Terry A. Miller

Imaging ultrafast molecular dynamics with laser-induced electron diffraction

Establishing the structure of molecules and solids has always had an essential role in physics, chemistry and biology. The methods of choice are X-ray and electron diffraction, which are routinely used to determine atomic positions with sub-ångström spatial resolution. Although both methods are currently limited to probing dynamics on timescales longer than a picosecond, the recent development of femtosecond sources of X-ray pulses and electron beams suggests that they might soon be capable of taking ultrafast snapshots of biological molecules and condensed-phase systems undergoing structural changes. The past decade has also witnessed the emergence of an alternative imaging approach based on laser-ionized bursts of coherent electron wave packets that self-interrogate the parent molecular structure. Here we show that this phenomenon can indeed be exploited for laser-induced electron diffraction (LIED), to image molecular structures with sub-ångström precision and exposure times of a few femtoseconds. We apply the method to oxygen and nitrogen molecules, which on strong-field ionization at three mid-infrared wavelengths (1.7, 2.0 and 2.3 μm) emit photoelectrons with a momentum distribution from which we extract diffraction patterns. The long wavelength is essential for achieving atomic-scale spatial resolution, and the wavelength variation is equivalent to taking snapshots at different times. We show that the method has the sensitivity to measure a 0.1 Å displacement in the oxygen bond length occurring in a time interval of ∼5 fs, which establishes LIED as a promising approach for the imaging of gas-phase molecules with unprecedented spatio-temporal resolution.

A Study of the Optical Emission from an rf Plasma during Semiconductor Etching

Spectroscopic analysis of optical emission during rf plasma etching of semiconductor materials has been used to gain a better understanding of the plasma chemistry involved in these systems. The emission was studied principally in CF4-O2 gas mixtures, but other gases were observed as well. It is known that the addition of a relatively small percentage of O2 to CF4 yields a much faster etching rate for silicon and silicon nitride. With the addition of 02 to CF4 discharges we have studied emission from atomic O and molecular CO with a large increase in the emission of atomic F. When the plasma is actively etching silicon or silicon nitride, the emission intensities of both F and O atoms are significantly lower. The etching process can be monitored by observing the intensities of these lines. Analysis of the emission features has also been used to determine abnormal conditions which can adversely affect the etching process.

Optical Techniques in Plasma Diagnostics

Recent developments in optical techniques for plasma diagnostics are critically reviewed. The primary emphasis is on plasma-induced emission and laser-induced fluorescence probes of radical and ionic concentrations and temperatures in low pressure discharges like those used in microelectronic fabrication. Other techniques such as optogalvanic, infrared, spontaneous and stimulated Raman, and multiphoton spectroscopy are also discussed briefly. Generally, emission techniques are seen to be useful primarily for qualitative analysis and characterization of the electron energy distribution. However, in some systems, emission actinometry, where an inert gas is used to account for changes in electron density, appears to be a valid means of determining relative changes in concentration. Laser-induced fluorescence spectroscopy has only recently been utilized for in situ measurements of ground state ions and radicals but has already yielded insight into mechanisms for formation, destruction, energy transfer, and maintenance of the discharge. By and large the other techniques show great promise but remain to be exploited as diagnostic tools for plasmas.

Radiative decay and radiationless deactivation in selectively excited CN

Individual vibrational levels (v=3–9) of the A 2Π state of the CN radical are excited by a tunable dye laser. The time and wavelength resolved fluorescence of the selectively excited levels is then recorded as a function of the Ar carrier gas pressure. It is found that the decay of many levels is, particularly at lower pressures, strongly double exponential, with the longer component time constant considerably in excess of the CN A 2Π radiative lifetime. The short time constant portion of this exponential is interpreted in terms of a model which includes fast collisional equilibration between the excited v′A 2Π level and a nearby v″, X 2Σ level. The slower part of the decay consists of quenching and radiative decay from the coupled levels. Approximate values for the rate constants governing these processes are derived.

