Patricia M. Dehmer

Very high resolution study of photoabsoprtion, photoionization, and predissociation in H2

Relative photoabsorption and photoionization cross sections for H2 (para and ordinary) have been measured at 78°K from 715 to 805 A for para‐H2 and from 745 to 805 A for ordinary H2, with a wavelength resolution of 0.016 A. This resolution represents a factor of 3 improvement over the previous data reported from this laboratory, and in addition, the new data have significantly improved statistics. This enables observation and identification of the R (0) npσ and npπ Rydberg series in para‐H2 to principal quantum numbers of approximately 40 for series converging to H2+(2Σg+, v=1–6). Linewidths and relative intensities were measured for a large number of these levels and the results are compared to calculations using a quantum defect theory (QDT) approach. Two‐channel QDT is used to assign nearly all the prominent structure in the para‐H2 spectrum. The strengths and limitations of the simple two‐channel theory and the necessity for extension to multichannel calculation for a complete understanding of the spe...

High resolution study of photoionization processes in O2

Relative photoionization efficiency curves were determined for the production of O+2, O+, and O− from molecular oxygen in the wavelength region of the helium continuum. The photoionization efficiency curve for O+2, determined at a wavelength resolution (FWHM) of 0.07 A throughout most of the region from 1030–580 A, shows rotationless autoionization structure which is broadened due to the processes of predissociation and/or autoionization. The photoionization effciency curves for fragment ions from ion pair formation and from dissociative ionization were determined at wavelength resolutions of 0.07 and 0.15 A, respectively. The data show considerable autoionization structure and provide information about the mechanisms for these dissociation processes. The ion pair photoionization efficiency curve shows steplike structure at threshold resulting from direct ionization from individual rotational levels of the ground state of O2. A value of the electron affinity of atomic oxygen of 1.462±0.003 eV was determin...

Photoionization of argon clusters

Argon clusters were produced in a free supersonic molecular beam expansion of pure argon at room temperature and the photoionization efficiency curves of the trimer through hexamer were measured in the wavelength regions from threshold to 700 A. A study of the Ar+3 photoionization efficiency curve as a function of nozzle stagnation pressure shows that fragmentation of heavier clusters can dominate the spectrum, even near threshold, and even when the nozzle conditions are such that the Ar+4 intensity is only a small fraction of the Ar+3 intensity. The Ar+3 photoionization efficiency curve, obtained using nozzle stagnation conditions such that no heavier ions were detected, exhibits several broad peaks near threshold which show similarities to bands of the dimer. At high nozzle stagnation pressures, the photoionization efficiency curves for Ar+3 to Ar+6 are nearly identical due to the effects of fragmentation. These spectra exhibit two very broad features which are similar to features observed in the solid....

Photoelectron spectra of Ar2 and Kr2 and dissociation energies of the rare gas dimer ions

The photoelectron spectra of Ar2 and Kr2 were determined at 584 A with resolutions of 18 and 21 meV, respectively, using an apparatus which combines a supersonic molecular beam source with a hemispherical photoelectron spectrometer. Dissociation energies were determined for the weakly bound B 2Π(3/2)g and C 2Π(1/2)u excited states of the dimer ions, and were found to average approximately 30% greater than those predicted by recent theoretical calculations. Optimum potentials were determined for the Ar+2 and Kr+2 ground states by fitting calculated Franck–Condon distributions to the observed photoelectron peaks. As in the case of Xe+2, the optimum Rg+2 potentials retain the shapes determined by theoretical calculations, but they are modified by scaling their dissociation energies to the experimentally determined values. The Morse function is shown to be an inadequate potential form for the Rg+2 ground states, since it is too attractive at large internuclear distance.

Resonant multiphoton ionization of H2 via the B 1Σ+u, v=7, J=2 and 4 levels with photoelectron energy analysis

We report measurements of photoelectron spectra of an electronically excited state of H2 (B 1Σ+u) in selected vibrational and rotational levels. The excited state was prepared by resonant three photon excitation of H2 X 1Σ+g, v=0,  J=3 and subsequently was ionized by a single additional photon. Partially resolved rotational structure is observed in the photoelectron spectra and is discussed in terms of the selection rules for direct photoionization and the partial wave composition of the ejected photoelectrons. An additional peak is observed in the spectra, which is tentatively interpreted as arising from photoionization of H*(n=2) formed by photodissociation of H2 in the B 1Σ+u state.

