A New Electronic System for Detecting Microwave Spectra

Abstract

I N an earlier report 1 we described a single-crystal detecting system for microwave spectroscopy which employs a low frequency cut-off filter to eliminate the contour of the oscillator mode and the spurious signals caused by reflections in the microwave line. We have now improved the sensitivity of our system by a modulation technique which allows the signal to be amplified at higher frequencies.Upon the slowly varying sawtooth voltage used to sweep the microwave oscillator over its mode we superimpose a radiofrequency voltage (^100 kc) of low amplitude which causes the oscillator to be frequency modulated over a small range of frequencies, at a rate determined by the superimposed radiofrequency. An absorption line of a gas then acts as a discriminator to produce intensity modulation of the output radiation at a frequency corresponding to that of the modulating voltage. A single-crystal detector is used with an amplifier tuned to the modulating frequency. After pre-amplification at this frequency a second detector is employed followed by the filter mentioned above and by an audio amplifier. The filter we find effective against reflections. There are sharp, strong signals caused by the abrupt beginning and ending of the mode which are not eliminated by the filter. These are easily identified and are useful as indicators for crystal tuning and as markers to define the limits of tube oscillation.Using the method described here with a 3.6-meter K-band wave-guide cell we easily detected the stronger lines of N 15 H 3 in its naturally occurring concentrations of 0.3 percent in ordinary NH 3 . A photograph of the 3,3 line of N 15 H 3 thus obtained is shown in Fig. 1. The photograph of the 3,3 line of N 14 H3 in Fig. 2 demonstrates that good FIG. 1. Cathode-ray scope display of the 3,3 line of N 15 Ha at natural concentrations of 0.3 percent in normal ammonia. Cell length, 3.6 meters. Pressure, 2 X10~3 mm of Hg. Modulation frequency 100 kc. FIG. 2. Cathode-ray scope display of the 3,3 line N 14 H3 showing satellite structure. Cell length, j3.6 meters. Pressure, 4X10~s. Modulation frequency 100 kc.resolution can also be obtained. The distortions apparent in these photographs are npt caused by the modulation but by the narrow band pass of the receiver used. Objectionable distortions can, of course, be produced by an incorrect modulation, but the adjustment of the modulation voltage is not critical.No greater sensitivity can be claimed for the system described above than for the Stark-effect modulation method of Hughes and Wilson. 2 In both systems the reception depends upon an intensity modulation produced by a high frequency motion of the signal in time (or components of the signal in the Stark-effect case). The present method has the advantage of being convenient to use with a cell of indefinite length. It is in general easier to modulate the oscillator than the molecules. The^ Hughes-Wilson method has the advantage of not requiring a filter to eliminate reflections.A more complete description of the system will be repo...

Accurate equilibrium structures of some challenging molecules: FNO, ClNO, HONO, FNO2, and N2O

“…[59] There is another source of uncertainty originating from the electronic correction. [60] To estimate it, the g-tensor was calculated at the B3LYP/6-311 + G (3df,2pd) level of theory. It gives a very large correction for A, about 78 MHz, whereas it is quite small for B (1.2 MHz) and C (0.3 MHz).…”

“…[5] A first confirmation is given be the use of the Kraitchman's equations. [60,62] Employing the A and B semiexperimental rotational constants gives r e (ON) = 1.1259 Å whereas the value is r e (ON) = 1.1286 Å with the B and C constants. As C is expected to be much more accurate than A, the later value is likely to be more reliable.…”

Microwave molecular spectroscopy

“…This method of detection has been described by W. D. Hershberger and by W. Gordy and M. Kessler. 6 If the variation of the oscillator frequency from its mean value is small compared with the width of the absorption line, the appearance of the absorption line will be as in Z), Figure 3. If a phase sensitive amplifier is used, together with a lowfrequency filter, the appearance is similar to that in C, Figure 3.…”