Frequency-stabilized cavity ring-down spectroscopy

Chemical Physics Letters 536 (2012) 1–8

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

FRONTIERS ARTICLE

Frequency-stabilized cavity ring-down spectroscopy D.A. Long a,⇑, A. Cygan b, R.D. van Zee a, M. Okumura c,⇑, C.E. Miller d, D. Lisak b, J.T. Hodges a,⇑ a

Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA ´ , Poland Instytut Fizyki, Uniwersytet Mikołaja Kopernika, ul. Grudziadzka 5/7, 87-100 Torun c Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA d NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA b

a r t i c l e

i n f o

Article history: Available online 21 March 2012

a b s t r a c t We describe frequency-stabilized cavity ring-down spectroscopy (FS-CRDS), an ultraprecise refinement of conventional CRDS. We review the technique and highlight some recent studies that have utilized FS-CRDS to perform precision measurements of molecular transitions in the near-infrared. We describe system enhancements that are currently under implementation, including Pound–Drever–Hall locking and optical frequency comb-stabilization, which have the potential to reduce the uncertainty in both the absorption and frequency axes of our spectra by more than an order of magnitude. Finally, we describe high impact applications of this capability that can exploit frequency axis uncertainty at the 10 kHz level and signal-to-noise ratios exceeding 200 000:1. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years numerous fundamental physical and chemical applications have emerged that require ultraprecise measurements of spectra and spectroscopic parameters. These applications span such varied measurement targets as greenhouse gas monitoring [1], laser-based isotope ratio measurements [2], and optical determinations of the Boltzmann constant [3]. They are united by their need for sub-1% measurement uncertainties; in many cases the required relative uncertainty is orders of magnitude lower than 1%. For example, typical targets for greenhouse gas sensing, laserbased isotope ratio measurements, and Boltzmann constant determinations are 0.25% [4], 0.01% (0.1‰) [5], and 0.0001% (1 part in 106) [3], respectively. Achieving these daunting measurement targets requires spectroscopic techniques that exhibit ultrahigh sensitivity, linearity, and stability. In addition, special attention must be paid to the determination of sample conditions (e.g., pressure, temperature, mixing ratio), each of which must be known with an uncertainty less than the total measurement target. Furthermore, at these sub-1% levels, precise measurement of the spectral line shape is of the utmost importance. In many cases the dominant uncertainty can be the choice of line profile [6]. These line shape effects are particularly important if spectroscopic reference data are to be extrapolated or interpolated to new conditions. At this level,

assessing line shape effects and quantifying the corresponding parameters requires a spectroscopic technique that exhibits ultrahigh resolution and spectral fidelity in order to minimize biases and spectrum distortion (as is commonly seen with Fourier-transform techniques for example). These urgent and challenging measurement targets motivated the development of a quantitative spectroscopic technique that exhibits ultrahigh sensitivity, resolution, stability, reproducibility, and robust automated operation. This effort resulted in frequency-stabilized cavity ring-down spectroscopy (FS-CRDS), an ultraprecise refinement of conventional continuous-wave cavity ring-down spectroscopy (cw-CRDS). We have used FS-CRDS to measure spectroscopic parameters of O2, CO2, and H2O transitions while carefully quantifying the measurement uncertainties. This Letter describes FS-CRDS, compares it to other cw-CRDS techniques, and notes the attributes of FS-CRDS that make it applicable to a broad range of spectroscopic measurements where high precision is required. In addition, we discuss significant enhancements we are making to our FS-CRDS spectrometers which will yield orders of magnitude increases the signal-to-noise ratio (SNR) and reductions in the frequency uncertainty. We conclude by highlighting research areas in which these new capabilities may have major impacts.

