We demonstrate coherent dual frequency-comb spectroscopy for detecting variations in greenhouse gases. High signal-to-noise spectra are acquired spanning 5990 to 6260 cm -1 (1600 to 1670 nm) covering ~700 absorption features from CO 2 , CH 4 , H 2 O, HDO, and 13 CO 2 , across a 2-km open-air path. The transmission of each frequency comb tooth is resolved, leading to spectra with <1 kHz frequency accuracy, no instrument lineshape, and a 0.0033-cm -1 point spacing. The fitted path-averaged concentrations and temperature yield dry-air mole fractions. These are compared with a point sensor under well-mixed conditions to evaluate current absorption models for real atmospheres. In heterogeneous conditions, timeresolved data demonstrate tracking of strong variations in mole fractions. A precision of <1 ppm for CO 2 and <3 ppb for CH 4 is achieved in 5 minutes in this initial demonstration. Future portable systems could support regional emissions monitoring and validation of the spectral databases critical to global satellitebased trace gas monitoring.

We demonstrate a high-accuracy dual-comb spectrometer centered at 3.4 μm. The amplitude and phase spectra of the P, Q, and partial R branches of the methane ν 3 band are measured at 25 to 100 MHz point spacing with resolution under 10 kHz and a signal-to-noise ratio of up to 3500. A fit of the absorbance and phase spectra yields the center frequency of 132 rovibrational lines. The systematic uncertainty is estimated to be 300 kHz, which is 10 −3 of the Doppler width and a 10-fold improvement over Fourier transform spectroscopy. These data quantify the accuracy and resolution achievable with direct comb spectroscopy in the midinfrared.

Small gas-phase clusters (ion pairs) of the ionic liquid [emim](+)[Tf2N](-) have been generated in a supersonic expansion. Clusters are investigated via UV photofragmentation and time-of-flight mass spectrometry. Spectra between 42,000 and 45,000 cm(-1) reveal dynamical branching between direct dissociation of the ion pair to the cation and anion and to radical species. The IR spectrum between 2800 and 3200 cm(-1) was measured by action spectroscopy. Multiple conformations of the ion pair are found to be present in the molecular beam, leading to broad spectral features, further complicated by hydrogen bonding and Fermi resonances. The measured and theoretical spectra compare well, and the jet-cooled ion pair structures present in the molecular beam are strongly hydrogen bonded "stacked" conformers.

We demonstrate a dual-comb spectrometer using stabilized frequency combs spanning 177 to 220 THz (1360 to 1690 nm) in the near infrared. Comb-tooth-resolved measurements of amplitude and phase generate over 4×10(5) individually resolved spectral elements at 100 MHz point spacing and kilohertz-level resolution and accuracy. The signal-to-noise ratio is 100 to 3000 per comb tooth. Doppler-broadened phase and amplitude spectra of CO(2), CH(4), C(2)H(2), and H(2)O in a 30 m multipass cell agree with established spectral parameters, achieving high-resolution measurements with optical bandwidth generally associated with blackbody sources.