D. L. Lynch

Studies of the photoionization cross sections of acetylene

We have studied the photoionization cross sections and photoelectron asymmetry parameters for all molecular orbitals of acetylene. These cross sections were obtained using accurate frozen‐core Hartree–Fock continuum orbitals in the final‐state wave function. The Hartree–Fock continuum equations were solved using the iterative Schwinger variational method. These fixed‐nuclei cross sections are compared with available experimental data and with results obtained using the Stieltjes moment theory approach and the continuum multiple scattering method. The possible role of a shape resonance in the 1πu→kπg channel and the resonant structure of the σu continuum are discussed in some detail.

Rotationally resolved (3 + 1) rempi of H 2 via the B1Σ+u state

In the paper, we compare the results of our ab initio calculations for the ro-vibrational branching ratios resulting from a (3+1) REMPI of H_2 via the B^1Σ^+_u state with the experimental data of Pratt, Poliakoff, Dehmer and Dehmer. These results indicate that non-Franck-Condon effects are less important here than in the (3+1) REMPI of H_2 via the C^1Π_u state. We observe that the ΔJ = ±3 peaks in the photoelectron spectrum are of negligible strength and that the ratio of ΔJ = +1 to ΔJ = −1 peak is independent of the ionic vibrational state. A detailed analysis indicates that these features arise as a result of a dynamic interference between the do and dμ ionization channels and do not imply either the smallness of the d-wave or the smallness of the j_t = 3 angular momentum coupling terms.

Resonance effects in the 5σ photoionization of CO

We report vibrational branching ratios and vibrationally resolved photoelectron angular distributions for resonant photoionization of the 5sigma level of CO leading to X^2Σ+CO+. These results were obtained using frozen-core Hartree–Fock photoelectron continuum orbitals. These studies provide a clear quantitative picture of the role of the kσ shape resonance in the observed non-Franck–Condon behavior and useful insight into the interplay between this shape resonance and a valence-like autoionizing resonance around 23 eV.

(2+1) resonant enhanced multiphoton ionization of H2 via the E,F 1Σ+g state

In this paper, we report the results of ab initio calculations of photoelectron angular distributions and vibrational branching ratios for the (2+1) REMPI of H_2 via the E, F^(1)Σ^+_g state, and compare these with the experimental data of Anderson et al. [Chem. Phys. Lett. 105, 22 (1984)]. These results show that the observed non‐Franck–Condon behavior is predominantly due to the R dependence of the transition matrix elements, and to a lesser degree to the energy dependence. This work presents the first molecular REMPI study employing a correlated wave function to describe the Rydberg–valence mixing in the resonant intermediate state.

Shape resonances in the photoionization of cyanogen

We have studied the photoionization cross sections and photoelectron asymmetry parameters for ionization of the 1pig(X 2Pig), 5sigmag(A 2Sigma + g ), and 4sigmau(B 2Sigma + u ) levels of cyanogen using frozen-core Hartree–Fock photoelectron continuum orbitals. The main purpose of these studies has been to extend our understanding of the dynamics of shape resonances from earlier studies of diatomic and smaller polyatomic molecules to a larger polyatomic system. The results do, in fact, reveal a rich shape resonant structure in the electronic continuum of this polyatomic system. There is a low-energy sigmau resonance which, as expected, is the C–C analog of the l=3 shape resonance seen in N2(3sigma - 1 g ) and several other diatomics. In contrast to this diatomic-like behavior, the presence of the two CN groups in C2N2 results in a second sigmau and a sigmag resonance corresponding to linear combinations of a l=3 shape resonance localized on the CN sites. Moreover, our results also show a pronounced shape resonant behavior in the piu continuum, which, to our knowledge, has not been seen in smaller molecules.

Photoionization cross sections of rovibrational levels of the B 1Σ+u state of H2

We report theoretical cross sections for direct photoionization of specific rovibrational levels of the B ^1Σ^+_u electronic state of H_2. The calculated cross sections differ considerably from values recently determined by resonant enhanced multiphoton ionization (REMPI) studies. In an attempt to understand the disagreement, we analyze in detail the REMPI dynamics and find that the multiphoton ionization probability is extremely sensitive to the spatial and temporal profiles of the laser pulses. Accurate characterization of laser profiles and their jitter is therefore necessary for a comparison between theory and experiment.

Photoelectron spectroscopy of excited molecular states

Results of studies of ion rotational and vibrational distributions for resonance enhanced multiphoton ionization are discussed.(AIP)Results of studies of ion rotational and vibrational distributions for resonance enhanced multiphoton ionization are discussed.(AIP)

Theoretical Studies of Resonantly Enhanced Multiphoton Ionization Processes in Molecules

Multiphoton absorption and ionization processes add a new dimension to the study of excited state dynamics in atomic and molecular systems. The extreme energy- and state-selectivity achieved through the use of lasers enables one to access specific excited states which would otherwise be inaccessible in single photon spectroscopy due to the lack of appropriate light sources or due to dipole selection rules. Photoionization out of these excited states combined with photoelectron energy analysis yields detailed information about the excited state and the ionization continuum. Recent experiments [1–14] on multiphoton ionization of diatomic molecules H2, N2, NO and CO have revealed several interesting features, such as the state selectivity in the residual ion, non-Franck-Condon behavior, Rydberg-valence mixing, rotational and vibrational state dependence of photoelectron angular distributions, and vibrational and rotational autoionization. Multiphoton ionization has also been used to study van der Waal’s complexes of rare gas atoms [15].