We describe collision studies on the two-photon excitation and fluorescence of the xenon 6p[1/2,0] excited state. Molecular nitrogen is the major collision partner. The 250-nm second harmonic of a line-narrowed Hansch-type dye laser excites the 6p state of xenon by two-photon absorption. Sample mixtures contain xenon at a partial pressure of 1 Torr and nitrogen at partial pressures of up to several hundred Torr. Time-resolved and time-integrated measurements of the intensity of the 828-nm 6p[1/2,0]—6s[3/2,1] allowed transition are obtained with photomultipliers, a transient digitizer, and a computerized data acquisition system. We extract pressure-dependent and pressure-independent components of the fluorescence lifetime and fluorescence yield from measurements over a range of nitrogen pressures. When the excitation power density is of the order of 104 W/cm2, the excited xenon atoms decay spontaneously. For spontaneous fluorescence, we obtain an inelastic (quenching) collision cross section of ~10–14 cm2 for xenon-nitrogen collisions, and a pressure-broadening coefficient of ~3 × 107 Hz/(Torr N2) for the two-photon transition rate. For excitation intensities above 2.6 × 106 W/cm2, the xenon atoms radiate cooperatively in a subnanosecond pulse. The fluorescence yield of this cooperative emission increases with nitrogen pressure rather than decreasing as in the spontaneous case. Mechanisms are discussed.
Excitation of xenon near the two-photon resonance between the ground state and the 6p(1/2,0) state produces forward conical emission at a wavelength close to the 828-nm 6p(1/2,0)—6s(3/ 2,1) excited-state transition. We describe experimental features of this process as excited by 600-ps pulses of 250-nm radiation from a frequency-doubled dye laser with intensity up to 1.3 × 108 W/ cm2 in xenon at pressures of 1-20 Torr. The conical emission occurs most prominently when the driving field is a few gigahertz above the frequency at which spontaneous emission from the excited 6p state peaks. The cone angle increases with xenon pressure in the 1-10-Torr range. The dependence appears consistent with the predictions based on noncollinearly phase-matched four-wave mixing. In this interpretation, the fourth wave would be near resonance with the 146.9-nm first resonance transition of xenon and detuned to the low-frequency side. The conical emission co-exists (and may compete with) an axial stimulated process which may be either two-photon driven amplified spontaneous emission or stimulated hyper-Raman scattering. The different tuning characteristics for the two processes make the extent of their competition unclear.
Forward- and backward-directed, stimulated emissions have been observed following two-photon pumping of the 5p states of krypton and the 6p', 7p, 8p, and 4f states of xenon. Multiphoton ionization spectra are also obtained and compared with the photon-detected excitation spectra. In krypton coherent emissions from the 5p states to the 5s are observed, and for xenon many p\ensuremath{\rightarrow}s, d\ensuremath{\rightarrow}p, and f\ensuremath{\rightarrow}d cascade emissions are observed. By analogy to the well-studied alkali-metal and alkaline-earth examples, the emissions are assigned as amplified spontaneous emission. Several intensity anomalies in the xenon p\ensuremath{\rightarrow}s emissions are not understood at present. Interference effects due to coherent cancellation between competing excitation pathways may be occurring.
The destruction rates of the xenon $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by collisions with ground-state xenon atoms have been measured by studying the temporal dependence of the number densities of the $6{s}^{\ensuremath{'}}$ and
6p atoms. The studies have been made both during the earlier afterglow of a xenon pulsed discharge and after the selective production of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms by a laser excitation in the late afterglow. An original optical pumping technique, using a tunable pulsed dye laser, was employed to populate the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ levels by excitation of the
6s[23]2 long-lived metastable atoms to $6{p}^{\ensuremath{'}}$ levels. The pressure dependence of the decay frequencies of these $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms was analyzed in the xenon pressure range of 0.04 to 1 torr and a kinetic model describing the evolution of the densities of these atoms after their production was established. The rate coefficients for the relaxation of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by two-body collisions involving a ground-state xenon atom are (8.45\ifmmode\pm\else\textpm\fi{}0.85) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}12}$
cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$ and (6.65 \ifmmode\pm\else\textpm\fi{} 1.00) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}11}$
cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. Results show that, in fact, at least 75% of this relaxation is a transfer of population to the
6p[21]1 level following ${\mathrm{Xe}}^{*}(6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1})+\mathrm{Xe}\ensuremath{\rightleftarrows}{\mathrm{Xe}}^{*}(6p{[\frac{1}{2}]}_{1})+\mathrm{Xe}\ensuremath{-}\ensuremath{\Delta}E$. These collisional transfer processes have been explained by means of Mulliken's xenon molecular potential curves.
The metastable atoms produced by the photodissociation of are detected by observing the enhanced fluorescence of the transition - in the buffer gas xenon. The photodissociation experiments are carried out under conditions of low-pressure (about 100 mtorr) in high-pressure Xe (20-200 torr). The fluorescence spectrum of ) in the wavelength region 510-560 nm is measured. A strong production of is found at wavelengths between 120 and 140 nm in the state, and a small production observed in the range 105-120 nm. Both spectra seem continuous. The Rydberg states in this wavelength region produce very little O(1S).
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.