Frequency measurements and self-broadening of sub-Doppler transitions in the v1 + v3 band of C2H2

Abstract

Frequency comb-referenced measurements of sub-Doppler laser saturation dip absorption lines in the v 1 + v 3 band of acetylene near 1.5 µm are reported. These measurements include transitions involving higher rotational levels than previously frequency measured in this band. The accuracy of the measured frequencies is typically better than 10 kHz. Measurements of the observed sub-Doppler line widths as a function of pressure showed that the self pressure-broadening coefficients are about 3.5 times larger than those derived from conventional pressure broadening of unsaturated Doppler-limited spectra. This is attributed to the contribution of velocity-changing collisions to the total dephasing rate in the low pressure sub-Doppler measurements.At higher pressures, when the homogeneous broadening becomes comparable to the typical Doppler shift per elastic collision, the velocity changing collisions cease to contribute significantly to the incremental pressure broadening. A time-dependent soft collision model is developed to illustrate the transition between low and high pressure regimes of sub-Doppler pressure-broadening.

Comb coherence-transfer and cavity ring-down saturation spectroscopy around 1.65 μm: kHz-accurate frequencies of transitions in the 2ν3 band of 12CH4

“…This increase of the pressure broadening slope at very low pressure may be attributed to the contribution of elastic velocity changing collisions, which becomes much smaller at higher pressure and for Doppler limited line profiles. 41 It is interesting to compare to results recently published by Twagirayezu et al 42 for saturation dips obtained in a combination band of acetylene in the near infrared, for the same pressure range as here. Their findings are very similar to ours, down to the actual values of shift and broadening coefficients.…”

Frequency metrology of molecules in the near-infrared by NICE-OHMS

“…The disagreements between low-pressure Lamb-dip measurements and high-pressure Doppler broadened spectra were found for both the pressure broadening and pressure shift coefficients. It has been discussed in our previous work [44], and also was recently investigated by Sears et al [46] indicating rich physics in the collision dynamics of molecules at low pressures. Other systematic effects -We have also investigated other experimental parameters which may potentially introduce systematic shifts, such as the laser power and the fiber EOM driving power (modulation index).…”

Dipole saturated absorption modeling in frequency modulation spectroscopy: Dealing with a Gaussian beam, resonance narrowing

“…Furthermore, C 2 H 2 can be used as a molecular target for the spectroscopic determination of the Boltzmann constant by means of Doppler Broadening Thermometry (DBT). [35] It is also a benchmarking molecule for VCC/PCC [36,37,38] while F. Constantin also identified a few transitions with strong sensitivity to the electron-to-proton mass ratio on purpose of testing Physics beyond the standard model. [39] To validate our models (actually four models depending mainly on the number of integrations performed), we selected two transitions of acetylene belonging to two different vibrational combination modes, but, belonging to same polyad ∆P = 11, and exhibiting a different line intensity.…”