Sub-Doppler optical-optical double-resonance spectroscopy using a cavity-enhanced frequency comb probe.

Accurate parameters of molecular hot-band transitions, i.e., those starting from vibrationally excited levels, are needed to accurately model high-temperature spectra in astrophysics and combustion, yet laboratory spectra measured at high temperatures are often unresolved and difficult to assign.

Polarization-dependent intensity ratios in double resonance spectroscopy.

Double Resonance is a powerful spectroscopic method that unambiguously assigns the rigorous quantum numbers of one state of a transition. However, there is often ambiguity as to the branch (ΔJ) of that transition. Spectroscopists have resolved this ambiguity by using the dependence of the double resonance intensity on the relative polarization directions of pump and probe radiation.

Two-photon absorption line shapes in the transit-time limit.

A weak excitation transit-time resolution limited analytic line shape is derived for a Doppler broadening-free degenerate two-photon transition from a standing wave with a TEM transverse profile. This approximation is appropriate when the collisional mean free path is much larger than the transverse width of the TEM beam. It is considerably simpler than the two-photon absorption line shape previously published, Bordé, C.

Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe.

We report the first measurement of sub-Doppler molecular response using a frequency comb by employing the comb as a probe in optical-optical double-resonance spectroscopy. We use a 3.3  μm continuous wave pump and a 1.

Doppler-Free Two-Photon Cavity Ring-Down Spectroscopy of a Nitrous Oxide (NO) Vibrational Overtone Transition.

We report Doppler-free two-photon absorption of NO at = 4.53 μm, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse , and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb.

Resonance enhanced two-photon cavity ring-down spectroscopy of vibrational overtone bands: A proposal.

This paper presents an analysis of near-resonant, rovibrational two-photon spectroscopy and the use of cavity ring-down spectroscopy for its detection. Expressions are derived for the photon absorption rate of a three-level system, correct to all orders and the simpler expressions that result from various approximations. The analysis includes the angular momentum projection degeneracies and linear or circular polarization of the exciting field.

Step-modulated decay cavity ring-down detection for double resonance spectroscopy.

A method of measuring double resonant two-photon signal and background from a single cavity ring-down decay is introduced. This is achieved by modulating the double resonance loss via one of the light sources exciting the transition. The noise performance of the method is characterized theoretically and experimentally.

Influence of spatial degeneracy on rotational spectroscopy: Three-wave mixing and enantiomeric state separation of chiral molecules.

Pulse flip angles are calculated for three-wave mixing, three-state cycles of chiral molecules to produce optimized free induction decay amplitudes proportional to the enantiomeric excess of a sample and to produce optimized degrees of state-specific enantiomeric separation. The calculations account for the spatial degeneracy of the levels involved and the resulting inhomogeneous distribution of transition dipole moments. Cycles of transitions that include R followed by Q followed by P branch transitions display only modest reductions of the calculated optimal signals compared to those calculated if every M component was optimally polarized.

The gas-phase structure of the asymmetric, trans-dinitrogen tetroxide (NO), formed by dimerization of nitrogen dioxide (NO), from rotational spectroscopy and ab initio quantum chemistry.

We report the first experimental gas-phase observation of an asymmetric, trans-NO formed by the dimerization of NO. In additional to the dominant NO species, rotational transitions have been observed for all species with single N and O substitutions as well as several multiply substituted isotopologues. These transitions were used to determine a complete substitution structure as well as an r structure from the fitted zero-point averaged rotational constants.

Hydrogen Isotopic Composition of Arctic and Atmospheric CH Determined by a Portable Near-Infrared Cavity Ring-Down Spectrometer with a Cryogenic Pre-Concentrator.

In this study, near-infrared continuous wave cavity ring-down spectroscopy was applied to the measurement of the δH of methane (CH). The cavity ring-down spectrometer (CRDS) system consisted of multiple DFB laser diodes to optimize selection of spectral line pairs. By rapidly switching measurements between spectral line peaks and the baseline regions, the long-term instrumental drift was minimized, substantially increasing measurement precision.

Lightweight Raman spectroscope using time-correlated photon-counting detection.

Raman spectroscopy is an important tool in understanding chemical components of various materials. However, the excessive weight and energy consumption of a conventional CCD-based Raman spectrometer forbids its applications under extreme conditions, including unmanned aircraft vehicles (UAVs) and Mars/Moon rovers. In this article, we present a highly sensitive, shot-noise-limited, and ruggedized Raman signal acquisition using a time-correlated photon-counting system.

Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy.

We describe a three-wave mixing experiment using time-separated microwave pulses to detect the enantiomer-specific emission signal of the chiral molecule using Fourier transform microwave (FTMW) spectroscopy. A chirped-pulse FTMW spectrometer operating in the 2-8 GHz frequency range is used to determine the heavy-atom substitution structure of solketal (2,2-dimethyl-1,3-dioxolan-4-yl-methanol) through analysis of the singly substituted (13)C and (18)O isotopologue rotational spectra in natural abundance. A second set of microwave horn antennas is added to the instrument design to permit three-wave mixing experiments where an enantiomer-specific phase of the signal is observed.

