S. N. Dixit

Circular dichroism in photoelectron angular distributions as a probe of atomic and molecular alignment

In this paper we show that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe atomic and molecular alignment in the gas phase. Careful choice of photon (left or right circularly polarized) propagation and photoelectron collection directions breaks the cylindrical symmetry of the target, giving rise to dichroic effects. CDAD exists in the electric dipole approximation. We illustrate the sensitivity of CDAD to alignment by considering photoionization of the A 2Sigma+ state of NO. Most of the cases of alignment we consider are created by multiphoton absorption while the others, more general, might be created in fragmentation, desorption, etc. The alignment created by n-photon absorption quickly reaches a classical limit which is reflected in the CDAD spectrum. Finally, we show that CDAD is also a sensitive probe of gas phase atomic state alignment by considering photoionization of the 7P3/2 state of cesium created by single photon absorption from the ground state.

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.

(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.

(2+1) REMPI of NO via the D 2Σ+ state: rotational branching ratios

Recent photoelectron spectroscopic studies in a (2 + 1) REMPI of NO via the Rydberg D^2Σ^+ state have revealed anomalous ionic rotational branching ratios. We have performed ab initio calculations of these branching ratios and find that the molecular nature of the ionization continuum plays an essential role in the dynamics. Even though the bound orbital is very atomic-like (> 98% p-like), the photoelectron continuum wavefunction is quite sensitive to the non-spherical nature of the molecular ionic potential and causes a strong persistence of the p-partial wave which, in turn, leads to a large ΔN = 0 peak.

Extraction of alignment parameters from circular dichroic photoelectron angular distribution (CDAD) measurements

In a previous paper, we showed that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe alignment in gas phase atoms and linear molecules. Often this alignment is parametrized through the moments of alignment A(2), A(4), etc., which are commonly extracted from fluorescence polarization measurements. In this paper we show how these can be simply extracted from CDAD spectra. This technique can be used in principle to extract the moments to any order.

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.

(1+ 1) CDAD: A new technique for studying photofragment alignment

We report a new technique for measuring photofragment alignment in the gas phase by observing circular dichroism in photoelectron angular distributions (CDAD). This technique is well suited for determining the gas phase alignment of linear molecules. The experiment involves excitation of the photofragment with linearly polarized light followed by photoionization with left or right circularly polarized light. The difference between the photoelectron angular distributions for these two cases is the CDAD spectrum. By measuring CDAD through two different excitation branches, one can obtain the ground state photofragment alignment A (2) 0 using a simple analytical formula independent of the photoionization dynamics.

Comment on ‘‘Three photon resonance ionization of H2’’ [J. Chem. Phys. 77, 877 (1982)]

Ritchie et al’s1 Paper on three photon resonance ionization of hydrogen is commented upon. Their three‐photon angular distribution relation based on the extended two−level rate theory of Eberly and O’Neil2, is found to be incorrect. (AIP)

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].