Photoelectron spectrum of Xe2 and potential energy curves for Xe+2

The photoelectron spectrum of the Xe2 van der Waals molecule was determined at 584 A with a resolution of 20 meV, using an apparatus which combines a supersonic molecular beam source with a hemispherical photoelectron spectrometer. This experiment gives new information on the potential energy curve for the bound Xe+2 ground state at large internuclear separation. An optimum potential was determined for this state by fitting the observed photoelectron peak to a calculated Franck–Condon distribution. The calculations show specifically that the Morse function is not an adequate approximation to the potential at large internuclear distances. This experiment also gives new information on the potential energy curves for the weakly bound Xe+2 excited states in the region of their potential minima, thus complementing Xe–Xe+ scattering experiments which probe these potentials in the region of their repulsive walls. Potential energy curves for all of the Xe+2 states dissociating to Xe(1S0)+Xe+(2P3/2 or 2P1/2) are c...

Photoionization of excited molecular states. H2 C 1Πu

Abstract We present photoelectron spectra for H 2 in the excited C 1 Π u , υ = 0–4. J = 1 levels prepared by multiphoton excitation. In accordance with the Franck-Condon principle, the H 2 + vibrational state distribution is dominated by Δυ = 0 transitions from the C 1 Π u state, illustrating a useful method for preparing vibrationally state-selected molecular ions. Equally important observed systematic departures from Franck-Condon factors, which provide detailed information on excited-state photoionization dynamics of molecules.

Zero‐kinetic‐energy photoelectron spectroscopy from the à 1Au state of acetylene: Renner–Teller interactions in the trans‐bending vibration of C2H+2 X̃ 2Πu

Double‐resonance excitation via the A 1Au state is used to record zero‐kinetic‐energy photoelectron spectra of acetylene. The analysis of these spectra leads to an improved value of 91 952±2 cm−1 for the adiabatic ionization potential to the C2H2+ X 2Πu ionic ground state. Because the A 1Au intermediate state has a trans‐bent geometry, transitions from it readily populate the trans‐bending vibration of the ground state ion, leading to new information about this mode and its Renner–Teller interactions. The relative intensities of the Renner–Teller components and of the rotational structure within each component also provide information on the dynamics of the photoionization process.

Photoionization of ArKr, ArXe, and KrXe and bond dissociation energies of the rare gas dimer ions

The relative photoionization cross sections for the heteronuclear rare gas dimers ArKr, ArXe, and KrXe are reported at a wavelength resolution of 0.15 to 0.28 A in the energy region between the molecular ionization threshold and the atomic 2P°1/2 ionization limit of the lighter rare gas atom. The dimer ionization potentials are 13.484±0.015 eV for ArKr, 11.968±0.012 eV for ArXe, and 11.763±0.011 eV for KrXe. Combining these values with the known values of the atomic ionization potentials and the neutral ground state dissociation energies yields values for the A 2Σ+1/2 ionic ground state dissociation energies of 0.528±0.015 eV for ArKr, 0.176±0.012 eV for ArXe, and 0.385±0.011 eV for KrXe. Molecular Rydberg structure in the region between the atomic fine structure thresholds is analyzed in terms of Rydberg series converging to the B 2Π1/2 or D 2Σ+1/2 states of the dimer ions. Molecular Rydberg structure in other regions of the spectrum is extremely complex and, in many cases, long vibrational progressions ...

High resolution photoionization study of ion‐pair formation in H2, HD, and D2

A new photoionization mass spectrometer which is significantly superior to previous instruments in both wavelength resolution and ion intensity is described. It has been used in the study of photon‐induced ion‐pair formation at 78 °K in para‐H2, ordinary H2 and D2 (the ortho‐para equilibrium mixtures), and HD in the wavelength region from 718 to 700 A, at a wavelength resolution (FWHM) of 0.035 A for para‐H2, 0.07 A for ordinary H2 and D2, and 0.15 A for HD. The threshold for formation of ion pairs from para‐H2 occurs at 715.753+0.046−0.008 A (17.3223+0.0002−0.0011 eV) and this value together with the accurately known values of the dissociation energy of H2 and the ionization potential of atomic hydrogen yields a value of the electron affinity of the hydrogen atom of 0.7542 −0.0004 +0.0013 eV in excellent agreement with the theoretical value of 0.75421 eV calculated by Pekeris. The observed thresholds for ion‐pair formation in D2 and HD are also in excellent agreement with the calculated positions. For pa...