Measurement of the sixth overtone band of nitric oxide, and its dipole moment function, using cavity-enhanced frequency modulation spectroscopy

Abstract

We applied cavity-enhanced frequency modulation absorption spectroscopy (also known as noise-immune cavity-enhanced optical heterodyne molecular spectroscopy) to perform high-resolution spectroscopy of the sixth overtone band of nitric oxide near 797 nm. By using novel high-bandwidth balanced detectors, baseline variations caused by residual amplitude modulation were significantly reduced. The ultrahigh sensitivity (2 x 10(-10) cm(-1) minimum detectable absorption at 1 Hz detection bandwidth) of our spectrometer allowed accurate measurements of 15 individual line intensities of P- and R-branch transitions in the 2Pi(1/2)-2Pi(1/2) subband. A vibrational transition moment of 3.09(6) muD+/-13% and Herman-Wallis coefficients of a = -0.0078(26) and b = 0.001 25(45) were found by fitting the line intensities. Based on our measured transition moment, and those of other transitions from the literature, a new parametrization for the electric dipole moment function (EDMF) of nitric oxide, valid for 0.91 < or = r < or = 1.74 A, has been extracted. The residuals of this fit demonstrate that the derived EDMF is the most accurate representation to date of the dipole moment function. The new EDMF will be valuable for accurate intensity prediction of transitions that cannot be easily measured experimentally.

Model for in-coupling of etalons into signal strengths extracted from spectral line shape fitting and methodology for predicting the optimum scanning range—demonstration of Doppler-broadened, noise-immune, cavity-enhanced optical heterodyne molecular spectroscopy down to 9 × 10^−14 cm^−1

“…In its Doppler-broadened (Db) mode of operation it has repeatedly demonstrated a spectral NEAL in the 10 −12 -10 −13 cm −1 Hz −1∕2 range [9][10][11]. This mode of operation has mostly been used for trace gas applications, e.g., for detection of species such as O 2 [12], N 2 O [13], NO [14,15], C 2 H 2 [10,16], and CH 4 [17,18]. The so far highest (best) detection sensitivity for Db detection is 4 × 10 −13 cm −1 Hz −1∕2 , which recently was achieved by the use of a well-documented system consisting of a high-finesse cavity and an Er-doped fiber laser [16].…”

Doppler broadened NICE-OHMS beyond the triplet formalism: assessment of optimum modulation index

“…However, often because of practical limitations, but sometimes also due to conceptional simplifications, it is often chosen or assumed to be well below unity, typically in the 0.3-0.5 range [1,2,[7][8][9]. There are basically two reasons for the latter: (1) it gives rise to a pure triplet (i.e., only one pair of sidebands) for which the modulation conditions agree with the original view of FMS [10,11], and (2) it still provides a reasonably large FM response.…”

Shot-noise-limited Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry

“…In this case, a well-stabilized fixed-frequency Nd:YAG laser was locked to a cavity with a finesse of 10 5 while addressing a sub-Doppler feature of C 2 HD at 1064 nm. Despite the fact that the technique, since then, has been constructed around a variety of tunable lasers in various configurations [5][6][7][8][9][10][11][12][13][14][15][16][17], no system that has provided shot-noise-limited conditions has been realized.…”