Radiative and radiationless vibronic deactivation rates in selectively excited CO

A tunable dye laser has been used to selectively excite particular (v=0–3) vibrational levels of A 2Π CO+. The CO+ is produced in a He bath by Penning ionization of CO by He metastables. By studying the time resolved emission of CO+ from selected vibrational levels as a function of He pressure, values have been obtained for the radiative decay rates and the velocity averaged cross‐sections for CO+ vibrational relaxation upon He collision. The cross sections are all rather large (≳0.1 A2) and increase considerably with increasing v. Time resolved spectra of the vibrationally relaxed emission require the postulation of a three‐level system to fit their form. It is proposed that both of these effects are consistent with a vibrational relaxation mechanism wherein the A 2Π CO+ vibrational levels relax via intermediate, highly excited, vibrational levels of the ground X 2Σ state of CO+.

Theoretical study of Jahn–Teller distortions in C6H+6 and C6F+6

Ab initio molecular orbital theory with the 6–31G basis set is used to study Jahn–Teller distortion effects in the benzene cation (C6H+6) and the perfluorobenzene cation (C6F+6). π‐electron correlation is included in these calculations. Completely optimized Jahn–Teller distorted geometries are obtained for the ground electronic states of both C6H+6 and C6F+6. The stabilization energies resulting from these distortions are calculated for both systems. The stabilization is partitioned into contributions from C–C stretch, C–H or C–F stretch, C–C–C bend, and C–C–H or C–C–F bend coordinates. Detailed comparison is made between C6H+6 and C6F+6 to consider the effects of substitution. The calculated geometries and stabilization energies for C6F+6 are compared to values derived from laser‐induced fluorescence experiments. The overall calculated distortion parameters for C6F+6 are in good agreement with the experimentally derived distortions, though the calculated C–C bond length change is somewhat larger than the...

Rotational, fine, and hyperfine structure in the high‐resolution electronic spectrum of ArOH and ArOD

A number of vibrational bands of the A 2Σ+↔X 2Π electronic spectrum of both ArOH and ArOD have been investigated by laser induced fluorescence with a high‐resolution, pulsed laser system yielding linewidths ≲250 MHz in the UV. This spectrum not only displays completely resolved rotational structure, but also fine and hyperfine structure. The hyperfine constants and precise interatomic distances derived from the rotational constants provide a very interesting picture of the electronic and geometric structure of the complex. The bonding is incipiently chemical in the A state with clear evidence for at least some electronic reorganization between Ar and the open‐shell OH radical in the complex. Conversely, the X state appears to be bound almost solely by physical van der Waals interactions characteristic of systems containing only closed‐shell species.

Gas‐Phase Electron Resonance Spectra of SF and SeF

The electron resonance spectra of the gaseous radicals SF and SeF have been detected and analyzed. The derivation of an effective Hamiltonian which operates in the rotational subspace of the ground vibronic state is described. Use of this Hamiltonian yields the rotational constants, the fine‐structure splittings, and the axial components of the 19F hyperfine interaction for both radicals.

High-resolution fluorescence excitation spectra of jet-cooled benzyl and p-methylbenzyl radicals

Abstract High-resolution, rotationally resolved, laser-induced, fluorescence excitation spectra of the A 1 and 6a 1 0 bands of benzyl and the 0 0 0 band of p -methylbenzyl radicals were obtained in supersonic expansions. All three spectra were assigned and fit, using the rigid rotor Hamiltonian as well as methyl group internal rotation theory. The results of the rotational analysis provide good rotation constants for benzyl and p -methylbenzyl and establish unambiguously that the symmetry of the excited electronic state in this transition of p -methylbenzyl is 2 A 2 (in C 2v ). The heights of torsional barriers that hinder the internal rotation of the methyl group in p -methylbenzyl also are determined. The torsional results are compared to those obtained previously for this radical in a vibrational analysis and to other open shell radicals.

Detection and characterization of alkyl peroxy radicals using cavity ringdown spectroscopy

Cavity ringdown spectra of the A 2A′−X 2A″ electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both the A and X states of CH3O2⋅. An estimate for the absorption cross section is determined from the CRDS absorption and the rate of radical–radical recombination.