A pulsed microwave Fourier transform spectrograph is described in detail. The increase in sensitivity and resolution obtained in the time domain relative to standard spectroscopy in the frequency domain is described theoretically and experimentally. Fast switching microwave diodes with a large dynamic range for the generation of high-power microwave pulses are essential for the operation of the spectrograph. A 512-point analog–digital converter and averager with a repetition rate of up to 30 kHz is used for signal-to-noise improvement. The observed emission signals in the time domain are Fourier transformed to the frequency domain on-line by a small laboratory computer. The spectra obtained for 13CH2O and CD2O show a marked improvement in signal-to-noise ratio and resolution. A 50-MHz bandwidth can be covered by a single pulse train.
We report a pulse sequence measurement of T1 in the J=0 → J=1 rotational transition in OCS in the pure gas, OCS–He mixtures, and OCS–CH3F mixtures at low pressures. This pulse sequence method is similar to the π, τ, π/2 pulse sequence used in nuclear magnetic resonance to measure T1. The pulse method of measuring T1 in rotational systems has several advantages over the analysis of transient absorption (transient nutation) results or the analysis of power broadened steady state absorption lines. We find T1=T2 for the J=0 → J=1 OCS transitions in pure OCS, OCS–He mixtures, and OCS–CH3F mixtures.
Transient experiments have been completed to measure T1 and T2 as a function of pressure for the inversion transitions in several (J,K,‖M‖ =J) states in 15NH3 and in the J=1→J=2, M=±1 transitions in OCS. In OCS, T1=T2 for both the J=0→J=1, M=0 and J=1→J=2, M=±1 transitions. In NH3, however, the ratio T2/T1 varies over the range 1.0?T2/T1?2. These results are interpreted using a modified form for the Anderson theory of line broadening. Comparisons are made between the results of this work and earlier steady state double resonance work.
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