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ALT'23 The 30th International Conference on Advanced Laser Technologies
LD-I-6
Fast sensitive laser absorption spectroscopy
V.V. Lagunov, V.N. Ochkin, A.I. Volkova
P.N.Lebedev Physical Institute Russian Academy of Sciences 119991 Russia, Moscow, Leninskiy prosp., 53
ochkinvn@lebedev. ru
To achieve the high sensitivity of molecule densities measurements by means the optical absorption spectroscopy one needs to use the technique with fast frequency tuning and high resolution. The practice shows that for the real single-record low signal/noise ratios ~10-2, absorption coefficient k~10-5 cm-1, spectral resolution Rs~10-3 cm-1 and time resolution of At~1 s the frequency tuning rates have to be in the order of r~(105-107) cm-1s-1 for a free space single-pass schemes or r~(102-104) cm-1s-1 for multi-pass cavities [1]. Such kind of parameters can be implemented using conventional diode (DL) and/or quantum cascade (QCL) semiconductor lasers. But, as it was first shown in [2] in free space experiments with DL the limitations of fundamental nature appear. At such tuning rates it was observed that the non-harmonic oscillations disturb the classical spectral line profile and those were interpreted as a non-stationary coherent effect due to interference of the incident and induced radiation from matter. Much later (e.g. [3]) this effect was studied in more detail by many groups using fast-tuned QCL and it was related to those that were predicted [4] and observed [5] in radiofrequency spectral range for nuclear magnetic resonance NMR at variable magnetic field. Other spectra tuning rate dependent distortions of non-oscillatory type have been observed in recent experiments with high-quality optical cells and related to finite photon lifetime in intracell absorbing media [6].
Though the frequency tuning rate-dependent spectra distortions are now widely discussed the central question on the possibility to use the absorption as a quantitative technique is in a shadow. Some attempts [7] to measure the concentration of absorbing particles based on classic absorption losses in the fast frequency tuning mode failed. This situation was analyzed, and the main results are the following:
- in a single pass case the spectrum is not stationary since the recording time is comparable or shorter than the phase relaxation time of absorbing particle excitations. The non-perturbed stationary spectrum can be restored from the non-stationary one and the basic absorption laws can be applied for quantitative measurements within the processing scheme provided - (i) a single wing of the line profile is used in the frequency range corresponding to the onset of interaction between the probe radiation and the molecular resonance during the frequency tuning. This leads to the restoration of the convolution of static profile and instrumental function; (ii) to reconstruct the static Voigt profile after the deconvolution it is necessary to account for: a) Lorentz and Doppler broadening, b) the broadening by optical transition saturation, c) the broadening caused by finite time of light-matter interaction due to flight of particles across the laser beam.
- in case of measurements in high-quality external optical cells, the main mechanism of spectra distortion due to finite intracavity photon lifetime manifests itself at the frequency tuning times longer than the particles phase relaxation times. That is why even for strong spectra distortion the real line profile remains static and the problem of quantitative measurements is reduced to the exclusion of a single instrumental function in the experimental spectrum deconvolution procedure.
- if the integral version of absorption measurements is used, then for both of the abovementioned cases of tuning the spectra depending on the velocity the classic Kravetz integral relation is valid for the static component of the spectrum, which can be applied not only to real physical profile, but also to the experimentally observed one. It is shown that the result of measuring the particles density does not depend on the instrumental function form, so the profile deconvolution is not required.
This work was supported by a grant from the Russian Science Foundation (project No.19-12-00310, https://rscf.ru/en/project/19-12-00310/).
[1] V. Lagunov, V.Ochkin, A.Volkova Determination of particle concentrations from absorption spectra with fast frequency tuning. ALT-2022 conference abstracts, pp. 192-192 (2022).
[2] I. I. Zasavitskii, M. A. Kerimkulov, A. I. Nadezhdinskii,V. N. Ochkin, S. Yu. Savinov, M. V. Spiridonov,and A. P. Shotov, "Coherent nonstationary effects under quick recording of absorption-spectrum," Opt.Spectrosc. 65 (6), 706-709 (1988).
[3] G. Duxbury, et al., Rapid passage induced population transfer and coherences in the 8 micron spectrum of nitrous oxide, Molecular Physics, 105.57, 741-754, (2007).
[4] F. Bloch, "Nuclear induction," Phys. Rev. 70 (7-8), 460-474 (1946).
[5] N. Blombergen, E. Parcell, and R. Pound, "Relaxation effects in nuclear magnetic resonance absorption, "Phys. Rev. 73 (7), 679-712 (1948).
[6] V. V. Lagunov, I. V. Nikolaev, and V. N. Ochkin, "High-resolution and high-sensitivity absorption spectroscopy in external optical resonators with fast frequency tuning," Spectrochim. Acta, Part A 246, 119060 (2021).
[7] J. H. Van Helden, S. J. Horrocks, G. A. D. Ritchie, Application of quantum cascade lasers in studies of low-pressure plasmas: Characterization of rapid passage effects on density and temperature measurements, Applied Physics Letters, 92(8), 081506, (2008).