2. Background ⇑ Corresponding authors. E-mail addresses: [email protected] (D.A. Long), [email protected] (M. Okumura), [email protected] (J.T. Hodges). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.03.035

The general technique of cavity ring-down spectroscopy (CRDS) has been reviewed previously [7–10]. Readers are directed to those Letters for details on the history and development of traditional

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Figure 1. Schematic of a typical frequency-stabilized cavity ring-down spectrometer. The probe laser (an external-cavity or distributed feedback diode laser) is locked to a particular TEM00 cavity mode, ensuring quantitative ring-down signals. The shown cavity modes were recorded by a CCD camera which was placed after the optical cavity. The cavity’s length is actively stabilized to an external frequency reference (either a frequency-stabilized or I2-stabilized HeNe laser) that is co-resonant with and counterpropagating within the cavity. The cavity is stabilized by varying the voltage applied to a piezoelectric transducer (PZT) to which one of the mirrors is mounted. Note that this servo ensures that the HeNe laser is resonant with a given TEM00 cavity mode. By stabilizing the cavity’s length we stabilize the entire comb of cavity transmission modes that serves as the frequency axis for our spectra. Also shown are dichroic optics (DO) that are reflective at the reference wavelength but transparent at the probe wavelength.

CRDS and its variants. Briefly, CRDS relies upon a measurement of the rate of decay of power within an optical cavity [11]. The absorption coefficient as a function of optical frequency, x, is then given as:

aðxÞ ¼

1 1  csðxÞ cso ðxÞ

ð1Þ

where s and s0 are the ring-down decay times with and without the absorbing medium, respectively. This technique has significant advantages over traditional direct absorption spectroscopy. Firstly, the use of an optical cavity leads to a large effective pathlength, and correspondingly high sensitivity. In addition, to first order CRDS is independent of laser intensity fluctuations. Early demonstrations of CRDS were performed with pulsed lasers [11]. While this approach allowed for highly sensitive characterizations of short-lived species with a very simple configuration [12], significant limitations were quickly realized for quantitative measurements. The wide spectral bandwidth and multi-mode character of these sources led to multi-exponential ring-down decays (i.e., nonquantitative operation) [13–16], while low pulse repetition rates (generally near 10 Hz) led to low data acquisition rates. Both constraints ultimately limited detection sensitivity. As a result, narrow bandwidth (and rapidly switchable) cw-lasers were introduced for single-mode excitation of cavity resonances [17,18]. To build up optical power within a ring-down cavity, the narrow line widths of these cw-lasers must overlap a given cavity resonance. For near-infrared external-cavity and distributed feedback diode lasers line widths are generally between 100 kHz and 1 MHz. However, with a 74 cm long cavity (free-spectral range of 202 MHz), the cavity line widths are 13 and 1.3 kHz for mirror reflectivities of 99.98% and 99.998%, respectively. To overcome these large disparities in widths, the simplest solution is to dither either the laser frequency or the cavity length to ensure overlap.

However, this technique leads to spectral distortion as well as greatly reduces the measurement duty cycle and coupling efficiency. These difficulties can be alleviated by actively locking the laser frequency to a given cavity transverse mode (i.e., single-mode or SM-CRDS). As individual cavity transverse modes are excited, the resulting ring-down decays are single exponentials with relative standard deviations as low as 0.03% and acquisition rates in excess of 10 kHz [19]. In a typical cw-CRDS experiment, the spectrum frequency axis is measured by a wavelength-meter. However, these wavelength meters generally exhibit an uncertainty greater than 30 MHz (1  103 cm1), which is inadequate for most high resolution measurements. In addition, the update rate of these devices is generally well below 10 Hz. This is insufficient to account for many variations in the cavity resonance frequencies and leads to frequency noise (and corresponding amplitude noise) in SMCRDS spectra. These limitations led us to develop FS-CRDS, a refinement of conventional SM-CRDS in which the optical cavity is frequency stabilized to an external reference [20]. FS-CRDS exploits the optical cavity to simultaneously enhance the detection sensitivity, frequency axis accuracy and stability, and spectrum reproducibility. 3. Frequency-stabilized cavity ring-down spectroscopy In FS-CRDS, the cavity’s length is actively stabilized with respect to an external optical frequency reference, which in turn stabilizes its entire comb of optical resonances. These resonances provide an extremely linear, stable, and accurate frequency axis for FS-CRDS spectra. Figure 1 illustrates these basic principles of FS-CRDS as well as a conception of the experimental setup. Our realizations of FS-CRDS employ a frequency-stabilized HeNe laser as the external reference, yielding a frequency axis precision of ±1 MHz or ±12 kHz, for polarization or I2-stabilized lasers,

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Figure 2. Short- and long-term measurement statistics [27]. (a) Empty cavity Allan deviation (square-root of Allan variance) for a typical FS-CRDS spectrometer as a function of averaging time, Dtav at a given probe laser frequency. For an optimum averaging time of 100 s, the minimum detectable absorption coefficient was 4  1011 cm1. (b) Reduction in the root-mean-square (rms) baseline noise level as a function of the number of co-added spectra. Even for averaging times as long as 9.7 h (100 co-added spectra) a roughly ergodic (n1/2) reduction in the baseline noise level is observed. Importantly, due to the low uncertainty frequency axis of FS-CRDS this spectrum co-adding procedure leads to negligible instrumental broadening.

