A well-isolated vibrational state of CO2 verified by near-infrared saturated spectroscopy with kHz accuracy.

Quantitative determination of atmospheric CO2 concentration by remote sensing relies on accurate line parameters. Lamb dips of the lines up to J'' = 72 in the 30013-00001 band at 1605 nm were measured using a comb-locked cavity ring-down spectrometer, and the positions were determined with an accuracy of a few kHz. A simple effective Hamiltonian model can fit the rotational energies in the 30013 state ideally within the experimental accuracy, indicating that the vibrational state is well-isolated and can be regarded as free from perturbations. From a comparison between other bands using a similar analysis, we conclude that the transitions in the 30013-00001 band could be more suitable as reference lines for sensing applications with the potentially improved line parameter accuracy.

Communication: Molecular near-infrared transitions determined with sub-kHz accuracy.

Precise molecular transition frequencies are needed in various studies including the test of fundamental physics. Two well isolated ro-vibrational transitions of 12C16O at 1.57 µm, R(9) and R(10) in the second overtone band, were measured by a comb-locked cavity ring-down spectrometer. Despite the weakness of the lines (Einstein coefficient A≃0.008 s-1), Lamb-dip spectra were recorded with a signal-to-noise ratio over 1000, and the line positions were determined to be 191 360 212 761.1 and 191 440 612 662.2 kHz, respectively, with an uncertainty of 0.5 kHz (δν/ν=2.6×10-12). The present work demonstrates the possibility to explore extensive molecular lines in the near-infrared with sub-kHz accuracy.

Ultrasensitive, self-calibrated cavity ring-down spectrometer for quantitative trace gas analysis.

A cavity ring-down spectrometer is built for trace gas detection using telecom distributed feedback (DFB) diode lasers. The longitudinal modes of the ring-down cavity are used as frequency markers without active-locking either the laser or the high-finesse cavity. A control scheme is applied to scan the DFB laser frequency, matching the cavity modes one by one in sequence and resulting in a correct index at each recorded spectral data point, which allows us to calibrate the spectrum with a relative frequency precision of 0.06 MHz. Besides the frequency precision of the spectrometer, a sensitivity (noise-equivalent absorption) of 4×10-11 cm-1 Hz-1/2 has also been demonstrated. A minimum detectable absorption coefficient of 5×10-12 cm-1 has been obtained by averaging about 100 spectra recorded in 2  h. The quantitative accuracy is tested by measuring the CO2 concentrations in N2 samples prepared by the gravimetric method, and the relative deviation is less than 0.3%. The trace detection capability is demonstrated by detecting CO2 of ppbv-level concentrations in a high-purity nitrogen gas sample. Simple structure, high sensitivity, and good accuracy make the instrument very suitable for quantitative trace gas analysis.

Precision spectroscopy of atomic helium.

Helium is a prototype three-body system and has long been a model system for developing quantum mechanics theory and computational methods. The fine-structure splitting in the 23P state of helium is considered to be the most suitable for determining the fine-structure constant α in atoms. After more than 50 years of efforts by many theorists and experimentalists, we are now working toward a determination of α with an accuracy of a few parts per billion, which can be compared to the results obtained by entirely different methods to verify the self-consistency of quantum electrodynamics. Moreover, the precision spectroscopy of helium allows determination of the nuclear charge radius, and it is expected to help resolve the 'proton radius puzzle'. In this review, we introduce the latest developments in the precision spectroscopy of the helium atom, especially the discrepancies among theoretical and experimental results, and give an outlook on future progress.

Optical-Optical Double-Resonance Absorption Spectroscopy of Molecules with Kilohertz Accuracy.

Selective pumping and probing of highly excited states of molecules are essential in various studies but are also challenging because of high density of states, weak transition moments, and lack of precise spectroscopy data. We develop a comb-locked cavity-assisted double-resonance spectroscopy (COCA-DR) method for precision measurements using low-power continuous-wave lasers. A high-finesse cavity locked with an optical frequency comb is used to enhance both the pumping power and the probing sensitivity. As a demonstration, Doppler-free stepwise two-photon absorption spectra of CO2 were recorded by using two milliwatt diode lasers (1.60 and 1.67 µm), and the rotation energies in a highly excited state (CO-stretching quanta = 8) were determined with an unprecedented accuracy of a few kilohertz.