New microwave spectrometer combines speed with sensitivity

Microwave spectroscopy, one of the most accurate ways to measure molecular structure, is widely used in chemical analyses. Today, most microwave spectrometers use pulsed supersonic jets as the source of cold molecules, which put a constraint on the instrument’s overall duty cycle due to the physical limitations of vacuum pumps. Additionally, these spectrometers either have a narrow frequency bandwidth with high sensitivity, as with cavity-enhanced instruments, or a wide frequency coverage but with low sensitivity, such as in a broadband configuration.

Porterfield et al. present an improved instrument for microwave spectroscopy that uses cryogenic buffer gas cooling. With this instrument, the sample molecules are cooled via collisions with helium atoms at 4-7 K. The first-stage amplifiers and protection switches are also cryogenically cooled, resulting in a system temperature of around 30-35K. The molecular sample is then probed with chirped pulse microwave spectroscopy at up to 50 kHz.

The new design has an extended operating range of 12-26GHz compared to the previous 12-18GHz, allowing for more molecular species to be simultaneously characterized. The measurements are also more sensitive due to the cryogenically cooled low-noise amplifiers and switches. In comparison with cavity-enhanced instruments, the new method was found to have a comparable detection sensitivity, but with an acquisition rate 3,000 times faster and just a moderate increase in sample consumption rate. When compared to wide-band spectrometers, the new instrument can acquire spectra nearly 75 times faster, also accompanied by an increased rate of sample consumption.

Source: “High sensitivity microwave spectroscopy in a cryogenic buffer gas cell,” by Jessica P. Porterfield, Lincoln Satterthwaite, Sandra Eibenberger, David Patterson, and Michael C. McCarthy, Review of Scientific Instruments (2019). The article can be accessed at https://doi.org/10.1063/1.5091773 .