Alexander M. Zolot

Direct-comb molecular spectroscopy with accurate, resolved comb teeth over 43 THz.

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.

Quantum state resolved scattering dynamics of F+HCl→HF(v,J)+Cl

State-to-state reaction dynamics of the reaction F+HCl-->HF(v,J)+Cl have been studied under single-collision conditions using an intense discharge F atom source in crossed supersonic molecular beams at Ecom=4.3(1.3) kcal/mol. Nascent HF product is monitored by shot-noise limited direct infrared laser absorption, providing quantum state distributions as well as additional information on kinetic energy release from high resolution Dopplerimetry. The vibrational distributions are highly inverted, with 34(4)%, 44(2)%, and 8(1)% of the total population in vHF=1, 2, and 3, respectively, consistent with predominant energy release into the newly formed bond. However, there is a small [14(1)%] but significant formation channel into the vHF=0 ground state, which is directly detectable for the first time via direct absorption methods. Of particular dynamical interest, both the HF(v=2,J) and HF(v=1,J) populations exhibit strongly bimodal J distributions. These results differ significantly from previous flow and arrested-relaxation studies and may signal the presence of microscopic branching in the reaction dynamics.

Crossed jet reactive scattering dynamics of F+H2O→HF(v,J)+OH:HF(v,J) product quantum state distributions under single-collision conditions

Reactive scattering dynamics of F+H(2)O-->HF+OH have been investigated under single-collision, crossed, supersonic jet conditions at 5.4(1.3) kcalmol, and nascent HF(v,J) rovibrational populations (v<or=2, J<or=17) have been observed via high resolution IR laser absorption methods. Vibrational populations are highly inverted, with 75(2)% and 21(5)% of the observed HF product formed in v=1 and 0, respectively. Small but finite branching [5(1)%] into the v=2 manifold is observed, which is accessible only via the additional center of mass collision energy in the crossed jets. Despite energy constraints associated with substantial vibrational excitation, the reaction dynamics also lead to rotationally hyperthermal HF, with populations observed up to near the energetic limit for each of the v=0,1,2 vibrational manifolds. The trends in these nascent product rovibrational distributions are interpreted in terms of ab initio predictions of a strongly bent, early-barrier transition state and long-range dipole-dipole interactions in the exit channel.

Quantum-state resolved reaction dynamics at the gas-liquid interface: Direct absorption detection of HF(v,J) product from F([sup 2]P)+squalane

Exothermic reactive scattering of F atoms at the gas-liquid interface of a liquid hydrocarbon (squalane) surface has been studied under single collision conditions by shot noise limited high-resolution infrared absorption on the nascent HF(v,J) product. The nascent HF(v,J) vibrational distributions are inverted, indicating insufficient time for complete vibrational energy transfer into the surface liquid. The HF(v=2,J) rotational distributions are well fit with a two temperature Boltzmann analysis, with a near room temperature component (T(TD) approximately equal to 290 K) and a second much hotter scattering component (T(HDS) approximately equal to 1040 K). These data provide quantum state level support for microscopic branching in the atom abstraction dynamics corresponding to escape of nascent HF from the liquid surface on time scales both slow and fast with respect to rotational relaxation.

Reactive scattering dynamics in atom+polyatomic systems: F+C2H6→HF(v,J)+ C2H5

State-to-state scattering dynamics of F+C2H6-->HF(v,J)+C2H5 have been investigated at Ecom=3.2(6) kcalmol under single-collision conditions, via detection of nascent rovibrationally resolved HF(v,J) product states with high-resolution infrared laser absorption methods. State-resolved Doppler absorption profiles are recorded for multiple HF(v,J) transitions originating in the v=0,1,2,3 manifold, analyzed to yield absolute column-integrated densities via known HF transition moments, and converted into nascent probabilities via density-to-flux analysis. The spectral resolution of the probe laser also permits Doppler study of translational energy release into quantum-state-resolved HF fragments, which reveals a remarkable linear correlation between (i) HF(v,J) translational recoil and (ii) the remaining energy available, Eavail=Etot-E(HF(v,J)). The dynamics are interpreted in the context of a simple impulsive model based on conservation of linearangular momentum that yields predictions in good agreement with experiment. Deviations from the model indicate only minor excitation of ethyl vibrations, in contrast with a picture of extensive intramolecular vibrational energy flow but consistent with Franck-Condon excitation of the methylene CH2 bending mode. The results suggest a relatively simple dynamical picture for exothermic atom+polyatomic scattering, i.e., that of early barrier dynamics in atom+diatom systems but modified by impulsive recoil coupling at the transition state between translationalrotational degrees of freedom.

CO2 phase and amplitude spectra measured over 2 km outdoor path with a dual-comb spectrometer

We present dispersive dual-comb spectroscopy of atmospheric CO2 across a 2-km open-air path. By sending a single comb through the open-air path, both molecular phase spectrum and conventional absorbance spectrum are obtained. The measured phase spectra match expected molecular lineshape models.

A method for comparing remote optical clocks over a free-space optical link

We demonstrate a method to compare optical clocks approaching 10-17 uncertainties through the exchange of optical pulses from phase-locked frequency combs. We discuss results over a 120 m air path and prospects for longer distances.

Dual-comb techniques for precision measurement

Dual asynchronous comb techniques for precision measurement are coming of age. In spectroscopy such systems a can cover 42 THz of optical spectra with 100 MHz point spacing and signal to noise ratios (SNR) better than 3000. Careful examination of the systematic errors has begun and Doppler broadened molecular line centers can be determined reliably to within 200 kHz. In related systems dual and single comb calibrations of swept cw lasers can facilitate rapid (0.5 ms) distance measurement with sub-micron precisions off a diffuse target.

Precision spectroscopy with frequency combs at 3.4 μm

We discuss precision spectroscopy with a comb-based spectrometer at 3.4 μm. Our goal is to explore comb-based spectroscopy as an alternative method for fast, highly resolved, accurate measurements of gas line shapes. The spectrometer uses dual 1.5 μm frequency combs down converted to 3.4 μm via difference frequency generation (DFG) with a stabilized 1 μm fiber laser. One 3.4 μm comb is transmitted through methane and heterodyned against the second, offset comb to measure the gas absorption and dispersion. Doppler-broadened methane spectral lines are measured to below 1 MHz uncertainty. We also discuss the higher sensitivity alternative of a comb-assisted swept-laser DFG spectrometer.