Ortho-Para-Dependent Pressure Effects Observed in the Near Infrared Band of Acetylene by Dual-Comb Spectroscopy

We demonstrate that dual-comb spectroscopy, which allows one to record broadband spectra with high frequency accuracy in a relatively short time, provides a real advantage for the observation of pressure-broadening and pressure-shift effects. We illustrate this with the ${\nu }_1+{\nu }_3$ vibration band of ${}^{12}{\mathrm{C}}_2{\mathrm{H}}_2$. We observe transitions from $P(26)$ to $R(29)$, which extend over a 3.8 THz frequency range, at six pressures ranging up to 2654 Pa. Each observed absorption line profile is fitted to a Voigt function yielding pressure-broadening and pressure-shift coefficients for each rotation-vibration transition. The effectiveness of this technique is such that we are able to discern a clear dependence of the pressure-broadening coefficients on the nuclear spin state, i.e., on the ortho or para modification. This information, combined with the pressure-shift coefficients, can facilitate a detailed understanding of the mechanism of molecular collisions.

Figure 1

Setup of present dual-comb spectrometer: PBS (polarization-dependent beam splitter), $\lambda /2$ (half-wave plate), BD (balanced detector).

Figure 2

(a) Transmittance spectrum around 197.2 THz with a sample pressure of 60 Pa. (b) Normalized absorbance of the $R(9)$ transition with different sample pressures. The horizontal axis is the frequency relative to the transition frequency measured with sub-Doppler resolution spectroscopy [10]. The Doppler width (half width at half maximum) for this transition is 238 MHz. Because of the large absorbance, small noise near the line center is enhanced in this type of plot.

Figure 3

The pressure effects of the $R(9)$ transition plotted as a function of the sample pressure. (a) Lorentzian width. (b) Line-center shift.

Figure 4

Pressure coefficients plotted as a function of $m$, where $m$ is $J^{\prime \prime }+1$ for the $R$-branch transitions and $-J^{\prime \prime }$ for the $P$-branch transitions. (a) Pressure-broadening coefficient. The spin-independent curve is from Eq. (4) with parameters of $b_1=46.0(6)\mathrm{kHz}/\mathrm{Pa}$, $b_2=-0.72(3)\mathrm{kHz}/\mathrm{Pa}$, $b_3=31(10)\mathrm{kHz}/\mathrm{Pa}$, and $b_4=0.81(18)$. The fitted line contains the spin-dependent part of Eq. (5) with $c_1=0.205(14)\mathrm{kHz}/\mathrm{Pa}$. The assumed Boltzmann distribution in Eq. (6) has peaks at $m=-9$ and $+10$. (b) Pressure-shift coefficient. The fitted line is from Eq. (7) with parameters of $d_{-1}=3.3(9)\mathrm{kHz}/\mathrm{Pa}$, $d_0=-2.17(12)\mathrm{kHz}/\mathrm{Pa}$, $d_1=-0.063(11)\mathrm{kHz}/\mathrm{Pa}$, and $d_2=-0.0028(5)\mathrm{kHz}/\mathrm{Pa}$.