Chris A. Michaels

Carbon nanotube tipped atomic force microscopy for measurement of <100 nm etch morphology on semiconductors

The use of carbon nanotubes as tips in atomic force microscopy for a systematic study of dry etching pattern transfer in GaAs is described. The GaAs samples are patterned via electron beam lithography and then etched using magnetron reactive ion or chemically assisted ion beam processing. The technique allows diagnosis, in air, of etched features with scale sizes of <100 nm.

Molecular supercollisions: Evidence for large energy transfer in the collisional relaxation of highly vibrationally excited pyrazine by CO2

The temperature dependence of the collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been investigated for the temperature range 243–364 K using high resolution time resolved diode laser spectroscopy. Particular emphasis is placed on vibration to rotation‐translation (V→R/T) energy transfer which leaves the CO2 vibrations unexcited and occurs predominantly through short‐range repulsive forces. Vibrationally hot pyrazine is prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. For the range of experimental cell temperatures used here, the nascent rotational population distributions of the 0000 ground state of CO2 resulting from collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. The CO2 translational recoil velocity was also measured for individual rotational levels of the 0000 state. In addition, temperature dependent rate constants and probabilities were determined for energy transfer from the vibrationally hot pyrazine into individual rotational levels of the 0000 state of CO2. The rotational distributions, velocity recoils, and quenching rates exhibit a very weak temperature dependence for production of CO2 high J states, indicating that the CO2 molecules involved in these energy transfer events originate from rotational levels only slightly greater than the thermal mean J value. Based on these results, values for ΔE, the energy transfer from hot pyrazine to CO2 resulting in final CO2 0000 states J=58 through J=82, are estimated to range from 2550 to 7090 cm−1 in a single collision.The temperature dependence of the collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been investigated for the temperature range 243–364 K using high resolution time resolved diode laser spectroscopy. Particular emphasis is placed on vibration to rotation‐translation (V→R/T) energy transfer which leaves the CO2 vibrations unexcited and occurs predominantly through short‐range repulsive forces. Vibrationally hot pyrazine is prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. For the range of experimental cell temperatures used here, the nascent rotational population distributions of the 0000 ground state of CO2 resulting from collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. The CO2 translational recoil velocity was also measured for individual rotational levels of the 0000 state. In addition, temperature dependent rate constants and probabili...

Translational and rotational excitation of the CO2(0000) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene: Evidence for impulsive collisions accompanied by large energy transfers

The relaxation of highly vibrationally excited perfluorobenzene (C6F6) by collisions with CO2 molecules has been investigated over the temperature range 243–364 K using diode laser transient absorption spectroscopy. Particular focus is placed on understanding both the dynamical features and the kinetics of collisions which are accompanied by large energy transfers into the CO2 rotational and translational degrees of freedom. Vibrationally hot perfluorobenzene (Evib=41 822 cm−1) was prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions (J=64–80) of the 0000 ground state of CO2 resulting from collisions with hot perfluorobenzene were probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the distributions of CO2 recoil velocities for individual rotational levels of the 0000 state. In addition, the temperature dependence of the state resolved, absolute rat...

Connecting quantum state resolved scattering data directly to chemical kinetics: Energy transfer distribution functions for the collisional relaxation of highly vibrationally excited molecules from state resolved probes of the bath

An energy transfer probability distribution function, P(E,E), for the collisional relaxation of a highly vibrationally excited donor molecule (C6F6, pyrazine) is constructed for the first time from experimental data on the bath (CO2) energy gain. A prescription for mapping bath quantum state resolved data onto P(E,E) is described in detail. Analysis of earlier experimental data allows a calculation of the high ΔE=E−E region (−7000 cm−1<E−E<−1500 cm−1) of P(E,E) for the above systems. Comparison of the P(E,E) functions reveals that C6F6 is a more efficient donor molecule than pyrazine, in agreement with previous experiments and trajectory calculations. In addition, resonance like structures in the P(E,E) functions arising from long range force mediated, V–V excitation of the carbon dioxide ν3 mode are discussed. These results indicate that accurate P(E,E) functions can be determined from experiments involving probes of the bath energy gain. This technique can be expected to provide stringent tests of curre...

