Albert M. F. Lau

Nonlinear Gaussian expansions of stochastic processes: super-Gaussian effects in coherent anti-Stokes Raman spectroscopy

We propose a new approach to the analysis of stochastic processes (such as fluctuating laser fields) with super-Gaussian statistics: the expansion of stochastic processes in terms of first and higher powers of Gaussian components, which are used like a basis set. This approach is applied to treat the super-Gaussian correlation effects observed in coherent anti-Stokes Raman scattering experiments using frequency-doubled pump laser fields. Our results give much better agreement with experimental data than previous theories do.

Molecular transition moment determination by Autler-Townes spectroscopy: D 1Πu-E,F 1Σg+ bands of H2

Abstract We investigate theoretically the determination of vibronic transition moments between two electronic states of a molecule by using the Autler-Townes effect (also known as ac, optical or dynamical Stark splitting) in a double optical resonance experiment. We calculate the Autler-Townes spectra of some representative bands of H2(D 1Πu-E,F 1Σg+) and examine their dependences on laser detuning, intensity, bandwidth, and the Doppler effect. As input to our calculation, the D-E,F electronic transition moment and the (ν′=0–15, ν=3, 6 and 7) vibronic transition moments have been computed. Our investigation shows that with moderate laser requirement, the Autler-Townes splitting of a large number of D-E,F vibronic transitions in H2 should be observable so that their transition moments may be deduced experimentally.

Field statistical effects on Autler-Townes spectroscopy of molecules using phase-diffusion lasers

Abstract Laser-field correlation effects on the Autler-Townes spectroscopy of the D 1Πu(ν′= 2, J′ = 2)-E,F 1Σg+ (ν = 6, J= 1) transition in H2 are examined theoretically using the phase-diffusion model for the laser-field fluctuations. Increase in effective laser band-width due to correlation effects in the two-photon E,F(ν = 6, J= 1)-X(ν″ = 0, J= 1) excitation step reduces some spectral contrast of the Autler-Townes doublet, but it does not modify the doublet peak separation in the strong saturated regime. The peak separation may be used as the main optimization condition in the fitting procedure to deduce transition moments.

Measurement of transition moments between molecular excited electronic states using the Autler‐Townes effect

This paper reports the observation of the Autler‐Townes splitting in the two‐color four‐photon ionization of hydrogen molecules, and the good agreement of theory with experiment. From the theoretical fits to the spectral profiles, the value of the electric dipole transition moment between the excited vibronic states E,F1Σ+g, v=6 and D1Πu, v’=2 is determined to be 2.0±0.5 a.u.

The Autler‐Townes effect in hydrogen at low pressure

The Autler‐Townes effect (also known as ac, optical or dynamical Stark splitting) on the Balmer‐α transition of hydrogen in 0.5–5 torr of helium is investigated. In the double optical resonance scheme, H(3p) is excited by absorption of two 243‐nm photons and a 656‐nm photon via the 2s resonant intermediate level. Calculated results show that in a Doppler‐free or a ‘‘Doppler‐reduced’’ beam configuration, the required intensity of the 656 nm laser (0.2 cm−1bandwidth) for observing the Autler‐Townes doublet has to be greater than 2×104 W/cm2 and 2×105 W/cm2 for a 243‐nm laser bandwidth of 0.1 cm−1 and 0.5 cm−1, respectively.

Nonlinear Gaussian expansions of stochastic processes: Non‐Gaussian effects in coherent anti‐stokes Raman spectroscopy

We propose a new approach to the analysis of stochastic processes (such as fluctuating laser fields) with super-Gaussian statistics: the expansion of stochastic processes in terms of first and higher powers of Gaussian components, which are used like a basis set. This approach is applied to treat the super-Gaussian correlation effects observed in coherent anti-Stokes Raman scattering experiments using frequency-doubled pump laser fields. Our results give much better agreement with experimental data than previous theories do.

Theory of molecular transition moment determination by Autler–Townes spectroscopy: D 1Πu−E,F1Σ+g bands of H2

We investigate the determination of vibronic transition moments between two electronic states of a molecule by the Autler–townes effect (also known as ac, optical or dynamical Stark splitting). For some representative bands of H2(D 1Πu−E,F 1Σ+@ig0, we calculate their Autler–Townes spectra and examine their dependences on laser detuning, intensity, bandwidth, and the Doppler effect. As input to our calculation, the D−E,F electronic transition moment and the (v’=0–14, v=1,4 and 6) vibronic transition moments have been computed. Our investigation shows that with moderte laser requirement, the Autler–Townes splitting of a large number of D−E,F vibronic transitions in H2 should be observable so that their transition moments may be deduced.

Analysis of Super-Gaussian Statistics of N-Harmonic Fields by Nonlinear Gaussian Expansions

In this paper, we propose a new approach to the analysis of stochastic processes (such as fluctuating laser fields) with super-Gaussian statistics: the expansion of stochastic processes in terms of first and higher powers of Gaussian components, which are used like a basis set. We apply this approach to treat the super-Gaussian correlation effects observed in coherent anti-Stokes Raman scattering experiments using frequency-doubled pump-laser fields. Our results give much better agreement with experimental data than previous theories do.

Double optical resonance at high pressure: collisional broadening and Doppler effects on ac Stark splitting

Abstract The article presents the density matrix theory describing a three-level system in single- or multi-photon resonant interaction with two finite-bandwidth lasers and subject to high pressure. Under the steady-state and the weak-probe approximations, analytic solutions for the excitation spectra and the conditions for resolving the ac Stark splitting (also known as the Autler-Townes effect) are given. Calculations show that the resolution of the ac Stark doublet, observed in the two-color, three-photon 1s→3p excitation spectra of atomic hydrogen produced in H 2 /air atmospheric-pressure flames, is greatly improved by eliminating the Doppler contribution of the 1s→2s two-photon transition. It is found that as pressure is increased, the asymmetry parameter of the doublet decreases; and at high enough pressure (e.g. several atm), the doublet becomes one broad resonance.