Cavity ring-down spectroscopy measurements of sub-Doppler hyperfine structure

Abstract

Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) was used to measure magnetic dipole transitions in the b 1 + g ← X 3 − g (0,0) band of O 2 . The 17 O-containing isotopologues show unresolved hyperfine structure due to magnetic hyperfine splitting in the ground state. The sensitivity and stability of FS-CRDS allow for quantitative sub-Doppler measurements of the hyperfine constants, even when the hyperfine splittings are much smaller than the Doppler width. Unlike saturation spectroscopy, this linear absorption technique can be applied to weak transitions and employed to quantitatively measure intensities and line shapes. This method may be an attractive approach for measuring unresolved hyperfine structure in excited electronic states. Recent advances in near-infrared spectroscopy have enabled measurements with frequency precision previously only possible in the microwave regime [1][2][3]. Unfortunately, large near-infrared Doppler widths often obscure any hyperfine structure. While saturation spectroscopy can resolve individual hyperfine components [2,4], it is by its very nature a nonlinear technique. As a result, saturation spectroscopy leads to distortion of the spectral line shape and cannot be employed to quantitatively measure line-shape parameters [5][6][7]. In addition, saturation spectroscopy is less general than traditional absorption spectroscopy and cannot easily be applied to the study of weak transitions, such as those described herein.Here we describe an alternate approach to the measurement of hyperfine structure in the near-infrared region: the use of a linear and highly sensitive cavity ring-down spectrometer. In particular, the hyperfine splitting of the 17 O-containing isotopologues of O 2 were measured for magnetic dipole transitions in the b 1 + g ← X 3 − g (0,0) band. While individual hyperfine transitions are unresolved, the spectra exhibit a sufficiently large signal-to-noise ratio to enable quantitative measurements of hyperfine coupling constants. We note that despite the large number of recent studies which have measured spectroscopic parameters of the b 1 + g ← X 3 − g (0,0) band for 16 O 2 and the rare isotopologues [9], observation of hyperfine splitting has not previously been reported.Measurements were made using the frequency-stabilized cavity ring-down spectrometer (FS-CRDS) located at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. The FS-CRDS system has been described in detail previously [10,11], and therefore only the most important details are given. FS-CRDS differs from other single-mode cw-CRDS spectroscopies by actively stabilizing the intracavity length to an external frequency reference. This length stabilization in turn stabilizes the cavity's free spectral range (FSR). The probe laser frequency is then stepped between successive TEM 00 cavity modes, resulting in an accurate, stable, and linear frequency axis. FS-CRDS has been utilized to measure transition frequencies to better than 0.5 MHz [1] and line intensities to bet...

The air-broadened, near-infrared CO2 line shape in the spectrally isolated regime: Evidence of simultaneous Dicke narrowing and speed dependence

“…In addition, FS-CRDS is a single-mode technique that enables the accurate measurement of sample absorption coefficient in terms of observed ring-down cavity decay time constants. FS-CRDS has been employed to measure ultra-weak transitions, 39,40 hyperfine structure 37 and rare isotopes, 42,43,49 and to produce reference standard spectroscopic data. 36,38,42,43,50 For the measurements described herein the probe laser was an external cavity diode laser with an output power of ∼15 mW and the reference laser was a frequency-stabilized HeNe laser with a long-term stability of 1 MHz (8 h).…”

“…As previously demonstrated, the sensitivity and spectral fidelity of FS-CRDS make it an ideal technique for detailed study of subtle line shape effects. [36][37][38][39][40][41][42][43] Furthermore, the use of multi-spectrum fitting, [44][45][46] which simultaneously fits spectra across a wide pressure range, can enable the collisional narrowing and speed-dependent effects to be separated and quantified. Below, we present FS-CRDS line shape measurements of an air-broadened near-infrared CO 2 transition near 6360 cm −1 , and we compare our measured Lorentzian broadening parameters to literature values.…”

Hyperfine structure of the transitions of 16O17O, 17O18O and 17O2 by CRDS at 80K

“…The observation of the ground state hyperfine structure by absorption spectroscopy in the near infrared or visible is particularly difficult as the Doppler width is significantly larger than the hf splitting. Very recently, Long et al [14] could measure the ground hyperfine structure from an excess of the broadening of some transitions of the b 1 + þ g À X 3 + À g band near 760 nm. The sub Doppler hyperfine structure could be evidenced from a detailed study of the line profile recorded by frequency-stabilized CRDS [14].…”

“…Very recently, Long et al [14] could measure the ground hyperfine structure from an excess of the broadening of some transitions of the b 1 + þ g À X 3 + À g band near 760 nm. The sub Doppler hyperfine structure could be evidenced from a detailed study of the line profile recorded by frequency-stabilized CRDS [14]. It is important to note that the evidenced extra broadening of the b 1 + þ g À X 3 + À g band transitions is due to the ground state hf structure as the b 1 + þ g upper electronic state is a singlet R state with negligible hf structure.…”

“…The global rovibrational analysis including the complete line list, description of the assignment procedure and derivation of the spectroscopic parameters from a global fit will be published in a separate contribution. In the present Letter, we limit the discussion to the hf structure of the 17 [5,[13][14][15][16]. The observation of the ground state hyperfine structure by absorption spectroscopy in the near infrared or visible is particularly difficult as the Doppler width is significantly larger than the hf splitting.…”