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Journal of the Optical Society of America B

Ionization efficiency of Doppler-broadened atoms by transform-limited and broadband nanosecond pulses: one-photon resonant two-photon ionization of muoniums

Rakesh Mohan Das, Souvik Chatterjee, M. Iwasaki, Takashi Nakajima

Published May 27, 2015• 6 citations
Publisher

Abstract

We theoretically study the ionization efficiency of Doppler-broadened atoms through a one-photon resonant two-photon ionization process using nanosecond pump and ionizing laser pulses. Under the presence of significant (> tens of GHz) Doppler broadening, the bandwidth of the transform-limited nanosecond pump pulse is far smaller than the Doppler width, and the use of the broadband nanosecond pulse rather than the transform-limited nanosecond pulse seems to be much more efficient to pump and eventually ionize atoms. It turns out, however, that the transform-limited nanosecond pump pulse with sufficient high intensity can outperform the broadband nanosecond pump pulse in the high intensity regime for the ionizing pulse, at which the other broadening mechanisms because of the strong pump and fast ionization play more important roles than the bandwidth of the pump pulse. Using a set of density matrix equations, we present specific numerical results for muoniums (μ+e−) with enormous Doppler widths (80 and 230 GHz), which support the above argument.

DOI
10.1364/josab.32.001237

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We theoretically study the ionization efficiency of Doppler-broadened atoms through a one-photon resonant two-photon ionization process using nanosecond pump and ionizing laser pulses. Under the presence of significant (> tens of GHz) Doppler broadening, the bandwidth of the transform-limited nanosecond pump pulse is far smaller than the Doppler width, and the use of the broadband nanosecond pulse rather than the transform-limited nanosecond pulse seems to be much more efficient to pump and eventually ionize atoms. It turns out, however, that the transform-limited nanosecond pump pulse with sufficient high intensity can outperform the broadband nanosecond pump pulse in the high intensity regime for the ionizing pulse, at which the other broadening mechanisms because of the strong pump and fast ionization play more important roles than the bandwidth of the pump pulse. Using a set of density matrix equations, we present specific numerical results for muoniums (μ+e−) with enormous Doppler widths (80 and 230 GHz), which support the above argument.
We study the spin polarization of Doppler-broadened atoms by broadband nanosecond (ns) pulses, and compare the polarization efficiency with that by the transform-limited picosecond (ps) pulses that have the same spectral bandwidth. Specific calculations are performed for the case of muonium with a set of density matrix equations and with rate equations using the uncoupled basis states. We find that the polarization efficiency with the broadband ns laser pulses is rather good, although it is not as good as that with the transform-limited ps pulses. Our results imply that, depending on the available laser system, we can use either broadband ns or transform-limited ps laser pulses to polarize almost any Doppler-broadened atoms.
Photoexcitation and photoionization of atoms, the central part of atom vapor laser isotope separation (AVLIS), relate to the ionization yield and isotope selectivity directly. Doppler broadening of absorption lines is one of the parameters that influence the photoexcitation and photoionization process of atoms. When evaporation temperature is high or beam equipment is absent, Doppler broadening has obvious influence on the ionization yield because most of atoms are non-resonantly excited. In this paper, we study the influences of Doppler broadening of absorption lines on a multi-step photoexcitation and photoionization process of atoms according to the facts of AVLIS. A Doppler-broadened three-level atom system with two resonant lasers is investigated. The interaction between laser field and atoms is described by a density matrix, which is calculated by fourth-order Runge-Kutta numerical method with variable steps. The results show that the ionization yield of atoms decreases with the increase of Doppler broadening of absorption lines under the same laser parameters. At a constant laser power, the ionization yield reaches its maximum value at the best laser bandwidths, which is different from that obtained without Doppler broadening, as reported in published papers. Meanwhile, the best laser bandwidth increases with the increase of Rabi frequency and Doppler broadening when other parameters are constant. Moreover, the best bandwidth of the second laser is smaller than that of the first laser in a multi-step photoionization process of atoms. Therefore, lasers should work at the best bandwidths in AVLIS for highest ionization yield. It is advantageous to make laser bandwidths slightly bigger than the best bandwidths technically for smaller fluctuation of ionization yield, owing to incoercible stochastic volatility in laser bandwidths. The ionization yield increases with the decrease of Doppler broadening, especially at the best laser bandwidths. Therefore, it is necessary to reduce Doppler broadening of atom vapor in laser ionization zone.
This paper reports the four-photon ionization of caesium atoms when the laser frequency is tuned through the resonant three-photon transition 6S → 6F. This experiment was performed by using a tunable-wavelength bandwidth-limited subnanosecond laser pulse at 1.06 μm, in the 108-109 W.cm-2 laser intensity range. Pulse widths of 1.5 ns, 50 ps, and 15 ps were used. The resonant character of the multiphoton ionization process was observed, even with the shortest pulse of 15 ps. Nevertheless the influence of a temporal effect is demonstrated according to theoretical predictions. The resonance shift ΔE of the 6S → 6F transition energy was found to be linear with the laser intensity I within the range 108-109 W.cm-2. ΔE = α I, with α = 2 cm-1/GW.cm-2. This result confirms previous measurements performed with single-mode 35 ns laser pulses and is in very good agreement with calculated resonance shifts.
An investigation of the resonant interaction of the rubidium atoms with an intensity (10 kWcm-2 ≤ I ≤ 2 MWcm-2) and a wavelength close to that of the D1 and D2 transitions of the rubidium atom (5S1/2 → 5P3/2 or 5S1/2 → 5P1/2, λD1 = 780 nm, λD2 = 795 nm), which has passes through rubidium vapor with density (1011 - 1014 cm-3) been studied theoretically. The system of equations describing the processes of Collisional ionization and multiphoton ionization of rubidium vapour resonantly excited with nanosecond pulsed laser is solved. The dependence of the ion density on the laser intensity and the atomic density of rubidium are considered. The result of calculations revealed that, both quadratic ion density dependence on laser intensity and linear behaviour of the ion density versus rubidium density for 5S1/2 → 5P3/2 transition is due to photoioization process. In contrast, for 5S1/2 → 5P1/2 transition, the ion density dependence is nonlinear and indicates that the collisional processes play the major contribution in the total ionization. Also, the obtained results showed reasonable agreement with the experimentally measured values of the ion density dependence given by Bakhramov et al. In addition, the analysis of mutual competition between the different ionization processes considered for the ion yield as a function of both laser intensity and atoms density are also presented this work.
We carry out the theoretical study for the generation of vacuum-ultraviolet pulses with a Doppler-broadened gas utilizing high atomic coherence. It is essentially a difference-frequency generation scheme induced by the two-photon near-resonant pump and probe pulses, where the key point is to generate high atomic coherence between the ground and two-photon near-resonant states through a variant of stimulated Raman adiabatic passage with a time-dependent detuning. The advantage of our scheme is that the degree of coherence is sensitive to neither the exact amount and even sign of the detuning, nor the exact timing between the pump, auxiliary, and probe pulses. Hence our scheme is practically insensitive to Doppler broadening. As a specific example, we consider the generation of picosecond Lyman-α pulses with a Kr gas, and quantitatively study the influence of Doppler broadening as well as the intensity and incident timing of the picosecond probe pulse with respect to the pump pulse. The numerical results indicate that our scheme has a certain advantage over the conventional scheme which utilizes two-photon resonant excitation.