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Annual Meeting Optical Society of America

Extreme pressure broadening of three-photon resonances in noble gases

W. R. Ferrell, M. G. Payne, W. R. Garrett

Published Jan 1, 1985• 1 citation
Publisher

Abstract

A pressure-broadening study of three-photon and one-photon resonances was performed using counterpropagating, linearly polarized laser beams. Self-broadening, as well as the collision-induced effects due to the presence of a second noble gas, was investigated for the 6s, 6s′, 5d and 7s of xenon and the 5s′ of krypton. A multiphoton ionization scheme in a calibrated proportional counter is the basis of a new technique by which collisional processes can be investigated for high pressure regimes which have previously been inaccessible to investigation, particularly for self-broadening where the mean free path of the photons is of the order of 10−3 cm for pressures as low as 1 Torr. Using this technique, we have recorded the collision-induced widths and shifts of the resonance for pressures up to 1000 Torr. In the hundreds of torr range, asymmetric, self-broadening line shapes were observed with full widths and shifts which were greater than five times the laser bandwidth (0.002-0.005 nm).

DOI
10.1364/oam.1985.tux2

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A pressure-broadening study of three-photon and one-photon resonances was performed using counterpropagating, linearly polarized laser beams. Self-broadening, as well as the collision-induced effects due to the presence of a second noble gas, was investigated for the 6s, 6s′, 5d and 7s of xenon and the 5s′ of krypton. A multiphoton ionization scheme in a calibrated proportional counter is the basis of a new technique by which collisional processes can be investigated for high pressure regimes which have previously been inaccessible to investigation, particularly for self-broadening where the mean free path of the photons is of the order of 10−3 cm for pressures as low as 1 Torr. Using this technique, we have recorded the collision-induced widths and shifts of the resonance for pressures up to 1000 Torr. In the hundreds of torr range, asymmetric, self-broadening line shapes were observed with full widths and shifts which were greater than five times the laser bandwidth (0.002-0.005 nm).
Three-photon excitation and multiphoton ionization techniques have been used to measure self-broadening for several resonance lines in xenon and krypton. Tabulations of widths and shifts of the xenon lines at 146.9, 129.6, 119.2, and 117.0 nm were made for pressures between 50 and 900 Torr. Similar measurements were made for the krypton resonance line at 116.5 nm. For pressures less than 500 Torr the widths were found to be in reasonable agreement with theoretical results, which were extrapolated into regions of concentration where the criteria for their validity are at best questionable. We have also found the lines to be shifted to the high-energy side of the unperturbed line. For four of the five lines studied, the blue shifts increased nearly linearly with increasing pressure. In the four cases where the shift could be measured the ratio of shift to width was close to (1/2. Our experimental technique is very simple, but it has yielded what we believe to be the first resonance broadening data based on absorption under conditions where there is strong trapping of the resonance radiation.
Three-photon excitation of a dipole-allowed atomic transition in rare gases is cancelled at an elevated gas pressure due to the well-known destructive interference of the three- and one-photon transition amplitudes in resonant atomic excitation. This cancellation effect is independent of laser light intensity, but is strongly modified in the presence of counterpropagating light. It has been shown that under three-photon resonant pumping at 1010 W cm−2 the multiphoton ionization through the cancelled 6s[3/2] J = 1 resonance of xenon can be initiated by counterpropagating light with an intensity as low as 103 W cm−2, which is by many orders of magnitude less than normally required for the onset of multiphoton excitation and ionization processes in rare gases. Some peculiarities of such triggered multiphoton ionization are discussed.
A competition between resonant multiphoton ionization (MPI) and third harmonic generation (THG) involving three-photon resonances was first reported by Miller et al.1 Recently, a similar competing process involving four-photon resonances was observed in the multiphoton ionization of Hg2. In this paper we report a similar observation in the three- and four-photon excitation spectra of Kr and Ar. Both the multiphoton ionization and the VUV radiation generated in the focal volume are measured over a broad range of gas pressures (0.01-100 Torr) and laser fluences. The 5s, 5f, and 6p′ levels of Kr and the 4s, 4p′, and 7p levels of Ar are investigated. The spectral lines are observed to broaden and to shift from line center as a function of increasing gas pressure and laser intensity. The effect observed in three-photon resonant lines is explained by the occurrence of a competition between MPI and THG.1 A similar effect observed in the four-photon resonances is interpreted as the result of THG enhanced by the four-photon levels—a five-photon process. However, it is not clear why the four-photon spectral line-width becomes narrower when the gas pressure is further increased; this is discussed.
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 normalized fluorescence spectrum of the Li 6708-\AA{} resonance line perturbed by the noble gases has been measured for 100 \AA{} either side of the resonance line, at a temperature of 400 \ifmmode\pm\else\textpm\fi{} 10\ifmmode^\circ\else\textdegree\fi{}C. The noble-gas pressure range of 100-1800 Torr was investigated. It was observed that the line wings scale linearly with this pressure for $2<|\ensuremath{\Delta}\ensuremath{\lambda}|<100$ \AA{}, and it was was demonstrated that a Lorentzianbroadened line core can be observed for this pressure range. The Li density was \ensuremath{\sim} 1010{10}^{10}1010/cm3{\mathrm{cm}}^{3}cm3 and was varied to establish the fluorescence spectrum in the limit of no radiation entrapment. The central region of the line has been measured with high resolution and compared to calculated profiles due to the various fine structure and isotopic line components, each broadened by a convolution of Lorentz, Doppler, and instrumental profiles. The shift and Lorentz broadening rates, as well as the wavelengths for transition to non-Lorentzian behavior, are reported. In contrast to the heavier alkali-metal-noble-gas cases, no distinctive far-wing features such as satellites or sharp inflections are observed. This is consistent with the much larger collision velocities for the Li case.