n>3), and comparisons between the line shapes are given for several features. [ABSTRACT FROM AUTHOR]