Chirped Pulse Spectrometer Operating at 200 GHz

TL;DR

This paper presents the development and validation of a 200 GHz chirped pulse spectrometer capable of high-resolution gas phase rotational spectroscopy, demonstrating its sensitivity, linearity, and data analysis advantages.

Contribution

It introduces a detailed design of a 200 GHz chirped pulse spectrometer, including phase stability, data accumulation, and analysis methods, advancing high-frequency microwave spectroscopy.

Findings

Detection of multiple OCS isotopologue transitions.

Confirmed linear response over four orders of magnitude.

Demonstrated line broadening analysis without Fourier transform.

Abstract

The combination of electronic sources operating at high frequencies and modern microwave instrumentation has enabled the recent development of chirped-pulse spectrometers for the millimetre and THz bands. This type of instrument can operate at high resolution which is particularly suited to gas phase rotational spectroscopy. The construction of a chirped pulse spectrometer operating at 200 GHz is described in detail while attention is paid to the phase stability and the data accumulation over many cycles. Validation using carbonyl sulphide has allowed the detection limit of the instrument to be established as function of the accumulation. A large number of OCS transitions were identified using a 10 GHz chirped pulse and include the 6 most abundant isotopologues, the weakest line corresponding to the fundamental R(17) transition of 16 O 13 C 33 S with a line strength of 4.3 x 10-26 cm-1 /(molec.cm-2). The linearity of the system response for different degrees of data accumulation and transition line strength was confirmed over 4 orders of magnitudes. A simple analysis of the time domain data was demonstrated to provide the line broadening coefficient without the need for conversion by a Fourier transform. Finally, the pulse duration is discussed and optimal values are given for both Doppler limited and collisional regimes.