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AIP conference proceedings

Multiwave mixing and multiphoton ionization in strontium vapor

K. Böhmer, Jürgen Reif, Eckart Matthias

Published Jan 1, 1987• 0 citations
Publisher

Abstract

The coherent interaction of a two‐photon‐resonant, pulsed dye laser with strontium vapor leads to both multiphoton ionization and the emission of coherent light simultaneously at several other frequencies. For a laser intensity of about 4×108 W/cm2 and a vapor pressure around 103 Pa, these frequencies are all of the same order of intensity, about three orders weaker than the laser. Interferences of different channels are observed, which render the usual characterization of non‐linear optical processes rather ambiguous.

DOI
10.1063/1.36727

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•Jan 1, 2010•0 citations
The coherent interaction of a two‐photon‐resonant, pulsed dye laser with strontium vapor leads to both multiphoton ionization and the emission of coherent light simultaneously at several other frequencies. For a laser intensity of about 4×108 W/cm2 and a vapor pressure around 103 Pa, these frequencies are all of the same order of intensity, about three orders weaker than the laser. Interferences of different channels are observed, which render the usual characterization of non‐linear optical processes rather ambiguous.
Atomic strontium vapor in a thermionic diode is irradiated by the light from a tunable dye laser at two-photon resonance on the 5s5d1D2 state. At a laser intensity of ~ 108 W/cm2 and a vapor pressure of several torrs, not only three-photon ionization occurs but also the generation of new frequencies due to a nonlinear polarization is observed. Simultaneously, strong and strictly directed beams are generated which are resonantly enhanced by the 5s5p1P1 and 5s6p1P1 states, respectively. Close to the first resonance, four-wave mixing causes braodband two-photon emission, the bandwidth of which is determined by the region of negative dispersion due to phase matching. On the other hand the process via the 6p state is exactly resonant as can be shown by comparison in every case with the respective fluorescence light. At higher pressures (~ 10Torr), however, a second line appears, which is also shifted to the side of negative dispersion.
Two independently tunable, linearly polarized dye lasers have been used to study multiphoton ionization of gaseous xenon. One laser is tuned near three-photon resonance with the allowed 6s[3/2]J=1° resonance, which is not observed with one laser at these pressures because of the destructive interference between the three-photon-excitation and third-harmonic-excitation pathways to the 6s[3/2]J=1° state. The second laser is tuned to resonance with higher allowed np and np′ states of xenon. The missing ionization signal is seen to be dramatically restored by the addition of the second laser beam; however, the signal again disappears as the pressure is increased. These results are discussed in relation to recent experimental and theoretical studies of cancellation effects between three-photon excitation and third-harmonic generation. Spectroscopic information obtained from tuning each dye laser, together with data on the angular distributions resulting from rotating the plane of polarization of one laser relative to that of the second laser, clearly shows that two different ionization mechanisms are operative. In addition, these experiments unambiguously illustrate the importance of dissociative ionization of quasi-dimers of xenon over a rather broad frequency range.
The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.
An experiment has been performed to demonstrate a third-order nonlinear optical effect involving the ground state of the strontium ion. Evidence of strong and slightly tunable parametric generation from a doubly resonant four-wave mixing process is provided when a single-wavelength laser is used to two-photon ionize strontium vapor and simultaneously pump close to the5s rightarrow 6stwo-photon resonance of the ion. This result appears to conform with a semiclassical wave equation description, although there is evidence that a population rate equation analysis will also be necessary if the process is to be modeled more accurately.
Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.