Midinfrared Cavity-Enhanced Two-Photon Absorption Spectroscopy for Selective Detection of Trace Gases.

Detection of trace gases, such as radioactive carbon dioxide, clumped isotopes, and reactive radicals, is of great interest and poses significant challenges in various fields. Achieving both high selectivity and high sensitivity is essential in this context. We present a highly selective molecular spectroscopy method based on comb-locked, mid-infrared, cavity-enhanced, two-photon absorption.

Profiling a pulsed molecular beam with cavity-enhanced absorption spectroscopy.

The molecular beam plays an important role in chemical dynamics experiments. The density in the beam is one of the critical factors influencing the reaction rate in these studies. Here we present a method based on laser-locked cavity-enhanced absorption spectroscopy to measure the molecular density in the beam.

Preparation of High Vibrational States in the Entire Molecular Beam.

Preparing highly excited molecules is of great interest in chemistry, but it has long been a challenge due to the high laser power required within the narrow line width to excite a weak transition. We present a cavity-enhanced infrared excitation scheme using a milliwatt laser. As a demonstration, about 35% of CO molecules in a ground-state rotational level were excited to the highly excited = 3 state in the entire pulsed supersonic beam, as confirmed by the depletion of molecules in the ground state.

Mid-infrared Doppler-free saturation absorption spectroscopy of the Q branch of CHν= 1 band using a rapid-scanning continuous-wave optical parametric oscillator.

We have developed a mid-infrared Doppler-free saturation absorption spectroscopy apparatus that employs a commercial continuous-wave optical parametric oscillator (CW OPO), complemented by a home-built automation and wavelength scanning system. Here, we report a comprehensive spectral scan of the Q branch transitions of the ν= 1 band of methane (CH) with an average linewidth (FWHM) of 4.5 MHz.

Mid-infrared-near-infrared double-resonance spectroscopy of molecules with kilohertz accuracy.

Precision measurements of molecular transitions to highly excited states are needed in potential energy surface modeling, state-resolved chemical dynamics studies, and astrophysical spectra analysis. Selective pumping and probing of molecules are often challenging due to the high state density and weak transition moments. We present a mid-infrared and near-infrared double-resonance spectroscopy method for precision measurements.

Precision spectroscopy of molecular hydrogen.

Precision measurements on the hydrogen molecule are of fundamental importance in understanding molecular theory. Comparison of accurate experimental data and theoretical results are used to test the quantum electrodynamics theory and determine physical constants used in the calculation. We review recent advances and perspectives in the precision spectroscopy of molecular hydrogen, representing state-of-the-art molecular spectroscopy methods and cutting-edge high-precision calculations.

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.

Analysis of krypton-85 and krypton-81 in a few liters of air.

Long-lived radioactive krypton isotopes, (81)Kr (t1/2 = 229,000 year) and (85)Kr (t1/2 = 10.76 year), are ideal tracers. (81)Kr is cosmogenic and can be used for dating groundwater beyond the (14)C age.

Ultrasensitive near-infrared cavity ring-down spectrometer for precise line profile measurement.

A cavity ring-down (CRD) spectrometer is built with a continuous-wave Ti:sapphire ring laser. Using a pair of R approximately 0.999 95 high-reflective mirrors, the noise-equivalent minimum detectable absorption loss reaches 7 x 10(-11)/cm over the spectral range of 780-830 nm.

A frequency-stabilized difference frequency generation laser spectrometer for precise line profile studies in the midinfrared.

A midinfrared laser spectrometer is built up based on the difference frequency generation (DFG) of a Nd:YAG (yttrium aluminum garnet) laser and a tunable Ti:sapphire (Ti:Sa) laser. Tuning the Ti:Sa laser and operating properly with the periodically poled lithium niobate crystal, the DFG emission is tunable in the spectral range of 2.3-5.