Figure 3. Examples of recent measurements which have exploited the stable frequency axis of FS-CRDS. (a) Measured PDH-locked FS-CRDS spectrum and speed-dependent Nelkin–Ghatak profile fit of the R7Q8 16O2 B-band magnetic dipole transition at 933 Pa [35]. This spectrum is the average of 1040 spectra with two etalon corrections. To the best of our knowledge, the signal-to-noise ratio of 220 000:1 is the highest ever reported. The signal-to-noise ratio was calculated as the ratio of the peak absorption to the standard deviation of the fit residuals in the spectrum baseline. (b) Measured FS-CRDS 16O17O A-band magnetic dipole transition at 236 Pa [29]. The spectrum shows clear evidence of unresolved hyperfine structure. Note the failure of the Voigt profile with constrained Doppler width (DW) to model the observed spectrum. Also shown is a fit which includes six hyperfine components (HC) as shown (F00 = 1.5, 2.5, . . . , 6.5; components with larger F00 have higher relative intensities). This fit is able to model the spectrum to within the instrumental noise level. Note that the only two floated parameters were the overall spectral center-of-mass and total intensity, no parameters were floated for the individual components. (c) FS-CRDS spectrum of the P-branch of the 16O2 A-band at 28 Pa [25]. Absolute transition frequencies were measured with uncertainties as low as 1 MHz (3  105 cm1) by tying the stabilized comb of cavity resonances to the hyperfine components of 39K (see insets). Note m0 = 389 285 000 and 391 015 000 MHz for the offset frequencies in the D1 and D2 insets.

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Table IA Component standard (1r) uncertainties in the absolute frequency axis for the original FS-CRDS spectrometer whereby frequencies were referenced to the hyperfine components of 39K [25]. The frequency reference utilized in the length stabilization servo was a frequency-stabilized HeNe laser. A low bandwidth servo was utilized to lock the probe laser to the cavity resonances. Quantity

Uncertainty (kHz)

Long term stability of the frequency-stabilized HeNe reference laser Fit positions of 39K D2 hyperfine lines Differential power shifting of D1 and D2 hyperfine lines Fit positions of 39K D1 hyperfine lines Uncertainty in AOM frequency Full width at half maximum of the cavity resonance Upper bound for dispersion of cavity modes Uncertainty in determination of the free spectral range (FSR) Combined uncertainty

500 250 140 100 20 16 15 0.075 586

respectively. When we previously used an I2-stabilized HeNe laser as our frequency reference, we monitored a beat note between the I2-stabilized HeNe and a higher power HeNe laser [21]. By incorporating a high power I2-stabilized HeNe, we will be able to use it directly as the frequency reference. An acousto-optic modulator (AOM) in a double-pass configuration frequency modulates the reference laser to facilitate a simple transmission lock to the optical cavity. As a result of this lock, the frequencies of each cavity resonance can be known with an uncertainty approaching that of the optical reference. We use dichroic ring-down cavity mirrors with ultrahigh reflectivity (>99.98%) for the spectroscopic probe wavelength and modest reflectivity (95%) for the reference laser. The low finesse of the cavity at the reference wavelength leads to relatively wide cavity resonances and greatly simplifies implementation of the transmission lock. The AOM can also be utilized to shift the frequency of the reference laser (and in turn the frequencies of the cavity resonances), thus, allowing for high point density (i.e., sub-free spectral range step sizes) measurements [20]. This technique has been used to observe Lamb dips in weak near-infrared H2O transitions [21] (see Figure 5). A thorough discussion of the implementation and experimental details of FS-CRDS can be found in Ref. [20] . An important element of all of the FS-CRDS spectrometers described herein is that they have been fully automated (see Ref. [22] for a detailed discussion of the automation procedure). As a result, spectra may be collected for several days without user intervention, thus, greatly increasing the effective duty cycle of the FSCRDS instrument. We have used this capability to collect spectra containing 10 000’s of points spanning up to 100 cm1 (for example see Figure 3c). Automation was crucial in our comprehensive study of the O2 A-band [b1Rg+ X 3Rg(0, 0)] [23–30]. FS-CRDS spectra were used to determine transition frequencies [25], line shape parameters (i.e., intensities, pressure broadening parameters, collisional narrowing parameters) [24,30], and pressure shifting parameters [23]. Similar studies have been performed for the rare isotopologues of O2 [26,31]. These rather involved studies of wide spectral regions could not have been performed expediently without a robust and automated spectrometer. Over the course of our measurements of positions for O2 A-band transitions, we produced an uncertainty budget for the absolute frequency axis which was referenced to hyperfine components of 39 K [25] (see Table IA). The dominant uncertainty was due to the long-term stability of the frequency-stabilized HeNe laser used as the frequency reference in our length stabilization servo (500 kHz). In addition, fit uncertainties for the hyperfine components of 39K and differential power shifting of these transitions contributed to a combined uncertainty in our measurement of

