Roger D. van Zee

Laser bandwidth effects in quantitative cavity ring-down spectroscopy

We have investigated the effects of laser bandwidth on quantitative cavity ring-down spectroscopy using the (r)R transitions of the b(ν = 0)?X(ν = 0) band of molecular oxygen. It is found that failure to account properly for the laser bandwidth leads to systematic errors in the number densities determined from measured ring-down signals. When the frequency-integrated expression for the ring-down signal is fitted and measured laser line shapes are used, excellent agreement between measured and predicted number densities is found.

Response of a ring-down cavity to an arbitrary excitation

An eigenmode analysis of the response of an empty ring‐down cavity to an arbitrary laser excitation is presented. By explicitly taking into account both the mode structure of the ring‐down cavity and the spectral content of the laser pulse, it is found that the complicated ring‐down signals commonly observed in the laboratory can be interpreted in terms of cavity mode beating. Some conclusions drawn from this analysis are verified experimentally by measurements of the time and frequency response of empty cavities. These observations provide clear evidence for the existence of longitudinal and transverse mode structures in ring‐down cavities.

Pulsed, single-mode cavity ringdown spectroscopy

We discuss the use of single-mode cavity ringdown spectroscopy with pulsed lasers for quantitative gas density and line strength measurements. The single-mode approach to cavity ringdown spectroscopy gives single exponential decay signals without mode beating, which allows measurements with uncertainties near the shot-noise limit. The technique is demonstrated with a 10-cm-long ringdown cavity and a pulsed, frequency-stabilized optical parametric oscillator as the light source. A noise-equivalent absorption coefficient of 5 x 10(-10) cm(-1) Hz(-1/2) is demonstrated, and the relative standard deviation in the ringdown time (sigma(tau)/tau) extracted from a fit to an individual ringdown curve is found to be the same as that for an ensemble of hundreds of independent measurements. Repeated measurement of a line strength is shown to have a standard deviation <0.3%. The effects of normally distributed noise on quantities measured using cavity ringdown spectroscopy are discussed, formulas for the relative standard deviation in the ringdown time are given in the shot- and technical-noise limits, and the noise-equivalent absorption coefficient in these limits are compared for pulsed and continuous-wave light sources.

Tracing electronic pathways in molecules by using inelastic tunneling spectroscopy

Using inelastic electron tunneling spectroscopy (IETS) to measure the vibronic structure of nonequilibrium molecular transport, aided by a quantitative interpretation scheme based on Greens function-density functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. We show that the IETS observations directly index electron tunneling pathways along the given normal coordinates of the molecule. One can then interpret the maxima in the IETS spectrum in terms of the specific paths that the electrons follow as they traverse the molecular junction. Therefore, IETS measurements not only prove (by the appearance of molecular vibrational frequencies in the spectrum) that the tunneling charges, in fact, pass through the molecule, but also can be used to determine the transport pathways and how they change with the geometry and placement of molecules in junctions.

Photoacoustic Spectrometer with a Calculable Cell Constant for Measurements of Gases and Aerosols

We benchmark the performance of a photoacoustic spectrometer with a calculable cell constant in applications related to climate change measurements. As presently implemented, this spectrometer has a detection limit of 3.1 × 10(-9) W cm(-1) Hz(-1/2) for absorption by a gas and 1.5 × 10(-8) W cm(-1) Hz(-1/2) for soot particles. Nonstatistical uncertainty limited the accuracy of the instrument to ∼1%, and measurements of the concentration of CO(2) in laboratory air agreed with measurements made using a cavity ring-down spectrometer, to within 1%. Measurements of the enhanced absorption resulting from ultrathin (<5 nm), nonabsorbing coatings on nanoscale soot particles demonstrate the sensitivity of this instrument. Together, these measurements show the instruments ability to quantitatively measure the absorption coefficient for species of interest to the climate and atmospheric science communities. Because the system constant is known, in most applications the acoustic response of this instrument need not be calibrated against a sample of known optical density, a decided advantage in field applications. Routine enhancements, such as improved processing of the photoacoustic signal and higher laser beam power, should further increase the instruments precision and sensitivity.

