High-resolution sub-Doppler Lamb dips of the ν2 fundamental band of H3(+)

Highly accurate experimentally determined energy levels of H3. J Chem Phys 2019;150:214303. [PMID: 31176322 DOI: 10.1063/1.5099454]

A sub-Doppler rovibrational spectroscopic survey of H₃ ⁺ has been conducted which included 36 transitions in the ν₂ ← 0 fundamental band, 15 transitions in the 2ν₂ ²←ν₂ hot band, and 7 transitions in the 2ν₂ ²←0 overtone band, improving the uncertainties of most transitions by more than an order of magnitude to ∼4 MHz. Combination differences were used to determine relative energy levels and forbidden rotational transitions up to J = 6. A fit of the ground state to an effective Hamiltonian was used to connect ortho and para states, and to determine the absolute energy levels relative to the forbidden (0, 0) state. Ultimately, 62 rovibrational energy levels in the ground, ν₂, and 2ν₂ ² states were determined with ∼10 MHz uncertainty. Comparing the experimentally determined energy levels with ab initio calculations revealed an unexpected dependence of the residuals on the quantum number G.

Precision saturated absorption spectroscopy of H3. J Chem Phys 2018;148:124310. [PMID: 29604856 DOI: 10.1063/1.5017080]

In our previous work on the Lamb-dips of the ν₂ fundamental band transitions of H₃⁺, the saturated absorption spectrum was obtained by third-derivative spectroscopy using frequency modulation with an optical parametric oscillator (OPO). However, frequency modulation also caused errors in the absolute frequency determination. To solve this problem, we built a tunable offset locking system to lock the pump frequency of the OPO to an iodine-stabilized Nd:YAG laser. With this improvement, we were able to scan the OPO idler frequency precisely and obtain the saturated absorption profile using intensity modulation. Furthermore, ion concentration modulation was employed to subtract the background noise and increase the signal-to-noise ratio. To determine the absolute frequency of the idler wave, the OPO signal frequency was locked to an optical frequency comb. The absolute frequency accuracy of our spectrometer was better than 7 kHz, demonstrated by measuring the wavelength standard transition of methane at 3.39 μm. Finally, we measured 16 transitions of H₃⁺ and our results agree very well with other precision measurements. This work successfully resolved the discrepancies between our previous measurements and other precision measurements.

Improving cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers

The performance of sensitive spectroscopic methods in the mid-IR is often limited by fringing due to parasitic etalons and the background noise in mid-infrared detectors. In particular, the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS), which is capable of determining the frequencies of strong rovibrational transitions of molecular ions with sub-MHz uncertainty, needs improved sensitivity in order to probe weaker transitions. In this work, we have implemented up-conversion detection with NICE-OHVMS in the 3.2 - 3.9 µm region to enable the use of faster and more sensitive detectors which cover visible wavelengths. The higher bandwidth enabled detection at optimized heterodyne frequencies, which increased the overall signal from the H3+ cation by a factor of three and was able to resolve sub-Doppler features which had previously overlapped. Also, we demonstrate the effectiveness of Brewster-plate spoilers to remove fringes due to parasitic etalons in a cavity enhanced technique. Together, these improvements reduced the instrument's noise equivalent absorption to 5.9×10^-11 cm^-1 Hz^-1/2, which represents a factor of 34 improvement in sensitivity compared to previous implementations of NICE-OHVMS. This work will enable extended high-precision spectroscopic surveys of H3+ and other important molecular ions.

Sub-Doppler resolution mid-infrared spectroscopy using a difference-frequency-generation source spectrally narrowed by laser linewidth transfer

The spectral linewidth of a 3.28 μm difference-frequency-generation source has been reduced to 3.5 kHz using a laser linewidth transfer technique [Opt. Express21, 7891 (2013)]. We use an optical frequency comb with a broad servo bandwidth to transfer a narrow linewidth of a pump laser, a 1.06 μm Nd:YAG laser, to a signal laser, a 1.57 μm external-cavity laser diode. This source enables us to record the Lamb dip of the ν3 band R(2) E transition of methane with a molecular spectral linewidth of 21 kHz while the frequency axis is absolutely calibrated.

A novel frequency control scheme for comb-referenced sensitive difference-frequency-generation spectroscopy

We present a novel scheme of frequency scan and wavelength modulation of a difference-frequency-generation source for comb-referenced sensitive spectroscopy. While the pump and signal frequencies are phase-locked to an optical frequency comb (OFC), the offset frequency between the signal wave and the nearest comb tooth is modulated to apply a wavelength-modulation technique, and the idler wave frequency is repeatedly swept for signal accumulation by changing the repetition frequency of the OFC. The spectrometer is applied to absolute frequency measurement of weak hyperfine-resolved rovibration transitions of the ν(1) band of CH(3)I, and the uncertainty in frequency determination is reduced by one order of magnitude in compared with that of the previous work published in Optics Express 20, 9178-9186 (2012).