A three-photon process using radiation at 2430 and 3660 A and with a 1s-2s two-photon resonance is used to excite atomic hydrogen from the ground state to Rydberg states of high principal quantum number. Collision-induced ionization is used to monitor the excitation.
Two-photon-resonant, three-photon ionization of atomic hydrogen and deuterium is performed at 243 nm, and the ions are detected by time-of-flight mass spectrometry. The power dependence of the 1S-2S two-photon excitation and one-photon ionization of 2S hydrogen and deuterium are determined. The ionization step saturates at 4 x 10(6) W/cm(2). The two-photon step-power dependence varies with laser linewidth, with saturation occurring between 1 x 10(7) and 3 x 10(7) W/cm(2). The extraction electric field in the mass spectrometer does not affect the 2S excitation. Strong signals are recorded at atomic concentrations of 10(5) atoms/cm(3). Nonresonant ionization of background gases producing H(+) and D(+) is minimized.
Tunable coherent ultraviolet radiation has been used to excite selectively the n=3 level in hydrogen and deuterium atoms via two-photon absorption from the ground state. The resulting Balmer-$\ensuremath{\alpha}$ fluorescence at 656 nm was observed as well as resonant three-photon ionization. This work demonstrates several advantages over other techniques for selective detection of neutral H and D atoms, including three-dimensional spatial resolution and a remote monitoring capability afforded by the use of laser-induced-fluorescence detection.
Tunable coherent ultraviolet radiation has been used to excite selectively the n=3 level in hydrogen and deuterium atoms via two-photon absorption from the ground state. The resulting Balmer-$\ensuremath{\alpha}$ fluorescence at 656 nm was observed as well as resonant three-photon ionization. This work demonstrates several advantages over other techniques for selective detection of neutral H and D atoms, including three-dimensional spatial resolution and a remote monitoring capability afforded by the use of laser-induced-fluorescence detection.
Rydberg states of atomic helium with principal quantum numbers ranging from n=20 to n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable 23S1 state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number $|{m}_{\ensuremath{\ell}}|=2$ were prepared in zero electric field and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg--Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-$|{m}_{\ensuremath{\ell}}|$ hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.
We have measured the resonance-enhanced four-photon ionization of atomic hydrogen. The degenerate four-photon process occurs via a three-photon resonance between the 1s and the 2p levels, with subsequent one-photon ionization near threshold. We have studied the highly asymmetric resonance-enhanced profile, i.e., the photoion yield as a function of laser detuning from three-photon resonance between the 1s and 2p levels. In particular, we have determined the width, shift, peak, and asymmetry of the profile as a function of laser intensity. The experimental results are compared to theoretical models. These models involve both the properties of the atom in an intense near-resonant radiative field, and a detailed model for the multimode laser field, particularly the field fluctuations due to mode beating. The asymmetric resonance-enhanced photoionization profile is well reproduced by both a random-phase and a chaotic-light model.