the 39K transitions of 300 kHz. The combined standard uncertainty in our absolute frequency axis was found to be 600 kHz. We have examined both short- and long-term measurement statistics for our FS-CRDS spectrometers. In the short-term, a measurement of the Allan deviation (a measure first developed to quantify the stability of atomic clocks) [32] can be utilized to ascertain the optimum averaging time and to identify the timescale of system drifts. In Figure 2a we have presented a typical Allan deviation plot [27]. In this plot it can be seen that system drift limits the optimum averaging time to 100s. For averaging times less than 100s we observe a roughly ergodic (n1/2) reduction in the noise level. This averaging reduces our minimum detectable absorption coefficient from 8  1010 to 4  1011 cm1 [27]. Low uncertainty in the FS-CRDS frequency axis allows for the long-term averaging of entire spectra with negligible instrumental broadening [27] (see Figure 2b). This type of averaging is effective in reducing the impact of slowly thermally-varying etalon features in the spectrum baseline. We have demonstrated the ability to average spectra for as long as 10 h and still observe a nearly ergodic reduction in the baseline noise level [27]. This long-term averaging reduced our minimum detectable absorption coefficient to 1.8  1011 cm1 and allowed us to quantitatively measure ultraweak electric quadrupole transitions with intensities as low as 3  1030 cm molec.1, twelve orders of magnitude weaker than that of a typical mid-infrared vibrational transition [33]. 4. Measurements Over the past 7 years, we have utilized FS-CRDS to perform a variety of high precision measurements of near-infrared molecular transitions. These studies have each exploited the stable frequency axis and ultrahigh detection sensitivity that FS-CRDS provides. 4.1. Stable frequency axis The hallmark of FS-CRDS is its stable, linear, and precise frequency axis in which the entire comb of cavity resonances is actively stabilized to an external optical frequency reference. We have utilized this capability for ultraprecise measurements of spectroscopic line shape parameters for O2 [23–31,34–37], CO2 [6,38], and H2O [21,39–44] near-infrared transitions. These measurements have produced parameters with relative uncertainties well below 1% and revealed the presence of higher order line shape effects such as Dicke narrowing [6,21,23–26,28,30,31,35,38,40–44], speed dependence [6,21,26,35,41,43,44], and line mixing [6]. Figure 3 illustrates a few examples of recent FS-CRDS studies which have exploited the stable frequency axis including measurements of absolute transition frequencies [25], unresolved hyperfine structure [29], and extremely high SNR line shapes (up to 220 000:1) [35]. Absolute transition frequencies were measured by linking the comb of stabilized cavity resonances to the hyperfine transitions of 39K, an absolute frequency reference, yielding uncertainties less than 1 MHz (3  105 cm1) [25]. We have also utilized the stable frequency axis of FS-CRDS for precision measurements of pressure shifting parameters [23]. In many of these studies we found that the uncertainty in the spectrum frequency axis is limited by the stability of our frequency reference laser [25,27,29]. In the New Technologies section we will discuss enhancements to our FS-CRDS spectrometers that will significantly reduce the overall frequency uncertainty. 4.2. Ultrahigh detection sensitivity The detection sensitivity of FS-CRDS, as with all CRDS techniques, arises primarily from the use of a high finesse optical

