E. D. Poliakoff

Non‐Franck–Condon 2σ−1u vibrational distributions in N+2. An interchannel‐coupled shape resonance observed by dispersed fluorescence

We report evidence for a molecular interchannel‐coupled shape resonance in N2 photoionization, observable in the vibrational branching ratios for the N+2(B 2Σ+u) state. The vibrational branching ratios for the N+2(B 2Σ+u) state are determined from vibrationally resolved dispersed fluorescence measurements on the N+2(B 2Σ+u→X 2Σ+g) band system. The results demonstrate that the partial cross section for the N+2(B 2Σ+u,v’=1) level is selectively enhanced in the photon energy range 28 <hνexc<30 eV. A shape resonance occurs in this range for the 3σg→eσu channel, and the observed enhancement is attributed to coupling between this channel and the 2σu→eσg channel. Vibrational branching ratio curves for the excited state ion also show a pronounced peak at hνexc=22 eV, which shifts for the curves that sample the v’=1 and v’=2 levels of the ion, suggesting a few interpretations. The utility of coupling monochromatized fluorescence detection with synchrotron radiation excitation for extracting relative partial photoi...

Vibrational branching ratios and photoionization dynamics for N2O

Abstract Vibrational resolved constant-ionic-state spectroscopy is used to study photoionization of N 2 O We determine the ratio of partial photoionization cross sections for alternative vibrational levels of the N 2 O + (A 2 σ+) state and these vibrational branching ratios are dependent on excitation energy, implying that there is a breakdown of the Franck-Condon approximation. These results may be sampling the wings of a shape resonance.

Constant ionic state spectroscopy of N2O. Dispersed fluorescence as a probe of molecular autoionization

We report electronic autoionization studies of N2O using vibrationally resolved constant ionic state (CIS) spectroscopy. Vacuum ultraviolet synchrotron radiation is the excitation source, and we determine the relative partial photoionization cross‐section curves for alternative vibrational levels (v’) of the ion by detecting dispersed fluorescence [N2O+(A 2Σ+,v’→X2Π,v‘)] from the ion. Excitation spectra sampling different vibrational levels reveal significant changes in the 3pπ resonance profile, including shifts of the resonance minima, and previously unobserved features. Analysis of the v’=(0,1,0) CIS spectra demonstrate that this level of the ion is produced predominantly by photoionization of the target molecules in the (0,1,0) level, i.e., via hot band excitation. These results are discussed in detail, as well as possible extensions and further studies.

Photoionization of aligned molecular excited states

Photoelectron angular distributions of several excited states of NO have been measured in an effort to better elucidate the role of alignment in resonant multiphoton excitation processes of molecules. In contrast to previous molecular REMPI measurements on NO, (2+1) angular distributions taken for low rotational levels of the E 2Σ+ (4sσ) Rydberg state of NO exhibit complex angular behavior which is characteristic of strong spatial alignment of the optically prepared levels. Photoelectron angular distributions were also found to be strongly branch and J dependent with the lowest rotational levels of the R21+S11 branch exhibiting the full anisotropy expected for an overall three‐photon process. Fluorescence anisotropies extracted from complementary two‐photon fluorescence angular distribution measurements reveal small, but nonzero alignment in all rotational levels with J>1/2, in contrast to the photoelectron results. Additional photoelectron angular distributions taken for (1+1) REMPI via the A 2Σ+ (3sσ), ...

Rotational distributions of molecular photoions following resonant excitation

We demonstrate that the photoelectron energy mediates the rotational energy distribution of N+2 ions created by photoionization, and conversely, that rotational energy determinations probe resonant excitation in molecular photoionization. Experimentally, this is accomplished by monitoring the dispersed fluorescence from N+2 (B 2Σ+u) photoions to determine their rotational energy distribution. These results demonstrate that while dipole selection rules constrain the total angular momentum of the electron–ion complex, the partitioning of angular momentum between the photoelectron and photoion depends on the photoejection dynamics. Implications for photoionization and electron impact ionizatin studies are discussed.

Multichannel interactions in the resonant photoionization of HCl

Vibrational state distributions of the A 2Σ+ excited state of HCl+ were measured by dispersed fluorescence following resonant photoionization. Autoionization of levels excited at the NeI resonance line strongly influence the vibrational branching ratios of the A 2Σ+ state although not in accord with the propensity rule expected for vibrational autoionization. Other measurements utilizing total fluorescence yields and synchrotron radiation confirm the presence of competing dissociation channels for autoionizing Rydberg states converging to the A 2Σ+ limit. These results are discussed in terms of the multichannel interactions responsible for determining the observed ion and fragment product distributions.

Vibrationally resolved electronic autoionization of core–hole resonances

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.

Fluorescence excitation studies of molecular photoionization in external electric fields

Abstract Using molecular nitrogen as an example, we show that fluorescence excitation spectroscopy can be used to measure partial photoionization cross sections of free molecules in external electric fields. The production of the N2+(B 2 v u+) state was studied and the threshold for this process was found to shift linearly with the square root of the applied field. This behavior is compared with the hydrogenic case and with previously studied systems.

Fluorescence polarization studies of autoionization in CS2

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 A 3), and comparisons between the line shapes are given for several features.

Alignment of excited states following multiphoton excitation

We report the measurement of the fluorescence anisotropy from excited neutral NO*(A 2∑+) molecules produced by two‐photon excitation, thus providing a measure of their alignment. The degree of fluorescence anisotropy depended on the excitation pathway, and the results for the fluorescence angular distributions following excitation via the O12 rotational branch were more anisotropic than the superimposed R12,Q22 branch results. Results are given for other rotational branches as well and they should serve as useful aids in photoelectron asymmetry studies following multiphoton ionization, where the alignment of the neutral target state directly influences the photoelectron asymmetries in the ionization step.