Detection of S-nitroso compounds by use of midinfrared cavity ring-down spectroscopy.

S-Nitroso compounds have received much attention in biological research. In addition to their role as nitric oxide donors, there is growing evidence that these compounds are involved in signaling processes in biological systems. Determination of S-nitrosylated proteins is of great importance for fundamental biological research and medical applications.

A high-finesse broadband optical cavity using calcium fluoride prism retroreflectors.

A high-finesse broadband optical cavity has been developed for use in the ultraviolet and visible region using Brewster-angle calcium fluoride (CaF₂) prism retroreflectors. Prior to prism construction, optical loss measurements of CaF₂ windows were performed using cavity ring-down spectroscopy at 250 nm. Total optical loss showed high spatial correlation with crystal birefringence, which was partially mitigated by orienting the <111> crystal axis with the laser beam.

Measurement of the 13C/12C of atmospheric CH4 using near-infrared (NIR) cavity ring-down spectroscopy.

A near-infrared (NIR) continuous-wave-cavity ring-down spectrometry (CW-CRDS) device was developed with the goal of measuring seasonal changes in the isotopic composition of atmospheric CH4 on Earth and eventually on Mars. The system consisted of three distributed feedback laser diodes (DFB-LDs), two of which were tuned to the absorption line peaks of (12)CH4 and (13)CH4 at 6046.954 cm(-1) and 6049.

Sensitivity limits of continuous wave cavity ring-down spectroscopy.

An optimized nonlinear least-squares fit algorithm for data processing in cavity ring-down spectroscopy (CRDS) is discussed, which improves the calculation efficiency substantially over using a general purpose fitting package. Theoretical absorption sensitivity limits for both the detector noise and the shot noise limited situations are derived and compared with experimental results. The effect of limiting the bandwidth of detection system on ring-down signal is discussed and compared with real ring-down data.

A rigid, monolithic but still scannable cavity ring-down spectroscopy cell.

A novel cell for continuous wave cavity ring-down spectroscopy (cw-CRDS) is described and tested. The cell is monolithic and maintains a rigid alignment of the two cavity mirrors. Two high-resolution and high-force piezoelectric transducers are used to sweep the length of the cell by elastic deformation of the 2.

Sensitivity limit of rapidly swept continuous wave cavity ring-down spectroscopy.

The averaged transmitted intensity of a cavity excited by a linearly frequency swept laser with finite line width is derived and presented as a sum over passes, analytical integrals (where the sum of passes is converted to a continuous time variable), and an approximate but computationally more stable stationary phase approximation expression. The transmitted waveform is used to derive the bias in extraction of the cavity decay rate from such a cavity transient for three different fitting models. Numerical simulation of cavity excitation gives statistical fluctuations in the transmitted intensity that leads to noise in the cavity decay rate.

Long-term stability in continuous wave cavity ringdown spectroscopy experiments.

Allan variance has been used to characterize the slow drift of a near-IR distributed feedback laser-based continuous wave cavity ringdown spectroscopy (CW-CRDS) system. Long-term drift in the cavity loss rate, highly correlated with changes in ambient pressure but not temperature, is observed. With differential measurement of on- and off-peak decay rates, the drift between them largely cancels out, but some residual drift remains if the lasers are detuned more than a few hundred megahertz from each other.

(HCN)(m)-M(n) (M = K, Ca, Sr): vibrational excitation induced solvation and desolvation of dopants in and on helium nanodroplets.

Infrared (IR) laser spectroscopy is used to probe the rotational and vibrational dynamics of the (HCN)(m)-M(n) (M = K, Ca, Sr) complexes, either solvated within or bound to the surface of helium nanodroplets. The IR spectra of the (HCN)(m)-K (m = 1-3), HCN-Sr, and HCN-Ca complexes have the signature of a surface species, similar to the previously reported spectra of HCN-M (M = Na, K, Rb, Cs) [Douberly, G. E.

Brewster angle prism retroreflectors for cavity enhanced spectroscopy.

The design of a high finesse optical cavity made from two prism retroreflectors is fully described. Optical beam propagation calculations to determine the specification of prism angles and relative dimensions, the size of the astigmatic TEM00 beam as it propagates in the cavity, and the sensitivity of the optic axis to changes in prism alignment and fabrication errors are presented. The effects of material dispersion are also quantified for three different materials: fused silica, calcium fluoride, and barium fluoride.

Cavity enhanced absorption spectroscopy using a broadband prism cavity and a supercontinuum source.

We report the design and construction of a cavity enhanced absorption spectrometer using broadband Brewster's angle prism retroreflectors and a spatially coherent 500 nm to >1.75 microm supercontinuum excitation source. Using prisms made from fused silica an effective cavity reflectivity of >99.