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Lasers, Sources, and Related Photonic Devices

High-spectral-resolution Two-photon Pump Polarization Spectroscopy Probe Technique for H-atom Detection

A.H. Bhuiyan, Aman Satija, S.V. Naik, Robert P. Lucht

Published Jan 1, 2012• 0 citations
Publisher

Abstract

We report the development of a two-photon pump (TPP) polarization spectroscopy probe (PSP) technique for detection of atomic Hydrogen using two single-mode laser sources. Significant effect of laser parameters on atomic transitions has been observed.
DOI
10.1364/lacsea.2012.lw5b.6

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Fluorescence-dip spectroscopy based on an optical double resonance is used to probe the Stark splitting in highly excited Rydberg states of atomic hydrogen. After Doppler-free two-photon excitation to n=3n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3n=3, fluorescence at Balmer $\ensuremath{\alpha}$ is observed. When the radiation of a second laser is tuned over the resonance between n=3n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3n=3 and a Rydberg state, the Stark splitting manifests itself as a series of dips in the Balmer- $\ensuremath{\alpha}$ fluorescence intensity. Rydberg states up to n=55n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}55n=55 can be identified. The minimum detectable electric field is 5V/cm5\mathrm{V}/\mathrm{cm}5V/cm. This is about an order of magnitude more sensitive than other laser-induced fluorescence techniques.
Two synchronised pulsed single-mode tunable laser systems are used to excite atomic hydrogen in an atomic beam. A vacuum ultraviolet (VUV) laser populates first the 2p 2 P 3/2 sublevel from the ground 1s 2 S 1/2 state. A second-step UV laser selects substates of the n=33 manifold in the presence of a weak electric field (23.6 V cm -1 ). Stark patterns of this manifold are recorded for different polarisation configurations of the exciting light beams. The n=44 substates are detected by the standard field ionisation technique. The measured relative intensities of the Stark components agree with the theoretical excitation probabilities in the two-step process, calculated to the first order of perturbation theory.
We investigate the excited-state dynamics of hydrogen (H) atoms in flames by using a femtosecond (fs) pump-probe scheme and measuring directional emission signals. An approximately 100-fs pump pulse at 205.1 nm excites H atoms through a two-photon process (n = 3 ←← n = 1), which is followed by detection of the forward emission signals induced by a broadband fs probe pulse near 656 nm. Above a certain threshold, we observe a quadratic dependence of the emission signal on the pump laser energy. Moreover, the linewidth of the forward emission signal varies with the probe delay and the probe laser energy. This behavior can be explained in terms of superradiance. We perform a theoretical analysis and compare the experimental results with the theory, and conclude that, within the conditions of our experiment, the behavior of the atomic system involves atomic coherence, which is produced non-adiabatically and corresponds to a superradiant process. Variations in the duration of the gain time window and lifetime of the excited-state H atoms in flames are explored at different flame conditions (i.e., equivalence ratio and heights above the burner). The fs pump-probe technique demonstrated here can also be extended to characterize the time-resolved population dynamics and the corresponding collisional energy transfer rates for energy levels involved in laser-induced fluorescence detection of H atoms in flames relevant to practical combustion applications.