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Figure 4. Examples of recent measurements which have exploited the ultrahigh sensitivity of FS-CRDS. (a) FS-CRDS spectrum and Galatry fit of the P51P51 16O2 A-band magnetic dipole transition at 12.5 kPa [28]. This ultraweak transition has an intensity of only 1.10(31)  1030 cm molec.1 at 296 K [28]. The shown spectrum is an average of 50 individual spectra. (b) FS-CRDS spectrum and Voigt fit of the TS(5) 16O2 A-band electric quadrupole transition at 12.7 kPa [27]. This ultraweak transition has an intensity of only 2.05(9)  1029 cm molec.1 [27]. The shown spectrum has a signal-to-noise ratio of 50:1 and is the average of 10 individual spectra. (c) FS-CRDS spectrum of the H2O transition located at 7181.156 cm1 at a H2O mole fraction of 4  109 and 46.5 kPa [42]. In this spectral region the minimum detectable H2O vapor mole fraction is 350  1012.

Figure 5. FS-CRDS spectrum of three blended H2O transitions centered at 7022.720 cm1 at a pressure of 0.09 Pa. The insets show Lamb dips which can be utilized to produce precision measurements of transition frequencies and therefore, deconvolve blended spectra. The three blended transitions are located at 7022.72122, 7022.70910, 7022.72849 cm1 with intensities of 6.253  1022, 4.639  1022, 1.452  1022 cm molec.1, respectively [33]. The relative positions measured through the use of Lamb dips show large deviations (30 MHz, 0.001 cm1) from those found in the HITRAN 2008 database [33].

cavity, which results in a long effective optical path length. Furthermore, FS-CRDS achieves ultrahigh detection sensitivity through single-mode operation and special attention to system stability that enables long-term spectral averaging. As a result we have achieved a minimum detectable absorption coefficient as low as 1.8  1011 cm1, corresponding to a line intensity of 2.5  1031 cm molec.1 [27]. As shown in Figure 4, this detection sensitivity enabled quantitative measurements (uncertainties as

low as 5%) of spectral parameters for ultraweak features including electric quadrupole transitions [27] and high-J magnetic dipole transitions (up to J = 51) [28] in the O2 A-band, and doubly-substituted isotopologues of CO2 at natural isotopic abundance [38]. This sensitivity has also allowed for precision measurements of air-broadened CO2 line shape parameters at near atmospheric concentrations [6], unlike less sensitive techniques, such as Fourier-transform spectroscopy, which rely upon extrapolations

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Table IB Expected component standard (1r) uncertainties in the absolute frequency axis for the OFCS-CRDS spectrometer (which we are presently constructing) whereby frequencies are referenced to a self-referenced optical frequency comb. This frequency comb will also be utilized in the length stabilization servo. A Pound– Drever–Hall high bandwidth servo [48] will be utilized to lock the probe laser to the cavity resonances. This servo will narrow the diode laser line width to a value commensurate with the width of the cavity resonances. Note that the expected combined uncertainty in this case is 60 times smaller than was demonstrated with the original FS-CRDS instrument. Quantity

Uncertainty (kHz)

Stability of length stabilization servo Full width at half maximum of the cavity resonance Stability of the self-referenced optical frequency comb (1 s) Combined uncertainty

