Vibrationally resolved shape resonant photoionization of N2O

Vibrationally resolved photoionization dynamics of CF4 in the DA12 state

Vibrationally resolved photoelectron spectroscopy of the CF4+ (D 2A1) state is studied for the first time over an extended energy range, 26.5<or=hnu<or=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.

Photoelectron trapping in N2O 7sigma-->ksigma resonant ionization

Vibrationally resolved photoelectron spectroscopy of the N2O+(A 2Sigma+) 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< or =hv< or =55 eV. There are three single-channel resonances in this range, two in the 7sigma-->ksigma channel and one in the 7sigma-->kpi 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< or =hv< or =40 eV, there are two shape resonances (ksigma and kpi). The ksigma ionization resonance is clearly visible in vibrationally resolved measurements at hv=35 eV, even though the total cross section in this channel is dwarfed by the cross section in the degenerate, more slowly varying 7sigma-->kpi channel. This ksigma resonance is manifested in non-Franck-Condon behavior in the approximately antisymmetric v3 stretching mode, but it is not visible in the branching ratio curve for the approximately symmetric v1 stretch. The behavior of the 35-eV ksigma resonance is compared to a previously studied N2O 7sigma-->ksigma shape resonance at lower energy. The mode sensitivity of the 35-eV ksigma 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 7sigma-->ksigma 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 ksigma 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.

Electronically forbidden (5σu→kσu) photoionization of CS2: Mode-specific electronic-vibrational coupling

Vibrationally resolved photoelectron spectroscopy of the CS(2) (+)(B (2)Sigma(u) (+)) state is used to show how nontotally symmetric vibrations "activate" a forbidden electronic transition in the photoionization continuum, specifically, a 5sigma(u)-->ksigma(u) 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</=hnu</=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 hnu approximately 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, 5sigma(u)-->kpi(g), 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.

Intrachannel vibronic coupling in molecular photoionization

We discuss the excitation of forbidden vibrational transitions accompanying photoionization of linear triatomic molecules. Excitation of a single quantum of the antisymmetric stretching vibration is observed for mole cules with inversion symmetry, as is the bending mode. Photoelectron spectra of the N2O+(A2Π), CO2+(C2Σg+), and CS2+(B2Σu+) states obtained over a range of ionization energies exhibit contrasting behavior for the relative intensities of the forbidden vibrations. These energy-dependent vibrational branching ratios are shown to result from an intrachannel vibronic coupling mechanism. Moreover, this intrachannel coupling can be further divided into two cases, one in which the photoionization cross section is sensitive to geometry changes, and a second case in which it is not. These different cases can be distinguished by comparing the experimental and theoretical results for all three molecules.Key words: photoelectron spectroscopy, vibronic coupling, photoionization.PACS Nos.: 33.60.Cv, 33.20.Ni, 33.20.Wr, 33.80.Eh

Observation of the symmetry-forbidden 5sigmau-->ksigmau CS2 transition: a vibrationally driven photoionization resonance

Vibrationally resolved photoelectron spectroscopy and Schwinger calculations are used to characterize a new resonance phenomenon in the 5sigma(u)-->ksigma(u) photoionization of CS2. This resonant channel is symmetry forbidden, yet is observable because it is activated by the antisymmetric stretching vibration. In addition, we show that a Franck-Condon breakdown occurs even though the energy dependence of the cross section is insensitive to geometry changes, which is unprecedented in photoionization.

Vibrational branching ratios in photoionization of CO and N2

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 N2 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 N2.

Mode-specific photoelectron scattering effects on CO2+(C 2Σg+) vibrations

Using high-resolution photoelectron spectroscopy, we have determined the energy dependent vibrational branching ratios for the symmetric stretch [v+ = (100)], bend [v+ = (010)], and antisymmetric stretch [v+ = (001)], as well as several overtones and combination bands in the 4sigmag(-1) photoionization of CO2. 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+ =( 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 4sigmag-->ksigmau 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.

Shape-resonance-enhanced vibrational effects in the angular distributions of C 1s photoelectrons from fixed-in-space CO molecules

Angular distributions of C 1s photoelectrons from fixed-in-space CO molecules have been measured with vibrational resolution. A strong dependence of the angular distributions on the vibrational states of the residual molecular ion has been found for the first time in the region of the shape resonance. Calculations in the relaxed core Hartree-Fock approximation have reproduced the angular distributions fairly well in the general shapes of the angular distributions due to the correct description of nuclear motion as an average of the internuclear-distance-dependent dipole amplitudes.