Speaker
Description
Molecular line lists for gases at high temperatures are crucial in fields ranging from combustion to astrophysics. However, due to the lack of experimental data, some of the energy levels that constitute the lists are inaccurate or even missing, and their theoretical models need validation. Addressing these gaps is especially important for methane, a key greenhouse gas and one of the biosignatures that the James Webb Space Telescope (JWST) can readily detect on terrestrial exoplanets [1].
Analyzing absorption bands for gases at high temperatures is a major challenge due to the extreme density of lines that cannot be resolved in the spectrum of a heated sample. As temperature rises, multiple excited states (known as hot-bands) are populated, and Doppler broadening increases, so the transitions starting from these states overlap.
Optical-optical double-resonance spectroscopy (DR) overcomes this challenge by pumping room-temperature molecules to a single excited state with a monochromatic, continuous wave (CW) laser. Then, a laser probe excites transitions from this single state to the final upper states. The probe transitions are Doppler-free, because only a narrow velocity group of molecules is excited by the pump. Previous DR techniques on molecules used CW probes, with limited accessible bandwidth [2]. Instead, we use an optical frequency comb, which consist of thousands of equidistant, stable, and narrow laser lines, as a probe with large bandwidth and high resolution [3,4].
To measure and assign transitions in the $3\nu_{3} \leftarrow \nu_{3}$ band of methane we pump with a high-power CW laser, locked to a transition in the $\nu_{3}$ band of methane, at around 3.3 $\mu$m. As probe, we use a frequency comb generated by a mode-locked Er:fiber laser with 250 MHz repetition frequency, and tunable from 1.65 $\mu$m to 1.8 $\mu$m.
To calibrate the system, the comb offset and repetition frequency are stabilized and referenced to frequency standards. To enhance the interaction length and thus the absorption sensitivity, the sample of pure methane is contained in an optical cavity, which is resonant with the probe. The probe spectra are measured using a Fourier transform spectrometer with resolution limited by the comb mode width [5,6]. From these spectra, we can simultaneously retrieve center frequencies of transitions in multiple hot-bands with high selectivity, and unambiguously assign final states. The assignment allows us to validate theoretical predictions from the TheoReTS [7] database with unprecedented accuracy.
The validated theoretical models for methane will be an important tool to extract information about the conditions of hot exoplanetary atmospheres from the infrared spectra that will be measured by JWST. Furthermore, DR with frequency combs has the potential to investigate the hot-bands of other molecules in a wide range of frequencies.
References
[1] M. A. Thompson et al., “The case and context for atmospheric methane as an exoplanet biosignature”, Proceedings of the National Academy of Sciences 119, e2117933119 (2022).
[2] W. Demtröder, “Optical pumping and double-resonance techniques”, in Laser spectroscopy 2: experimental techniques (Springer Berlin Heidelberg, Berlin, Heidelberg, 2015), pp. 225–269.
[3] A. Foltynowicz et al., “Measurement and assignment of double-resonance transitions to the 8900-9100 cm−1 levels of methane”, Physical Review A 103, 022810 (2021).
[4] A. Foltynowicz et al., “Sub-doppler double-resonance spectroscopy of methane using a frequency comb probe”, Physical Review Letters 126, 063001 (2021).
[5] L. Rutkowski et al., “Optical frequency comb fourier transform spectroscopy with subnominal resolution and precision beyond the Voigt profile”, Journal of Quantitative Spectroscopy and Radiative
Transfer 204, 63–73 (2018).
[6] P. Maslowski et al., “Surpassing the path-limited resolution of Fourier-transform spectrometry with frequency combs”, Physical Review A 93, 021802(R) (2016).
[7] M. Rey et al., “TheoReTS–an information system for theoretical spectra based on variational predictions from molecular potential energy and dipole moment surfaces”, Journal of Molecular Spectroscopy 327, 138–158 (2016).