10 1.3 0.1 10.1

of measurements made at mixing ratios of several percent [45]. Further, FS-CRDS has been applied to measurements of the vapor pressure of ice at temperature down to 100 °C [46], demonstrating a minimum detectable H2O vapor mole fraction of 350  1012 [42]. 5. New technologies The single-mode operation and stabilized cavity length in FSCRDS provide exceptional precision for the spectral y- and x-axes, respectively. We are presently implementing significant system improvements to our FS-CRDS spectrometers that should provide orders of magnitude improvement in both the absorption and frequency axes. In our original FS-CRDS implementation the probe beam was locked to a TEM00 cavity resonance using a low bandwidth (300 Hz) transmission locking procedure [20]. While this approach was effective in achieving single-mode operation, ring-down signals were generated randomly [47], thus greatly reducing the transmitted signal amplitude, measurement duty cycle and acquisition rate. We recently implemented a 2 MHz bandwidth Pound– Drever–Hall (PDH) lock [48] which increases the data acquisition rate 500-fold [36]. We have demonstrated SNRs as high as 220 000:1 for weak O2 magnetic dipole transitions using PDHlocked FS-CRDS with an optical cavity of moderate finesse (F  12 000) [35]. Long-term averaging of spectra was possible due to active offset correction of the PDH-error signal [49]. As noted above, the uncertainty in the FS-CRDS spectral frequency axis is dominated by uncertainty in the optical frequency reference. We have recently acquired a two-octave spanning, self-referenced optical frequency comb (OFC) which offers relative frequency stability of 5  1013 (1s). We are incorporating the OFC into a FS-CRDS spectrometer to create the first OFC-stabilized cavity ring-down spectrometer (OFCS-CRDS). In addition, the OFC will provide an absolute frequency measurement at each point within the recorded spectrum, thus, producing a metrology-level frequency axis. As can be seen in Table IB, the OFCS-CRDS spectrometer is expected to have a combined standard (1r) uncertainty in the absolute frequency axis of 10 kHz (3  107 cm1). This uncertainty is dominated by the stability of the cavity length stabilization servo, whose bandwidth is limited by the response of the piezoelectric transducer to which one of the cavity mirrors is mounted. Nevertheless, the uncertainty in the OFCS-CRDS frequency axis is expected to be 60 times lower than was achieved with our earlier FS-CRDS instrument which was referenced to 39K [25] (see Table IA). In many respects, OFCS-CRDS is complimentary to direct frequency comb spectroscopy (DFCS) (see Refs. [50–52] for recent reviews). While DFCS offers multiplexed detection, its sensitivity is orders of magnitude less than can be commonly

achieved with FS-CRDS (due to either intensity fluctuations or the low optical power of a given comb tooth). 6. Future science OFCS-CRDS is expected to offer an ultrastable and linear frequency axis, SNRs > 200 000:1, and frequency uncertainties near 10 kHz. These instrumental attributes are well suited to a wide variety of applications which require low uncertainties (e.g., tests of the symmetrization postulate [53] and measurements of variations in fundamental constants [54]). We highlight a few applications that we find particularly exciting. The precision and accuracy of FS-CRDS could enable future partial pressure (i.e., concentration) standards. For example, longterm monitoring, reporting, and verification of greenhouse gases requires a primary standard based upon the International System of Units (SI). Currently, this link is achieved through prepared gas cylinders that are used to calibrate field sensors, an expensive, cumbersome, and opaque process. The stability and metrology-level quality of FS-CRDS potentially offers a path to direct traceability to the SI [55]. Given the high coupling efficiency and intracavity power levels of PDH-locked CRDS, the described instrument can be applied to sub-Doppler molecular eigenstate spectroscopy. The measurement of Lamb dips can allow for metrology-level measurements of transition frequencies and be utilized to deconvolve blended spectra. Figure 5 shows Lamb dips recorded with FS-CRDS for three blended water transitions. The measured transition frequencies can then be utilized to constrain the fitting of these three transitions, thus allowing them to be deconvolved. The addition of PDH-locking will allow for the measurement of Lamb dips for far weaker transitions. Application to near-infrared CH4 and H2O transitions would be particularly interesting, as these measurements could greatly aid in the assignments of these transitions, information which is particularly crucial to the study of planetary systems (e.g., [56–59]). The extremely high SNR spectra of OFCS-CRDS offers the promise of direct potential fitting [60] experiments, in which the scattering potential is directly determined from a multispectrum fit of high-resolution, high-SNR, absorption line shapes. This type of analysis moves beyond the approximations inherent in contemporary line profiles and has recently been applied to CO2 spectra in the mid-infrared [61–63]. Line shape effects such as speed dependence, collisional narrowing, and line mixing naturally result from the scattering dynamics and do not require empirical parameters or a priori assumptions. The use of absorption spectroscopy as a direct probe of the intermolecular potentials could open new frontiers in experimental chemical physics in much the same manner as the introduction of crossed molecular beams [64–66] and femtosecond laser spectroscopy [67,68]. Acknowledgements We would like to acknowledge all of those who have contributed to the development and application of FS-CRDS including: David J. Robichaud, Daniel K. Havey, Liwei Yuan, Jeremie B. Courtois, Roman Ciuryło, Katarzyna Bielska, Piotr Masłowski, Linda R. Brown, Laurence Y. Yeung, Gregory J. Rosasco, J. Patrick Looney, James R. Whetstone, Gregory E. Scace, W. Wyatt Miller, Howard P. Layer, Ryszard S. Trawin´ski, Szymon Wójtewicz, Jolanta Domysławska, and Mariusz Piwin´ski. We acknowledge continual support from the National Institute of Standards and Technology (NIST), Gaithersburg, MD, including the NIST Greenhouse Gas Measurements and Climate Research Program, which made much of the work described herein possible.

