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We have calculated ac polarizabilities of the 2 3 S and 2 1 S states of both 4 He and 3 He in the range 318 nm to 2.5 μm and determined the magic wavelengths at which these polarizabilities are equal for either isotope. The calculations, only based on available ab initio tables of level energies and Einstein A coefficients, do not require advanced theoretical techniques. The polarizability contribution of the continuum is calculated using a simple extrapolation beyond the ionization limit, yet the results agree to better than 1% with such advanced techniques. Several promising magic wavelengths are identified around 320 nm with sufficient accuracy to design an appropriate laser system. The extension of the calculations to 3 He is complicated due to the additional hyperfine structure, but we show that the magic wavelength candidates around 320 nm are predominantly shifted by the isotope shift.

Ionization of hydrogen atoms with principal quantum number n =32, 40, and 51-74^by a 9.92-GHz electric field F(/) = z/ocoscof was studied with a superimposed static electric field F s = 0, 2, 5, and 8 V/cm. The measured field strengths Fo(\0°/o) at which 10% of the atoms were ionized are in excellent agreement with classical calculations in both one and two spatial dimensions. Covering finer detail as well as gross structure of the n dependence of FoOOo/o), the agreement supports the application of classical dynamics to the analysis of this strongly perturbed quantum system.

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Eight transitions in the Eu I-spectrum connecting the ground state with configuration 4f 7 6S 2 with states of the configuration 4f 7 6s 6p were studied with high resolution laseratomic-beam spectroscopy. CW dye lasers operating in the wavelength regions 435--470nm and 560-630nm were used for this study. New data for the hyperfine structure in lS3Eu were obtained as well as new and more accurate values for the isotope shifts between lSlEu and 153Eu. The existing data for the hyperfine structure in ~SlEu were reproduced with an exception for the level z~'P>2 .

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We present the first experimental results of quenching (ionization) of H(«o-43-49) atoms by a linearly polarized microwave electric field consisting of two frequencies (7.58 and 11.89 GHz). Adding a second frequency increases the quenching probability over that for only one frequency, especially where there is local stability at scaled frequencies near certain rational fractions, e.g., nfico -z and j. Some, but not all, two-frequency quench curves display a rich structure that appears to be related to that observed in some single-frequency quench curves.PACS numbers: 32.80.Rm, 42.50.Tj This Letter addresses a basic problem-how a bound quantal system responds to driving forces strong enough to rip it apart. Laser multiphoton ionization of atoms and dissociation of molecules are two prominent examples. We report here the first results 1 for ionization of hydrogen atoms H(A*O) with principal quantum number no^l by microwave electric fields consisting of two frequencies. Both fields may be strong. Lacking a theory directly relevant to these first "two-frequency" (2f) ionization experiments, we focus on data that we can interpret in the light of previous "single-frequency" (If) experiments 2 " 8 and theory. 9 " 22 Though this limits our present understanding to cases where one driving field is strong and the other significantly weaker, we expect it to stimulate interest in theory that is challenged by driving forces that rival the Coulomb binding force and by the need to treat final continuum states.Quantal "Floquet" theory 23 is useful for one periodic driving force with constant amplitude. Recently developed for finite -level systems, "multimode Floquet" theory 23 treats (rationally related) polychromatic driving forces, but there is some theoretical controversy 24 about the (chaotic?) response of even simple model systems. It appears that theory is not well advanced when, e.g., a continuum is involved, the driving frequencies are not rationally related, or the driving amplitudes vary in time.Let us recall some previous If ionization results. In atomic units the binding energy of level no is l/2«(?. A scaled frequency 9 ca/no~3 =n$co is the microwave frequency measured in units of the classical Kepler frequency (approximately the average of the no^>\ to no±l transition frequencies). This Letter reports data for no values between 43 and 49 and two fixed frequencies, a>i/2/r=7.5816(6) GHz and <»2/2;r = l 1.8893(6) GHz. Taken together these mean n §a> < 0.25, or more than four photons for spanning the no to no ± 1 interval, and a much larger number (l/ln^/co^no/Kn^co) > 100 for reaching the free-atom continuum. A scaled field 9 F/nQ~4 =noF is the microwave electric field amplitude measured in units of the initial Coulomb binding field. Let noF(X%) be that scaled field producing an ionization probability P[ on =*X%.Previous 9.92-GHz If ionization data 5 " 8 for « 0 == 32-90 covered the range n$co =0.05-1.1. Curves of noF(X%) vs nfico exhibited alternating local maxima (local stability) and minima (local instability)...

We present experimental results for the ionisation of excited hydrogen atoms, with initial principal quantum number no in the range 32 5 no 5 48, by low frequency microwaves in the scaled frequency range 0.05 < R < 0.2. We compare these with results of a classical Monte Carlo calculation showing that there is structure in the experimental ionisation curves of quantal origin. A further comparison with a simple one-dimensional quantal theory, using an adiabatic basis, shows that the observed structure can be explained in terms of resonances between a few adiabatic states. A very simple two-state model is shown to possess all the relevant features of the system. Further analysis of this onedimensional model shows that when the field frequency tends to zero the static field ionisation limit is approached through infinitely many such resonances. at R 2 I/p, of width O(e-P), p being a large integer.