Long‐ and short‐range interactions in the temperature dependent collisional excitation of the antisymmetric stretching CO2(0001) level by highly vibrationally excited pyrazine

The relaxation of highly vibrationally excited pyrazine, C4H4N2, by collisions with CO2 that produce molecules in the vibrationally excited antisymmetric stretch state (0001) has been investigated using high resolution infrared transient absorption spectroscopy at a series of ambient cell temperatures. The vibrationally hot (Evib≊5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the vibrationally excited CO2 molecules are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions, strongly suggest that the vibrational excitation of CO2 occurs via two mechanisms. The vibrational energy transfer is dominated by a long‐range attractive force interaction, which is accompanied by almost no rotational and translational excitation. However, the CO2(0001) product s...

The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide: Excitation of the infrared inactive (1000), (0200), and (0220) bath vibrational modes

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (1000) and (0200), along with the unmixed overtone bend state (0220). The vibrationally hot (Evib≈5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements ...

Near-Field Infrared Imaging and Spectroscopy of a Thin Film Polystyrene/Poly(ethyl acrylate) Blend

The application of broadband, near-field infrared microscopy to the characterization of the mesoscale structure of a thin film polymer blend is described. Key features of this instrument, which couples the nanoscale spatial resolution of scanning probe microscopy with the chemical specificity of vibrational spectroscopy, include broad tunability and bandwidth, parallel spectral detection for high image acquisition rates, and infrared-transparent aperture probes. Near-field spectral transmission images of a thin film of polystyrene/poly(ethyl acrylate) acquired in the C–H stretching region are reported. An assessment of the relative importance of transmission image contrast mechanisms is a significant aim of this work. Analysis of the near-field infrared spectra indicates that the image contrast in the C–H stretching region is largely due to near-field coupling and/or scattering effects. Identification and differentiation of the operative contrast mechanisms on the basis of their relative dependence on wavelength is discussed. Analysis of the contrast attributed to absorption is consistent with the chemical morphology of this sample derived from previous chemical modification/atomic force microscopy studies.

Stress mapping of micromachined polycrystalline silicon devices via confocal Raman microscopy

Stress mapping of micromachined polycrystalline silicon devices with components in various levels of uniaxial tension was performed. Confocal Raman microscopy was used to form two-dimensional maps of Raman spectral shifts, which exhibited variations on the scale of the component and on the scale of the microstructure. Finite element analysis models enabled direct comparison of the spatial variation in the measured shifts to that of the predicted stresses. The experimental shifts and model stresses were found to be linearly related in the uniaxial segment, with a proportionality constant in good agreement with calculations based on an opto-mechanical polycrystalline averaging analysis.

Indentation device for in situ Raman spectroscopic and optical studies

Instrumented indentation is a widely used technique to study the mechanical behavior of materials at small length scales. Mechanical tests of bulk materials, microscopic, and spectroscopic studies may be conducted to complement indentation and enable the determination of the kinetics and physics involved in the mechanical deformation of materials at the crystallographic and molecular level, e.g., strain build-up in crystal lattices, phase transformations, and changes in crystallinity or orientation. However, many of these phenomena occurring during indentation can only be observed in their entirety and analyzed in depth under in situ conditions. This paper describes the design, calibration, and operation of an indentation device that is coupled with a Raman microscope to conduct in situ spectroscopic and optical analysis of mechanically deformed regions of Raman-active, transparent bulk material, thin films or fibers under contact loading. The capabilities of the presented device are demonstrated by in situ studies of the indentation-induced phase transformations of Si thin films and modifications of molecular conformations in high density polyethylene films.

Competition between photochemistry and energy transfer in ultraviolet-excited diazabenzenes. II. Identifying the dominant energy donor for “supercollisions”

CO 2 bath molecules scattered into J=72 of the 00 0 0 vibrational state at short times after 248 or 266 nm UV excitation of pyrazine are probed using high resolution time resolved IR diode laser spectroscopy as a function of UV laser fluence from ∼3 to 80 mJ/cm2. The implications of pyrazine photodissociation for the interpretation of these collisional energy transfer experiments are considered. Specifically, the possibility that translationally hot HCN resulting from pyrazine dissociation may be the source of excitation for collisions that impart a large amount of rotational and translational energy to CO 2 molecules is examined. Transient absorption measurements probing rotationally and translationally excited CO 2 molecules produced following excitation of pyrazine are analyzed within the context of a kinetic scheme incorporating pyrazine photodissociation, as well as excitation of CO 2 by both translationally hot HCN and vibrationally excited pyrazine. This analysis indicates that vibrationally hot pyrazine, which has sufficient energy to dissociate, is the source of excitation in collisions imparting large amounts of rotational and translational energy to CO 2 .