Mechanism of the reaction, CH4+O(1D2)→CH3+OH, studied by ultrafast and state-resolved photolysis/probe spectroscopy of the CH4⋅O3 van der Waals complex

The mechanism of the reaction CH4+O(1D2)→CH3+OH was investigated by ultrafast, time-resolved and state-resolved experiments. In the ultrafast experiments, short ultraviolet pulses photolyzed ozone in the CH4⋅O3 van der Waals complex to produce O(1D2). The ensuing reaction with CH4 was monitored by measuring the appearance rate of OH(v=0,1;J,Ω,Λ) by laser-induced fluorescence, through the OH A←X transition, using short probe pulses. These spectrally broad pulses, centered between 307 and 316 nm, probe many different OH rovibrational states simultaneously. At each probe wavelength, both a fast and a slow rise time were evident in the fluorescence signal, and the ratio of the fast-to-slow signal varied with probe wavelength. The distribution of OH(v,J,Ω,Λ) states, Pobs(v,J,Ω,Λ), was determined by laser-induced fluorescence using a high-resolution, tunable dye laser. The Pobs(v,J,Ω,Λ) data and the time-resolved data were analyzed under the assumption that different formation times represent different reaction...

OH (ν, J) distributions from the reaction between CH4 and O (1D2), initiated in CH4·O3 clusters

Abstract The nascent OH rovibrational distribution for the reaction CH4·O3+hν266 nm→CH3+OH+O2 are reported. The population ratio between the vibrational levels was found to be ν(0)/ν(1) = 1.0±0.2 (2σ), ν(1)/ν(2) = 0.67±0.26, and ν(2)/ν(3) = 1.5±0.2; and significant rotational excitation was observed for all vibrational states. The asymmetric lambda doublet is populated to a greater extent than expected statistically, but no measurable spin—orbit propensity was observed. Both the rotational and vibrational distributions are somewhat cooler than those of the analogous ‘free’ reaction, CH4+O(1D2)→CH3+OH.

Measuring pressure with cavity ring-down spectroscopy

This paper discusses recent research at NIST aimed towards the use of cavity ring-down spectroscopy as an absolute partial pressure sensor. The motivation for this work is reviewed, the concepts behind cavity ring-down spectroscopy are outlined, and recent results are summarized. The paper ends with a look at future metrological uses of this technique.

Construction of a high power OPO laser system for differential absorption LIDAR

Our goal is to develop and characterize optical measurement technology to enable accurate quantification of greenhouse-gas emissions from distributed sources and sinks. We are constructing a differential absorption LIDAR (DIAL) system that will be sensitive to the three primary greenhouse gases, carbon dioxide, methane, and nitrous oxide. Our system uses a high energy optical parametric oscillator (OPO) operating from 1585 nm to 1646 nm. Here we describe this OPO system and initial characterization of its output. The OPO uses a Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) design. The commercially available RISTRA cavity is machined from a solid block of aluminum. The compact single piece cavity design requires no mirror adjustments and image rotation provides efficient light conversion efficiency and excellent beam quality. The injection seeded OPO has demonstrated total output energy of 50 mJ/pulse when pumped with 220 mJ/pulse of 1064 nm radiation. The pump laser has a repetition rate variable from 1 Hz to 100 Hz and a temporal pulse width of 4.2 ns. In the current configuration the seed laser is locked to a mode of the cavity.

Single-Mode Cavity Ring-Down Spectroscopy for Line Shape Measurements

The precise measurement of absorption line shapes and line strengths is demonstrated using single-mode cavity ring-down spectroscopy. This technique utilizes a pulsed laser to excite a single transverse electromagnetic mode of a high-finesse, stable resonator. From the measurement of the decay time constant of this mode, the optical losses can be determined. As the cavity mode is tuned through an absorption resonance, the variation of the decay time constant yields a measure of the absorptive losses in the cavity. Spectra of the weak transitions of the molecular oxygen A-band are used to demonstrate the capability of this technique to measure absorption profiles and absolute line strengths. Repeated measurements of the line strength have a standard deviation <0.3%.