Randall E. Murphy

Time-resolved Fourier spectroscopy*†

Fourier spectroscopy has enjoyed considerable success in the measurement of sources that are stationary with respect to time. A technique for the study of time-varying spectral features, by use of Fourier spectroscopy, is presented. With this technique, the multiplex advantage is retained and the available measurement time is utilized efficiently. The technique consists of obtaining a series of interferograms, separated sequentially in time, according to the time-resolution capability of the detection system. The interferogram signal is recorded over a time interval, Δt, corresponding to a particular time tj, in the evolution of the source. The entire series of interferograms is obtained in a single scan of a Michelson interferometer. The S/N may be improved by continued exposure at the same optical path difference, corresponding to the same tj in the evolution of the source. The time-sequenced interferograms are transformed to obtain the recovered spectra at successive time intervals. As a demonstration of the technique, the spectral evolution of a N2/O2 gas mixture subjected to high-energy-electron irradiation is given in 50 µs intervals. The spectra clearly show the change in time of the fundamental sequence of NO (X2Π, Δν = 1), and the (001 → 000) vibration–rotation bands of N2O and NO2.

Improvements in time resolved Fourier spectroscopy

The technique of Fourier spectroscopy, the full realization of its advantage, has been applied to a time-dependent study. The principle of the technique, the SNR expected, and the instrumentation involved are discussed together with some recent results.

The vibrational relaxation of NO(v = 1–7) by O2

Highly vibrationally excited NO(v = 1–10) has been created by irradiation of N2/trace O2 mixtures with 36 keV electrons. The resulting spectrally‐resolved time histories of the NO fundamental vibration/rotation band have been analyzed to determine the room temperature rate constants for the processes NO(v)+O2→NO(v−1)+O2, v = 1–7. The rate constants increase monotonically with v.

A study of the vibrational level dependent quenching of CO(v =1–16) by CO2

The technique of time resolved Fourier spectroscopy has been used to determine rate constants for the processes CO(v)+CO2→CO(v−1)+CO2, where the vibrationally excited CO is created through electron irradiation of Ar/CO2 mixtures. The CO production mechanism, predominantly dissociative recombination of CO2+, is found to produce CO excited to as much as v=19. The CO(v) deactivation rate constants are deduced from examination of the time histories of the vibrational population distribution. From a Stern–Volmer analysis, the residual quenching not due to CO2 is attributed entirely to CO(v=0) relaxation of CO(v) and radiative decay. Experimentally determined upper bounds for the CO(Δv=1) transition probabilities for spontaneous emission have been obtained for levels 7–12.

Off-axis effects in a mosaic Michelson interferometer

The interferogram equations for a Michelson interferometer operated with a mosaic array of detectors are derived. The effects on the instrumental line shape function (ILF) due to an individual detector field subtense and its displacement from the optical axis of the interferometer have been numerically computed from the interferogram equations. Theoretical predictions of the dependence of the ILF on various system parameters are presented as well as comparisons of the theoretical ILF with those measured in the laboratory.

On self‐trapping of CO2 (ν3) fluorescence

Spectrally resolved measurements are presented for ν3 band fluorescence of CO2. A pulsed 30 KeV electron beam is used to excite the fluorescence of CO2 in a low pressure chamber. The fluorescence is viewed at right angles. A comparison between measured spectrum and predicted spectrum is presented. (AIP)