When three-photon atomic excitation is produced by two laser beams of angular frequencies ${\mathrm{\ensuremath{\omega}}}_{\mathit{L}1}$ and ${\mathrm{\ensuremath{\omega}}}_{\mathit{L}2}$, which are crossed at an angle \ensuremath{\theta}, large cooperative pressure-dependent shifts can be produced in the peak position of the excitation line shape. Such shifts occur when the transition between states is dipole allowed in both one and three photons. When \ensuremath{\theta} or ${\mathrm{\ensuremath{\omega}}}_{\mathit{L}2}$/${\mathrm{\ensuremath{\omega}}}_{\mathit{L}1}$ is small, these shifts are very large compared to the pressure broadened width of the line. Our analysis shows that for small \ensuremath{\theta} or small ${\mathrm{\ensuremath{\omega}}}_{\mathit{L}2}$/${\mathrm{\ensuremath{\omega}}}_{\mathit{L}1}$, where the shift is large, the excitation is suppressed near the unperturbed resonance due to a destructive interference between the three-photon excitation by the laser and one-photon excitation due to the four-wave-mixing field, whereas the shifted resonance appears at the position where the atoms interact through ``resonant'' four-wave-mixing photons that are phase matched with the laser field. By studying an experimental situation in xenon we show that the theoretical predictions are in precise agreement with experiment for several angles and a wide range of pressures.
Partial cancellation of the ionization signal in the four-photon-resonant five-photon ionization via J=2 4f and 5f resonances in Xe has been observed at increased gas pressures. Experiments with one and two counterpropagating laser beams provide strong evidence that cancellation is due to destructive interference at the three-photon step between the coherent three- and one-photon excitation channels, the latter involving one vacuum ultraviolet photon of the third-harmonic generation that occurs above the 6s[3/2
]1o state. The well-known destructive interference for the ``on-resonance'' case is now shown to occur also for large detuning from the resonance (up to 2000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) probed by four-photon-resonant five-photon ionization. This type of cancellation may provide a new insight in interference effects involved in multiphoton-excitation processes.
Resonantly enhanced multiphoton ionization signals through the
6s[23]J=1 state of Xe rapidly disappear at pressures above 0.30 Torr; concurrently, intense, vacuum-ultra-violet third-harmonic radiation is detected in the forward direction. Laser excitation spectra of the vacuum ultraviolet light exhibit increasing blue shifts and bandwidths with increasing pressure. The observed competition between ionization and third-harmonic generation is discussed in terms of a model involving fast collective emission.
The nonresonant, three-photon ionization spectrum of atomic Xe in the region between the
Xe+ 23/2 and
21/2 fine-structure limits is reported. Using linearly polarized light, four autoionizing Rydberg series are observed converging to the
Xe+ 21/2 limit, corresponding to
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)ns'[(1/2)]\ifmmode^\circ\else\textdegree\fi{}
1, (2P\ifmmode^\circ\else\textdegree\fi{}
1/2)nd'[(3/2)]\ifmmode^\circ\else\textdegree\fi{}
1,
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)nd'[(5/2)]\ifmmode^\circ\else\textdegree\fi{}
3, and
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)ng'[(7/2)]\ifmmode^\circ\else\textdegree\fi{}
3; as expected, using circularly polarized light only the J=3 series are observed. These results are in contrast to earlier experimental work in which no resonant structure was observed. Photoelectron angular distributions are reported for the
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)9s'[(1/2)]\ifmmode^\circ\else\textdegree\fi{}
1 and the
(2P\ifmmode^\circ\else\textdegree\fi{}
1/2)7d'[(3/2)]\ifmmode^\circ\else\textdegree\fi{}
3 autoionizing resonances. The results are compared with the recent theoretical calculations by P. Gangopadhyay et al. [Phys. Rev. A 34, 2998 (1986)].
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.