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D.A. Long et al. / Chemical Physics Letters 536 (2012) 1–8 David A. Long is presently a research scientist at the National Institute of Standards and Technology in Gaithersburg, MD. He earned his Ph.D. in chemistry from the California Institute of Technology in 2011. He was advised by Prof. Mitchio Okumura and supported by NSF and NDSEG graduate fellowships. He performed his undergraduate studies at Kenyon College where he graduated as valedictorian and summa cum laude. He is a member of the Orbiting Carbon Observatory (OCO-2) Science Team. His primary research interests are cavityenhanced spectroscopy, molecular line shape effects, greenhouse gas monitoring, and atmospheric remote

sensing.

Agata Cygan obtained her M.Sc. degree in experimental physics from Nicolaus Copernicus University (NCU) in 2008. She is now a fourth-year Ph.D. student at NCU under the supervision of Prof. Ryszard S. Trawin´ski. She was a visiting researcher at the National Institute of Standards and Technology, USA. Her research interests includes application of high-resolution cavity ringdown spectroscopy to molecular spectral line shape analysis.

Roger D. van Zee joined NIST in 1992 as a National Research Council Postdoctoral Fellow in the Molecular Physics Division. He later moved to the Process Measurements Division, where he worked on developing quantitative spectroscopic methods and instruments. In 2006, van Zee was made a Department of Commerce Science and Technology Fellow. Currently, he is a manager in the Material Measurement Laboratory at NIST.

Mitchio Okumura received his Ph.D. from the University of California at Berkeley under Yuan T. Lee in 1986. After postdoctoral research at The University of Chicago with Takeshi Oka, he joined the faculty in 1988 of the California Institute of Technology. He is currently Professor of Chemical Physics in the Chemistry and the Environmental Science and Engineering departments. His research interests are in the applications of high sensitivity laser techniques to laboratory studies of the kinetics and spectroscopy of atmospheric radicals relevant to air pollution chemistry, and to atmospheric spectroscopy relevant to remote sensing.

Charles E. Miller received a Ph.D. in Chemical Physics from the University of California, Berkeley. He is a Project Scientist with the Jet Propulsion Laboratory, California Institute of Technology. His research covers highresolution molecular spectroscopy, atmospheric photochemistry, and carbon cycle science, with an emphasis on developing new solutions for satellite remote sensing of greenhouse gases and megacity CO2 emissions. He is Principal Investigator of the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE), a member of the Orbiting Carbon Observatory (OCO-2) Science Team, a member of the Greenhouse Gases Observing Satellite (GOSAT) RA Science Team; he was Deputy Principal Investigator of the OCO mission.

Daniel Lisak received his doctorate from Nicolaus Copernicus University (NCU). He joined the group of Atomic, Molecular and Optical Physics at NCU. He was a visiting researcher at University of Naples, Italy and at the National Institute of Standards and Technology, USA. His main scientific interest is investigation of atomic and molecular spectral line shapes in the gas phase. Recently he is involved in high-resolution cavity ring-down spectroscopy experiments.

Joseph T. Hodges has worked as a research engineer at the National Institute of Standards and Technology in Gaithersburg, Maryland since 1991. His research interests include: laser-based combustion diagnostics, metrology to underpin low concentration humidity standards and cavity-enhanced laser spectroscopy of atmospheric gases such as water vapor, oxygen, carbon dioxide and methane. In the late 1990s, Dr. Hodges and coworkers pioneered cavity ring-down spectroscopy (CRDS) for quantitative measurements of weakly absorbing media.