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Conference on Lasers and Electro-Optics

Ground-state neutral atomic beam generated by pulsed CO2 laser irradiation

Haibo Meng, M. R. Wang

Published Jan 1, 1982• 0 citations
Publisher

Abstract

Al and ln cylindrical targets were irradiated by a homemade UV excited CO2 laser beam. A ground-state populated neutral atomic beam was generated, and the time profile and absolute densities at different positions from the target surface were measured by laser resonance fluorescence. Compared with atomic beams which are generated by ordinary methods such as oven or gasdynamic, this atomic beam has the following properties:

DOI
10.1364/cleo.1982.fk6

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The Journal of Chemical Physics•Feb 4, 2013•21 citations
Al and ln cylindrical targets were irradiated by a homemade UV excited CO2 laser beam. A ground-state populated neutral atomic beam was generated, and the time profile and absolute densities at different positions from the target surface were measured by laser resonance fluorescence. Compared with atomic beams which are generated by ordinary methods such as oven or gasdynamic, this atomic beam has the following properties:
Our work describes two most common methods of generating neutral atoms needed for experiments with trapped and cooled ions. The easiest but not most effective way of the neutral atoms generating is to use a heated oven. A better method allowing a fine control of generating atoms is to use a pulsed laser ablation. A vacuum chamber with Ca atoms source and necessary optics was assembled. Detection of generated particles was achieved by laser-induced fluorescence and sensitive camera. The paper consists of measurement of both method of generating the atoms.
The pulsed Li atomic beam produced in our experiment is based on controlled transversely-excited-atmospheric CO2{\mathrm{CO}}_{2}CO2​ laser--induced ablation of a Li metal target. The atomic beam is propagated in vacuum or in CO2{\mathrm{CO}}_{2}CO2​ gas at low pressure. Atoms in the beam are probed by laser-induced fluorescence spectroscopy. This allows the determination of time-of-flight and velocity distributions. Li Rydberg states (n=5--13) are populated in the beam by two-step pulsed-laser excitation. The excited atoms interact with CO2{\mathrm{CO}}_{2}CO2​ molecules. l- and n-changing cross sections are deduced from the time evolution of the resonant or collision-induced fluorescence following this selective excitation. l-changing cross sections of the order of 104{10}^{4}104 A˚2{\r{}}^{2}˚2 are measured; they increase with n as opposed to the plateau observed for Li∗{\mathrm{Li}}^{\mathrm{*}}Li∗ colliding with a diatomic molecule. This behavior is qualitatively well explained in the framework of the free-electron model. n\ensuremath{\rightarrow}n' changing processes with large cross sections (10--100 A˚2{\r{}}^{2}˚2) are also observed even in the case of large electronic energy change (\ensuremath{\Delta}Enn′{E}_{\mathrm{nn}\mathcal{'}}Enn′​>103{>10}^{3}>103 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). These results can be interpreted in terms of resonant-electronic to vibrational energy transfers between Li Rydberg states and CO2{\mathrm{CO}}_{2}CO2​ vibrational modes.
The method of atomic-resonance fluorescence has been applied to the detection of very small concentrations of radioactive 20Na atoms (<3 × 109 m−3). The 20Na isotope is produced by the reaction 20Ne(p, n)20Na by passing a 20 MeV proton beam through a neon gas target of density 3.5 × 1024 atoms/m3. A beam of a continuous-wave dye laser tuned to the D2 line of sodium is transmitted through the production region and the fluorescence light is detected by means of photon counting. A digital synchronous-detection technique has been applied to measure the time-dependent behavior of the 20Na atom density shortly after a proton irradiation. This behavior is determined both by radioactive decay and by diffusion of the atoms out of the production region. The absolute 20Na atom density has been estimated, using Rayleigh scattering of the laser beam on the neon gas for calibration of the optical system. The density of neutral 20Na atoms appeared to be an order of magnitude lower than the density produced. The detection method has also been used to measure the temperature dependence of saturated sodium-vapor density down to 3 × 1011 atoms/m3 (292 K).
By using Stark-induced adiabatic Raman passage (SARP) with partially overlapping nanosecond pump (532 nm) and Stokes (683 nm) laser pulses, 73% ± 6% of the initial ground vibrational state population of H(2) (v = 0, J = 0) is transferred to the single vibrationally excited eigenstate (v = 1, J = 0). In contrast to other Stark chirped Raman adiabatic passage techniques, SARP transfers population from the initial ground state to a vibrationally excited target state of the ground electronic surface without using an intermediate vibronic resonance within an upper electronic state. Parallel linearly polarized, co-propagating pump and Stokes laser pulses of respective durations 6 ns and 4.5 ns, are combined with a relative delay of ~4 ns before orthogonally intersecting the molecular beam of H(2). The pump and Stokes laser pulses have fluences of ~10 J/mm(2) and ~1 J/mm(2), respectively. The intense pump pulse generates the necessary sweeping of the Raman resonance frequency by ac (second-order) Stark shifting the rovibrational levels. As the frequency of the v = 0 → v = 1 Raman transition is swept through resonance in the presence of the strong pump and the weaker delayed Stokes pulses, the population of (v = 0, J = 0) is coherently transferred via an adiabatic passage to (v = 1, J = 0). A quantitative measure of the population transferred to the target state is obtained from the depletion of the ground-state population using 2 + 1 resonance enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The depletion is measured by comparing the REMPI signal of (v = 0, J = 0) at Raman resonance with that obtained when the Stokes pulse is detuned from the Stark-shifted Raman resonance. No depletion is observed with either the pump or the Stokes pulses alone, confirming that the measured depletion is indeed caused by the SARP-induced population transfer from the ground to the target state and not by the loss of molecules from photoionization or photodissociation. The two-photon resonant UV pulse used for REMPI detection is delayed by 20 ns with respect to the pump pulse to avoid the ac Stark shift originating from the pump and Stokes laser pulses. This experiment demonstrates the feasibility of preparing a large ensemble of isolated molecules in a preselected single quantum state without requiring an intermediate vibronic resonance.